2. Metal Heat Treating
Topics of Presentation
⢠What Is Metal Heat Treating?
⢠Where Is It Used?
⢠Why and How It Is Done?
⢠What Processes & Equipment Are Used for
Heat Treating?
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3. What is Heat Treating ?
Controlled Heating And Cooling of Metal to Change Its Properties
and Performance.
Through:
⢠Change in Microstructure
⢠Change in Chemistry or Composition
Temperature
Holding
(soak)
Time
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4. A Few Heat Treating Facts
⢠Heat Treating of Metals Represents
Approximately 100 BCF Gas Load Nationwide.
⢠Heat Treaters Use Natural Gas to Supply
About 2/3 of the Energy Used for Heat
Treating (induction, vacuum & commercial
atmospheres main competition).
⢠Current Share of Gas Decisions is about 50 /
50 Between Gas & Electric.
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5. Why Use Heat Treating ?
In simple TermsâŚ.
â˘
Soften a Part That Is Too Hard.
â˘
Harden a Part That Is Not Hard Enough.
â˘
Put Hard Skin on Parts That Are Soft.
â˘
Make Good Magnets Out of Ordinary
Material.
â˘
Make Selective Property Changes Within
Parts.
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6. Who uses Heat Treating ?
⢠Aircraft Industry
⢠Automobile
Manufacturing
⢠Defense Sector
⢠Forging
⢠Foundry
⢠Heavy Machinery
Manufacturing
⢠Powder Metal Industries
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7. What Industrial Sectors Use Heat Treating
?
SIC
Industry
331
332
34
Steel Mills
Iron and Steel Foundries
Metal Fabrication
Machinery and
Electrical/Electronic Equipment
Transportation Equipment
Commercial Heat Treating
Steel Service Centers
35 & 36
37
3398
5051
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8. Types of Heat Treaters
⢠Commercial Heat Treaters
â Heat Treating of Parts As âJob-shopâ.
â Reported Under SIC Code 3398.
â Approx. 10% of All Heat Treating Production Is by Commercial Heat
Treaters.
â Usually There Are 4 to 5 Captive Heat Treaters for Each
Commercial Heat Treater Shop.
⢠Captive Heat Treaters
â Usually a Part of Large Manufacturing Business.
â They Usually Produce âProductsâ Rather Than Parts.
â Captive Heat Treating Is Scattered Through All Manufacturing SIC
Codes (DEO has over 100 individual SICâs for Heat Treaters).
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9. Commonly Heat Treated Metals
â˘
Ferrous Metals
â˘
Non-ferrous Metals
â
â
â
â
â
Steel
Cast Iron
Alloys
Stainless Steel
Tool Steel
â
â
â
â
Aluminum
Copper
Brass
Titanium
Steel Is the Primary Metal Being Heat Treated.
More Than 80% of Heat Treating Is Done for Steel
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12. Commonly Used Equipment
for Heat Treating Operations
â˘
â˘
Metal Cleaning (Wash-Rinse) Equipment
Gas fired furnaces
â
â
â
â
â˘
Direct fired using burners fired directly into a furnace
Indirect fired furnaces: radiant tube, muffle, retort etc.
Molten salt (or lead) bath
Fluidized bed
Electrically heated Furnaces
â Induction heating
â Electrical resistance heating
â Other (i.e. Laser, electron-beam etc.)
â˘
â˘
â˘
Quench or cooling equipment
Material handling system
Testing and quality control laboratory equipment
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13. Gas Fired Metal Heat Treating Furnaces
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14. Electrically Heated Equipment for Metal
Heating
Electric Atmosphere Furnace
Vacuum Furnace
Induction Equipment
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15. Types of Heat Treating Furnaces
Box
4560
Induction
Carbottom
Bell, Hood, tipup
100
150
150
Vertical Pit
60
Vacuum
13425
Box
100
7700
Salt Bath
Lead pot (cont.)
Rolelr hearth
Barre-roller
Cont. stripline
Cont. induction
200
Conveyor
4395
Car Bottom
6000
Pusher
Pusher
Fluidized bed
Salt-bath
3195
Conveyor
4890
Bell, hood, tip-up
50
330
5
55
130
4765
Vertical Pit
Rotary hearth, shaker hearth
Plasma
Induction
Laser
2510
Vacuum
Electric beam
Flame heads
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16. Heat Treating Furnaces
Two Primary Types
⢠Atmospheric
â Operated at ambient (atmosphere) pressure.
â Load is heated and cooled in presence of air or
special gases (process atmospheres), in liquid
baths or in a fluidized bed.
⢠Vacuum
â Operated at vacuum or sub-atmospheric
pressure.
â May involve high pressure gas cooling using
special gases.
â Includes ion or plasma processing equipment.
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17. Heat Source for Gas Fired Furnaces
⢠Direct Fired Burners *
⢠Radiant Tubes *
⢠Muffle or Retort Heated by
Outside Burners/Electrical
Elements
⢠Hot Oil or Steam Heating
Muffle
Burners
Direct fired muffle furnace
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* These could be directly exposed to the work
or can be outside a muffler a retort.
18. Nonferrous Heat Treating Furnaces
Types of Furnaces
Coil/foil Annealing Furnaces
Rod/wire Annealing Furnaces
Log Homogenizing Furnaces
Ingot Preheating Furnaces
Aging Furnaces
Indirect Heating
(Radiant Tubes or Electrical Resistance)
Temperature Range 350°F to 1150°F
Atmosphere With Dew Point Control
May Includes Water Quench or
Controlled Cooling
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19. Why Use Protective Atmospheres?
⢠To Prevent Oxidation, Loss of Carbon
(Decarburizing), and Avoid Corrosion.
â Most Gases Containing Oxygen (i.e. Air, Water Vapor
[H2O], Carbon Dioxide [CO2] React With Iron,
Carbon and Other Elements Present in Steel and
Other Metals.
â Reactivity Depends on Temperature and Mixture of
Gases in Contact With Steel.
⢠To Avoid and Eliminate Formation of Flammable or
Explosive Mixtures
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20. Types of Process Atmospheres
⢠Protective
â To Protect Metal Parts From Oxidation or Loss of
Carbon and Other Elements From the Metal Surfaces.
⢠Reactive
â To Add Non-metallic (i.e., Carbon, Oxygen, Nitrogen) or
Metallic (i.e., Chromium, Boron, Vanadium) Elements to
the Base Metal.
⢠Purging
â To Remove Air or Flammable Gases From Furnaces
or Vessels.
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21. Importance of Protective Atmospheres
in Heat Treating
â˘
Proper composition and concentration in a furnace is
required to give the required surface properties for the
heat treated parts.
â˘
Loss of atmosphere âcontrolâ can result in unacceptable
parts and result in major economic penalty - it can cost a
lot!
â˘
Atmospheres contain potentially dangerous (explosive, life
threatening) gases and must be treated with ârespectâ.
â˘
New advances in measurement and control of atmospheres
in heat treating allow precise control of atmospheres to
produce quality parts.
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22. Commonly Used Atmospheres in Heat Treating
Protective and Purging
â Endothermic gases
⢠Lean â high and low dew
point
⢠Rich - high and low dew
point
â Nitrogen
â Mixture of N2 and
small amount of CO
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Reactive
â Exothermic gases
â Mixture (or individual)
of gases: Hydrogen,
CO, CH4, Nitrogen and
other hydrocarbons
â Dissociated Ammonia
(H2 + N2)
23. Source of Atmospheres
Requirement:
A Mixture of Gases (CO, H2, CO2, H2O and N2) That
Give the Required Composition for the Processing
Atmosphere.
⢠Natural Gas (Hydrocarbon) Air Reaction
⢠Natural Gas - Steam
Reaction
⢠Ammonia Dissociation or
Ammonia-air Reaction
Or:
⢠Mixture of Commercial Gases
(N2, H2 and Hydrocarbons)
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24. Use of Atmospheres in a Plant
Requirement:
A Mixture of Gases (CO, H2, CO2, H2O and N2) That
Give the Required Composition for the Processing
Atmosphere.
⢠Most plants have an in-house, centrally
located, atmosphere gas generators for
different types of atmospheres required in
the plant
⢠In some cases one or more generators may be
located for each âshopâ or production area
⢠In many cases other gases (i.e. N2, H2, NH3)
are piped from storage tanks located within
the plant premises and distributed by a piping
system to furnaces.
⢠Gas flow is mixed, measured and controlled
prior to its injection in the furnace.
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25. ANNEALING
⢠Annealing, involves heating to a predetermined
temperature, holding at this temperature, and finally
cooling at a very slow rate.
⢠The temperature, to which steel is heated, and the
holding time are determined by various factors such
as the chemical composition of steel, size and shape
of steel component and final properties desired.
26. Purpose of Annealing
i. Relieve internal stresses developed during
solidification, machining, forging, rolling, or
welding;
ii. Improve or restore ductility & toughness;
iii. Enhance machinability
iv. Eliminate chemical non-uniformity;
v. Refine grain size; and
vi. Reduce the gaseous contents in steel.
27. CLASSIFICATION
Annealing treatment can be classified into groups based on the
following:
1. Heat treatment
temperature
⢠Full annealing
⢠Partial annealing
⢠Sub-critical
annealing
2. Phase
transformation
⢠First-order
annealing
⢠Second-order
annealing
3. Specific
purpose
⢠Full annealing
⢠Isothermal
annealing
⢠Diffusion
annealing
⢠Partial annealing
⢠Recrystallization
annealing
⢠Spheroidisation
annealing
28. 1.1 In full annealing the steel is heated above the critical
temperature(A3) and then cooled very slowly.
1.2 Partial annealing, also known as incomplete annealing
or intercritical annealing, involves heating of steel to a
temp. lying between lower critical temperature(A1) and
upper critical temperature (A3 or Acm).
1.3 Subcritical annealing is a process in which the
maximum
temp. to which is heated is always less than the lower
critical temperature(A1).
29. Classification based on phase
transformation features.
2.1 First-order annealing is performed on steel with the sole aim of
achieving some properties. Any change in the characteristics of steel
achieved by this type of annealing is not correlated to phase
transformation.it can be performed at a wide range of temperatures
above or below the critical temperatures.
2.2 The second-order annealing differs from the former in the sense
that the end results in the former are essentially due to phase
transformation which takes place during the treatment.
30. Types of annealing based on
specific purpose
3.1 Full annealing
â˘
â˘
â˘
In this, steel is heated to its 50°C above the austenitic temperature and
held for sufficient time to allow the material to fully form austenite or
austenite-cementite grain structure. The material is then allowed to cool
slowly so that the equilibrium microstructure is obtained.
