The “Quenching and Partitioning” Process: Background and Recent Progress Fernando Rizzo Seminar Cambridge, 09-12-04

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

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

“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 +

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

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

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

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

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

Thermodynamics of Carbon Partitioning

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

“True” Metastable Equilibrium g g + a a + Fe3C % Carbon Temperature g + Fe3C

“True” Metastable Equilibrium

“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!

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

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

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

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

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

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

Martensite Formation During Final Quench

Calculated Final Austenite Fraction in High-Al Steel

Effect of Intercritical Annealing Step

Effect of Manganese Content

Effect of Carbon Content

Some Experimental Results

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 [ C 50 100 150 200 250 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

9260 Bar Steel (10 s - 500oC)

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

Retained Austenite in Fe-C Alloys

Fine Structure a b 190C-120 s

TRIP Sheet Steel

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)

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)

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

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

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

2-Step Partitioning Kinetics (aIC=25%)

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

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

Uniform Elongation, Sub-sized tensile samples

Comparison With Other High Strength Steels

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