1 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Processo Q&P (Quenching and Partitioning) Estudo de caso completo Fernando Rizzo

2 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Projeto de Cooperação Internacional J.G. Speer, D.K. Matlock, A. Streicher – Colorado School of Mines, USA F. Rizzo, A. R. Aguiar – PUC, Rio de Janeiro, Brazil D.V. Edmonds, Kejian He – University of Leeds, UK NSF-CNPq (CIAM), NSF-EPSRC

3 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Quenching and Partitioning : - Background - Fundamental Issues - Recent Results J.G. Speer, D.K. Matlock, B.C. De Cooman and J.G. Schroth, Acta Mater., 51 (2003) J.G. Speer, A.M. Streicher, D.K. Matlock, F.C. Rizzo and G. Krauss, Austenite Formation and Decomposition, ed. E.B. Damm and M. Merwin, TMS/ISS, Warrendale, PA, USA, 2003, pp John G. Speer, David V. Edmonds, Fernando C. Rizzo, David K. Matlock, Current Opinion in Solid-State and Materials Science, 8 (2004)

4 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012

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

6 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 The “Q&P” Process    Step 1.Austenitize or Intercritically Anneal    - more austenite - lower C  - higher M s - less austenite - higher C  - lower M s

7 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Step 2.Cool (quench?) below M s - M s -T Q controls martensite formation - intercritical annealing has more stable austenite and higher carbon martensite       Austenitize + Quench Intercritical Anneal + Quench

8 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Step 3. Diffuse Carbon from Supersaturated Martensite - Phase compositions change - Phase boundaries stationary      

9 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Q&P Process Schematic

10 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 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

11 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Thermodynamics of Carbon Partitioning

12 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Important Questions How much can we enrich the austenite? That is…what are the “equilibrium” martensite and austenite compositions? Or…when does partitioning stop?

13 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 “True” Metastable Equilibrium     Fe 3 C % Carbon Temperature  Fe 3 C

14 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 “True” Metastable Equilibrium

15 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 “True” Metastable Equilibrium CANNOT Apply!!     Fe 3 C % Carbon Temperature  Fe 3 C X alloy XX XX - The equilibrium phase fractions are fixed by the lever rule - The actual phase fractions were fixed by cooling below Ms!

16 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 A New Equilibrium Condition was Hypothesized “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

17 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Properties of “Constrained Carbon Equilibrium” - Not a unique condition at any temperature! - Depends on initial phase fractions/compositions

18 Workshop Aplicações da Termodinamica Computacional a Siderurgia Austenite may be more enriched or less enriched than ortho- or para- equilibrium T0T0 A3A3 Properties of “Constrained Carbon Equilibrium”

19 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Properties of “Constrained Carbon Equilibrium” Carbon Constrained Equilibrium: Mass balance:

20 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Key Characteristics of CCE - Almost all of the carbon should partition to austenite - Enrichment levels are potentially very high (Fe-0.5C)

21 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Example CCE Calculations - 1.0%C Initial Austenite

22 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 We have all the pieces to predict microstructure… Example Steel Composition: C=0.15 Mn=1.0 Si=1.5 M s ( o C)= (%C)-30.4(%Mn)-12.1(%Cr)-17.7(%Ni)-7.5(%Mo) M s =445 o C (Steel) Intercritical Annealing T=810 o C f  ~ 22%C  ~ 0.68 wt. %M s ~222 o C f  ~ 78%    T IA =810 o C

23 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Quench T = 150 o C Fraction of Martensite (Koistinen and Marburger) Final Microstructure f  ~ 10% f M ~ 12% f  ~ 78% Phase Compositions After 450 o C Partitioning C  ~ 1.5% C M ~.0019%       T q =150 o C T p =450 o C

24 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Q&P Process Design Methodology ASSUME: - Complete partitioning of carbon to austenite - No competing reactions (carbide formation)

25 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Calculations for Experimental Al-Steel (at QT)  IA =0.5

26 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Martensite Formation During Final Quench

27 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Calculated Final Austenite Fraction in High-Al Steel

28 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Effect of Intercritical Annealing Step

29 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Effect of Manganese Content

30 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Effect of Carbon Content

31 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Example of DICTRA Simulation Solid-Solid Phase Transformations in Inorganic Materials 2005 Edited by J. Howe TMS (The Minerals, Metals & Materials Society), 2005 CARBON ENRICHMENT OF AUSTENITE AND CARBIDE PRECIPITATION DURING THE QUENCHING AND PARTITIONING (Q&P) PROCESS F.C. Rizzo 1, D.V. Edmonds 2, K. He 2, J.G. Speer 3, D.K. Matlock 3 and A. Clarke 3 1 Department of Materials Science and Metallurgy; Pontifícia Universidade Católica-Rio de Janeiro; RJ , Brazil 2 School of Process, Environmental and Materials Engineering; University of Leeds; Leeds LS2 9JT, United Kingdom 3 Advanced Steel Processing and Products Research Center; Colorado School of Mines; Golden, CO 80401, USA

32 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Simulation Conditions Steel composition: 0.19C-1.59Mn-1.63Si wt% Heat treatment: Fully austenitized at 900 o C, quenched to 293 o C to produce 68% martensite and partitioned at 400 o C. The thickness of the ferrite and austenite plates used in the simulation were 0.30 and 0.14 microns, respectively (obtained by TEM).

33 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Carbon Concentration Profiles for ferrite and austenite

34 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Carbon concentration profiles in  and  during partitioning under CCE at 400C, for a 0.19C-1.59Mn-1.63Si steel

35 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Average carbon concentration as a function of time for  (0.30m) and  (0.14m) plates during partitioning at 400 o C

36 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Variation of (a) carbon flux and (b) carbon activity at the interface during partitioning. Time plotted in a log scale

37 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Carbon concentration (wt%) at  and  interfaces as a function of time during partitioning at 400 o C

38 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Carbon flux and concentration in the center of (a) ferrite and (b) austenite plates as a function of time

39 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 CONCLUSIONS 1.For the scale of microstructure investigated, carbon depletion from the ferrite during partitioning at 400C occurs quite rapidly, around seconds, while the austenite takes much longer, around 10 seconds, to achieve a uniform concentration. 2.Due to its rapid depletion, the carbon concentration in the center of the ferrite plate starts to decrease after seconds. After this time the driving force for carbide precipitation is gradually reduced. 3.Carbon enrichment of the austenite will promote, initially, a substantial increase in the carbon concentration at the interface and a progressive stabilization of the plate, advancing from the interface to the center. Full stabilization is achieved when the composition of the central region reaches a carbon concentration corresponding to room temperature M s.

40 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Some Q&P Experimental Results

41 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 TEM micrographs of the Q&P microstructure produced in a 0.19%C-1.59%Mn-1.63%Si TRIP steel composition Quenching to 260°C and partitioning at 400°C for 100 s: (a) bright-field image and (b) dark-field image using a (200) austenite reflection. (a) (b)

42 Workshop Aplicações da Termodinamica Computacional a Siderurgia 2012 Total elongation vs. ultimate tensile strength for TRIP, Dual phase (DP), martensitic (M), and Q&P sheet steel products