30CrNiMo steel has been widely used in mining machinery due to its high hardenability, good comprehensive mechanical properties and excellent cold and hot workability. The production process of 30crnimo steel castings produced by a factory includes as cast forming and normalizing (890 ℃) × 1 h, air cooling), quenching and tempering (heat preservation at 880 ℃ for 1 h, water quenching, tempering at 600 ℃ for 2 h) treatment. During the performance test, the impact absorption energy was unqualified (the test value was less than 10 J, far lower than the lower limit of the required value of 34 J). We were entrusted to carry out the test. The impact samples of steel castings are taken from single cast test blocks, and the sampling positions are shown in the figure. By means of scanning electron microscope (SEM) and optical microscope (OM) in the laboratory, the causes of unqualified impact absorbed energy of steel castings were determined.
1) Through the test and analysis of impact fracture of 30CrNiMo steel castings, it is found that the direct cause of unqualified impact energy absorption is the non-equilibrium transformation structure and grain boundary inheritance caused by alloy element segregation.
2) In view of the non-equilibrium transformation structure such as bainite in steel castings after normalizing, normalizing is adjusted to isothermal annealing to avoid non-equilibrium transformation, which can reduce the genetic degree of grain boundary and effectively improve the impact properties of steel castings.