Acta Materialia: Revealing carbide precipitation effects and their mechanisms during quenching-partitioning-tempering process by QPT-LE model
Acta Materialia（Volume 217, 15 September 2021, 117176） published the research study from Professor Nailu Chen’s group （Shanghai Jiao Tong University）in cooperation with Professor Jian Lu’s group (City University of Hong Kong) and Professor Hao Chen’s group (Tsinghua University) with the title of “Revealing carbide precipitation effects and their mechanisms during quenching-partitioning-tempering of a high carbon steel: Experiments and Modeling”. Professor Nailu Chen, Professor Jian Lu and Professor Hao Chen are the co-corresponding authors. PhD student Jiazhi Zhang under the Joint Ph.D. programme offered by Shanghai Jiao Tong University-City University of Hong Kong and Zongbiao Dai from Tsinghua University are the first authors.
In recent decades, quenching and partitioning (Q&P) steels, as typical third generation advanced high strength steels (AHSSs), have attracted significant attention due to its excellent balance of strength and ductility. As theory for Q&P process, constrained carbon equilibrium (CCE) model can predict the quenching temperature corresponding to the maximum retained austenite volume fraction (VRA) in steel, which provides theoretical support for Q&P process. However, the VRA and/or carbon content in retained austenite (Cγ) measured by experiments are often found to be much lower than the values predicted by CCE model. The discrepancies may be in large part due to austenite decomposition and in part due to incomplete carbon partitioning during the partitioning step. In view of the importance of accurate prediction on VRA and Cγ, Dai et al. proposed a quenching and partitioning-local equilibrium (QP-LE) thermo-kinetic model, in which the martensite/austenite interface is assumed to be in local equilibrium, to investigate the kinetics of martensite/austenite interface migration and carbon partitioning during the Q&P process. The VRA and Cγ predicted by the QP-LE model were in agreement with the experimental results of a series of low carbon Q&P steels. However, due to the ignorance of carbide precipitation and carbon trapping at dislocations, the QP-LE model cannot be applied to medium/high carbon steels with mass carbides precipitation, especially the quenching-partitioning-tempering (Q-P-T) steels with emphasis on carbide precipitation.
In this study, Professor Nailu Chen’s group, Professor Jian Lu’s group and Professor Hao Chen’s group systematically investigated the microstructure (including primary martensite, retained austenite, secondary martensite and carbides) and carbon partitioning between martensite and austenite (Fig. 1 to 4) of a high carbon Q-P-T steel. And they established a concise QPT-LE (Local Equilibrium) thermo-kinetic model with dual interfaces (martensite/carbide and martensite/austenite) migration to predict the evolution of austenite fraction and its carbon content based on the consideration of austenite decomposition and carbide precipitation (Fig. 5). The effects of carbide precipitation on the VRA and Cγ were revealed and the QPT-LE model can better predict the experimental results compared with popular CCE model and QP-LE model (Fig. 6). Moreover, since the model has no carbon content limit, it is expected to be applied to the composition and process design of low carbon, medium carbon and high carbon Q-P-T or Q&P steels for high strength-ductility.
Fig. 1 TEM images of the Q-P-T (170-400-600s) specimen: (a) BF image indicating η-transition carbides precipitated from dislocation-type martensite, (b) DF image of η-transition carbides with inserted SAED pattern.
Fig. 2 (a) Three dimensional atom probe maps of Fe, C, Mn, Si and Nb distribution in the Q-P-T (170-400-600s) sample; (b),(c) C, Mn, Si and Nb concentration profiles along the red (η-carbide) and yellow (NbC) arrows, respectively, indicated in the C atom map in (a).
Fig. 3 (a) Three dimensional atom probe maps of Fe, C, Mn, Si, and Nb of the Q-P-T (170-400-600s) sample; (b) C, Mn, Si and Nb profiles across the austenite/martensite interface, indicated in the C atom map in (a).
Fig. 4 EBSD images of the Q-P-T samples: (a) Q-P-T (150-400-600s); (b) Q-P-T (170-400-600s) and (c) Q-P-T (190-400-600s); (d) Volume fractions of various phases measured by EBSD and XRD.
Fig. 5 Sketch of the QPT-LE model with dual interfaces migration: (a) initial state and (b) finishing state during partitioning/tempering. η: transition carbide; γ austenite; α’ in (a) refers to primary martensite, α’ in (b) refers to primary martensite or bainite ferrite formed during partitioning/tempering process, which has a similar BCC structure.
Fig. 6 (a) carbon content in retained austenite, (b) retained austenite fraction and (c) secondary martensite fraction of Q-P-T samples meausred by experiments and predicted by the QPT-LE, QP-LE and CCE models.
This work was supported by the National Natural Science Foundation of China (51771114, 51371117 and U1808208), the National Key R&D Program of China (2017YFA0204403) and the Technology Innovation Cooperation Zone Shenzhen Park Project (HZQB-KCZYB-2020030).