A Notably Increased Toughness without Losing Strength on a Low Carbon Martensite Steel Via a Short Stay at Low Temperature
It is well known that a low carbon martensite steel,treated via quenching and tempering process,possess an excellent strength and toughness properties.Recently years,a quenching and partitioning (QP) process propounded by Speer et al.has been widely applied on medium and low carbon steels for it can increase the toughness of steel.A notably increased amount of carbon-rich retained austensite is responsible for the increased toughness.However,the strength of martensite is reduced when it is carbon-depleted after the partitioning process,and the quenching temperature is usually at a high level between Ms and Mf,resulting in a lower strength of steel after the QP process as compared with steel after the traditional quenching and tempering (QT) process.Recently,we found an interesting phenomenon on a low carbon steel,that is a notably increased toughness without losing tensile strength obtained via quenching to room temperature with a short stay at 210℃ and followed by tempering at a low temperature (QST),as compared with the QT steel.The martensite start temperature of the steel is ~370℃.As compared with QT steel,the tensile strength of QST steel kept at ~1700MPa,however the yield strength reduced from 1360MPa to 1250MPa,and elongation increased from ~6.0% to 9.5%,the impact toughness increased form 65 J/cm2 to 108 J/cm2.Detailed characterization on the impact process shows that as compared with the QT specimen,the energy value needed for the crack initiation Ei and the energy for crack propagation Ep of the QST specimen account for ~78% and ~22%,respectively,of the total improvement,as shown in Fig.1.This reveals that the microstructure of the QST specimen possesses not only a high crack propagation resistance,but a much higher crack initiation resistance,which contributes most to the improved toughness of the studied steel.An improved crack initiation resistance can basically improve the safety of structural parts,which is extremely important.XRD results did not show any obviously change on the amount of retained austensite,but a carbon-richer in retained austensite of QST steel.TEM results show a notably refined plate thickness of ferrite for QST steel,which probably due to the fast quenching rate of molten salt and the stopped growth process.Although about ~87 vol.% martensite has formed during the cooling process,the dilatation curve also showed a gradually increased C-gauge during a short stay at 210℃,after a sharply increased C-gauge for martensite transformation,as shown in Fig.2,indicating an occurrence of bainite transformation after a comprehensive considering on the carbon content in untransformed austenite and the relative martensite transformation temperature of the untransformed austenite.Then,some low temperature bainite ferrite formed during the short stay process,which is high strength.Apart from the microstructure factors,the short stay treatment at 210℃ not only relieves the previously formed martensite stress generated during the quenching process and phase transformation,but can decrease the micro-cracks generated by the quenching stress,which is beneficial to the ductility and toughness.According to these results,we can conclude that introducing a short stay at a low temperature during the quenching process,not only can refine the microstructure,introduce high strength low temperature bainite,but can relieve the stress and decrease the micro-cracks,all of which result in a notably improved mechanical properties,especially the toughness of steel.We think that this QST process has a potential of applying onindustry.
low carbon steel austempering phase transformation mechanical properties
YANG Zhinan ZHANG Fucheng JI Yunlong WANG Yanhui WU Dongdong
National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan Uni State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao
国际会议
成都
英文
750-751
2016-11-16（万方平台首次上网日期，不代表论文的发表时间）