Effect Of Microstructure Of 42CrMo4 Heavy Forgings On Low Temperature Impact Properties

In the actual production of large forgings,there is often a case where the low-temperature impact properties can not meet the requirements In this paper,based on the microstructures of large forgings,through the continuous decomposition of the radius direction of the cross section of the 42CrMo4 wind power spindle and the comparison and analysis of the fracture of the impact specimens with different low temperature impact toughness,the distribution of microstructure and the relationship between microstructure and low temperature impact properties of large forgings before and after heat treatment are preliminarily obtained.The effect of non martensitic microstructure on the impact of low temperature on low temperature impact was discussed by changing the isothermal temperature and time by the salt bath isothermal test of small samples of the same material.The results showed that the main microstructures of 42CrMo4 forged spindle were bulk ferrite and pearlite.While with subsurface depth increased,the quantity of feiirte increased at first and then decreased,and reached maximum at about subcutaneous 90mm.On the other hand,there were also some ribbons,pseudo eutectoid structures and a small amount of mixed grains in the spindle after forged.And the main microstructure of 42CrMo4 tempered spindle was tempered sorbite,but there were some "white block" which slightly lighter than tempered sorbite appeared since subcutaneous 30mm,and gradually gathered with depth.Compared with the impact specimens which had different low-temperature impact toughness in farcture morphology and microstructure,we found that the specimens with higher impact energy had dimple morphology in it fracture while more cleavage morphology was found in the specimens with lower impact.As Akv which means impact energy decreased,the proportion of radiation area and the white blocks gradually increased.The hardness of the white block is slightly lower than that of surrounding tempered sorbite,and the difference is about 2-3HRC.At the same time,a thin layer of cabide which was not completely continuous was found at the edge of the white block region.Keeping the simples of 42CrMo4 steel in salt baths at 460?,350?,and 260?for 10s,20s,100s and 600s,respectively,it was obvious that the microstructure after isothermal at 460? were upper bainite,martensite and retained austenite,while the upper bainite is distributed like a strip.After tempered at 680?,there were some white block developed in the microstructure,and its distribution corresponded to that of upper bainite.In addition,with the extension of isothermal time,the number of blocks increased and the area became larger,while the low-temperature impact toughness of the samples declined a lot.The simples of 42CrMo4 steel isothermal treated at 350 ? developed lower bainite,martensite and retained austenite.The microstructure of that was more homogeneous,and no white block occurs after tempering at 680 ?.A small amount of lower bainite produced by isothermal heating at short time can not increase the impact toughness of the sample at low temperature but decrease slightly.However,as the isothermal time continues to extend,the impact properties of the samples gradually improve.The simples of 42CrMo4 steel isothermal treated at 260 ? developed martensite,less bainite and retained austenite.During a long period of isothermal process,Continuous cooling transformation and isothermal transformationof supercooled austenite,the diffusion of carbon from martensite and bainite to the surrounding austenite,and self tempering process of a small amount of martensite occur successively.And there were no white block appeared after tempered in 680 ?.At the initial stage of isothermal treatment,the impact toughness of the samples decreased slightly,but with the extension of isothermal time,the impact toughness at low temperature increased generally.Therefore,focusing on the low-temperature impact properties of large forgings,attention should be paid to microstructure control during the optimization of heat treatment process and related parameters.It is possible to reduce and even avoid the appearance of upper bainite during the quenching process by alternately cooling water and air,so as to obtain more lower bainite as much as possible,so that the large forgings have a stable and excellent low temperature impact toughness.