Stainless Steel Casting Production Process
Stainless steel castings, is a variety of stainless steel production of steel products in general, mainly used for a variety of media corrosion conditions.
As early as 1910 has been found, steel in the Cr content of more than 12%, there is good corrosion and oxidation resistance. Typical stainless steel contains, in addition to Cr12% or more, one or more other alloying elements such as Ni, Mo, Cu, Nb, Ti and N2.
Classification of chemical composition Stainless steel Cr stainless steel and Cr, Ni stainless steel two categories. The corrosion resistance of stainless steel is mainly C content and precipitation of carbides, so corrosion-resistant stainless steel containing C as low as possible, usually C ≤ 0.08%, but the high temperature mechanical properties of heat-resistant steel is determined by its organization in the stability Of the carbide precipitation phase, so the heat-resistant steel containing C are higher, generally carbon content of 0.20% or more.
According to metallographic classification, stainless steel is divided into ferritic stainless steel, martensitic stainless steel, austenitic stainless steel and biphasic (ferritic in austenitic matrix) stainless steel:
(1) ferritic stainless steel
With chromium as the main alloying element, the amount of Cr is generally between 13% and 30%. Has a good resistance to oxidation of the medium corrosion resistance and resistance to air oxidation at high temperatures, can also be used as heat-resistant steel. The welding performance of this kind of steel is poor. When the chromium content is more than 16%, the as-cast structure is coarse, and long-term insulation between 400-525 ℃ and 550-700 ℃, there will be "475 ℃" brittle phase and σ phase, the steel brittle. The brittleness at 475 ° C is related to the ordering of Cr-containing ferrite. 475 ℃ brittle phase and σ phase brittle, can be heated to 475 ℃ above and then fast to improve. The brittleness at room temperature and the brittleness of the post-weld heat affected zone are also one of the basic problems of ferritic stainless steel. Vacuum refining can be used to add trace elements such as boron, rare earth and calcium, or austenite forming elements such as Ni, Mu, N, Cu, etc.) approach to be improved. In order 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 grain growth in the heat affected zone. Commonly used ferrite steel has ZGCr17 and ZGCr28. The impact toughness of this type of steel is low, in many cases by high nickel austenitic stainless steel replaced. Ni content of more than 2%, containing more than 0.15% N of ferritic steel has a good impact performance.
(2) martensitic stainless steel
Martensitic stainless steels include martensitic stainless steel and precipitated hardened stainless steel. In engineering applications, mechanical properties are the main purpose. Although this type of steel in the atmospheric corrosion and more moderate corrosive media (such as water and some organic media) has good corrosion resistance, but its corrosion performance is often not as a test project. The chemical composition of the range is: Cr13% -17%, Ni2% -6%, C ≤ 0.06%. Metallic microstructure is mainly low carbon slab martensite, therefore, has excellent mechanical properties, strength index is more than twice the austenitic stainless steel, but also has a good process performance, especially welding performance. So in the important engineering applications occupy a very important position, is casting an important branch of the field of stainless steel.
(3) austenitic stainless steel
Cr-Ni-Mo, Cr-Ni-Cu or Cr-Ni-Mo-Cu; Cr-Mn-N and Cr-Ni-Mn-Cr; N series. Cr-Ni is represented by the famous "18-8". Cr-Ni-Mo, Cr-Ni-Cu and Cr-Ni-Mo-Cu were added on the basis of Cr-Ni system with 2% to 3% molybdenum and copper (or both) to enhance the resistance to sulfuric acid Of the corrosive, but molybdenum is a ferrite forming elements, in order to ensure austenitizing, plus molybdenum containing Ni should be appropriate to increase. Cr-Mn-N is a Ni-saving alloy. When the amount of Cr is greater than 15%, the addition of the individual alone can not get the ideal austenite structure, must be added 0.2% -0.3% nitrogen, to get a single austenite must be added more than 0.35% nitrogen. As a result of the high N content of the casting is often caused by pores, loose and other defects, and by adding the right amount of N and a small amount of Ni, you can get a single austenite, which appears Cr-Ni-Mn-N system. Of course, to get austenite, ferrite complex structure, do not need to add more N and Ni.
(4) austenite - ferrite duplex stainless steel
The microstructure of the composite steel is usually containing 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 high-chromium alloy steel, with good resistance to sulfuric acid corrosion, can be used to manufacture castings. On this basis, the development of controllable ferrite steels, with high strength, and in the sulfate have good resistance to stress corrosion, commonly used in the oil industry equipment.
The so-called investment casting process, simply that is made of fusible material (such as wax or plastic) made of meltability model (referred to as investment or model), on which a number of layers of special refractory coating, after drying and hardening Forming a whole shell, and then steam or hot water from the shell melt off the model, and then put the shell in the sand box, filled with dry sand in its shape, and finally the mold into the roasting furnace after high temperature Roasting (such as the use of high-strength shell, you can not shape the mold after the shell directly roasting), casting or shell by firing, pouring molten metal in which castings.
Casting die size accuracy is high, generally up to CT4-6 (sand casting for the CT10 ~ 13, die-casting for the CT5 ~ 7), of course, due to the complex investment casting process, the impact of casting size accuracy factors, such as mold The shrinkage of the material, the deformation of the investment mold, the change in the amount of the shell during the heating and cooling, the shrinkage of the alloy, and the deformation of the casting during the solidification process. Therefore, although the dimensional accuracy of the conventional investment casting is higher Consistency still needs to be improved (using medium and high temperature wax casting size uniformity to improve a lot).
When pressing the mold, the cavity surface finish is high, so the surface finish of the investment mold is relatively high. In addition, the shell is made of high-temperature special adhesive and refractory coating refractory coating coated on the investment in the mold made in direct contact with the molten metal cavity surface finish high. Therefore, the casting surface roughness than the general casting of the high, generally up to Ra.1.6 ~ 3.2μm.