The austenitising temp is a function of carbon content of the steel and
can be generalized as:
⢠For hypoeutectoid steels and eutectoid steel
Âť Ac3+(20-40oC) [to obtain single phase austenite]
⢠For hypereutectoid steels
Âť Ac1+(20-40oC) [to obtain austenite+ cementite]
31. Purpose of full annealing
â˘
â˘
â˘
â˘
â˘
To relieve internal stresses
To reduce hardness and increase ductility
For refining of grain size
To make isotropic in nature in mechanical aspects
For making the material having homogeneous
chemical composition
⢠For making the material suitable for high machining
processes
⢠To make steel suitable for undergoing other heat
treatment processes like hardening, normalizing etc.
32. ⢠The grain structure has coarse
Pearlite with ferrite or Cementite
(depending on whether hypo or
hyper eutectoid). The steel
becomes soft and ductile.
33. ⢠The formation of austenite destroys all structures
that have existed before heating. Slow cooling
yields the original phases of ferrite and pearlite
in hypoeuetectoid steels and that of cementite
and pearlite in hypereutectoid steels.
34. 3.2 Isothermal Annealing
⢠It is a process in which hypoeutectoid steel is
heated above the upper critical temperature and
this temperature is maintained for a time and then
the temperature is brought down below lower
critical temperature and is again maintained. Then
finally it is cooled at room temperature. This
method rids any temperature gradient.
⢠The prefix âisothermalâ associated with annealing
implies that transformation of austenite takes place
at constant temperature.
35. The closer the temp of isothermal holding is to A1,
coarser is the pearlite, softer is the steel, but longer
is the time of isothermal transformation.
36. Advantages:
â˘
â˘
â˘
â˘
Improved machinability.
Homogeneous structure and better surface finish.
Time required for complete cycle is comparably less.
The process is of great use for alloy steels as the steels
have to be cooled slowly.
Limitation:
It is suitable only for small-sized components. Heavy
components cannot be subjected to this treatment
because it is not possible to cool them rapidly and
uniformly to the holding temperature at which
transformation occurs. Thus structure wont be
homogeneous mechanical properties will vary across
the cross-section.
37. 3.3 Diffusion Annealing
⢠This process, also known as homogenizing annealing, is employed
to remove any structural non-uniformity like dendrites, columnar
grains and chemical inhomogeneity which promote brittleness and
reduce ductility and toughness of steel.
⢠Process:
ď Steel is heated sufficiently above the upper critical temperature (say,
1000-2000oC), and held at this temperature for 10-20 hours,
followed by slow cooling.
ď Segregated zones are eliminated, and a chemically homogeneous
steel is obtained by this treatment as a result of diffusion.
ď Heating to such a high temp. results in considerable coarsening of
austenitic grains & heavy scale formation. The coarse austenite thus
obtained further transforms to coarse pearlite on cooling, which is
not a desirable structure as mechanical properties are impaired.
38. ď§
The main aim of homogenising annealing is to make the
composition uniform, i.e to remove chemical
heterogeneity.
ď§
The impact energy and ductility of the steel increase as
the homogenizing temperature increases and the
hardness, yield strength and tensile strength decrease
with an increase in the homogenizing temperature.
ď§
Homogenizing annealing has a few shortcomings as
well. It results in:
ďą
ďą
ďą
Grain coarsening of austenite, thereby impairing the properties
Thick scales on the surface of steels
It is an expensive process
39.
40. 3.4 Partial Annealing
⢠Partial annealing, also known as inter-critical
annealing or incomplete annealing, is a process in
which steel is heated between A1 and Acm and is
followed by slow cooling.
⢠Generally, hypereutectoid steels are subjected to
this treatment. The resultant microstructure
consists of fine pearlite and cementite instead of
coarse pearlite and a network of cementite at grain
boundaries.
⢠As low temperatures are involved in this process, it
is less expensive than full annealing.
41. ⢠Hupoeutectoid steels are subjected to this
treatment to improve machinability. However,
steels with coarse structure of ferrite and pearlite
or with widmanstätten structure are not suitable
for this treatment. This is because only a
considerable amount of ferrite remains
untransformed, and only a part of it along with
pearlite transforms to austenite.
⢠This coarse or accicular untransformed ferrite
results in poor mechanical properties.
42. 3.5 Recrystallization
Annealing
⢠The process consists of heating steel above
the recrystallization temperature, holding at
this temperature and cooling thereafter.
⢠It is used to treat work-hardened parts made
out of low-Carbon steels (< 0.25% Carbon).
This allows the parts to be soft enough to
undergo further cold working without
fracturing.
43.
44. ⢠Recrystallization temp(Tr) is given by:
⢠Tr= (0.3-0.5)Tmp
⢠As little scaling and decarburization occurs in
recrystallization annealing, it is preferred over
full annealing.
⢠No phase change takes place and the final
structure consists of strain-free, equiaxed
grains of fine ferrite produced at the expense
of deformed elongated ferrite grains.
⢠However It would produce very coarse grains
if the steel has undergone critical amount of
deformation. In such cases, full annealing is
preferred.
47. 3.5 Spheroidization annealing
⢠Spheroidization annealing consists of heating, soaking and
cooling, invariably very slowly to produce spheroidal pearlite
or globular form of carbides in steels.
⢠To improve the machinability of the annealed hypereutectoid
steel spheroidize annealing is applied.
⢠Hypereutectoid steels consist of pearlite and cementite. The
cementite forms a brittle network around the pearlite. This
presents difficulty in machining the hypereutectoid steels.
⢠This process will produce a spheroidal or globular form of a
carbide in a ferritic matrix which makes the machining easy.
⢠Prolonged time at the elevated temperature will completely
break up the pearlitic structure and cementite network. The
structure is called spheroidite.
48. Spheroidising Process:
⢠Heat the part to a temperature just below the FerriteAustenite line, line A1 or below the AusteniteCementite line, essentially below the 727 ºC (1340
ÂşF) line. Hold the temperature for a prolonged time
and follow by fairly slow cooling. Or
⢠Cycle multiple times between temperatures slightly
above and slightly below the 727 ÂşC (1340 ÂşF) line,
say for example between 700 and 750 ÂşC (1292 1382 ÂşF), and slow cool. Or
⢠For tool and alloy steels heat to 750 to 800 ºC (13821472 ºF) and hold for several hours followed by slow
cooling.
49. ⢠All these methods result in a structure in
which all the Cementite is in the form of
small globules (spheroids) dispersed
throughout the ferrite matrix. This structure
allows for improved machining in continuous
cutting operations such as lathes and screw
machines. Spheroidization also improves
resistance to abrasion.
53. TEMPERING
â Martensite is a very strong phase, but it is normally very
brittle so it is necessary to modify the mechanical
properties by heat, treatment in the range 150â700°C.
â Essentially, martensite is a highly Supersaturated solid
solution of carbon in iron which, during tempering,
rejects carbon in the form of finely divided carbide
phases.
â The end result of tempering is a fine dispersion of
carbides in an Îą-iron matrix which often bears little
structural similarity to the original as-quenched
martensite.
54. NORMALIZING
ďą The normalizing of steel is carried out by
heating above the UCT (Upper Critical
Temperature) to single phase austenitic
region to get homogeneous austenite,
soaking there for some time and then
cooling it in air to room temperature.
ďą The austenitising temperature range are:
ďFor hypoeutectoid steels and eutectoid
steel
⢠Ac3 + (40-60oC)
ďFor hypereutectoid steels
⢠Acm + (30-50oC)
55. ďą During normalising we use grain refinement
which is associated with allotropic
transformation upon heating ÎłâÎą
ďą Parts that require maximum toughness and
those subjected to impact are often
normalized.
ďą When large cross sections are normalized,
they are also tempered to further reduce
stress and more closely control mechanical
properties.
ďą The microstructure obtained by normalizing
depends on the composition of the castings
(which dictates its hardenability) and the
cooling rate.
56. Figure below shows the normalizing temperatures
for hypoeutectoid and hypereutectoid steels
57. AIMs OF NORMALIZING
⢠To produce a harder and stronger steel than
full annealing
⢠To improve machinability
⢠To modify and/or refine the grain structure
⢠To obtain a relatively good ductility without
reducing the hardness and strength
⢠Improve dimensional stability
⢠Produce a homogeneous microstructure
⢠Reduce banding
⢠Provide a more consistent response when
hardening or case hardening
60. COMPARISON OF ANNEALING
AND NORMALIZING
ď The metal is heated to a higher temperature and then
removed from the furnace for air cooling in normalizing rather
than furnace cooling.
ď In normalizing, the cooling rate is slower than that of a
quench-and-temper operation but faster than that used in
annealing.
ď As a result of this intermediate cooling rate, the parts will
possess a hardness and strength somewhat greater than if
annealed.
ď Fully annealed parts are uniform in softness (and
machinability) throughout the entire part; since the entire part
is exposed to the controlled furnace cooling. In the case of the
normalized part, depending on the part geometry, the cooling
is non-uniform resulting in non-uniform material properties
across the part.
ď Internal stresses are more in normalizing as compared to
annealing.
ď Grain size obtained in normalizing is finer than in annealing.
ď Normalizing is a cheaper and less time-consuming process.
62. Comparison of timetemperature cycles for
normalizing and full annealing
The slower cooling of annealing results in higher temperature
transformation to ferrite and pearlite and coarser microstructures than
does normalizing.
63. effect of annealing and normalizing on
ductility of steels
Annealing and normalizing do not present a significant
difference on the ductility of low carbon steels. As the
carbon content increases, annealing maintains the %
elongation around 20%. On the other hand, the ductility of
the normalized high carbon steels drop to 1 to 2 % level.
64. effect of annealing and normalizing
on the tensile strength AND YIELD
POINT of steels
ď The tensile strength and the yield point of the
normalized steels are higher than the
annealed steels.
ď Normalizing and annealing do not show a
significant difference on the tensile strength
and yield point of the low carbon steels.
ď However, normalized high carbon steels
present much higher tensile strength and
yield point than those that are annealed. This
can be illustrated from the figures.
65.
66.
67. effect of annealing and normalizing on the
hardness of steels
Low and medium carbon steels can maintain similar
hardness levels when normalized or annealed. However,
when high carbon steels are normalized they maintain
higher levels of hardness than those that are annealed.
