Effects of different heat treatments on the structure and properties of super martensitic stainless steel
Super martensitic stainless steel is a new type of martensitic stainless steel that strictly controls the carbon content below 0.03% on the basis of traditional martensitic stainless steel and increases the nickel content. Compared with the traditional low-carbon martensitic stainless steel, super martensitic stainless steel not only has good ductility toughness and higher strength and hardness, but also has higher fracture toughness, underwater fatigue strength and abrasion resistance. After the martensitic stainless steel is normalized, lath martensite can be obtained, and after tempering at a certain temperature, further tempered martensite can significantly affect and improve the overall properties of the material. Predecessors studied super martensitic stainless steel normalizing at 1050°C and tempering between 500°C and 700°C, focusing only on its microstructure and mechanical properties, and did not study its abrasion resistance. In the study, the super martensitic stainless steel 1.4314 (S41500) was normalized and tempered once and selected part of the temperature for secondary tempering. The relationship between the hardness, impact toughness and abrasion resistance of the material under 8 different heat treatments was discussed and studied in detail.
After tempering at 550℃-650℃, super martensitic stainless steel will produce inverted austenite, which is black strips and blocks under the transmission electron microscope, which are often distributed on the boundaries of martensite laths and austenite crystals. At the boundary, the length is 102nm-103nm and the width is about 100nm. Inverted austenite will reduce the strength and hardness of the material and increase toughness. When the primary tempering temperature reaches 700℃, the diffusion degree of Ni enriched near the reversed austenite increases, the segregation of Ni decreases, and the reversed austenite transforms into new martensite during the cooling process, which is almost non-existent in the material Inverted austenite, so the hardness increases.
After 1.4314 (S41500) stainless steel is tempered once at 500℃-700℃, the hardness value first decreases and then increases as the temperature rises. The hardness of the secondary tempering is generally lower than that of the primary tempering at the same temperature. In the range of 500℃-700℃, the toughness increases first and then decreases after the first tempering; the second tempering treatment has little effect on the toughness of steel.
The cumulative weight loss curve of 1.4314 (S41500) stainless steel is parabolic-like, the cumulative weight loss increases with the increase of time, and the cumulative weight loss rate continues to decrease. According to the law of material abrasion, for two-body wear, the hardness of the material determines its wear resistance. Therefore, the abrasion resistance of super martensitic stainless steel is closely related to the hardness of the material. Generally, the higher the hardness of the material, the lower the cumulative weight loss and the better the abrasion resistance. The study found that 1.4314 (S41500) steel has the highest hardness and poor toughness after tempering at 500°C, the second highest hardness and good toughness after tempering at 550°C, and the best abrasion resistance after tempering at 550°C, while the material after tempering at 500°C Its anti-abrasion performance ranks second. Therefore, it can be found that the abrasion resistance of 1.4314 (S41500) steel is mainly related to the hardness, and its impact toughness also affects the abrasion performance.
The hardness of 1.4314 (S41500) steel in the secondary tempering is slightly lower than that of the primary tempering at the same temperature, and the abrasion resistance is not improved. Considering the material properties and economy,1.4314 (S41500) steel normalizing at 1050℃ + one tempering at 550℃ is the best heat treatment process.