The Microstructure Evolution And Mechanical Properties Of High Carbon QPT Steels

In order to improve the efficiency of energy and mineral resource utilization, and also thesafety of heavy machine and equipment, it is necessary to develop advanced high strength steelswith the excellent combination of high strength, high ductility and good toughness. In theprocess of the quenching-partitioning-tempering (QPT) heat treatment the steel is quenched at acritical temperature between martensite-start (Ms) and martensite-finish (Mf) temperature,followed by a tempering heat treatment to enrich the residual austenite with carbon escapingfrom the supersaturated martensite and to form carbides. The advanced high strength steels withthe excellent combination of good strength and toughness can be obtained by adjusting thevolume fraction of martensite and retained austenite, and also by the types and size of carbides inQPT process. In1-step QPT, the partitioning and tempering is combined into one process; and in2-step QPT process, partitioning and tempering process is separately carried out at differenttemperatures.This paper briefly introduces the theory of QPT and recent experimental developments suchas the CCE model, which was proposed by Speer, the difference and relation of the CCE modeland experiments, carbon partitioning, carbide precipitation and interface movement, TRIPphenomenon and dislocation movement, as well as the effects of the quenching temperature,tempering temperature and tempering time on microstructure and mechanical properties are alsobriefly reviewed.The effects of quenching temperature and partitioning time on the microstructures andmechanical properties of the steel Fe-0.824C-1.50Si-1.79Mn-0.22Mo-0.90Cr-1.12Co-0.94Al-0.13Nb (wt.%) were investigated in the QPT process. According the calculation by JMatPro andCCE model, the heat treatment process was designed as follows: specimens were quenched to5°C,50°C and100°C, and then tempered at450°C for60-1800s. Results show that the volumefraction of austenite was0.06%,5.46%and30.34%in the speimens quenched to5°C,50°C and100°C, respectively. Nano-sized carbides were observed in the speimens quenched at50°C and100°C. The hardness of these speimens was almost the same as the speimens quenched to5°Cdue to nano-sized carbide precipitation. In the tempering process, for the specimens quenched at50°C and100°C, the nano-sized carbides were dissolved within120s, the carbon content ofmartensite was also reduced. Both of these two reasons led the decrease in hardness byapproximately200HV1. When the tempering time was prolonged to120-3600s, new carbideswere formed and the harnesses was increased again. When the tempering time exceeded3600s.,the harnesses were decreased due to the coarsening of precipitated carbides For the specimenquenched at5°C, the microstructure was almost fully martensite. the decomposition ofmartensite and the reduction of carbon content in the martensite resulted in the decrease inhardness during the whole tempering process.The effects of partitioning process of2-step QPT process were investigated utilizing thesteel of Fe-0.95C-0.91Si-1.30Mn-2.3Cr-0.99Mo-0.17Ti (wt.%). Specimens were partitioned at250°C and300°C for120s and5400s after quenching at-2°C, and then tempered at450°C for300s and1800s, followed by quenching into water at room temperature. In the range of theexperimental temperature and time, results show that the retained austenite did not obviously decompose in the specimen partitioned at a low temperature for a long time, and in the specimenpartitioned at a high temperature for a short time; in constrast, the retained austenite was partlydecomposed in the specimen partitioned at a low temperature for a short time, and also in thespecimen partitioned at a high temperature for a long time. Results also show that the specimenpartitioned at a high temperature for a short time had a best precipitation strengthening effectswhen tempered at450°C. However, there was no obvious precipitation strengthening in thespecimen partitioned at a low temperature for a long time. The specimen treated by1-step QPTwas compared with that treated by2-step QPT. Results show that the retained austenite wasobviously decomposed in the specimen treated by1-step QPT and was tempered at450°C for1800s; ant that the carbides in the specimen treated by2-step QPT was bigger in size and theprecipitation strengthening effect was not remarkable compared with that of1-step QPT.