68. ADVANTAGES OF NORMALIZING OVER
ANNEALING
ď Better mechanical properties
ď Lesser time-consuming
ď Lower cost of fuel and operation
ADVANTAGES OF ANNEALING OVER
NORMALIZING
ď Greater softness
ď Complete absence of internal stresses
which is a necessity in complex and
intricate parts
69. HARDENING
⢠It is the process of heating the steel to
proper austenitizing temperature , soaking at
this temperature to get a fine grained and
homogeneous austenite , and then cooling
the steel at a rate faster than its critical
cooling rate.
70. OBJECTIVES OF HARDENING
The aims of hardening are:
1. Main aim of hardening is to induce high hardness. The
cutting ability of a tool is proportional to its hardness.
2. Many machine parts and all tools are hardened to
induce high wear resistance higher is the hardness ,
higher is the wear and the abrasion resistance .For
example ,gears, shaft.
3. The main objective of hardening machine components
made of structural steel sis to develop high yield
strength with good toughness and ductility to bear high
working stresses.
71. Austenising Temperature for
Pearlitic Steels
⢠The steel is first heated to proper austenising
temperature to obtain a homogeneous and fine
grained austenite. This temperature depends on the
composition(carbon as well as alloying elements).
⢠The austenitising temperature of plain carbon steels
depends on the carbon content of the steel and is
generalised as :
⢠For hypo-eutectoid steels :Ac3 + (20 â 40°C)
⢠For hyper-eutectoid steels and eutectoid steel:Ac1 +
(20 â 40°C)
73. Austenising Temperature for
Pearlitic Steels
⢠Hypereutectoid steels, when heated in above
temperature range, to obtain homogeneous and finegrained austenite which on quenching transforms to finegrained (very fine needles/plates), and hard martensite as
is desired to be obtained.
⢠Heating these steels only up to critical range (between
Ac3 and Ac1) is avoided in practice.
⢠Steel then has austenitic and ferrite.
⢠On quenching, only austenite transforms to martensite,
and ferrite remains as it is, i.e., incomplete hardening
occurs .
⢠The presence of soft ferrite does not permit to achieve
high hardness, if that is the objective.
74. Austenising Temperature for
Pearlitic Steels
⢠If the aim is to get high strength by the process of
tempering ferrite does not permit this as it has low
tensile and yield strengths .
⢠In fact, ferrite forms the easy path to fracture.
⢠Quenching of hypoeutectoid steels from temperatures
much above the proper temperatures , when
austenite has become coarse, results in coarse
acicular form of martensite.
⢠Coarse martensite is more brittle, and a unit or two
lower in hardness. It lowers the impact strength even
after tempering, and is more prone to quenchcracking.
75. Austenising Temperature for
Pearlitic Steels
⢠Hypereutectoid steels, when heated in the above range,
i.e., just above Ac1 have fine grains of austenite and
proeutectoid cementite.
⢠On quenching austenite transforms to fine martensite and
cememtite remains unchanged.
⢠As the hardness of cementite (â 800 BHN) is more than
that of martensite (650-750 BHN), its presence increases
the hardness, wear and abrasion resistance as compared
to only martensitic structure.
⢠If temperature of austenitisation is much higher than Ac1
but still below Acm temperature, a part of proeutectoid
cementite gets dissolved to increase the carbon content
of austenhlc(> 0.77%)
76. Austenising Temperature for
Pearlitic Steels
⢠On quenching as-quenched hardness is less,
because :
1. Lesser amount of proeutectoid cementite is
present.
2. Larger amount of soft retained austenite is
obtained as the dissolved carbon of cementite has
lowered the Ms and Mf temperature.
3. A bit coarser martensite has lesser hardness.
77. Austenising Temperature for
Pearlitic Steels
⢠Heating hypereutectoid steels to a temperature
higher than Acm results in 100% austenite . It is very
coarse austenite as very rapid grain-growth occurs
due to dissolution of restraining proeutectoid
cementite .
⢠The as-quenched hardness is low because of:
1) Absence of harder cementite.
2) As more carbon has dissolved in austenite, more
retained austenite is obtained.
3) Coarser martensite is a bit less hard and more brittle.
⢠Thus, these temperatures are avoided in carbon
steels
78. PROCESS OF QUENCHING
⢠When a heated steel object (say at 840°C) is plunged into
a stationary bath of cold it has three stages as:
Stage A -vapour-blanket stage:
⢠Immediately on quenching, coolant gets vapourized as
the steel part is at high temperature, and thus, a
continuous vapour- blanket envelopes the steel part.
⢠Heat escapes from the hot surface very slowly by
radiation and conduction through the blanket of water
vapour.
⢠Since the vapour-film is a poor heat conductor, the
cooling rate is relatively low (stage A in fig ). This long
stage is undesirable in most quenching operations.
80. PROCESS OF QUENCHING
Stage B-Intermittent contact stage (Liquid-boiling stage):
⢠Heat is removed in the form of heat of vaporization in this
stage as is indicated by the steep slope of the cooling curve.
⢠During this stage, the vapour-blanket is broken intermittently
allowing the coolant to come in contact with the hot surface
at one instant, but soon being pushed away by violent
boiling action of vapour bubble.
⢠The rapid cooling in this stage soon brings the metal surface
below the boiling point of the coolant.
81. PROCESS OF
QUENCHING
⢠The vaporization then stops. Second stage
corresponds to temperature range of 500⌠to 100âŚc ,
and this refers to nose of the CCT curve of the steel ,
when the steel transforms very rapidly ( to non
martensite product ).
⢠Thus, the rate of cooling in this stage is of great
importance in hardening of steels.
Stage C-Direct-Contact stage (Liquid-cooling stage):
⢠This stage begins when the temperature of steel
surface Is below the boiling point of coolant.
⢠Vapours do not form. The cooling is due to convection
and conduction through the liquid. Cooling is slowest
here.
82. QUENCHING MEDIUMS
⢠As the aim is to get martensite, the coolant should have
quenching power to cool austenite to let it transform to
martensite. The following factors effect the quenching power of
the coolant :
⢠The cooling rate decreases as the temperature of water and
brine increases, i.e., it increases stage âAâ, i.e., helps in
persistence of the vapour blanket stage.
⢠The increased temperature brings it closer to its boiling point, and
thus, requires less heat to form vapour, specially above 60°C.
⢠Good range of temperature for water as coolant is 20-40°C.
⢠Oils in general, show increased cooling rates with the rise of
temperature, with optimum cooling rates in range 50°â80°C.
83. QUENCHING MEDIUMS
⢠In oils, the increase of temperature increases the persistence of
vapour-blanket, but this resulting decrease in the cooling rate is
more than compensated by the decrease of viscosity (with the
rise in temperature) to result in increase of rate of heat removal
through the oil.
⢠If the boiling point of a coolant is low, vapours form easily to
increase the âAâ stage of cooling. ÂĄt is better to use a coolant with
higher boiling point. A coolant with low specific heat gets heated
up at a faster rate to form vapours easily.
⢠A coolant with low latent heat of vapourisation changes into
vapour easily to promote âAâ stage, i.e., decreases the cooling
rate.
⢠A coolant with high thermal conductivity increases the cooling
rate. Coolants with low viscoity provide faster cooling rates and
decrease the âAâ stage.
84. QUENCHING MEDIUMS
⢠A coolant should be able to Provide rate of cooling fast
enough to avoid transformation of austenite to pearlite
and bainite . Plain carbon steel invariably require çooling
in water or brine. Whereby alloy steels are quenched
normally in oils.
⢠But milder the cooling medium , lesser the internal
stresses developed , and thus lesser the danger of
distortion , or cracks . An ideal quenching medium is one
which is able to provide very fast cooling rate near the
nose of the curve ( 650 -550°C)and at the same time it
should provide very considerable slower rate if cooling
within the range of martensitic transformation( 300 200°C) to minimize internal stresses .
85. WATER
⢠The oldest and still the most popular quenching medium, water meets
the requirements of low cost ,general easy availability, easy handling
and safety.
⢠The cooling characteristics change more than oil with the rise of
temperature, specially there is a rapid fall in cooling capacity as the
temperature rises above 60°C, because of easy formation of vapourblanket.
⢠The optimum cooling pover is when water is 2O-4O°C.
⢠Thc cooling power of water is between brine and oils.
⢠Water provides high cooling power to avoid the transformation of
austenite to pearlite/bainite, but the major draw back is that it also
provides high cooling rate in the the temperature range of martensite
formation.
⢠At this stage, the steel is simultaneously under the influence of
structural stresses (non-uniform change in structure) and thermal
stresses which increase the risk of crack formation.
86. BRINE
⢠Sodium chloride aqueous solutions of about 10% by weight are widely
used and are called brines.
⢠The cooling power is between 10% NaOH aqueous solution and water.
⢠These are corrosive to appliances.
⢠The greater cooling efficiency of brines, or other aqueous solutions is
based as :
⢠In brine heating of the solution at the steel surface causes the deposition
of crystal of the salt on hot steel surface .
⢠This layer of solid crystals disrupts with mild explosive violence, und
throws off a cloud of crystals. This action destroys the vapour-film from
the surface, and thus permits direct contact of the coolant with the steel
surface with an accompanying rapid removal of heat.
⢠Brines are used where cooling rates faster than water arc requited.
87. OILS
⢠Oils have cooling power between water at 40°C to water at 90°C.
⢠In oil-quench, considerable variation can be obtained by the use
of animal, vegetable, or mineral oil, or their blends.
⢠Oils should be used at 50- 80°C when these are more fluid, i.e
less VISCOUS, which increases the cooling power.
⢠As the oils used generally have high boiling points, moderate
increase of temperature of oil does not very much increase the
vapour blanket stage. However, oils in contrast to water, or brine,
have much lower quenching power .
⢠Its this relatively slow cooling rate in the range of martensitic
formation is atlvantageous as it helps in minimsing the danger to
crack formation.
⢠Oils with high viscosity are less volatile, and thus have decreased
vapour-blanket stage (increase thecooling rate). As lesser volatile
matter is lost, their cooling power is not affected much with use.
88. POLYMER QUENCHANTS
⢠polymer quenchants cool rapidly the heated steel to
Ms temperature, and then rather slowly when
martensite is forming .
⢠Polymer quenchants are water-soluble organic
chemicals of high ,molecular weights, and are
generally polyalkylene glycol-based, or polyvinyl
pyrolidene-based.
⢠Widely different cooling rates can be obtained by
varying the concentration of Organic additives in
water; higher the additions, slower is the cooling rate
of solution.
⢠There are little dangers of distortions and cracks.
89. SALT BATHS
⢠It is an ideal quenching medium for a steel of
not very large section but with good
hardenabilty.
