Study On Spheroidizing Annealing Process Of S7 Tool Steel

S7 shock-resistant tool steel still retains excellent toughness and abrasive resistance at high-strength condition; also with hardening resistance and high softening resistance; and excellent mechanical fatigue resistance and other characteristics. However, in the process of actual production found that:lamellar pearlite and reticular cementite structure were easy to be obtained after rolling air cooling after forging and conventional annealing. This organization is hard and brittle, not only difficult to cutting, is also easy to deformation and cracking in the subsequent quenching process. In this thesis, normalizing pretreatment and slow-cooling annealing, isothermal annealing, subcritical annealing test were used. All the samples were characterized by OM, SEM, TEM, EDS and so on. The influences of annealing parameters and annealing processes on carbides’spheroidization and hardness were researched. This thesis has reference value to optimize production process and enhance product quality for the actual production of S7 steel. And through experimental research and data analysis, the following major conclusions were obtained.1. The original annealing microstructure of S7 steel needs further spheroidizing annealing processes to improve the microstructure because of its lamellar pearlite structure, nonuniform organization and high Brinell hardness.2. Austenitizing temperature is the key factor to the pearlite spheroidization of S7 steel. Slow-cooling annealing and isothermal annealing were attributed to realize completely spheroidality of S7 steel when austenitized at 840℃. Comparing the two processes, carbides distribution more uniform when isothermal annealing. And subcritical annealing failed to significantly improve the lamellar pearlite structure of S7 steel executed at 740-780℃.3. Normalizing pretreatment+annealing process can realize completely spheroidality, but the uniformity of microstructure was reduced when normalizing temperature elevated to 960℃, grain coarsening obviously at 1000℃. S7 steel was normalized at 940℃then slow-cooling annealed or isothermal annealed at 820℃, shown a better spheroidized structure. Normalized at 94℃ then subcritical annealed at 760℃, Brinell hardness was 216 with a spheroidized structure, only need 10 hours.4. According to the results of TEM and Thermo-Calc, the main carbide types in S7 steel are M3C、M23C6、MC、M6C and M7C3.5. According to the research results and actual production of the factory, the optimal annealing process of S7 tool steel is isothermal annealing. The average particle diameter was about 0.61 um when executed at 840℃(±20℃); and fine particle obtained at 940℃ normalization+840℃(±20℃) isothermal annealing.