According to the results of chemical elements, the chemical composition of steel castings meets the requirements of ZG30CrNiMo steel in JB /t5000.6-2007 standard. The unqualified impact absorption energy caused by uneven hardness of the material can be excluded from the hardness test results. According to the observation results of fracture morphology, intergranular fracture appears in the fracture of impact specimen. The small plane size on the intergranular fracture is close to the original austenite grain size as cast, which indicates that there is grain boundary inheritance in the heat treatment process of steel castings.
Generally speaking, the driving force of grain boundary migration is inversely proportional to the curvature radius of the grain boundary. The larger the original grain is, the straighter the grain boundary is, and the curvature radius of the grain boundary is almost infinite. When heating later, the less likely it is to migrate, and the easier it is to retain the complete original grain boundary. It can be seen from the energy spectrum test results that the Nb content at the stripe is higher than that at the surrounding non stripe, indicating that there is NB segregation in the steel. NB is a strong carbide forming element, and most of it exists in the form of carbide NBC in the steel. When there are precipitates at the grain boundary of steel, it is difficult to eliminate them at the general heat treatment temperature because they cannot be completely dissolved.
According to the metallographic examination results, ferrite + pearlite and granular bainite were formed during normalizing; For 30CrNiMo steel, according to relevant research, due to the high content of Cr, Ni, Mo and other alloys, non-equilibrium transformation may also occur when the cooling rate is slow. Once bainite or martensite is formed, it may not be eliminated by subsequent normalizing or quenching and tempering according to the genetic phenomenon of steel structure, resulting in mixed grains or coarse grains in the specimen after heat treatment. Because NB can increase hardenability and inhibit the formation of pearlite, it indicates that the segregation of alloy elements in the steel can not be eliminated by normalizing, and the uneven mixed crystal phenomenon of subsequent quenched grains also indicates the existence of segregation of alloy elements.
The segregation of alloying elements at grain boundaries can change the migration characteristics of grain boundaries. On the one hand, the alloy elements segregated at the grain boundary can reduce the activity of the grain boundary and hinder the migration of the grain boundary. On the other hand, more second phase particles may be formed to pin the grain boundary and consolidate the inheritance of the grain boundary.