Heat treatment of corrosion-resistant stainless steel castings
1 martensitic corrosion-resistant stainless steel castings heat treatment martensitic corrosion-resistant cast steel chromium mass fraction of l3%, and high carbon content, good hardenability, after appropriate heat treatment, not only has a good overall mechanical properties, corrosion resistance is also good, so the steel is often supplied in a heat treatment state. Martensitic corrosion-resistant cast steel commonly used tempering process, usually choose 950 ~ 1050. C oil quenching or air cooling, and then 650 ~ 750. C tempering – that is, tempering treatment. General quenching should be tempered immediately after quenching to prevent quenching tissue stress and cracking of castings. Tempered state organization for tempering sothite and ferrite. Figure ll-49 for quenching temperature on w (Cr) = 13% of the martensitic corrosion-resistant stainless steel brittle transformation temperature. Table ll-20 shows the effect of four different heat treatment specifications on the mechanical properties of ZGl5Crl 3 cast steel. Table 11-21 shows the mechanical properties of ZGl5Crl 3 cast steel at different tempering temperatures. Table lI-22 shows the mechanical properties of ZG30Crl3 cast steel at different tempering temperatures. From Table li-20 to Table lI-22, it can be seen that ZGl5Crl3 and ZG30Crl3 cast steel will have tempering brittleness at 300. C to 600. C. Therefore, tempering in this brittle zone should be avoided as much as possible. Figure 11-50 shows the effect of tempering temperature on the mechanical properties of ZG06Crl3Ni4Mo (CA a 6NM). Table 11-23 for the martensitic corrosion-resistant stainless cast steel heat treatment ping specification. ①After annealing, furnace cooling. ② quenching holding time of at least 30rain, oil cooling or air cooling. ③ Tempering temperature shall not be used 370 ~ 595. C. ④ CA a 6NM for the United States steel, equivalent to ZG06Crl3Ni4M0. Containing a small amount of nickel, molybdenum, silicon and other alloying elements of high strength low carbon martensitic stainless steel castings, after normal tempering treatment has good overall mechanical properties, welding properties and anti-wear properties. Widely used in large hydraulic turbine whole casting or casting and welding impeller. Its usually selected heat treatment specifications for 950 ~ 1050. C normalizing + 600 ~ 670. C tempering. For large castings such as hydraulic turbine impeller heat treatment specification parameters (/ E fire tempering temperature, holding time and cooling rate, etc.) must be strictly controlled in order to obtain the specified performance requirements of the casting. Figure ll-51 ~ Figure ll-56 for the normalizing temperature, holding time, the cooling rate after normalizing, tempering temperature and molybdenum, silicon content on ZGl0Crl3Nil high-strength martensitic stainless steel castings performance effects. 2. Heat treatment of ferritic stainless steel castings Ferritic stainless steel chromium mass fraction is usually 16% to 30%, no phase change when heated, can not use heat treatment to strengthen, so the corrosion resistance requirements are not high castings, can be used in the cast state. To improve corrosion resistance and machinability, annealing can be performed. To avoid brittleness, annealing temperature should not be lower than 540. C, nor higher than 850. C, and annealing insulation end should be air-cooled or water-cooled, should not be cooled with the furnace. Ferrite stainless steel castings annealing heat treatment characteristics: (1) high chromium stainless steel castings heated easily caused by grain coarsening, and the castings become brittle, prone to intergranular corrosion, so should avoid overheating. (2) ferritic stainless steel castings in the high temperature above 850.C fast cooling is sensitized and intergranular corrosion. Therefore, the ferritic stainless steel casting annealing temperature is best below 850.C.. (3) ferritic stainless steel castings, especially when containing 7.22 (Cr) a z8%, in heating to 700 ~ 800. C d-phase and brittle. Therefore, after annealing, slow cooling is easy to become brittle. At the same time in 370 to 540. C temperature heating will also produce brittleness. 3. Austenitic stainless steel castings heat treatment of austenitic stainless steel castings are cast austenite + carbide or austenite + ferrite two-phase organization. In order to make the steel has the best corrosion resistance, the general castings are required to implement heat treatment. Its heat treatment characteristics are: (1) solution treatment: the casting will be heated to 950 ~ 1175. C, so that the carbide completely dissolved. After holding temperature quenching into water, oil or air, to get single-phase organization. The choice of solid solution temperature depends on the carbon content of the steel. The higher the carbon content, the higher the required solution temperature. Figure 11-57 shows the Fe-Ni-Cr alloy carbon solubility curve. Commonly used austenitic stainless steel solid solution treatment specifications are shown in Table ll-24. Austenitic stainless steel solid solution treatment of the heating method is appropriate to use the first low temperature preheating, and then quickly heated to the solid solution temperature process to reduce the heating process of the surface of the cast steel and the heart of the temperature difference. The holding time is determined by the wall thickness of the cast steel parts. Generally calculated according to each 25mm wall thickness insulation lh, to ensure that all sections of the castings can be heated through.
