Effects of Heat Treatments on Microstructure and Properties of Ni-alloyed and Modified Ultrahigh Carbon Steel
|School||Central South University|
|Keywords||UHCS quench-tempering austempering impact toughness|
Ultrahigh carbon steels (UHCS) have great potential to be used as tools, dies or wear-resistant parts owning to their high hardness and good wear resistance. However, it is difficult for UHCS to have a combination of high hardness and impact toughness in the previous studies. In the present study, Ni addition and proper heat treatment were operated to obtain UHCS with different microstructures. The effects of heat treatments on harness, impact toughness and wear resistance of UHCS were mainly studied. The results are summarized as follows:(1) The eutectoid transformation starting temperatures(A1temperature) of UHCS with0,0.5and1wt%Ni addition were measured by using quenching-hardness test. The A1temperature of UHCS without Ni addition was790℃, which was decreased by20℃and30℃by adding0.5wt%Ni and1wt%Ni, respectively.(2) The quenched UHCS were tempered for2h at different-temperatures. The microstructure and mechanical properties varied much when the tempering temperature changed. Tempered martensite with high hardness (about55HRC) while low impact toughness (10J/cm2) was obtained when the tempering temperature was250℃. Tempered troostite was obtained when the tempering temperature raised to400℃, it had lower hardness as well as lower impact toughness (<10J/cm2) than tempered martensite. Tempered sorbite with high impact toughness (>30J/cm2) while low hardness (30HRC) was obtained under600℃tempering.(3) The microstructure of UHCS after austempering consisted of bainitic ferrite, retained austenite and carbides, which show high hardness (>50HRC) combined with high impact toughness(about30J/cm2), and the impact toughness of UHCS improved with the increase of Ni content. UHCS with1wt%Ni addition showed the best comprehensive mechanical properties:impact toughness of39J/cm2and hardness of53HRC.(4) The sliding wear behavior of austempered UHCS under different loads showed to be quite different. When the vertical load was100N, the wear resistance of UHCS got worse with the increase of Ni addition, while a reverse result was obtained under load of200N.