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Kesri Steels Limited - Heat Resistance Steel Castings
Steels are now used for a wide variety of conditions entailing heat and corrosion under both static and dynamic stresses such as aero engine valves, furnace conveyers, retorts, oil cracking units and gas turbines.
Stainless steel castings are usually classified as either corrosion-resistant castings or heat-resistant. The usual distinction between heat-resistant and corrosion-resistant casting steels is based on carbon content.
Castings are classified as heat resistant if they are capable of sustained operation while exposed, either continuously or intermittently, to operating temperatures that result in metal temperatures in excess of 650oC.
Heat-resistant steel castings resemble high-alloy corrosion-resistant steels except for their higher carbon content, which imparts greater strength at elevated temperature.
The three principal categories of Heat Resistant Steel Casting steels, based on composition are:
- Iron-chromium alloys
- Iron-chromium-nickel alloys
- Iron-nickel-chromium alloys
In the cast stainless steels structures may be austenitic, ferritic, martensitic, or ferric-austenitic (duplex). Chromium (a ferrite and martensite promoter), nickel, and carbon (austenite promoters) are particularly important in determining microstructure. In general, straight chromium grades of high-alloy cast steel are either martensitic or ferritic, the chromium-nickel grades are either duplex or austenitic, and the nickel-chromium steels are fully austenitic.
Cast austenitic alloys usually have from 5 to 20% ferrite distributed in discontinuous pools throughout the matrix, the percent of ferrite depending on the nickel, chromium, and carbon contents.
Ferrite can be beneficial in terms of weldability because fully austenitic stainless steels are susceptible to a weldability problem known as hot cracking, or microfissuring. The presence of ferrite in duplex CF alloys improves the resistance to stress-corrosion cracking (SCC) and generally to intergranular attack.