The document discusses various heat treatment processes and their effects on steel properties. It begins by explaining the crystal structures of pure iron and how they change with temperature. It then discusses how carbon content affects the cooling of steel from an austenite state. Rapid cooling forms martensite, while slower cooling forms pearlite or cementite. Various steel grades and their applications in heat treatment are also covered, along with processes like carburizing, nitriding, and precipitation hardening. Microscope images demonstrate the microstructural changes from these treatments.

1. Heat Treatment
Presented to
Irish Light Aviation Society
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2. Crystal structure of Pure Iron
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α iron BCC and exists at room temp
Γ iron FCC BCC changes to FCC at 910C
Δ iron BCC at 1400C γ reverts to BCC

3. Iron and Steel – Alloy
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4. Phase diagram - Summary
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• Up to 2% carbon can dissolve in FCC iron
• Only 0.02% carbon can dissolve in BCC iron
• As FCC steel (Austenite) cools slowly and changes to
BCC any dissolved carbon in excess of 0.02%
precipitates and forms Cementite Fe3C
• If FCC is cooled rapidly the precipitation is prevented
and it is this characteristic that is the heart of
“Hardening of Steel by Heat Treatment”

5. Slow Cooling
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If a steel containing 0.4% carbon is slowly cooled from
above the austenising temperature
• small crystals of BCC start to form containing 0.02%
carbon and the carbon in the austenite increases.
• When the carbon content of the austenite reaches
0.8% carbon starts to precipitate in the form of
cementite.
• Ferrite and cementite form alternate layers known as
Pearlite

6. Slow Cooling - continued
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If a steel containing 0.8% carbon is slowly cooled from
above the austenising temperature
• small crystals of BCC start to form containing 0.02%
carbon and the carbon in the austenite increases.
• When the carbon content of the austenite reaches
0.8% ,carbon starts to precipitate in the form of
cementite.
• A mix of pearlite and cementite is formed

7. Microscopic Images
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Pearlite and ferrite, 0.3%C Pearlite with no ferrite, 0.8%C

8. Rapid Cooling - Quenching
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When austenite is rapidly cooled another structure called
Martensite is formed. This is not shown on the phase diagram
because it is not an equilibrium state. Rapid cooling prevented
the equilibrium state from forming.
In simple terms the FCC structure of austenite has changed to
BCC of ferrite but the carbon hasn’t diffused out of the structure.
In effect Martensite, is a highly stressed, supersaturated BCC
structure.
Martensite is very hard and very brittle.

9. Quenching Rate
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• For a plain carbon
steel to get
Martenite cooling
rate must be faster
than ½ speed on
sound
• Alloying elements
eg Nickel, Chrome,
Molybendum,
Manganese can be
added to move
the C curves to the
right – ie slower
cooling needed.

10. Microscope images of hardened steel
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Untempered Martensite Tempered martensite

11. Measurement of Hardness
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• Hardness is not an intrinsic material property dictated in terms of
fundamental units but people instinctively know know what it is.
• Hardness is the property that enable the material to resist plastic
deformation.
• Hardness is devined by the test method which is invariably an indent
method.
• Various methods, Rockwell A, B, C. scales.
• Vickers, Brinell, Microhardness.

12. Strength of Steel
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13. Steel Grades and Heat Treatment
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Mild steel – since no carbon cannot in itself be heat treated
unless carbon is added. (Carburising)
Plain Carbon steels - 0.3C to 0.5% common grades AISI
1030 to 1050 limited hardenability typically shafts hardness
range high 30 to low 50’s HRc
Spring steels 0.5 to 0.9%C – common grades 1060 (-.6%C)
High hardness, mid to high 50’s HRC for springs.
Low Carbon, medium alloy – common grade 8620 0.22%C.
Good for Shafts and suitable for carburising. Ductile centre
with hardness circa mid 30’s HRc. Oil hardenable with a
very hard case, high 50’s HRc.
Medium carbon, low alloy. 0.3 to 0.5%, Cr, Mo alloy. 4130,
4140. High strength up to 50’s HRc. High strength general
engineering and aerospace. EN16= 4130, EN 19=4140 Oil
harden.

14. Steel Grades and Heat Treatment
Continued
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Medium Carbon, higher alloys. Ni, Cr, Mo. 4340=EN24. Oil harden.
Nickel increases uniformity in hardness. High stress engineering
applications. Small pieces will air harden

15. Steel grades and Heat Treatment
Continued
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Tool Steels – AISI teminology
W – Water hardenable tool steel rarely seen now
S grades – Shock resistant rarely seen high Silicon give the
shock resistance
O grade – Oil Hardening O1 very common. Cold work, shear
blades etc. Max hardness 62HRc
A grades – Air hardening A2 commonly seen. Air hardening
and cold work applications.
D series – high carbon, high Cr. Cold work tool steel. D2. Dies
for car body parts.
H series – hot work tool steels. H13 commonly seen. Plastic
injection moulding. Hardness low 50’s. Vanadium give the
higher heat resistance

16. Steel grades and Heat Treatment
Continued
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High speed steels – Cutting tools
T series circa 0.75C with 18% Tungsten (W) Air hardenable, mid
60’s HRc.
M series - primarily Molybdenum (8.5%) + others . High 60;’s
HRC.
Martensitic Stainless Steels
4 series 410, 420, 430 & 440 with increasing hardness, ie
increasing carbon lower corrosion resistance. Low 40’s (410) to
high 50’s (440). Chrome is locked up by Carbon so less Cr for
corrosion resistnace.

17. Processes at NHTC
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Vacuum Hardening
Air hardening steel is heated in vacuum – therefor no oxygen – no corrosion
and quenched in nitrogen gas. Nitrogen gas ins circulated within chamber
heat is transferred in a heat exchange to water.

18. Process at NHTC
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19. Belt Furnace
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Suitable for small parts – oil / water hardening
Furnace is sealed, except opening and flushed continuously with
nitrogen to prevent corrosion

20. Carburising
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This process is used to case harden steels with little of no carbon.
Carbon is diffused into the steel at about 910C. Carbon is
obtained by cracking hydrocarbon, we use Propane. Case depth
depends on time circa 0.8mm after 3hrs.
Parts are then quenched.

21. Heat Treatment
previously and art – now a science
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22. Precipitation Hardening
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Certain grades of aluminium harden by precipitates forming
Solution treatment involves heating the parts to dissolve the precipitate
In the metal and then rapid (water) quenching. This locks the parts in
Solution.
The parts are precipitated slowly at room temperature (age hardening) or
The process is accelerated by higher temperature.
The PH range of steels can also be processed by this method.

23. Nitriding
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Nitriding is not a quenching process – hardening is achieved
by diffusing nitrogen atoms into the steel which form Nitrides
with the iron and these are very hard. If there is a little 1%
aluminium in the steel this improved nitride formation. NHTC
do this in a fluidised bed furnace. Temperatue is 525C and
hence little or no deformation. It is a surface hardening
process. Nitrogen is obtained from breakdown of NH3

24. Micro Images of Nitriding
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