Effects Of Rapid Tempering On The Microstructure And Cryogenic Impact Property Of Low Carbon Steel

To obtain the better toughness have been the tough question for the researcher all around the world. What they always do was decreasing the strengthness to improving the toughness by tempering after quenching. The fact is that conventional tempering process needs much more times, so it can consume more energy. This article is to develope a new rapid tempering process using thermal simulation tester. Basing on the industry products from the Iron and Steel corporation, the test steel were A36 steel and SS400 steel. The former was low carbon microalloy steel containing Nb, V and Ti alloy elements and the latter was plain carbon steel. The motivation were, in comparison to conventional tempering, to investigate the uniformly distributed precipitationes and whether they can make the cryogenic toughness better using rapid tempering at a heat rate of 20℃per second.The A36 steel was tempered at 620℃for holding time 70min in conventional resistance furnace, and tempered rapidly at 620℃for 40s,80s and 120s after quenching , respectively. Both microstructures and impact properties were also investigated. It was observed that, in rapid tempering process, the nucleation sites of nanosize cementite precipitations are at the dislocations of the steel. As the holding time increased, the sizes of the cementite precipitations are increased because of there being sufficient time for the particles growing up. When the holding time up to 120s, the cementite precipitations reach the maximum size in this test. The remarkable improvement in the toughness of test steel was associated with the nanosize precipitate particles, which was related to the rapid tempering process parameters at 620℃for holding 40s.For the SS400 steel, the conventional tempering and rapid tempering process were proceeded respectively, and both their samples are quenched in 10% agitated iced brine. The tempering temperatures are designed as 560℃, 620℃and 680℃, respectively. Rapid tempering specimens were heated at a heating rate of 20℃/s, and all samples of these three tempering temperatures are maintained 30s, 40s and 50s with the Gleeble 1500D tester, respectively. The impact toughness at -40℃, LOM microstructure examination and scanning electron microscope(SEM) were analyzed in detail. The experiment results show that the upper bainite, martensite and less retained austenite were obtained in the rapidly quenching samples. In comparison to conventional process, the matrix microstructure was changed from the larger size and bulk ferrite to refined banding structure, and the cementites were obviously refined and precipitated inside the grains and along the grain boundaries. The cryogenic impact test implies that the maximum values of impact toughness of holding time 30s or 50s are achieved at tempering temperature 620℃. Furthermore, a favorite value of impact toughness of holding time 40s is available.