30 Cr2ni4mov Steel Heat Treatment Process Of Mixed Crystal Formation Mechanism And Control

The heat treatment is the most key step to determine the large forgingsperformance. The processes of the forging heat treatment are varieties. Theheating and cooling processes are slow. And there is structure geneticcharacteristic in the heat treatment after forging process.So the mixed crystalcan’t be controlled effectively. The mixed crystals can bring down themechanical properties of the forgings.And when the forgings are by ultrasonictesting, the mixed crystals can jam the signal to obstruct the judgment of theinternal quality. So before the final heat treatment, the large forging must bedeal with the structure genetic to refine the crystal. From the results of the study,some methods such as the twice or several times high-temperature normalizing，the several times high-temperature tempering，the critical normalizing and theaustenitic recrystallization can remove the structure genetic.The30Cr2Ni4MoV large forging steel is the research object of this paper.This article uses three different methods that are the three timeshigh-temperature normalizing, the high-temperature tempering and the criticalnormalizing to do the experiments. Through observing the microstructure tofind the mechanism of the structure genetic and determine a best technique.It was denoted that when the martensitic structure is heated there are twotypes of austenitizing processes forming the plate and nodular austenite inA_C1-A_C3of the heating austenitization process. In the low temperature sidenear A_C1（690℃-750℃） the plate austenite forming is in a dominantposition.The plate austenite has the ability for restoring to the orientation oforiginal phase and when it is further developed, structural inheritance may becaused after full austenitizing.There are a large number of nodular austeniteforming in the high temperature side near A_C3（750℃-790℃）.The nodularaustenite can restrain the forming of the plate austenite to hack the graininheritance.The results of refining crystal heat treatment show that after three times’high-temperature normalizing the original grain size of level1can become level7;After two times of high temperature tempering the original grain size of level 1can become level5; After two times’ critical normalizing the grain size canreach level8.