Study On Pre-heat Treatment Technologyof New Ultra High Strength Secondaryhardening Steel

This paper was aiming at coarse-grain problem after forging,cracking problem after cooling&annealing,and problem that annealing hardness too high to machining for M54ultra high strength secondary hardening steel.The formation law and conditions of hot deformation coarse grain and its hereditary feature was studied by means of Gleeble-3800thermal simulated test machine;cooling after forging and the following annealing's affection on microstructure and mechanical property was simulated,to judge cracking tendency of cooling after forging and annealing.At last,the technology of lower the annealing hardness,and improve the machining property was studied,and the following conclusion was drawn:Through observation of microstructure change during compressive deformation between1000?to 850??cooling from 1250??,it was observed that small deformation?30%high fall?didn't occur dynamic recrystallization,coarse grain from 1250?was retained.Large deformation?70%high fall?occurred part of dynamic recrystallization with the temperature900?&950?,and nearly complete dynamic recrystallization with the temperature 1000?,remarkably fine the grain.It was predicted that if part of ingot and simi-finished forging product delay deformation after high-temp heating,it will form the coarse grain in part of the forging by the reason of the fall of temperature.The deformed coarse grain specimen?deformed at 850??reheating austenitizing at 950?,1000?,1050?and 1100?,results shows that coarse deformed grain was inherited at 950 and 1000?reheating austenitizing,uniformed sizing-flat polygon recrystallized austenite grain was formed at1050?.By the observation on microstructure and mechanical properties change during cooling after forging in laboratory simulation,it was shown that if experimental steel didn't totally cold after high temperature austenitizing and formed part of martensitic structure,the steel will be extremely brittle after annealing,therefore the forging which didn't totally cold before annealing got the larger tendency of cracking;if experimental steel directly get into 630?annealing furnace after high temperature austenitizing,its'phase didn't change,and got the hardness close to quenching state,weaken the austenitizing grain boundary and embrittled the steel,therefore forging directly“red deliver”form high temperature to annealing furnace got a bad annealing effect,and got a weaker grain boundary with a high hardness,increase the forging cracking tendency.Normalizing at 1075?by AMS standard recommendation,M54 got a hardness of HRC53.8;Lower the austenitizing temperature,residual M₂C?M₆C carbide rich in Mo and W in austenitic matrix will lead to the decline of C,Mo,W content in matrix,and hardness of air cooling is 2HRC lower than the hardness of normalized state.Annealing at 630?⁶45?for 5 hours after normalize at 1075?got a hardness of HRC44.3,and lower the austenitizing temperature with the same annealing regime got a hardness of HRC41.3;Long time annealing will lower the hardness below HRC40,therefore lower the austenitizing temperature will effectively lower the final annealing hardness.Raw material treatment with the new softening technology of“850?austenitizing air cooling+630?annealing”,followed by AMS standard quenching,cold treatment and secondary hardening tempering,its mechanical property correspond to the material didn't softened,proves that the new annealing soften technology didn't affect the final mechanical property.