1. The “Quenching and Partitioning” Process: Background and Recent Progress
Fernando Rizzo
Seminar Cambridge,

2. International Cooperation Project
J.G. Speer, D.K. Matlock
– Colorado School of Mines, USA
F. C. Rizzo – PUC, Rio de Janeiro, Brazil
D.V. Edmonds – University of Leeds, UK

3. Quenching and Partitioning:
- Background
- Fundamental Issues
- Recent Results

4. “Conventional” Processing of Steels with g CC and Isothermal
TRIP Steels
“Conventional”
Processing of
Steels with g
CC and
Isothermal
Transformations Ac 3
Temperature C
= C i g M F
Time S
B +

5. The “Q&P” Process Quenching and Partitioning
Provisional US Patent Application: September, 2003

6. The “Q&P” Process a g Step 1. Austenitize or Intercritically Anneal
more austenite
lower Cg
- higher Ms
less austenite
higher Cg
- lower Ms

7. a Step 2. Cool (quench?) below Msg a
Austenitize + Quench Intercritical Anneal + Quench
Ms -TQ controls martensite formation
intercritical annealing has more stable austenite
and higher carbon martensite

8. Phase compositions change Phase boundaries stationary
Step 3. Diffuse Carbon from Supersaturated Martensite g
Phase compositions change
Phase boundaries stationary

9. New Processing Concept (Sheet, Bar,…etc)
Use carbon partitioning intentionally…
from partially transformed martensite to
untransformed austenite.
Usually precluded because carbide precipitation occurs during tempering of martensite.
Result: carbon-enriched austenite

10. Thermodynamics of Carbon Partitioning

11. Important Questions How much can we enrich the austenite?
That is…what are the “equilibrium” martensite and austenite compositions?
Or…when does partitioning stop?

12. “True” Metastable Equilibriumg
g + a
a + Fe3C
% Carbon
Temperature
g + Fe3C

13. “True” Metastable Equilibrium

14. “True” Metastable Equilibrium CANNOT Apply!!g
g + a
g + Fe3C a
Temperature
a + Fe3C Xa
Xalloy Xg
% Carbon
The equilibrium phase fractions are fixed by the lever rule
The actual phase fractions were fixed by cooling below Ms!

15. A New Equilibrium Condition was Hypothesized
“Constrained Paraequilibrium” (CPE) or “Constrained Carbon Equilibrium” (CCE)
Iron atoms are completely immobile (the phase boundaries are stationary).
Carbon atoms are completely mobile.
Carbon diffuses until its chemical potential (activity) is equal in ferrite and austenite.
Assume…competing reactions are precluded by Si/Al
Acta Materialia, vol. 51, May, 2003

16. Properties of “Constrained Paraequilibrium”
Not a unique condition at any temperature!
Depends on initial phase fractions/compositions

17. A3 T0 - Austenite may be more enriched or less enriched
than ortho- or para- equilibrium

18. Key Characteristics of CPE
(Fe-0.5C)
- Almost all of the carbon should partition to austenite
- Enrichment levels are potentially very high

19. Q&P Process Design Methodology
ASSUME:
- Complete partitioning of carbon to austenite
No competing reactions (carbide formation)

20. Calculations for Experimental Al-Steel (at QT)
aIA=0.5

21. Martensite Formation During Final Quench

22. Calculated Final Austenite Fraction in High-Al Steel

23. Effect of Intercritical Annealing Step

24. Effect of Manganese Content

25. Effect of Carbon Content

26. Some Experimental Results

27. 9260 Bar Steel (0.6 C, 1.0 Mn, 2.0 Si) g g Q u e n c h T m p r a t [ C50
100
150
200
250 Q u e n c h T m p r a t [ o C ]
0.2
0.4
0.6
0.8 1
0.1
0.3
0.5
0.7
0.9 P s F i 2 4 6 8 A b w . g M i n i t i a l q u e n c h i n i t i a l q u e n c h M f i n a l q u e n c h % C a r b o n g f i n a l

28. 9260 Bar Steel (10 s oC)

29. 9260 Q&P (~ 25% g)
190C, 30 s 400C

30. Retained Austenite in Fe-C Alloys

31. Fine Structure a b
190C-120 s

32. TRIP Sheet Steel

33. Current Automotive Sheet Steel Families
Elongation (%) 10 20 30 40 50 60 70
400
600
1000
200
800
1200 IF
Mild
HSLA
DP, CP
TRIP BH
CMn IS
MART
Yield Strength (MPa)

34. Our Goal in Q&P Sheet Processing!
Elongation (%) 10 20 30 40 50 60 70
400
600
1000
200
800
1200 IF
Mild
HSLA
DP, CP
TRIP BH
CMn IS
MART
Yield Strength (MPa)

35. TRIP Sheet Property Exploration
Si-TRIP Steel Composition C Al Mn Si P N Cr S
0.19
0.036
1.59
1.63
0.013
0.0109
0.03
0.002
sub-sized tensile samples for microstructure/property study

36. 0.19C-1.59Mn-1.63Si Process Diagram
100
200
300
400 Q u e n c h T m p r a t ( ) , C
0.2
0.4
0.6
0.8 1 A s i F o 4 8 12 16 b w . % C g M Q T F I N A L R E S H

37. 2-Step Q&P Response (aIC=25%)

38. 2-Step Partitioning Kinetics (aIC=25%)

39. Q&P Microstructure (aIC=25%)
g=8.4%
IAT=820oC, 180s / QT=200oC, 10s / PT=400oC,10s

40. Higher Quench Temperature (aIC=25%)
QT=260°C, PT=400°C, 30s

41. Uniform Elongation, Sub-sized tensile samples

42. Comparison With Other High Strength Steels

43. Conclusions The Q&P Process “Works”
- substantial austenite fractions have been obtained
We have a process design methodology
- (currently based on full partitioning)
Much work remains – fundamental and applied
competing reactions (alloying effects on carbide formation)
fine scale microstructures
alloy / microstructure / property optimization
clarify benefits & incorporate mill processing considerations