⢠Addition of O.3-O.5â% water almost doubles
the cooling capacity. Normally holding time is
2-4 minutes/cm of section thickness.
⢠Salt baths used for austenitising keep the
steel clean.
90. INTERNAL STRESSES S DURING
QUENCHING
⢠internal stresses are produced due to non-uniform plastic
deformation. In quenching of steels ,this may be caused by
thermal stresses, structural stresses, or both, or even premature
failure of part in service.
⢠Cooling during quenching lakes place non-uniformly, i.e., causes
temperature gradient across the section.
⢠Surface layers contract more than the central portion.
⢠Contraction of surface is resisted by the central portion, and this
puts the central portion under the compressive stresses, and the
surface layers in tension .
⢠If the magnitude of stress becomes more than the yield stress of
steel (at that deformation occurs.
⢠These stresses that develop in a quenched part as a result of
unequal cooling are called thermal stresses.
91. INTERNAL STRESSES DURING
QUENCHING
⢠Structural stresses are the stresses which develop due to due to
phase change (mainly austenite to martensite), and at different
times.
⢠Structural stresses are developed due to two main reasons:
1. Austenite and its transformation products have unequal specific
volume i.e. change in volume occurs when transformation
occurs.
2. Phase changes occur at different times in the surface and in
centre.
⢠Under right conditions, both types of stresses get superimposed
to become larger than the yield strength to cause warping. but
when the tensile internal stresses become larger than the tensile
strength cracks appear.
⢠If an austenitised steel is quenched, it contracts thermally till Ms
temperature is attained .
93. INTERNAL STRESSES DURING
QUENCHING
⢠figure(a) illustrates this in stage 1 As surface cools faster than
centre, i.e., contracts more than centre distribution of stresses
across the section is illustrated in fig (b), i.e, the surface is under
tensile nature of stress, while centre is under compressive
stresses.
⢠Only thermal Stresses are produced in stage 2 , surface having
attained Ms temperature, transforms to martenSite, and thus
expands, while the centre is still contracting as it is getting
cooled.
⢠In stage Il, centre may get plastically deformed ,as it is still
ductile austenite.
⢠In stage 3, martensite of surface and austenite of centre continue
contracting leading to slight increase in stress levels.
94. INTERNAL STRESSES DURING
QUENCHING
⢠In stage IV, centre has attained M5 temperature, and begins
to expand as it forms martensite, while surface is still
contracting.
⢠The centre, as it expands, puts the surface in higher stress
levels .
⢠The surface has little deformation as it consists of brittle
martensie.
⢠It is during this stage, the greatest danger of cracking exists.
⢠Thus, stress levels are highest not in the beginning of the
quench, but when the centre is transforming to martensite.
95. INTERNAL STRESSES DURING
QUENCHING
⢠However, higher is the Ms temperature of the
steel, lesser is the expansion, there is reduced
danger of quench-cracking.
⢠Increase of carbon and alloying elements lower
the Ms temperature making the steel more prone
to quench cracking.
96. RETAINED AUSTENITE
⢠Martensitic transfomiation is essentially an athermal transformation.
⢠Austenite begins to transform to martensite at Ms, and the amount of
martensite formed increases as the temperature decreases to complete
at Mf temperature.
⢠Less than 1 % of austenite may not transform because of unfavourable
stress conditions.
⢠The Ms and Mf temperatures are lowered as the amounts of carbon
content and alloying elements(except cobalt and aluminium) increase in
the steel.
⢠In a quenched steel, the amount of martensite formed depends on the
location of Ms and Mf and the temperature of the coolant (which is
normally room temperature. As long as room temperature lies between
Ms and Mf temperatures, austenite does not transform completely to
martensite as it has not been cooled below Mf temperature.
⢠This untransformed austenite is retained austenite.
97. ADVANTAGES OF
RETAINED AUSTENITE
⢠10% retained austenite is normally desirable as its
ductility relieves some internal stresses developed
during hardening. This reduces the danger to
distortion or cracks.
⢠The presence of 30-40% retained austenite makes
straightening of components possible after hardening.
⢠Non distorting tools owe their existence to retained
austenite . It tries to balance transformational volume
changes during heating as well as cooling cycles of
heat treatment to produce little overall change in size
of the tools.
98. DISADVANTAGES
1) The presence of soft austenite decreases the
hardness of hardened steels.
2) As retained austenite may transform to lower
bainite, or martensite later in service ,increase
in dimensions of the part occurs.
3) This creates problems in precision gauges or
dies.
4) Stresses may develop in the part itself as well
as in adjacent pans. Grinding-cracks are
mainly due to retained austenite transforming
to martensite.
5) Austenite is non-magnetic, decreases the
magnetic properties of the steels.
99. ELIMINATION OF RETAINED
AUSTENITE
⢠Retained austenite is generally undesirable. It is eliminated by
one of the methods:
1. Sub-Zero treatment (cold treatment):
⢠It Consists of cooling the hardened steel (having retained
austenite) to a temperature below 0°C or its Mf temperature.
⢠There is no reason to cool a steel much below its Mf temperature.
⢠Sub-zero treatment is more effective if it is done immediately
after the quenching operation (normally done to room
temperature).
⢠Sub-zero treatment is done in a low temperature-cooling unit,
which consists of double-walled vessel.
⢠The interior is made of copper in which the parts to be deepfrozen are kept, and the exterior is made of steel provided with
good heat-insulation.
100. ELIMINATION OF RETAINED
AUSTENITE
⢠The space in between the vessels is filled with
some coolant.
⢠The sub-zero coolants could be, dry ice (Solid
CO2) + acetone (â 78°C); Ice + NaCl (â23°C);
liquid air (â183°C); liquid N2 (â 196°C); Freon
(â 111°C).
⢠Total time of cooling in this unit is 1/2 to 1 hour.
⢠As this treatment transforms austenite to
martensite, steels after sub-zero treatment have
high hardness, wear and abrasion resistance,
and have no danger of grinding-cracks.
101. ELIMINATION OF RETAINED
AUSTENITE
⢠The stresses are increased further and thus, tempering
should be done immediately after sub-zero treatment.
⢠Carburised steels, ball-bearing steels, highly alloy tool
steels, are normally given cold treatment.
2. Tempering:
⢠The second stage of tempering eliminates the retained
austenite in most steels.
⢠In high alloy steels, multiple tempering is able to
eliminate the retained austenite during cooling from the
tempering temperature.
102. DEFECTS IN HARDENING
⢠The main defects produced during hardening are:
1. Mechanical properties not up to specifications:
⢠The common defect in hardened tools, or component
is too low a hardness.
⢠One or more of the followings could be the cause of
such a defect.
⢠Insufficient fast cooling due to overheated or even
polluted coolant could be responsible for a defect.
⢠The presence of scale, or oil, etc. on the surface also
decreases the cooling rate.
⢠Circulation of coolant, or agitation of component may
also result in such defect.
103. DEFECTS IN HARDENING
â˘
A shorter austenitising time can also cause such a defect. Lower
austenitising temperature can also result in such a defect.
⢠Decarburisation can also result in low surface hardness. If too
high temperature had been used, which produces larger amount
of retained austenite can result in low surface hardness.
2. Soft Spots:
⢠Soft areas on the hardened surface are called âsoft spotsâ.
⢠The adhering scale, or decarburisation of some areas or
prolonged vapour-blanket stage due to overheated coolant or
insufficient agitation or circulation of coolant, or rough surface
could cause presence of soft spots surface.
104. ELIMINATION OF RETAINED
AUSTENITE
3. Quench Cracks:
⢠Quench cracks form as a result of internal stresses developed of tensile
nature exceeding the tensile strength of the steel.
⢠Steel with lower Ms temperature due to higher contents of alloying
elements are more prone to quench cracks. Higher carbon also results
in more brittle martensite.
⢠Quench cracks can form if there is more time lag between the process of
quenching and tempering.
⢠Overheating of steel or a wrong coolant which gave much faster rate of
cooling, or there is faulty design of the component with sharp corners
and sharp transition between sections, or a wrong steel has been
chosen.
⢠Presence of large amounts of retained austenite causes grinding cracks.
⢠The other defects could be distortion and warpage; change in
dimensions; oxidation and decarburisation
105. Tempering of plain carbon steels
â In the as-quenched martensite structure,the
laths or plates are heavily dislocated to an
extent that individual dislocations are very
difficult to observe in thin-foil electron
micrographs.
â A typical dislocation density for a 0.2 wt%
carbon steel is between 0.3 and 1.0 x 1012 cm
cm-3. As the carbon content rises above
about 0.3 wt%, fine twins about 5â10 nm
wide are also observed.
â Often carbide particles, usually rods or small
plates, are observed (Fig. 9.1).
106. Tempering of plain carbon steels
â These occur in the first-formed
martensite, i.e. the martensite formed
near Ms, which has the opportunity of
tempering during the remainder of the
quench.
â This phenomenon, which is referred to as
autĂł-tempering, is clearly more likely to
occur in steels with a high Ms.
108. Stages of Tempering
â On reheating as-quenched martensite, the
tempering takes place in four distinct but
overlapping stages:
â Stage 1, up to 250°C â precipitation of E-iron
carbide; partial loss of tetragonality in martensite.
â Stage 2, between 200 and 300°C â decomposition
of retained austenite .
â Stage 3, between 200 and 350°C â replacement
of &iron carbide by cementite; martensite loses
tetragonality.
â Stage 4, above 350°C â cementite coarsens and
spheroidizes; recrystallization of ferrite.
109. Tempering â stage 1
â Martensite formed in medium and high carbon
steels (0.3â1.5 wt% C) is not stable at room
temperature because interstitial carbon atoms can
diffuse in the tetragonal martensite lattice at this
temperature.
â This instability increases between room
temperature and 250°C, when âŹ-iron carbide
precipitates in the martensite (Fig. 9.2).
â This carbide has a close-packed hexagonal
structure, and precipitates as narrow laths or
rodlets on cube planes of the matrix with a welldefined
orientation relationship .
110. Tempering â stage 1
â At the end of stage 1 the martensite still possesses a
tetragonality, indicating a carbon content of around
0.25 wt%.
â It follows that steels with lower carbon contents are unlikely
to precipitate âŹ-carhide.
â This stage of tempering possess an activation energy of
between 60 and 80 kJ mo1, which is in the right range for
diffusion of carbon in martensite. The activation energy has
been shown to increase linearly with the carbon concentration
between 0.2 and 1.5 wt% C.
â This would be expected as increasing the carbon
concentration also increases the occupancy of the preferred
interstitial sites, i.e. the octahedral interstices at the midpoints of cell edges, and centres of cell faces, thus reducing
the mobility of C atoms.
