The strength, especially plasticity and toughness of steel castings are greatly reduced due to coarse grains, uneven microstructure and internal residual stress. The purpose of heat treatment is to eliminate or reduce casting stress, refine grain, uniform composition and improve microstructure, so as to obtain mechanical properties, wear resistance and special service properties that meet the service requirements. Therefore, steel castings must be heat treated. Due to the particularity of the structure of as cast steel castings, the heat treatment process is not exactly the same as that of rolled steel with the same composition. The heat treatment process of steel castings with different materials, sizes and service requirements is quite different. Therefore, a series of heat treatment processes have been carried out for steel castings with different materials, service conditions and performance requirements. The research on heat treatment process mainly focuses on the selection of process parameters such as heating speed, austenitizing temperature, holding time, cooling mode, tempering temperature and tempering time.
For example, after heat preservation and water cooling treatment at 900 ℃ and 1000 ℃ for 1h, M23 (C, b) 6 and M7 (C, b) 3 secondary boron carbides with dispersed distribution are precipitated in the matrix of Fe-Cr-B cast steel, which improves the corrosion resistance of the matrix to molten aluminum and reduces the corrosion rate of the steel by about 25.1% and 16.7% respectively compared with that as cast. 0Cr18Ni10Ti stainless steel adopts optimized 1020 ℃ × 1.5 h solution treatment + 890 ℃ × After 2 h stabilization treatment, a single and uniform austenite structure was obtained, and the precipitation of grain boundary carbides and grain boundary pitting pits were eliminated. After 196 h corrosion test in 5% sulfuric acid solution, the corrosion weight loss rate decreased from 0.0288g/h before treatment to 0.0207g/h, and the corrosion resistance was excellent. Ls8 (zg20crni2mo) cast steel is mainly a low-temperature material used to manufacture the lifting box of drilling rig. In order to meet the performance requirements in low-temperature environment, three heat treatment processes of complete quenching + high-temperature tempering, sub temperature quenching + high-temperature tempering and complete quenching + sub temperature quenching + high-temperature tempering were studied. The results show that ls8 steel has the best comprehensive mechanical properties after complete quenching, sub temperature quenching and high temperature tempering. The tensile strength and yield strength at 40 ℃ reach 760 MPa and 680 MPa respectively, and the elongation and impact energy reach 20% and 51 J respectively, which is better than the other two heat treatment processes. Practice has proved that ls8 steel casting can fully meet the performance requirements of drilling rig after complete quenching, sub temperature quenching and high temperature tempering. After studying the effects of austenitizing temperature, holding time, isothermal quenching temperature and isothermal quenching time on the microstructure and properties of high silicon cast steel, it was found that high silicon cast steel × 120 min austenitizing, 320 ℃ × 120 min isothermal quenching treatment can obtain Austenite Bainite duplex structure with alternating arrangement of carbon supersaturated lath bainite and carbon rich retained austenite, which has good comprehensive mechanical properties in combination with strength, toughness and hardness. Under the same conditions, the service life of the bucket teeth of high silicon cast steel excavator treated by the heat treatment process is 80% higher than that of high manganese steel and 21.6% higher than that of low alloy cast steel.
After being treated by the newly developed incomplete quenching isothermal distribution heat treatment process (holding at 840 ℃ for 2h, isothermal quenching at 210 ℃ for 0.5h, and then holding at 340 ℃ for 1H and quenching to room temperature), the surface of medium and low carbon chromium silicon manganese martensitic cast steel is hardened by laser, so that the cast steel obtains a multiphase structure composed of low-carbon martensite and carbon rich austenite, The tensile strength is greater than 1600mpa, the elongation is greater than 10%, and the average hardness of the matrix is HV 570 ~ 600. The comprehensive mechanical properties are significantly improved compared with the traditional quenching + tempering heat treatment process. The effect of repeated heat treatment times on the properties of cd3mn duplex stainless steel was studied. The results showed that the proportion of ferrite in cd3mn duplex stainless steel increased after repeated solution treatment at 1080 ℃ and 1120 ℃ for 2 ~ 4 times, respectively. When the solution temperature increases from 1080 ℃ to 1120 ℃, the ferrite content increases from about 30% to about 45% under the same solution treatment times, the proportion with austenite phase is more balanced, and M23C6 σ The pitting corrosion resistance of the steel in ferric chloride solution was increased by 148%.
