2. Isothermal Transformation Kinetic
s
time
Mat E 443 Ferrous Metallurgy
Iowa State University
R.E. Napolitano
T Time-Temperature-Transformation curve
(TTT-
curve)
Time to 50% completion
4. Isothermal TT
T
Curves
T
T1
time
1
f = fraction transformed
f 0.5
Mat E 443 F e0
rr.o1
us
Metallurgy
0 Iowa State University
R.E. Napolitano
10% 50% 90%
0.9
f = fract
M
7. CCT
from
cooling curves on log(t) scale
the TT
T
not isothermal
as you are cooling, time is passed at
higher temperatures where nucleation
start is far away
by the time you get to lower T,
nucleation gets “pushed” further right
8. Ms and Mf
Alloying additions affect Ms and Mf
determines where you have to cool to for complete martensite
Mf determines retained austenite
machining problems!
Totten, G.E. et. al. Quenchants and Quenching Technology
9. Retained Austenite Issues
Austenite = soft (decreased part hardness)
subsequent transformation to martensite during machining or end-use
expansion -> internal stresses -> loss of machining tolerance
hard martensite appearing in otherwise tempered microstructure
1% C, 1% Mn, and 0.4% Mo – 40% retained
γ
George Langford, Sc.D., Massachusetts Institute of
Technology
11. What does Hardenability Allow?
High Hardenability – Oil
Quench
Moderate Hardenability – Oil Quench
Low Hardenability – Water Quench
Totten, G.E. et. al. Steel Heat Treatment: Metallurgy and Techniques
Hardenability is an inherent property of the material
Hardening (hardness distribution or depth profile) is a function of processing factors
1. Shape and size of the cross section
2. Hardenability of the material
3. Quenching conditions
12. Sample Geometry
Sample geometry plays a large role in the heat treatment of real components
Thermal conductivity of steel is 54 W/mK @
25C
∞
better than air (0.024) or water (0.58), but
<<<<<
Outer surface may cool at “water quench” rate
inside will not
13. Distortion
Fundamental Causes
1) Residual stresses that cause shape change when they exceed the material yield
strength
2) Stresses caused by differential expansion due to thermal gradients. These stresses
will increase with the thermal gradient and will cause plastic deformation as the yield
strength is exceeded.
3) Volume changes due to transformational phase change. These volume changes will
be contained as residual stress systems until the yield strength is exceeded.
Totten, G.E. et. al. Steel Heat Treatment: Metallurgy and Techniques
14. Thermal Distortion – Residual Stresses
As heat is applied expansion will occur. The regions of
tension and compression that result are directly related
to this expansion. Upon cooling this situation will reverse
itself.
Constraining the piece can cause residual stresses to
exist.
Question: What residual stresses exist in these figures?
Chumbley, S. Mat E 444 - Corrosion and Failure Analysis. Iowa State University.
2009.
15. Thermal Distortion – Residual Stresses
Always remember that the last material to cool
will be in tension. Spot heat or partial heating of
a part (e.g. flame cutting a beam, welding) can
cause local stresses and distortions.
Do these residual stresses make sense?
Chumbley, S. Mat E 444 - Corrosion and Failure Analysis. Iowa State University.
2009.
16. Transformational Distortion
Vα(BCC)<Vγ(FCC)<Vα’
(BCT)
(well known volume changes)
However, all three events occur simultaneously.
Other factors:
heating/cooling rate
geometry
inconsistent material composition
In steels when hardening using the martensitic transformation the general rule is that
the metal that is the last metal to harden will be in compression
17. Quench Cracking
Example - cracked replacement retention pin cap for ripper tip wearout
Cracks were noticed after 32 hours in the field
Vetterick. et. al. Retention Pin Cap Failure Analysis. Mat E 444. Iowa State University.
2009
18. Quench Cracking
Example - cracked replacement retention pin cap for ripper tip wearout
Vetterick. et. al. Retention Pin Cap Failure Analysis. Mat E 444. Iowa State University.
2009
19. How do we Prevent Quench
Cracking?
Hardenability is an inherent property of the material
Hardening (hardness distribution or depth profile) is a function of processing factors
1. Shape and size of the cross section
2. Hardenability of the material
3. Quenching conditions
Totten, G.E. et. al. Steel Heat Treatment: Metallurgy and Techniques
20. Measuring
Grossmann’s Hardenability Concept
Hardenability
quench cylinders of a given material into a given quenchant
defined 50% martensite as criteria for hardenability
critical diameter Dcrit defined as the diameter where 50% martensite
Dcrit is valid only for the quenching media used
21. Measuring Hardenability
Grossmann’s Hardenability Concept
defined 50% martensite as criteria for hardenability
critical diameter Dcrit defined as the diameter where 50% martensite
Why?Very easy to determine using optical microscopy
Brooks, Charlie R. Principles of the heat treatment of plain carbon and low alloy steels. ASM International.
1996.
22. Quench Severity
Grossmann’s Hardenability Concept
introduced the quenching intensity (severity) factor H
describes quenching medium and its condition
defined ideal critical diameter DI
diameter of a given steel that would produce 50% martensite at the center
when quenched in a bath of quenching intensity H∞
H∞ indicates a hypothetical quenching
intensity that reduces the surface temperature
of the heated steel to the bath temperature in
zero time
23. Quench
Grossmann’s Hardenability Concept
Severity
Steel A:
quenched in still water
(H=1.0)
Dcrit = 28mm
Steel B:
quenched in oil
(H=0.4)
Dcrit = 20mm
However,
Steel A:
DI = 48mm
Steel B:
DI = 52mm
By definition, Steel B has a higher hardenability than Steel
A
independent of quenching medium
27. What determines Hardenability?
Prior austenite grain size
smaller grain size = greater pearlite nucleation
greater pearlite nucleation = lower hardenability
ASTM grain size No. 7 has 4x the grain-boundary area of ASTM No.
3
larger grains = increased brittleness, loss of ductility
increased quench cracking
Carbon content
>%C = increased hardenability
still very low
AISI 1045 ASTM No.
7
DI = 5.6mm (0.%%
in)
pearlite and proeutectoid phases
more difficult to form at higher C
austenitized in V + Fe3C nucleates pearlite
28. What determines Hardenability?
Alloying
all alloying elements increase hardenability in steel (to varying degrees)
exception: Cobalt
increases nucleation and growth of pearlite
alter the phase diagram
prevent decomposition of V -> α + Fe3C
(pearlite)
For 4340 – have 90 seconds to cool to Ms
