Stainless Steel & Its Advantages
Stainless steel, is a type of steel usually containing 10 to 30 percent chromium. In conjunction with low carbon content, chromium imparts remarkable resistance to corrosion and heat. Other elements, such as nickel, molybdenum, titanium, aluminum, niobium, copper, nitrogen, sulfur, phosphorus, or selenium, may be added to increase corrosion resistance to specific environments, enhance oxidation resistance, and impart special characteristics.
There are different corrosion resistant properties associated with different grades of stainless steel. Because of the invisible layer of chromium oxide that protects stainless steel materials, making them stain and corrosion resistant, stainless steel is also the ideal choice for hospital and other hygiene-conscious conditions.
Another major benefit of using stainless steel materials is the significant strength-to-weight advantage over other material options. Because of the unique ability of stainless steel to resist corrosion, heat damage and chemical damage, high strength duplex grades provide added strength, allowing for a reduced thickness in the material, providing a cost advantage over conventional grades of stainless steel.
Castability of Stainless Steel
Due to above advantages of stainless steel, it is widely used as the material of investment casting process. This type of castings is called stainless steel investment casting. Below we will discuss the castability of stainless steel:
Castability (Fluidity) of stainless steel is the ability of the molten stainless steel to flow easily without premature solidification is a major factor in determining the proper filling of the mold cavity.
The higher the castability of a molten stainless steel, the easier it is for that molten stainless steel to fill thin grooves in the mold and exactly reproduce shape of mold cavity, there by successfully producing the castings with thinner sections. Poor castability leads to casting defects such as incomplete filling or misruns especially in thinner sections of a casting. Because castability is dependent mainly upon the viscosity of molten metal, it is clear that higher temperature improves castability of molten metal and alloys, where as presence of impurities and non metallic inclusions adverse it.
Castability of stainless steel is nothing but producing a stainless steel casting with minimum cost, defects and time. This can be done by the high compatibility between the product requirement and process castability.
Castability is the ease of forming a casting. Castability of stainless steel can be thought of as how easy is it to cast a quality part. A very castable stainless steel part design is easily developed, incurs minimal tooling costs, requires minimal energy, and has few rejections.
There are a wide range of stainless steel casting grades and types, such as 304(L), 316(L), PH17-4, 2205 duplex stainless steel, etc. All these stainless steel grades can be cast in various shapes and applications.
Factors That Affect the Castability of Stainless Steel
Not all shapes of stainless steel components are castable. This is mainly as a result of the inherent problems associated with the solidification of molten stainless steel. Factors include the condition of the molten stainless steel and the design and the shape of the casting (and hence the moulds used).
The shape of the stainless steel casting can be modified and placement of risers to induce directional solidification, and delay the freezing of sections of the casting through which molten metal has to flow to feed another location may be used. Directional solidification is also induced using chills and insulators.
Contraction after solidification must also be catered for. If sections of the stainless steel casting are restrained, the shrinkage stresses can cause hot tears, particularly at changes of section size and profile.
This problem is best overcome by:
- Modifying the shape of the stainless steel casting to allow for
- A gradual change of cross section
- Large radii at change of profile
- Inducing directional cooling
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