111. Tempering â stage 2
⢠During stage 2. austenite retained during
quenching is decomposed usually in the
temperature range 230-300°C.
⢠In martensitiC plain carbon steels below 0.5
carbon. the retained austenite is often below 2%,
rising to around 6 % at 0.8 wt C and over 30 % at
1.25 wt C.
⢠The little available evidence suggests that in the
range 230-300°C, retained austenite decomposes
to bainitic ferrte and cementite, but no detailed
comparison between this phase and lower bainite
has yet been made.
112. Tempering â stage 3
â During the third stage of tempering, cementite
flrst appears in the microstructure as a
Widmanstatten distribution of plates which have a
well-defined orientation relationship with the
matrix which has now lost its tetragonality and
become ferrite.
â This reaction commences as low as 100°C and is
fully developed at 300°C, with particles up to 200
nm long and 15 nm in thickness.
â Similar structures are often observed in lower
carbon steels as quenched, as a result of the
formation of Fe3C during the quench.
113. Tempering â stage 3
â During tempering, the most likely sites for the
nucleation of the cementite are the âŹ-iron carbide
irterfaces with the matrix (Fig 9.2) and as the
Fe3C particles grow, the âŹ-iron carbide particles
gradually
disappear.
â The twins occurring in the higher carbon
martensites are also site for the nucleation and
growth of cementite which tends to grow along
the twin boundaries forming colonies of similarly
oriented lath shaped particles (Fig. 9.3) which
can be readily ditinguished from the normal
Widmanstatten habit.
114. Tempering â stage 3
â A third site for the nucleation of cementite is the
grain boundary regions (Fig, 9.4)of both the
interlath boundaries of martensite and
the original austenite grain b0unjaries.
â The cementite can form as very thin films which
are difficult to detect but which gradually
sp1eroidise to give rise to welI-defined particles
of Fe3C in the grain boundary regions.
â There is some evidence to show that these.
boundary cementite films can adversely affect
ductility. However it can be modified by addition
of alloying elements.
115. Tempering â stage 3
â During the third stage of tempering , the
tetragonality of thc matrix disappears and it is
then, essentially, ferrite, not supersaturated
with
respect to carbon.
â Subsequent changes in the morpriology of
cementite particles occur by process where the
smaller particles dissolve in the matrix providing
carbon for the selective growth of the larger
particles.
116. Tempering â stage 3
â During the third stage of tempering , the
tetragonality of thc matrix disappears and
it is then, essentially, ferrite, not
supersaturated with
respect to carbon.
â Subsequent changes in the morpriology of
cementite particles occur by process
where the smaller particles dissolve in the
matrix providing carbon for the selective
growth of the larger particles.
117. Tempering â stage 4
â It is useful to define a fourth stage of tempering in
which the cementite particles undergo a coarsening
process and essentially lose their crystallographic
morphology, becoming spheroidized.
â It commences between 300 and 400âŚC, while
spheroidizatiun takes place increasingly up to 700âŚC.
â At the higher end of this range of tempera.
ture the martensite lath boundaries are replaced by
more equi-axid fèrrite grain boundaries by a process
which is best described as recrystallization.
â The final result is an equi-axed array of ferrite
grains with coarse spheroidized particles of Fe3C (Fig.
9.5), partly, but not exclusively, by the grain
boundaries.
118. Tempering â stage 4
â The spherodisation of the Fe3C is encouraged by the
resulting decrease in surface energy.
â The particles which preferentially grew and spheroidize are
located mainly at interlath boundaries and prior
austenite boundaries, although some particles remain in the
matrix.
â The boundary sites are preferred because of the greater
ease of diffusion in these regions. Also, the growth of
cementite into ferrite is associated with a decrease in density
so vacancies are required to accommodate the growing
cementite.
â Vacancies will diffuse away from cementite particles which
are redissolving in the ferrite and towards cementite
particles which are growing, so that the rate controlling
process is likely to be the diffusion of vacancies.
119. Tempering â stage 4
â The original martensite lath boundaries remain stable up to about
600°C, but in the range 350â600°C. there is considerable
rearrangement of the dislocations within the laths and at those lath
boundaries which are essentially low angle boundaries.
â This leads to a marked reduction in the dislocation density and to
lath-shaped ferritic grains closely related to the packets of
similarly oriented laths in the original
martensite.
â This process, which is essentially one of recovery, is
replaced between 600 and 700°C by recrystallization which results
in the formation of equi-axed ferrite grains with spheroidal Fe3C
particles in the boundaries and within the grains.
120. Tempering â stage 4
â This process occurs most readily in carbon
steels.
â At higher carbon content, the increased
density of cementite particles is much more
effective in pinning the ferrite boundaries, so
recrystallisation is much more sluggish.
â The final process is the continued coarsening
of the cementite particles and gradual ferrite
grain growth.
121. Role of carbon content
â Firstly, the hardness of the as-quenched martensite is
largely influenced by the carbon content, as is the morphology
of the martensite laths which have a up to 0.3 wt% C,
changing at higher carbon contents.
â The Ms temperature is reduced as the carbon content
increases, arid thus the probability of the occurrence of
auto-tempering is less.
â During fast quenching in alloys with less than 0.2 wt % C, the
majority (up to 90%) of the carbon segregates to dislocations
and lath boundaries, but with slower quenching some
precipitation of cementite occurs.
â On subsequent tempering of low carbon steels up to 200°C
further segregation of carbon takes place. but no
precipitation has been observed.
123. Role of carbon content
⢠Under normal circumstances it is
difficult to detect any tetragonality in
martensitic in steels with less than 0.2
wt% C, a fact which can explained by the
rapid segregation of carbon during
quenching.
⢠The hardness change; during tempering
are also very dependent on carbon
content, as shown in figure for steels up
to 0.4 wt% C.
⢠Above this concentration, an increase in
hardness has been observed in
temperature range 50â150°C, as âŹcarbide precipitation strengthens the
martensite.
⢠However, the general trend is an overall
124. Mechanical properties of tempered
plain carbon steels
â The absence of other alloying elements means that the
hardenability of the steels is low, so a fully martensitic
structure is only possible in thin sections.
â However, this may not be a disadvantage where shallow
hardened surface layers are all that is required.
â Secondly, at lower carbon levels, the Ms temperature is
rather high, so autotempering is
likely to take place.
â Thirdly, at the higher carbon levels the presence
of retained austenite will influence the results.
â Added to these factors, plain carbon steels can exhibit
quench cracking which makes it difficult to obtain reliable
test results. This is particularly the case at higher carbon
levels, i.e. above 0.5 wt% carbon.
125. Tempering of alloy steels
â The addition of allying elements to a steel has a
substantial effect on the kinetics of the y âÎą
transformation, and also of the pearlite reaction.
â Most common alloying elements move the TTT
curves to longer times, with the result that it is
much easier to miss the nose of the curve during
quenching.
â This essentially gives higher hardenability, since
martensite structures can be achieved at slower
cooling rates and, in practical terms, thicker
specimens can be made fully martensitic.
â Alloying elements have also been shown to have a
substantial effect in depressing the Ms
temperature.
126. The effect of alloying elements
on the formation of iron
carbides
⢠It is clear that certain elements, notably
silicon
can stabilize the âŹ-iron carbide to such an
extent that it is still present in the
âmicrostructure after tempering at 400°C in
steels with 1-2 wt% Si, and at even higher
temperatures if the silicon is further
increased.
⢠The evidence suggests that both the
nucleation and growth of the carbide is
slowed down and that silicon enters into the
âŹ-carbide structure.
⢠It is also clear that the transformation of
âŹ-iron carbide to cementite is delayed
considerably.
127. The effect of alloying
elements on the formation of
iron carbides
⢠While the tetragonality of martensite
disappears by 300°C in plain carbon steels,
in steels containing some alloying elements,
e.g. Cr, Mo, W V, Ti, Si, the tetragonal
lattice is still observed after tempering at
450°C and even as high as 500°C .
⢠It is clear that these alloying elements
increase the stability of the
supersaturated iron-carbide solid
solution.
⢠In contrast manganese and nickel decrease
the stability.
128. The effect of alloying
elements on the formation of
iron carbides
⢠Alloying elements also greatly
influence the proportion of austenite
retained on quenching.
⢠Typically a steel with 4%
molybdenum, 0.2%C, in the
martensitic state contains less than
2% austenite, and about 5% is
detected in a steel with 1% vanadium
and 0.2%C.
⢠The austenite can be revealed as a
fine network around the martensite
129. The effect of alloying elements
on the formation of iron carbides
⢠On tempering each of the above steels at
300°C, the austenite decomposes to give thin
grain boundary
films of cementite which, in the case of the
higher concentrations of retained austenite,
can be fairly continuous along the lath
boundaries.
⢠It is likely that this interlath cementite is
responsible for tempered embrittlement
frequently encountered as a toughness
minimum in the range 300â350°C, by leading
to easy nucleation of cracks, which then
propagate across the tempered martensite
laths.
130. â˘
The effect of alloying elements on
the
formation also restrain the coarsening
Alloying elements canof iron carbides
of cementite in the range 400-700°C, a basic
process during the fourth stage of tempering.
⢠Several alloying elements, notably silicon,
chromium.
molybdenum and tungsten, cause the cementite to
retain its fine Widmanstatten structure to higher
temperatures, either by entering into the
cementite structure or by segregating at the
carbide-ferrite
interfaces.
⢠Whatever the basic cause may be, the effect is
to delay
significantly the softening process during
131. The effect of alloying
elements on the formation of
iron carbides
⢠This influence on the cementite dispersion
has other effects, in so far as the carbide
particles, by remaining finer, slow down the
reorganization of the dislocations inherited
from the martensite, with the result that
the dislocation substructures refine more
slowly.
⢠In plain-carbon Steel cementite particle
begin to coarsen in the temperature range
350 -400°C and addition of chromium,
silicon, molybdenum or tungsten delays the
coarsening to the range 500-550°C.
⢠It should be emphasized that up to 500°C
the only carbides to form are those of iron.
132. The formation of alloy carbides
Secondary Hardening
⢠A number of the familiar alloying elements in
steels form carbides, nitrides and borides
which are thermodynamically more stable
than cementite.
⢠It would therefore be expected that when
strong carbide elements are present in a
steel in sufficient concentration, their
carbides would be formed in preference to
cementite .
⢠Nevertheless during the tempering of all
ahoy steels, alloy carbides do not form until
the temperature range 500-600°C , because
below this the metallic alloying elements
cannot diffuse sufficiently rapidly to allow
alloy carbides to nucleate.
