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Alloys Make the Grade of Stainless Steels

The manufacture of quality stainless steel, from heat to heat and year to year, demands precise control of raw material ingredients and melting practices. Exact quantities of presorted scrap and alloying elements are delivered to the melting furnaces so that the heats or lots will be within specified composition ranges. Those composition ranges typically include a group of chemical elements for each grade of stainless steel.

Alloying Elements
Following is a brief look at the alloying elements found in stainless steels and their functions.
Chromium forms a surface film of chromium oxide to make the stainless steel corrosion resistant. It also increases the scaling resistance at elevated  temperatures.

Nickel stabilizes the austenitic structure and increases ductility, making stainless steel easier to form. It increases high temperature strength and corrosion resistance, particularly in industrial and marine atmospheres, chemical, food and textile processing industries.

Silicon increases scaling resistance by forming a tight initial scale, which will withstand cyclic temperature changes. It resists carburizing at high temperatures and slightly increases tensile strength and hardness. Small amounts of silicon are added to all grades of stainless for deoxidizing.

Manganese promotes the stability of austenite, at or near room temperature and improves hot working properties. Addition of up to 2% manganese has no effect on strength, ductility and toughness. Manganese is important as a partial replacement of nickel in 200 series stainless grades.

Molybdenum increases corrosion resistance, strength at elevated temperatures, and creep resistance. It expands the range of passivity and counteracts tendency to pit especially in chloride environments.

Aluminum is a very strong ferrite former and lowers the hardenability of stainless steel. It improves scaling resistance.

Carbon strengthens stainless steel but promotes the formation of precipitates harmful to corrosion resistance.

Columbium combines with carbon to reduce susceptibility to intergranular corrosion. It acts as a grain refiner and promotes the formation of ferrite.

Copper is added to stainless steels to increase their resistance to certain corrosive environments. It also decreases susceptibility to stress corrosion cracking and provides age-hardening effects.

Titanium combines with carbon to reduce susceptibility to intergranular corrosion. It acts as a grain refiner and promotes
the formation of ferrite.

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