Solid solution treatment of the cooling medium, can use water, oil or air. Among them, water is commonly used. Air cooling is only applicable to thin-walled castings. For the austenitic stainless steel that can not or is not suitable for solid solution treatment, can also be used 870 ~ 980. C insulation 24 ~ 48h after the air-cooling process. To improve the corrosion resistance of steel. But this process is not applicable to thin-walled castings of stainless steel with very low carbon content or castings that need to be welded after cutting and processing. (2) stabilization process: austenitic stainless steel castings (18Crl8Ni type) although the best corrosion resistance after solid solution treatment. But when reheated to 500 ~ 850. C or castings in this temperature when used, then the steel chromium carbide will again precipitate along the austenite grain boundaries, resulting in steel grain boundary corrosion damage or weld cracking defects. This phenomenon is called sensitization. In order to improve this type of austenitic stainless cast steel grain boundary corrosion resistance. Generally add titanium, niobium and other alloying elements, and in the solid solution treatment, and then reheated to 850 ~ 930. C, fast cooling. This titanium and niobium carbide first precipitation from the austenite, thus preventing the precipitation of chromium carbide, improving the steel in the above temperature heating when used to resist grain boundary corrosion performance. 4 precipitation hardening stainless steel castings heat treatment precipitation hardening martensitic stainless steel generally contains the formation of hardening phase of copper, lead, molybdenum, titanium and other alloying elements. These elements have a large solubility in austenite, while in martensite is very small. Therefore, precipitation-hardening martensitic stainless steel heat treatment is first solid solution treatment, so that the cast state precipitation of the hardened phase is fully dissolved. Then precipitation hardening treatment, so that the secondary hardening phase precipitation so as to improve the strength of stainless steel castings and make it both good corrosion resistance purposes. In addition, the solution treatment will also improve the cutting performance of cast steel parts. Precipitation hardening stainless steel castings heat treatment characteristics: (1) solution treatment before, it is best to slowly preheat to 650.C, and then quickly warm up. Castings can also be high-temperature loading furnace. Table lI-25 for different wall thickness castings the highest furnace temperature. (2) solution temperature is generally I020 ~ 1060. C, holding time is calculated by lh per 25mm wall thickness. The solid solution temperature should not be too high, otherwise it will be overheated to reduce the Ms point of steel, increasing the residual austenite in steel, reducing the strength of the casting. The shape of complex castings, the solid solution temperature can be reduced to 927. C. (3) in order to eliminate the cast steel parts (especially thick pieces) in the presence of dendritic organization and the composition of the inhomogeneity of the segregation, the best in the solid solution before the high temperature homogenization treatment. Homogenization of the temperature of l000 ~ 1150. C, holding time according to the casting wall thickness. Listed as different casting performance requirements of aging treatment specifications. Figure 11-58 shows the effect of aging temperature on the tensile and yield strength of ZGCrl7Ni4Cu2 (U.S. grade 7-4PH – see Chapter 6) precipitation hardening stainless steel. (5) In order to improve the mechanical properties of precipitation hardening stainless steel, the solution treatment is followed by 700 to 810. C × 2h air cooling, and then 620. C × 4h air cooling two-stage heat treatment, its cutting properties also improved significantly.