Stainless Steel Casting Dielectric Corrosion
Stainless steel castings, which are made of various stainless steel materials, are mainly used for the corrosion conditions of various media.
As early as 1910, when the Cr volume in steel exceeded 12%, good corrosion resistance and oxidation resistance were found. Typical stainless steel contains one or more other alloy elements, such as Ni, Mo, Cu, Nb, Ti and N2, in addition to Cr12 %.
According to chemical composition, stainless steel has Cr stainless steel and Cr and Ni stainless steel. Influence the corrosion performance of stainless steel are mainly C content and precipitation of carbides, so the corrosion resistance of stainless steel C content is lower, the better, usually C 0.08% or less, however, the high temperature of heat resistant steel mechanical performance is determined by its stable carbides precipitate phase in the organization, so the C content of heat resistant steel is higher, general carbon content above 0.20%.
According to the classification of metallographic structure, the stainless steel is divided into ferritic stainless steel, martensitic stainless steel, austenitic stainless steel and double phase (ferritic stainless steel in austenitic matrix).
(1) ferrite stainless steel
Chromium is the main alloy element, with Cr amount generally between 13% and 30%. The ability of good oxidation resistance and air oxidation resistance at high temperature can also be used as heat resistant steel. This kind of steel has poor welding performance. Chromium is more than 16%, the as-cast organization bulky, in 400-525 ℃ and between 550-700 ℃ heat preservation for a long time, there will be a "475 ℃" brittle phase and the sigma phase, make steel brittle. 475 ℃ brittleness associated with ordering containing Cr ferritic phenomenon. 475 ℃ brittle phase and the sigma phase brittleness, can be heated to above 475 ℃ and then quickly to improve the cold. Room temperature brittleness of brittleness and after welding heat affected zone is also one of the basic problems of ferritic stainless steel, vacuum refining, can be used to trace elements (such as boron, rare earth and calcium) or austenitic formation elements (such as Ni, Mu, N, Cu, etc.) can be improved. In order to improve the mechanical properties of the weld zone and the thermal impact zone, a small amount of Ti and Nb are also added to prevent the growth of grain in the heat affected zone. Common ferritic steel has ZGCr17 and ZGCr28. The steel has a low impact toughness and is replaced by the high nickel austenitic stainless steel on many occasions. Ferritic steel containing more than 2% of Ni and more than 0.15% of N has good impact performance.
(2) martensite stainless steel
Martensitic stainless steel includes martensitic stainless steel and precipitation-hardened stainless steel. In engineering application, mechanical properties are the main purpose. Although this type of steel has good corrosion resistance in atmospheric corrosion and moderate corrosive medium (such as water and certain organic media), its corrosion performance is often not a test item. Its chemical composition range is: Cr13%- 17%, Ni2%- 6%, C is less than 0.06%. Microstructure of low carbon plate strip martensite, therefore, it has excellent mechanical properties, strength index is more than 2 times of austenitic stainless steel, and also have good process performance, especially the welding performance. Therefore, it occupies a very important position in important engineering applications and is an important branch in the field of casting stainless steel.
(3) austenitic stainless steel
Austenitic stainless steel can be divided into four groups, namely Cr‐Ni series. Cr‐Ni‐ Mo, Cr‐Ni‐Cu or Cr‐Ni‐Cu series. Cr‐Mn‐N and Cr‐Ni‐Mn‐N systems. Cr‐Ni is represented by the famous "18-8". Cr ‐ Ni ‐ ‐ Mo, Cr Ni ‐ ‐ Ni, Cu, Cr ‐ Mo ‐ Cu system on the basis of Cr ‐ Ni is add 2% to 3% of molybdenum and copper join (or both), in order to improve the resistance to sulfate corrosion, but molybdenum ferrite formation elements, in order to guarantee the austenitizing, after add molybdenum content of Ni to appropriately increase. Cr‐Mn‐N is an alloy that saves Ni. When the Cr quantity is greater than 15%, the ideal austenite tissue cannot be obtained alone, and it must be added 0.2 to 0.3% of the nitrogen, and the single austenite must be added by 0.35% of the nitrogen. Because of the excessive amount of N, it often causes the casting to produce stoma and loose defects, and to add a moderate amount of N and a small number of Ni, the single austenite can be obtained, and Cr‐Ni‐Mn‐N system is present. Of course, to get the austenite, ferrite complex, you don't have to add more N and Ni.
(4) austenitic ferritic stainless steel
The metallographic structure of composite steel is usually a ferrite containing 5% to 40% to improve the welding property of the alloy, increase the strength and improve the stress corrosion resistance. For example, Cr28%-Ni10%-C0.30% high carbon high chromium alloy steel, has good corrosion resistance to sulphuric acid, can make casting use. The steel-controlled ferritic steel with high strength and good stress corrosion resistance in sulfate is used in the equipment of petroleum industry.
The production process
So-called investment casting process, simply be with fusible materials (such as wax material or plastic) into the meltability model (referred to as "investment pattern or model), on which the coated with several layers of tailor-made refractory coating, after drying and hardening type to form a whole shell, reoccupy steam or hot water from the melt in the shell model, and then put type shells in the sand, dry sand by filling in all round its modelling, finally will be cast into the furnace after high temperature roasting (such as high strength shell, can need not modelling and will release the shell directly after roasting), cast type or shell after roasting, and are casting in pouring molten metal.
Size precision investment castings, general of CT4-6 (sand casting is CT10 ~ 13, die-casting is CT5 ~ 7), of course, due to the complexity of investment casting process, factors influencing the dimension accuracy of castings is more, such as mold material shrinkage and deformation of casting, shell in the process of heating and cooling line quantity change, alloy of shrinkage and deformation of castings in the solidification process, so the average size although higher precision investment castings, but still need to improve the consistency (in medium and high temperature of the wax casting size consistency to improve a lot).
When pressing the melting mold, the surface finish of the cavity is high, so the surface finish is high. In addition, the shell is made of special adhesive and refractory material made of refractory materials, which is made of refractory coating. The surface of the cavity with direct contact with molten metal is high. Therefore, the surface finish of the molten mold casting is higher than that of ordinary casting parts, which can generally reach Ra.1.6~3.2 mu m.