Heat Treatment On The Organization And Performance Of Ultra-high Strength Bainitic Steel

Based on the design concept of the ultra-high strength steels, medium carbon ultra-highstrength steels containing high Mn and Cr were designed by means of low temperature bainitetransformation and quenching-partitioning-tempering (Q-P-T) process. The microstructure,hardness, volume fraction of retained austenite and carbon content of low temperature bainiticsteels and Q-P-T steels were characterized utilizing optical microscope (OM), scanning electronmicroscope (SEM), hardness tester, X-ray diffraction (XRD). The reason for ultra-high strengthwere analyzed.Nanostructured bainitic microstructure and carbon-enriched retained austenite have beenachieved by transforming austenite to bainite at low temperature ranging from200℃to250℃for a long time. The hardness of high Mn steel reached up to approximately636HV1bytransforming austenite to bainite at200℃for10days,and high Cr steel631HV1for12days.The hardness at200℃was higher than by50HV1that at250℃. Strong austenite and/or largedriving force at the low transformation temperature led to ultra fine bainitic ferrite plates. X-raydiffraction analysis indicated that low-temperature bainite transformation is an incompletereaction. The carbon content in carbon-enriched retained austenite was more than that T0′phaseboundary predicted.Tempering resistance and carbide precipitation of a low temperature super bainitic steelwas investigated during tempering over the temperature range200~400℃. Microstructuralobservations and hardness test show that the tempering resistance of high Mn steel and high Crsteel were not good. After tempering, the hardness was decreased by about60HV1, the contentof retained austenite was also decreased.Under different quenching temperatures or different time of partitioning and tempering,the transformed microstructures consisted of lath martensite, carbon-enriched retained austeniteand carbides. Microstructural observations and hardness test show that high Mn steel has higherhardness when quenched in oil at130℃, the hardness was544HV1, whereas the hardness ofhigh Cr steel had almost no change when quenched in oil from130℃to190℃and thehardness was535HV1. The hardness of high Mn steel reached up to approximately627HV1when quenched in oil at100℃, in constrast, the high Cr steel reached604HV1. The hardnesswas decreased with increasing partitioning and tempering time. The amount of retainedaustenite became reduced. High hardness was achieved in the investigated steel when thepartitioning and tempering time is60s.