133. The formation of alloy carbides
Secondary Hardening
⢠The metallic elements diffuse substitutionally, in
contrast to carbon and nitrogen which move
through the iron interstitially.
⢠With the result that the diffusivities of carbon
and nitrogen are of several orders of magnitude
greater in iron than those of the metallic alloying
elements.
⢠Consequently higher temperatures are needed for
the necessary diffusion of the alloying elements.
⢠It is this ability of certain alloying elements to
form fine alloy carbide dispersions in the range
500â600°C, which remain very fine even after
prolonged tempering, that allows the development
of high strength levels in many alloy steels.
134. The formation of alloy carbides
Secondary Hardening
⢠Indeed, the formation of alloy carbides
between 500 and 600°C is accompanied by a
marked increase in strength, often in excess
to that of the as-quenched martensite.
⢠This phenomenon, which is referred to as
secondary hardening, is best shown in steels
containing molybdenum, vanadium, tungsten,
titanium, and also in chromium steels at higher
alloy concentrations.
135. The formation of alloy carbides
Secondary Hardening
⢠This secondary hardening process is a type of
age-hardening reaction, in which a relatively
coarse cementite dispersion is replaced by a
new and much finer alloy carbide dispersion.
⢠On attaining a critical dispersion parameter,
the strength of the steel reaches a maximum,
and as the carbide dispersion slowly coarsens,
the strength drops.
136. The formation of alloy carbides Secondary Hardening
⢠The process is both time and
temperature dependent, so both
variables are often combined in a
parameter:
P = T(k + log t)
where:
T is the absolute temperature and
t the tempering time in hours,
while k is a constant which is about
20 for alloy steels, usually referred
to as the Holloman-Jaffe parameter.
137. Martempering
Interrupted quench from the A1
ďś Delay cooling just above
martensitic transformation for
a length of time to equalize T
throughout the piece
ďś Reduce thermal gradient
btw surface & center
ďś Minimize distortion, cracking,
and residual stress.
ďś
137
138. CASE HARDENING
⢠Low carbon steels cannot be hardened by
heating due to the small amounts of
carbon present.
⢠Case hardening seeks to give a hard outer
skin over a softer core on the metal.
⢠The addition of carbon to the outer skin is
known as carburising.
140. ⢠Case hardening or surface hardening is the process of
hardening the surface of a metal object while allowing the metal
deeper underneath to remain soft, thus forming a thin layer of
harder metal (called the "case") at the surface. For steel or
iron with low carbon content, which has poor to no hardenability
of its own, the case hardening process involves infusing
additional carbon into the case
⢠Case hardening is usually done after the part has been formed
into its final shape, but can also be done to increase the
hardening element content of bars to be used in a pattern
welding or similar process. The term face hardening is also used
to describe this technique, when discussing modern armour
⢠Because hardened metal is usually brittler than softer metal,
through-hardening (that is, hardening the metal uniformly
throughout the piece) is not always a suitable choice for
applications where the metal part is subject to certain kinds of
stress. In such applications, case hardening can provide a part
that will not fracture (because of the soft core that can absorb
stresses without cracking) but also provides adequate wear
resistance on the surface
141.
142.
143. Case Hardened Chisel
Chisel:
- cutting edge is hard and wear-resistant
- tang is tough and elastic
If the chisel would be hard throughout, it could break
when the hammer is striked onto it!
Figure - Cut through a hardened chisel - 1 cutting edge (hard),
2 twig (tough)
145. CARBURIZING
The highest hardness of a steel is obtained when its carbon content
ishigh, around 0.8 weight % C (Figure 1). Steels with such high
carbon content are hard, but also brittle, and therefore cannot be
used in machine parts such as gears, sleeves and shafts that are
exposed to dynamic bending and tensile stresses during operation. A
carbon content as high as 1% C also makes the steel difficult to
machine by cutting operations such as turning or drilling. These
shortcomings can be eliminated by using a low carbon content steel
to machine a part to its final form and dimensions prior to
carburizing and hardening. The low carbon content in the steel
ensures good machinability before carburizing. After carburizing
and quenching the part will have a hard case but a softer core that
will assure wear and fatigue resistance. The martensitic case attains
a hardness corresponding to its carbon content. The case is typically
0.1â1.5 mm (0.004- 0.060 inches) thick. The core of the part
maintains its low carbon concentration and corresponding lower
hardness.
146. A carburizing atmosphere must be able to
transfer carbon â and also nitrogen in the
case of carbonitriding â to the steel surface
to provide the required surface hardness. To
meet hardness tolerance requirements this
transfer must result in closely controlled
carbon or nitrogen concentrations in the steel
surface. The carbon concentration, as
indicated in Figure 3, can be controlled by the
ratio (vol% CO)2/(vol% CO2) in the furnace
atmosphere. The atmosphere nitrogen
activity, which plays an important role in
carbonitriding
147. PROPERTIES OF CARBURISED
STEEL
The gas-carburized (carbonitrided) part
can be said to consist of a composite
material, where the carburized surface is
hard but the unaffected core is softer
and ductile. Compressive residual stresses
are formed in the surface layer upon
quenching from the carburizing
temperature. The combination of high
hardness and compressive stresses results
in high fatigue strength, wear resistance,
and toughness.
148.
149. Maximum hardness for unalloyed steels is obtained
when the carbon concentration is about 0.8%C, as
was shown. Above that carbon concentration the
hardness decreases as the result of an increased
amount of retained austenite. The hardness curve
therefore often exhibits a drop in hardness close
to the surface, where the carbon concentration is
highest. Carbon, nitrogen and almost all alloying
elements lower the Ms-temperature (see
reference [2] for the definition of Ms
temperature). This leads to a retained austenite
concentration gradient that increases towards the
surface after carburizing and quenching.
150. ⢠To compensate for this effect, the surface carbon
concentration after carburizing that provides
maximum surface hardness has to be lowered as the
alloy content of the steel increases. Carbide forming
elements, such as chromium and molybdenum, can
counteract this effect and raise the surface carbon
concentration that provides maximum hardness. This is
because the formation of carbides leads to a lowered
carbon concentration in the austenite, although the
average carbon concentration is high. Table 1 gives
some examples of the relation between maximum
hardness and carbon concentration for different
types of steels. Mo-alloyed steels obtain the highest
surface hardness and Ni-alloyed steels the lowest. MnCr steels obtain an intermediate surface hardness
151.
152. Case and Carburizing depth
According to European standards [6], the case
depth is abbreviated to CHD (case hardened
depth) and defined as the depth from the surface
to the point where the hardness is 550HV, as
shown . Sometimes a hardness other than 550HV
is used to define the case depth
153. Attained case depth depends not only on
carburizing depth, but also on the hardening
temperature, the quench rate, the hardenability of
the steel and the dimensions of the part. This is
illustrated in the schematic CCT diagrams. The
hyperbolic temperature/ time-dependent parts of
the transformation curves depict the
transformation from austenite to ferrite/pearlite.
For a high hardenability steel these curves are
located far to the right in the diagram, ensuring
that the cooling curves do not cross the
ferrite/pearlite transformation
curve
154. Hardenability increases not only with base
steel alloy content but also with increased
carbon and nitrogen concentrations. The
carburized or carbonitrided case therefore
has higher hardenability than the base steel
155.
156.
157. In Figure âaâ the cooling curves for both âsurfaceâ
and âcenterâ cross the transformation line for the
base steel, the core. This means that the core will
transform to ferrite/pearlite upon cooling from
hardening temperature. If the cooling curves are
related to the âcaseâ instead, it can be seen that
the cooling line for the surface passes to the left
of the ferrite/pearlite transformation curve. Thus
theâsurfaceâ cooling line first crosses the Ms
(case) line, meaning that the austenite will
transform to martensite, as is the intention in
case hardening. The hardenability of steel number
1 in Figure âb âis too low to result in martensite
transformation even for the carburized case
158. â In Figure âcâ carbonitriding is a method for
achieving high enough hardenability to form a
martensitic case. (The âsurfaceâ cooling line
passes to the left of the carbonitrided
transformation curve.) Carbonitriding is a way to
make water-quench steels become oil hardening
steels.
â In Figure âdâ schematically shows the effect of
part dimensions on cooling rate. The bigger the
dimensions, the slower the cooling rate. Therefore
there is a certain maximum diameter for a certain
steel grade that can be hardened to form a
martensitic case. When a martensitic case is
formed the case depth will decrease with
159.
160. Carburizing depth is not standardized but is
nevertheless used in practice, and is defined as
the depth from the surface to the point
corresponding to a specified carbon concentration.
As a guideline, the case depth (CHD) for common
steels and part dimensions is approximately equal
to the carburizing depth to the point where the
carbon concentration is about 0.35%. The
carburizing depth depends on treatment time and
temperature. With prolonged carburizing time
carbon can diffuse to a greater depth into the
steel. Increasing the temperature increases the
rate of diffusion and thus increases the
carburizing depth
161.
162.
163.
164.
165. CHEMISTRY OF CASE
HARDENING
Carbon itself is solid at case-hardening
temperatures and so is immobile. Transport
to the surface of the steel was as gaseous
carbon monoxide, generated by the
breakdown of the carburising compound and
the oxygen packed into the sealed box. This
takes place with pure carbon, but unworkably
slowly. Although oxygen is required for this
process it is re-circulated through the CO
cycle and so can be carried out inside a
sealed box
166. â The sealing is necessary to stop the CO
either leaking out, or being oxidised to
CO2 by excess outside air.
â Adding an easily decomposed carbonate
"energiser" such as barium carbonate
breaks down to BaO + CO2 and this
encourages the reaction
â C (from the donor) + CO2 <â> 2 CO
167. Increasing the overall abundance of CO and the
activity of the carburising compound.[1] It is a
common knowledge fallacy that case-hardening was
done with bone, but this is misleading. Although
bone was used, the main carbon donor was hoof
and horn. Bone contains some carbonates, but is
mainly calcium phosphate (as hydroxylapatite).
This does not have the beneficial effect of
encouraging CO production and it can also
introduce phosphorus as an impurity into the steel
alloy.
168. Carbon transfer from gas to
surface
Possible carbon transfer reactions are
2CO â C+CO2
CH4 â C + 2H2
CO+H2â C+H2O
It has been shown that the last of these
reactions,is by far the fastest and is therefore
the rate-determining reaction in carburizing
atmospheres with CO and H2
as major gas components . The slowest carburizing
reaction is from methane, with a rate that is only
about 1% of the rate of carburizing from CO+H2
.
