Hardening Hardening cycle

1. Hardening

2.  Hardening treatment consist of heating to predetermined
temperature usually known as hardening temperature ,holding at
that temperature followed by rapid cooling such as quenching in
water ,oil or salt water .
 High tensile strength and hardness can be achieved by this
process.
 The high hardness is achieved by rapid cooling , rapid cooling
means cooling rate is equal to more than the upper critical
cooling rate.
 Rapid cooling results in the transformation of austenite at
considerably low temperature product called martensite which is
a hard microconstituent of steel.
Introduction

5.  Hypo eutectoid steel
 The hardening temperature depends on chemical
composition in plain carbon it depends on carbon
only .
 Hypo eutectoid steel is heated to about 30 to 50 ̊C
above the upper critical temperature.
 Ferrite and pearlite transform to austenite at
hardening temperature.
Hardening
Of Hypo And Hyper Eutectoid Steel

6.  The austenite transform to martensite on rapid quenching
.
 If hypo eutectoid steel is heated to a hardening
temperature equal to hyper eutectoid steel the structure
will consist of ferrite and austenite .
 This will transform to martensite and ferrite on quenching.
 Ferrite is very soft phase lows the hardness of hardened
steel .
 This is known as incomplete hardening
.
Count….

7.  Hyper eutectoid steel is heated to about 30 to 50 ̊C above the
lower critical temperature.
 The preferred hardening temperature for this steel lies between
A1 and Acm .
 there are two advantages in this temperature range .
 The first one is related to the presence of cementite in hardened
steel .
 The presence of cementite in martensitic matrix increase the
wear resistance because both the constituent are hard .
 The second advantage is to attainment of fine martensite in the
final structure.
Hyper Eutectoid Steel

8.  If we heat the hyper eutectoid steel above Acm for
hardening the coarsening of austenitic grains and
decarburization occur .
 Coarse austenite ill transform to coarse acicular martensite
which has poor mechanical properties .
 Decarburized surface respond poorly to hardening
treatment.
 In addition to these factors quenching from such a high
temperature will introduce several internal stresses into
the hardened steel
Count…..

9.  Important factors are
 Chemical composition of steel
 Size and shape of steel part
 Hardening cycle
 Homogeneity and grain size of austenite
 Quenching media
 Surface condition of steel part
Factors Effecting The Hardening
Process

10.  Critical hardening temperature depends on the chemical
composition of steel.
 In alloy steel hardening temperature has to be controlled closely
and depends on the nature and amount of the alloying element.
 Comparatively large amount of retain austenite is present in the
hardened steel contain austenitic stabilizing element due to
which strength reduces .
 If there is carbide forming element in a given alloy steel the
larger is the volume fraction of carbide in the hardened steel and
higher is the wear resistance.
 Other properties such as longer too life ,increase cutting ability
can be achieved if carbide are present in martensitic matrix .
Effect Of Chemical Composition

11.  Large parts with variable thickness of sections are heated
at a very slow rate as to avoid thermal gradient between
outer and inner layer
 These parts should be held at the hardening temperature
foe sufficient time to attain uniform temperature
throughout the volume .
 Cooling rate should also be appropriate. Sharp corner
should be avoided .
 For larger parts martensitic transformation will not
proceed throughout the volume this results in production
of high magnitude of internal stresses.
Effect Of Size And Shape

12.  For complete hardening, hardening temperature
should be such that homogenous austenite which
minimum grain size.
 Ideally cooling rate should be equal to the just upper
critical cooling rate
 cooling rate more than the critical cooling rate will
result in development of large internal stress
 Slow heating rate is desirable in some cases.
 However slow heating rates are to be avoided where
the problems of oxidation and decarburization is
encountered
Hardening cycle

13.  In such cases steel component is heated in two stages.
 To start with, it is heated to an intermediate temperature which is
lower than the austenitizing temperature.
 The component is socked at this temperature for a sufficiently
long time to avoid oxidation and decarburization .
 Now the steel part is transferred to other furnace to desire
temperature.
 In this way the problem of oxidation and decarburization is
minimum .
 Steel component which are heated to a hardening temperature
at slow heating rate required less holding time for
homogenization
Contind…..

14.  The properties of hardened steel depends to a large extent to a
nature of austenite because martensite in the quenched steel is
formed by the direct transformation of austenite.
 Homogeneity and grain size of austenite play an important role
as far as final properties are concerned.
 Quenching Media
 A quenching media is that is one which provides a cooling rate
higher than the critical cooling rate.
 Quenching media characteristics such as its temperature,
specific heat, thermal conductivity, and latent heat of
vaporization effect the cooling rate to a great extent
Homogeneity And Grain Size Of
Austenite & quenching media

15.  The presence of oil grease scale and other foreign
particles on the surface is not desirable.
 Oil and grease burn during heating and leave behind
residue which is bad conductor of heat.
 Thus the spots where this residue exists will cool at
slower rate than the rest of the surface.
 This may lead to incomplete hardening.
 Presence of scale and foreign particles also result in
variable cooling rate within in the mass.
Surface condition of steel part

16.  Following are the hardening method
 Convectional or direct quenching
 Quenching in stages
 Spray Quenching
 Quenching with self tempering
Hardening methods

17.  It consist of quenching the steel component from
hardening temperature to quenching media.
 The part is allow to cool up to to the temperature of
quenching bath.
 In addition to sever to internal stress steel part also
develop tendency towards the distortion and cracking due
to drastic cooling rate .
 Cooling rate can be controlled by other quenching media
like oil only smaller sections can be hardened in
quenchants having lesser quenching power
Direct Quenching

18.  Quenching in stages in sequence in different media consist
of quenching steel part from hardening temperature to a
bath maintained at a predetermined temperature which is
higher than the MS temperature.
 The medium used generally is water than it is transferred
quickly to a milder quenching medium where it s cooled to
room temperature such as oil and air.
 Internal stresses are less in this method due to two
reasons
1. The severity of cooling is reduced
2. Internal stresses develop are also reduced
Stage Quenching

19.  In this method steel part is cooled rapidly from
hardening temperature by spray quenchant
continuously
 In this case the rate of heat extraction from the steel
part is much higher as compare to direct quenching
process due to continuous stream of quenchant in
contact with steel surface
 This process is used for selective hardening
 Water is most commonly used quenchant
Spray Quenching

20.  Certain application specially those involving impact loading
require a soft and tough core with hardened and toughened
case.
 This condition is attained by quenching with self tempering.
 In this process steel part is quenched from the hardening
temperature.
 The part is withdrawn from the quenching bath after some
time without allowing it to cool completely so considerable
amount of heat will be retained in this central portion.
 The component is cooled in mild quenching media such as
oil/air.
Hardening with Self Tempering

21.  The first quenching results formation of martensite
 The depth up to which martensite forms will depend to a great
extend on the time period allowed to remain in the quenching
bath.
 Cooling during second quench will result in homogenization of
temperature
 The core will be cooled at much reduce rate.
 The core will undergo austenite to pearlite transformation in
place of martensite
 So already formed martensite will be tempered by without any
additional tempering during second cooling
 Thus the result case will get hardened and core get soft