Effect of Tempering Temperature on Microstructure and Precipitate of 960MPa Engineering Machinery Steel

Microstructure and change rules of precipitates of 960 MPa engineering machinery steel were studied by TEM after quenching and tempering.The results show that the microstructure is main lath martensite and a few twinned martensites after quenching at 920 ℃ and self tempering martensite occurred.After tempering below 400 ℃,the size and quantity of(Nb,Ti)(C,N) didn't vary obviously.After tempering at 500 ℃,(Fe,Mn,Cr)3C is precipitated near grain boundaries and subgrain boundaries.After tempering at 600 ℃,(Fe,Mn,Cr,Ni,Mo,V)3C is precipitated in grains and near grain boundaries.At the same time,Mo2C is precipitated in quantities.After tempering at 650 ℃,VC is precipitated in quantities.The recrystallization happenes and polygonal ferrites begin to form.