169. Interaction between Furnace Atmosphere and
Steel
In the above reaction, carbon monoxide (CO) and
hydrogen(H2) react so that carbon (C) is deposited on the
steel surface and water vapor (H2O) is formed. The
furnace atmosphere must contain enough carbon monoxide
and hydrogen to allow the carburizing process to proceed in
a uniform and reproducible fashion. The supply of fresh gas
must compensate for the consumption of CO and H2. A
higher gas flow is required in cases where the furnace
charge area is high, resulting in a high rate of carbon
transfer from gas to surface. In the initial part of a
carburizing cycle, there is also a high carbon transfer rate,
which may be compensated for by increasing the gas supply.
170. According to the fundamental principles of chemistry, the equilibrium
condition for the carburizing reaction 1 is described by an equilibrium
constant expressed by:
K1 = (ac¡ PH2O)/(PCO ¡ PH2)
The value of K1 is dependent on the temperatureand can be calculated
from the relationship:
log K1 = â7.494 + 7130/T
where T is the absolute temperature in Kelvin. ac is termed carbon
activity and is a measure of the âcarbon contentâ of the gas. We see
that ac can be calculated if K1 and the gas composition are known.
When the carbon activity of the gas, acg, is greater than that of the
steel surface, acs, there is a driving force to transfer carbon as
expressed by the following equation:
dm/dt = k ¡ (acg â acs)
dm/dt = kâ ¡ (ccg â ccs)
where: m designates mass, c concentration per unit volume, t time,
dm/dt expresses a carbon flow in units of kg/cm2 ¡ s or mol/m2 ¡ s, and
171.
172.
173.
174. The gradient dc/dx has its highest value at the beginning
of the cycle when carbon has only diffused to a thin depth.
This results in a high driving force for carbon flux by
diffusion into the steel. The rate of the carbon transfer
from gas to surface will therefore initially be the limiting
step. At the start of a carburizing cycle, the term ccg â ccs
has its highest value, and accordingly the driving force for
carbon transfer from gas to steel has its highest value.
The surface carbon Concentration ccs will increase with
increasing carburizing time. The driving force for carbon
transfer, ccg â ccs, will thus decrease. The carbon
concentration gradient, dc/dx, will decrease concurrently
as carbon diffuses into the steel. In conclusion, these
limitations will lead to a continuous reduction of carbon
flux into the steel
175.
176.
177.
178.
179. Carburizing Atmospheres
Endogas
A carburizing atmosphere can be achieved by
means of incomplete combustion of propane or
methane with air in accordance with one of the
reactions:
C3H 8 + 7.2 air â 5.7 N2 + 3CO + 4H2
CH4 + 2.4 air â 1.9 N2 + CO + 2H2
The mixing and combustion of fuel and air
takes place in special endothermicgas
generators
180. Nitrogen/Methanol Atmospheres
Introducing nitrogen and methanol directly into the
furnace chamber is a common way of creating the furnace
atmosphere. Upon entering the furnace, methanol cracks to
form carbon monoxide and hydrogen in accordance with the
following reaction:
CH 3 OH â CO + 2H 2
complete cracking of methanol into CO and H 2 only occurs
if the temperature is above 700-800°C (12921472°F),which is why methanol should not be introduced
into a furnace at a lower temperature. The cracking of
methanol into CO and H2 requires energy. This energy is
taken from the area surrounding the point of methanol
injection. There must therefore be sufficient heat flux
towards the injection point to ensure proper dissociation
181.
182. A high gas flow is desirable in the following
cases:
â At the beginning of a cycle when the furnace
is originally airfilled or has been contaminated
with air after a door opening. The higher the
gas flow is, the faster the correct gas
composition will be obtained.
â When carbon demand is great, i.e. at the
beginning of a process or in cases with a large
charge surface area.
183.
184. Low gas flow can be used in the
following cases:
â When the furnace is empty.
â When the carbon demand is low, i.e.
at the end of a process or in cases with
a small charge surface area
188. Pack carburising
â The component is packed
surrounded by a carbon-rich
compound and placed in the
furnace at 900 degrees.
â Over a period of time carbon
will diffuse into the surface
of the metal.
â The longer left in the
furnace, the greater the
depth of hard carbon skin.
Grain refining is necessary in
order to prevent cracking.
189. The gas-carburizing process
In gas carburizing the workpieces are heated in
contact with carbon containing gases such as the
hydrocarbons, methane, ethane, and propane. The
carburizing gases are diluted with an endothermic
carrier gas which consists mainly of nitrogen (N2)
and carbon monoxide (CO) along with smaller
amounts of carbon dioxide (CO2), hydrogen (H2),
and water (H2O). Of these gases, N2 is inert and
acts only as a dilutent. The carrier gas serves to
control the amount of carbon supplied to the steel
surface and prevents the formation of soot
residue.
190. The reactions involved in carburizing are as below. First, the methane
or propane enrichment of the carburizing-gas mixtures provides the
primary source for the carbon for carburizing by slow reactions such
as
CH4 + CO2 â2CO + 2H2 (14-1)
CH4 + H2O âCO + 3H2 (14-2)
These reactions decrease the concentrations of CO2 and H2O and
increase the amounts of CO and H2. Then the CO breaks down to
deposit and allow the carbon to diffuse into the steel surface by the
following overall reversible reactions:
2CO âC (in Fe) + CO2 (14-3)
CO + H2 âC (in Fe) + H2O (14-4)
The carbon-potential control during carburization is attained by
maintaining a steady flow of the carrier gas and varying the flow of the
hydrocarbon enrichment gas.
191. Induction: Why Induction Heat Treatment?
Advantages
Greatly shortened
Highly
Highly energy
Less-pollution
heat treatment cycle
selective
efficiency
process
Practical Problems
⢠Lack of systematic heating time and temperature distribution control inside WP.
⢠Nonlinear effect of material properties.
⢠Lack phase transformation data inside WP for hardness and residual stress determination.
⢠Evaluate combination effect of AC power density, frequency and gap on final hardness pattern.
⢠Trial and error, cost and design period.
Research content: FEM based electromagnetic/thermal analysis
Numerical modeling
may provide better
prediction
+ quenching analysis + hardening analysis
Research objective: (1) Provide T field, time history inside WP
(2) Determine formed content of martensite, pearlite and bainite.
(3) Determine hardness distribution in WP.
(4) Guidance for induction system design.
192. Introduction: Induction Hardening Process
⢠Induction heating:
metal parts heated to
austenite Phase
â˘Fast quenching
process transforms
austenite to
martensite phase
â˘Martensite
content determines
the hardness
â˘Martensitic
structure is the
most hardest
microstructure
workpiece
Inductor/coil
Heating
process
Joule heat
by
eddy
current
Electromagneti
c field
Induction
coil
High
freq.
AC
193. Principle: Electromagnetic and Thermal
Analysis
Electromagnetic Analysis
WP
Coil
Thermal Analysis with
finite element model
Input AC
power to coil
Calculation of
magnetic vector potential (A)
Calculation of
magnetic flux density (B)
Calculation of
magnetic field intensity (H)
I
0
A
4
B=
C
dl
r
(Gaussâ Law for
magnetic field)
A
(b) FEA model
(a) WP geometry
H=B/
QN
Calculation of
electric field intensity (E)
B
t
E
QN
(Faradayâs Law)
QW
QEt
QC
QE
QB
QR+ QCV
(Outside)
Calculation of
electric field density (D)
D=
E
QS
QS
(c) Interior element
Calculation of
current density (J)
Calculation of
Inducting heat (Qinduction)
Output:
Heat generation Qinduction in WP
H
D
J
t
(Ampereâs
Circuital Law)
(d) Surface element
Heat conduction
T
t
c
Qinduction = E J = J2/
c
T
t
2
k
k
2
T Qinduction
T Qinduction A
Induced Joule heat
4
F T 4 Tair
Heat radiation
A h T Tair
Heat convection
194. Case Study: Complex Surface
Hardening
Material: Carbon
Steel, AISI 1070
Automotive parts from
Delphi Inc.,
Sandusky,Ohio
â˘Concave and convex on
surface of workpiece make
the heating process not easy
to control.
concave
convex
Real spindle to be hardened
Geometry Model
â˘ANSYS system is employed
for the analysis.
â˘Mesh should be much finer
at locations of convex and
concave in both coil and
workpiece.
FEA model and B.C.
Mesh generated by ANSYS
195. Case Study: Material Properties -- AISI
1070
(a) Electromagnetic Properties
conductivity
WP relative
permeability
Electrical
Resistivity
(b) Thermal Properties
Specific
heat
Emissivity
Convection
coefficient
197. Effect of current density distribution
⢠Constant current distribution in
coil can not result in good heating
pattern, especially at concaves of
workpiece
⢠Better hardened pattern
resulted from modification of
Finer coil mesh and enhanced
coil current density at area
neighboring to surface concaves
of workpiece.
(a1) Constant current distribution in coil
(a2) heated pattern
(b1) Adjusted current distribution in coil
(b2) heated pattern
⢠Enhanced coil current density
suggests utilization of magnetic
controller at those area in coil
design process. Physically this
can be fulfilled by magnetic
controller.
198. Case Study:Temperature Variation with Time in Induction Heating
Process
t=0.5s
Total heating time
th = 7.05s
f=9600Hz
s=1.27mm
J=1.256e6 A/m2
t=4s
t=2s
199. Case Study: Heating Curves
Summary
⢠A finite element method based modeling system is developed to analyze
the coupled electromagnetic/thermal process in induction heating and
implemented in ANSYS package, with following capabilities.
⢠Provide electrical and magnetic field strength distribution.
⢠Provide instantaneous temperature field data in workpiece.
⢠Provide Temperature history at any location in heating process.
⢠Provide guidance for inductor/coil design based on adjustment of current
density distribution and desired heating patterns.
200. Quenching of carburized parts
Carburized parts are usually quenched from
the austenitic condition to produce a hard
case with a martensitic structure, as shown
201. Most gas-carburized parts are directly
quenched from the carburizing temperature
of about 925°C or from about 845°C without
being cooled to room temperature. The
decrease in temperature from about 925 to
845°C can be accomplished by allowing the
temperature of the carburizing to decrease,
moving the workpiece to a lower
temperature zone of the furnace, or by
transferring the workpiece to another
furnace
202. Tempering of carburized parts
Many carburized and hardened parts are placed into
service without tempering, especially if the applications for
the parts are not critical with respect to cracking and
chipping. On the other hand, many hardened carburized
parts are given a low-temperature temper treatment,
usually in the 150 to 190°C range, since in this temperature
range, hardening is not greatly reduced and toughness and
resistance to cracking is slightly increased
203. The diffusion of nitrogen into the surface layers of low
carbon steels at elevated temperature. The formation of
nitrides in the surface layer creates increased
mechanical properties.
