Stainless Steel

Stainless steel is an iron-based alloy that contains a minimum of 10.5% chromium, making it highly resistant to corrosion and rusting.

Composition and Properties

Stainless steel contains iron with chromium and additional elements like molybdenum, carbon, nickel and nitrogen. The chromium content forms a self-healing protective oxide film that prevents deeper corrosion. The material’s density ranges from 7.5 to 8.0 g/cm3, with melting points between 1,400-1,530°C.

Corrosion Resistance

The passive chromium oxide layer only affects the outer few atomic layers while protecting the underlying metal. Adding nitrogen improves resistance to pitting corrosion, while increasing chromium content above 11% and adding nickel to at least 8% enhances overall corrosion resistance.

Strength and Durability

Type 304, the most common grade, has a tensile yield strength of 210 MPa in annealed condition, which can increase to 1,050 MPa when cold worked. Precipitation hardening alloys like 17-4 PH can reach strengths up to 1,730 MPa through heat treatment.

Types and Classifications

Austenitic Stainless Steel

Makes up about two-thirds of all production. Contains high nickel content and maintains its structure at all temperatures. The 300-series, including popular Types 304 and 316, offers excellent corrosion resistance.

Ferritic Stainless Steel

Contains 10.5-27% chromium with minimal nickel. Magnetic and less expensive than austenitic grades. Common in automotive exhaust systems and kitchen equipment.

Martensitic Stainless Steel

Offers high strength but lower corrosion resistance. Used in turbines, tools, and surgical instruments. Can be heat treated to improve properties.

Applications and Uses

The material excels in:

• Food processing equipment
• Surgical instruments
• Building construction
• Chemical processing
• Marine environments

Drawbacks

• Higher initial cost compared to carbon steel
• Can still corrode under certain conditions
• Susceptible to stress corrosion cracking in high chloride environments
• Complex welding requirements

Historical Development

Harry Brearley discovered martensitic stainless steel in 1912 while researching gun barrel materials. By 1929, US production reached 25,000 tons annually. The development of the AOD process in the 1950s enabled large-scale, cost-effective production.

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Stainless Steel (Wikipedia)

Stainless steel, also known as inox, corrosion-resistant steel (CRES), and rustless steel, is an alloy of iron that is resistant to rusting and corrosion. It contains iron with chromium and other elements such as molybdenum, carbon, nickel and nitrogen depending on its specific use and cost. Stainless steel's resistance to corrosion results from the 10.5%, or more, chromium content which forms a passive film that can protect the material and self-heal in the presence of oxygen.

The alloy's properties, such as luster and resistance to corrosion, are useful in many applications. Stainless steel can be rolled into sheets, plates, bars, wire, and tubing. These can be used in cookware, cutlery, surgical instruments, major appliances, vehicles, construction material in large buildings, industrial equipment (e.g., in paper mills, chemical plants, water treatment), and storage tanks and tankers for chemicals and food products. Some grades are also suitable for forging and casting.

The biological cleanability of stainless steel is superior to both aluminium and copper, and comparable to glass. Its cleanability, strength, and corrosion resistance have prompted the use of stainless steel in pharmaceutical and food processing plants.

Different types of stainless steel are labeled with an AISI three-digit number. The ISO 15510 standard lists the chemical compositions of stainless steels of the specifications in existing ISO, ASTM, EN, JIS, and GB standards in a useful interchange table.