Study On The Martensite Formation And Strengthening And Toughening Of Steels

The martensitic transformation of several industrial steels has been studied in the article, such as 65Mn, 60Si2Mn, 9CrSi, CrWMn, 50CrMn,5CrMnMo and 42CrMo. The results show that, first martensite plate is formed at the boundaries of austenitic grains in the experiment among all of the low-carbon and high-carbon alloyed steels, and the shape of the martensite in grain boundary is lamellar, except the steels including Mo. The atomic arrangement, stress condition and high density dislocation in the boundaries of austenitic may supply conditions to martensite nucleate there. Compared with lath martensite, the lamellar has lower surface energy and it's higher strain energy can be delivery at boundaries, so the shape of the martensite in grain boundary is lamellar. The martensite of steels should be first formed at boundaries of austenitic grains and the self-catalysis can induction intracrystalline martensite to formation.The boundary martensite is one of organize factors which lead to temper brittleness. When the martensite in grain boundary analytical it will separate out some carbide, which will contribute to the low temperature temper brittleness. When high tempering,the martensite has reduced free carobatomic density which can rejection P segregation to the grain boundary. So it can be replace martensite with lower bainite in grain boundary to improve steels performance. The reason of the steels including Mo have lower temper brittleness is that Mo can reject martensite which first formed at the boundaries of austenitic grains.