Stainless steel castings are the general term for steel castings produced with various stainless steel materials, which are mainly used in various medium corrosive conditions.
As early as 1910, it was found that when the Cr content in steel exceeds 12%, it has good corrosion resistance and oxidation resistance. In addition to containing Cr12% or more, typically stainless steel also contains one or more other alloying elements, such as Ni, Mo, Cu, Nb, Ti, and N2.
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Classification
According to the chemical composition of stainless steel, there are two categories of stainless steel: Cr stainless steel and Cr and Ni stainless steel. The main factors affecting the corrosion performance of stainless steel are the C content and the precipitated carbides, so the lower the C content of the corrosion-resistant stainless steel, the better, usually C≤0.08%, but the high-temperature mechanical properties of the heat-resistant steel are determined by the stability of its structure. Therefore, the C content of heat-resistant steel is relatively high, and the carbon content is generally above 0.20%.
According to the metallographic classification, stainless steel is divided into ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, and duplex (ferrite in an austenitic matrix) stainless steel:
(1) Ferritic stainless steel
Chromium is the main alloying element, and the Cr content is generally between 13% and 30%. It has good corrosion resistance to oxidizing media and air oxidation resistance at high temperatures, and can also be used as heat-resistant steel. The welding performance of this steel is poor. When the chromium content is more than 16%, the as-cast structure is coarse, and if the temperature is kept between 400-525 °C and 550-700 °C for a long time, the “475 °C” brittle phase and σ phase will appear, making the steel brittle. The brittleness at 475℃ is related to the ordering phenomenon of Cr-containing ferrite. The brittle phase at 475°C and the sigma phase brittleness can be improved by heating to above 475°C and then rapidly cooling. The brittleness at room temperature and the brittleness of the heat-affected zone after welding are also one of the basic problems of ferritic stainless steel. Vacuum refining, adding trace elements (such as boron, rare earth, calcium, etc.) or austenite forming elements (such as Ni, Mo, N, Cu, etc.) method to improve. To improve the mechanical properties of the weld zone and the heat-affected zone, a small amount of Ti and Nb are usually added to prevent the grain growth of the heat-affected zone. Commonly used ferritic steels are ZGCr17 and ZGCr28. This type of steel has low impact toughness and is replaced by austenitic stainless steel containing high nickel on many occasions. Ferritic steels with a Ni content of more than 2% and an N content of more than 0.15% have good impact properties.
(2) Martensitic stainless steel
Martensitic stainless steel includes martensitic stainless steel and precipitation hardening stainless steel. In engineering applications, the main purpose is mechanical properties. Although this type of steel has good corrosion resistance in atmospheric corrosion and milder corrosive media (such as water and some organic media), its corrosion performance is often not used as an inspection item. The range of its chemical composition is Cr13%-17%, Ni2%-6%, and C≤0.06%. The metallographic structure is mainly low-carbon lath martensite. Therefore, it has excellent mechanical properties, and its strength index is more than twice that of austenitic stainless steel. At the same time, it has good process performance, especially welding performance. Therefore, it occupies an extremely important position in important engineering applications and is an important branch in the field of cast 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-Mo-Cu series; Cr-Mn-N series, and Cr-Ni-Mn- N series. The Cr-Ni series is represented by the famous “18-8”. Cr-Ni-Mo, Cr-Ni-Cu, and Cr-Ni-Mo-Cu series add 2%-3% molybdenum and copper (or both) based on the Cr-Ni series to improve the resistance to sulfuric acid However, molybdenum is a ferrite-forming element. To ensure austenitization, the Ni content should be appropriately increased after adding molybdenum. The Cr-Mn-N system is a Ni-saving alloy. When the content of Cr is greater than 15%, the ideal austenite structure cannot be obtained by adding manganese alone, and 0.2%-0.3% nitrogen must be added. To obtain single austenite, more than 0.35% nitrogen must be added. Since the N content is too high, defects such as pores and porosity are often formed in the casting, and single austenite can be obtained by adding an appropriate amount of N and a small amount of Ni, which results in a Cr-Ni-Mn-N system. Of course, to obtain austenite and ferrite complex structure, it is not necessary to add more N and Ni.
(4) Austenitic-ferritic duplex stainless steel
The metallographic structure of multiphase steel usually contains 5%-40% ferrite to improve the weldability of the alloy, increase the strength and improve the resistance to stress corrosion. For example, Cr28%-Ni10%-C0.30% high-carbon and high-chromium alloy steel has good sulfuric acid corrosion resistance and can be used in castings. The controllable ferritic section steel developed on this basis has high strength and good stress corrosion resistance in sulfate and is often used in equipment in the petroleum industry.
Production Process
The so-called investment casting process, simply put, is to use fusible materials (such as wax or plastic) to make a fusible model (referred to as investment mold or model), and apply several layers of special refractory coatings on it, which are dried and hardened. After forming an integral mold shell, the mold is melted from the mold shell with steam or hot water, and then the mold shell is placed in a sandbox, filled with dry sand molding around it, and finally, the mold is placed in a roasting furnace for high temperature. Roasting (for example, when a high-strength mold is used, the mold shell after demolding can be directly fired without molding), after the mold or mold is fired, molten metal is poured into it to obtain a casting.
The dimensional accuracy of investment castings is relatively high, generally reaching CT4-6 (CT10~13 for sand casting and CT5~7 for die casting). Of course, due to the complex process of investment casting, many factors affect the dimensional accuracy of castings, such as mold The shrinkage of the material, the deformation of the investment mold, the linear change of the shell during the heating and cooling process, the shrinkage rate of the alloy, and the deformation of the casting during the solidification process, etc., so the dimensional accuracy of ordinary investment castings is high, but its Consistency still needs to be improved (castings with medium and high-temperature waxes are much more dimensionally consistent).
When pressing the investment mold, a mold with a high surface finish of the cavity is used, so the surface finish of the investment mold is also relatively high. In addition, the shell is made of high temperature-resistant special binder and refractory paint prepared from refractory materials, which is coated and hung on the investment mold, and the inner surface of the cavity that is in direct contact with the molten metal has a high smoothness. Therefore, the surface finish of investment castings is higher than that of general castings, generally reaching Ra.1.6~3.2μm.
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