204. â˘Benefits of Nitriding
â˘Types of Nitriding
â˘Future of Nitriding
â˘Process Determination
â˘CVD Reaction
â˘Deposition Process
â˘Diffusion Depth
â˘Process Results
205. â˘Principal Reasons for Nitriding are:
â˘Obtain High Surface Hardness
â˘Obtain a Resistant Surface
â˘Increase Wear Resistance
â˘Increase Tensile Strength and Yield Point
â˘Improve Fatigue Life
â˘Improve Corrosion Resistance
(Except for Stainless Steels)
206. â˘Improves Mechanical Properties
â˘Surface Hardness
â˘Corrosion Resistance
â˘Chemical Reaction
â˘Nitrogen & Iron
â˘Core Properties Not Effected
â˘Temperature Range
â˘495 - 565 ÂşC
â˘Below Tempering Temperature
⢠White Layer By-Product
â˘Thin
â˘Hard Iron Nitride
207. â˘Process methods for nitriding include:
â˘Gas
â˘Liquid
â˘Plasma
â˘Bright
â˘Pack
***Lots of more nitriding methods for
specific applications***
http://www.nitriding.co.uk/np01.htm
209. Liquid nitriding:
â˘Thermo-chemical Diffusion Treatment
â˘Hardening Components With Repeatability.
â˘Salt Bath, at Less Critical Temperatures.
â˘Preserves Dimensional Stability
â˘Corrosion Protection
â˘Exhibit Long-Term Resistance to Wear,
Seizure, Scuffing, Adhesion and Fatigue.
210. â˘Vacuum Chamber
â˘Pressure = 0.64 Pa
â˘Pre-Heat Cycle
â˘Surface Cleaning
â˘Ion Bombardment
â˘Control Gas Flow
â˘N, H, CH4
â˘Ionization by Voltage
â˘Blue-Violet Glow
â˘Wear Resistant Layer
http://www.milwaukeegear.com/nitrid.htm
211. Mechanism of Nitriding
⢠Fe- N equilibrium diagram can be used to study nitriding
process.
⢠The solubility limit of nitrogen in iron is temperature
dependent, and at 450 °C the iron-base alloy will absorb
up to 5.7 to 6.1% of N.
⢠Beyond this, the surface phase formation on alloy steels
tends to be predominantly Îľ-phase.
⢠This is strongly influenced by the C-content of the steel;
the greater the carbon content, the more potential for
the Îľ phase to form
⢠As the temperature is further increased to the gamma
prime (Îłâ˛-nitride) phase temperature at 490 °C , the
âwindowâ or limit of solubility begins to decrease at a
temperature of approximately 680 °C.
212.
213. Effect of alloying elements
⢠Plain carbon steel form Iron nitride
(brittle & low hardness) on nitriding due to
absence of nitriding forming element.
⢠Strong nitriding elements- Al, Mo, Cr, Ti,
V etc form nitrides causes internal
precipitation of nitrides resulting high
surface hardness.
⢠Steel containing several alloying elements
have higher hardness than by a single
element
215. â˘Replacing Liquid Nitriding
â˘Environmental Effects
â˘Ease of Control
â˘More Complex Substrates
â˘Performed at Lower Temperatures
â˘Creates Higher Residual Stress
http://www.northeastcoating.com/
216. Gas Nitriding
â˘How do the variables of nitriding steel affect the
process and the mechanical properties of the
surface?
â˘The following variables were investigated:
â˘Time
â˘Temperature
â˘Gas Velocity
â˘Develop Process Model
226. Disadvantages
â It is an expensive process.
â Much more time is required to develop the
requisite case depth (due to low temp)
â Expensive gas ammonia is used in nitriding.
â Expensive alloy steels can only be nitrided
and are used.
â Nitriding is more expensive than
carburising and carbonitriding
227. Carbonitriding of Steels
Carbonitriding is a modified form of carburizing and is not a
form of nitriding. The modification in carbonitriding consists of
adding ammonia (NH3) to the carburizing gas so that nitrogen
diffuses in the steel case along with carbon. Carbonitriding is
usually carried out at a lower temperature and for a shorter
time than gas carburizing, and so a thinner case is usually
produced than by carburizing. Carbonitriding is principally used
to produce a hard, wear-resistant case in steels, normally from
0.075 to 0.75 mm thick. Nitrogen increases the hardenability of
steel, and so a carbonitrided case has higher hardenability than
a carburized case on the same steel. Also, since nitrogen is an
austenite stabilizer, high nitrogen levels can result in retained
austenite, particularly in alloy steels. Maximum hardness and less
distortion can be attained by carbonitriding since less drastic oil
quenching than for carburizing can be used.
229. Introduction
⢠LASER beams are invisible
electromagnetic radiation in infra-red
portion of spectrum.
⢠Used for surface hardening of ferrous
material
⢠Laser used for hardening:
â YAG- Solid state type
â CO2 Gas type
⢠CO2 Laser is commonly used for surface
hardening when the power required is
more than 500W.
230. Hardening MechanismHypoeutectoid
⢠Consider the microstructure of a
hypoeutectoid steel containing 0.35% carbon.
It consists of pearlite colonies surrounded by
proeutectoid ferrite.
⢠On heating,
â Pearlite ď Austenite (dissolution of the
cementite),
â Growth of the Austenite transformation front
into regions of high carbon concentration, at a
rate controlled by carbon diffusion between the
lamellae.
â Ferrite transforms by nucleation and growth of
austenite at internal ferrite grain boundaries,
(rate controlled by carbon diffusion
231.
232. ⢠The phase diagram shows that under
equilibrium conditions, pearlite begins to
transform to austenite at 723âŚC(Ac1), and
that transformation of ferrite is complete at
about (Ac3) temperature.
⢠The high heating rate experienced during
laser heating (on the order of 1000 K sâ1)
results in superheating of Ac1 and Ac3,
typically by about 30 and 100âŚC, respectively.
⢠The heat is conducted to the bulk at a very
fast rate which results in surface quenching
and martensitic hardening.
233. ⢠By selecting power density and the speed
of the laser spot a desired case depth can
be hardened.
⢠Case depth also depends on hardening
response of ferrous material (not more
than 2.5mm)
234.
235. Why LASER Hardening?
⢠Local surface hardening exactly where
required
⢠Low distortion and no rework
⢠Short wavelength enabling superior
absorption
⢠Closed-loop temperature control
⢠Highest process efficiency of all laser
types
⢠Extremely reliable for production
processes
⢠Highest level of process safety and
236.
237.
238. Advantages of LH vs other hardening
techniques
⢠Selective areas hardened without affecting surrounding material
⢠Possible automation and integration with other in-line production processes
⢠Quick turn-around time
⢠Treatment depth accurately controlled and highly reproducible due to direct
temperature control
⢠Superior hardness can be obtained compared to conventional processes
⢠(typically 20% higher hardness)
⢠No external quenching required â eliminates complex quench equipment
⢠Minimal heat input â limited distortion â no need for post treatment
machining â final machined components for laser hardening
⢠Phase transformation + volume expansion â residual compressive stresses into
surface â improves mechanical properties, e.g. wear and fatigue resistance,
lowers crack sensitivity
239. Laser hardening applications
⢠Steering gear assemblies
⢠Turbine blades
⢠Cutting edges and edges of dies for sheet metal forming
⢠Cam followers, Gear teeth and Shafts
⢠Rim geometries, e.g. Piston rings
⢠Plastic injection moulds at highly loaded areas on the surface
⢠Cylinder liners in diesel engines
⢠Heavy duty and ball bearing steels
⢠Tool steels
242. â TOOL STEEL are high quality steels made to
controlled chemical composition and processed to
develop properties useful for working and shaping of
other materials.
â The Carbon content in them is between 0.1 -1.6% . Tool
steel also contain alloying elements like, Chromium,
Molybdenum and Vanadium.
â Tool steel offers better durability, strength,
corrosion resistance and temperature stability, as
compared to the Construction & Engg. Steel.
â These are used in applications such as Blanking, die
forging, forming, extrusion and plastic molding etc..
243. OBJECTIVES OF HEAT
TREATMENT OF TOOL STEELS
â To obtain a desired microstructure and
properties suitable for machining or cold
deformation.
â To release residual stresses accumulated
during previous thermal and mechanical
treatments.
â To homogenize the microstructure with
globular carbides by a spheroidization
treatment.
â To dissolve by a normalizing treatment the
intergranular carbides that are detrimental to
the mechanical properties of tool steels.
244. CHARACTERISTICS OF TOOL
STEELS
â Require special heat treatment process.
â Higher cost than alloy steels.
â Better hardenability than most carbon
and alloy steels.
â Higher heat resistance.
â Easies to heat treat.
252. SHOCK RESISTING TOOL
STEELS
â Carbon content = 0.5-0.6%. Alloying elements â Cr, W ,
Mo.
â These are characterized by good toughness, hardness
and improved hardenability. These steels are generally,
water or oil- hardened.
â âLow temperature Temperingâ is carried out where,
toughness and hardness of the tool steel are of prime
importance, otherwise âHigh temperature Temperingâ is
preferred.
â Silicon-manganese steels (0.55% C, 2.0% Si, 1.0 % Mn)
are included in this group. Due to their high Si-content,
decarburization and grain coarsening takes place in these
type of steels.
253. ⢠HEAT TREATMENT PROCEDURE (in general) :â Annealing : Slow & uniform heating in the range of
790-800°C followed by furnace cooling at rate of
8-15°C/hr.
â Stress relieving : Heat to 650- 675°C and furnace
cooling.
â Hardening :
Preheating â warming to about 650°C &
holding for 20 minutes/ 25mm.
Austenitizing â heating to 900-950°C &
holding again for 20minutes/25mm.
â Tempering : Heating to 205-650°C, holding for 30
minutes/25mm and then, air cooling.
256. HOT WORKED TOOL STEELS
â Carbon content = 0.3-0.5% . These steels are used for high
temperature metal forming operation (except cutting), where
the temperature is around 200-800°C.
â These are characterized by high hot yield strength, high red
hardness , wear resistance, toughness, erosion resistance,
resistance to softening at elevated temperatures, good
thermal conductivity
â These are divided into 3 groups depending on the principle
alloying elements:
â Chromium based [H11- H19]
â Tungsten based [H20- H26]
â Molybdenum based [H41- H43]