This document discusses the effect of cooling rate and quenching on microstructure and hardness of steel. It covers various topics such as the different phases of iron and steel based on carbon content and temperature. It describes the microstructures that form from equilibrium and non-equilibrium cooling, including pearlite, bainite, and martensite. Factors that influence cooling rates during quenching are identified. Hardness testing methods are also briefly outlined. The document provides references for further reading on steel metallurgy topics.

1. Isfahan University of Technology
Materials Department
Heat treatment laboratory
Effect of cooling rate and Quench on microstructure
and hardness
Reza Rashidi
WWW.MATERIALS.IUT.AC.IR 1

2. I. Introduction
II.Experimental
III.Conclusion
IV.References
Contents :
REZA.RASHIDI2012@GMAIL.COM 2

3. Introduction: Pure IronPureIron
Ferrite Iron α (BCC) 0-912˚C
Austenite Iron γ(FCC) 912-1392˚C
Delta Iron δ(BCC) 1392-1536˚C
HISTORICAL NOTE
The first three letters of the Greek alphabet are alpha, beta, and
gamma (α, β, γ), but there is no structure of iron called beta iron.
When the structure of iron was being discovered in the late 19th
century, the magnetic transition in iron that occurs at 770 °C
(1420 °F) caused scientists to theorize a structure of iron they
called beta iron, which was later shown not to exist.
Fig .1. Temperature dependence of the
mean volume per atom in iron crystals
(Hume-Rothery,The Structure of Alloys of
Iron, Pergamon Press, Oxford, UK, 1966).
REZA.RASHIDI2012@GMAIL.COM 3

4. Introduction: Iron –Carbon (Steel)
Austenite Up to 2.11%C fcc
Ferrite Up to
0.022%C
bcc
Cementite 6.67%C orthorhombic
Equibriumphase
REZA.RASHIDI2012@GMAIL.COM 4

5. Introduction: Iron –Carbon (Steel)
Critical temperature
REZA.RASHIDI2012@GMAIL.COM 5

6. Microstructure of Steel :equilibrium cooling rate
hypo eutectoid eutectoid hypereutectoid
Pearlite
Coarse
Fine
TABLE .1 ■ The effect of carbon on the strength of steels
Slow Cooling (Coarse Pearlite) Fast Cooling (Fine Pearlite)
Carbon %
Yield
Strength
(psi)
Tensile
Strength
(psi)
%
Elongation
Yield
Strength
(psi)
Tensile
Strength
(psi)
%
Elongation
0.20 42,750 57,200 36.5 50,250 64,000 36.0
0.40 51,250 75,250 30.0 54,250 85,500 28.0
0.60 54,000 90,750 23.0 61,000 112,500 18.0
0.80 54,500 89,250 25.0 76,000 146,500 11.0
0.95 55,000 95,250 13.0 72,500 147,000 9.5
After Metals Progress Materials and Processing Databook, 1981.
REZA.RASHIDI2012@GMAIL.COM 6

7. Microstructure of Steel: disequilibrium cooling rate
1. Martensite
Austenite(fcc)
0.8%C
Slow cooling
rate
Enough time
for diffusion
Ferrite (bcc)
0.02%C
Austenite(fcc)
0.8%C
Rapid
cooling rate
There is not
enough time
for diffusion
Martensite
(bct)
As %C
increase
c/a
(tetragonalite)
Increase
Strength
increase
REZA.RASHIDI2012@GMAIL.COM 7

8. Microstructure of Steel : disequilibrium cooling rate
1. Martensite
Lath
Plate
Mixed
Lath Mixed plate
REZA.RASHIDI2012@GMAIL.COM 8

9. Microstructure of Steel :disequilibrium cooling rate
1. martensite
 Quench Temperature control the
amount of Martensite
 Ms and Mf, fall rapidly
as wt %C in austenite increases
 Retained austenite as a function
of carbon content in Fe-C alloys.
REZA.RASHIDI2012@GMAIL.COM 9

10. 2. Bainite (α + cem)
Upper bainite Lower bainite
 at fast cooling rates, there will be a competition
along the old austenite grain boundaries, with
pearlite forming in some places and bainite forming
in other places
REZA.RASHIDI2012@GMAIL.COM 10
Microstructure of Steel :disequilibrium cooling rate

11. Microstructure of Steel
Cooling curve :
REZA.RASHIDI2012@GMAIL.COM 11

12. Characterization of Quench Bath Cooling Performance
Factors Affecting Cooling Rates:
1. The ability of the heat to diffuse from the interior to the surface of the steel specimen
2. The ability of the quenching medium to remove heat from the surface of the part
Transfer of
heat
CONDUCTION
convection
Severity of Quench: H=F/K
F: heat transfer factor K: thermal conductivity
Cooling stage:
A:vapor blanket cooling stage
B:vapor transport cooling stage….most heat transfer rate
C:liquid cooling stage
Movement of piece Air Oil Water
None 0.02 0.3 1.0
Moderate --- 0.4—0.6 1.5—3.0
Violent ---- 0.6—0.8 3.0—6.0
REZA.RASHIDI2012@GMAIL.COM 12

13. The Hardness Test:
vickers
Rockwel
Brinell
• diamond indenter shaped in the form of a pyramid
REZA.RASHIDI2012@GMAIL.COM 13

14. References :
Steel Metallurgy for the Non-Metallurgist-2007
Steels Microstructure and Properties-2006
Steels Processing Structure and Performance-2005
The Science and Engineering of Materials - Askeland 6th Edition
Principles of the heat treatment of plain carbon and low alloy steels
REZA.RASHIDI2012@GMAIL.COM 14

15. Thanks For Your Attention
REZA.RASHIDI2012@GMAIL.COM 15