Effects Of Initial Quenched Martensite On Microstructure And Mechanical Properties Of Quenching And Partitioning Steel

Quenching and Partitioning(Q&P)is a new process of fabricate mixture microstructure of martensite and retained austenite.during which steels are partly quenched firstly and the carbon diffusion from initial martensite into untransformed austenite was controlled secondly.and quenched to room temperature lastly.it is an effective way to improve the ductility and toughness of high strength steels.A method of predicting the final phase fraction of Q&P steels according to initial martensite was proposed by Speer et al..while there is big difference between calculating results and experimental ones.a new model was proposed by Wang.which base on the inhomogeneous distribution of lath during martensitic transformation.explain the difference more reasonably.on the new modle basis,5 types of low alloy steel with carbon content of 0.2-0.5wt.% were studied in present research work,and the SEM,TEM.XRD,Nano indent and tension test were employed to study the effects of initial martensite fraction on the microstructure.especial for retained austenite.and mechanical properties.Mixture microstructure of initial martensite,fresh martensite and retained austenite were obtained after Q&P process,with the increasing fraction of initial martensite, the size and quantity of fresh martensite reduce gradually.and the fraction of retained austenite increase and then decrease,the maximum retained austenite fraction is obtained when the initial martensite fraction range from 30% to 50%,the relatively fine size and homogeneous distribution of untransformed austenite after initial quenching process induce to absorb more carbon and increase the its stability,which increase the microstructure and mechanical properties was detected in the experimental steels with different carbon content. the increasing fraction of initialWith the increasing fraction of initial martensite,the tensile strength increase gradually,and the yield strength and reduction increase gradually,the total elongation and the product of tensile strength to total elongation increase firstly and then decrease,the volume fraction and stability determine the ductility,the TRIP effect of retained austenite lead to high ductility and high value of product of tensile strength to total elongation.The nano hardness of both initial martensite and fresh martensite increase guadually,the higher carbon content and finer martensite lath result in the higher hardness of fresh martensite than initial martensite.The nucleation of microvoid under uniaxial tension mainly induced by large sized carbonitride and oxide precipitations,and the phenomenon of fresh martensite induce microvoid is unobserved.