Stainless Steel

There are more than 250 different stainless steels. These various grades of stainless are divided into five major families or classes. The general classes have been developed to consolidate the chemistries and mechanical properties required to meet specific customer application needs.

Martensitic Stainless Steels

These steels of the 400 series usually contain a minimum of 11.5% up to 18% chromium and have higher levels of carbon than ferritics. They are capable of being heat treated to a wide range of useful hard-ness and strength levels, and are used extensively in cutlery, sports knives and multipurpose tools.

Ferritic Stainless Steels

This group of steels in the 400 series contains 10.5% to 20% chro-mium for corrosion resistance and resistance to scaling at elevated temperatures. They are nonhardenable by heat treating and are always magnetic. Ferritic stainless is used in applications where resis-tance to corrosion is important, such as automotive emission control exhaust systems.

Austenitic Stainless Steels 

Austenitic stainless steels are the most specified grades produced because of their excellent formability and corrosion resistance. All 200 and 300 series steels are austenitic and con-tain 15% to 30% chromium and 2% to 20% nickel for enhanced surface quality, forma-bility and increased corrosion and wear resistance. They are non-magnetic in the annealed condition and depend-ing on the composition, primarily the nickel content, they become slightly magnetic when cold worked. These steels are used for automotive trim, cookware, pro-cessing equipment and a variety of industrial applications.

Precipitation-Hardening
Stainless Steels There are two general areas of PH grade stainless steels; martensitic and semi-austenitic. The martensi-tic group includes 17-4 PH®and 15-5 PH®chromium-nickel, with columbium and copper additions. They develop their high strength and hardness through heat treatment, which recipitates the copper. The marten-sitic PH steels are used in aerospace, chemical and petrochemical, and food processing applications.

The semi-austenitic grades are 17-7 PH®and PH 15-7 Mo®. They are austenitic in the annealed state, but martensitic in the hardened condition. 17-7 PH stainless has excellent high strength and fatigue properties, and is used in aerospace compo-nents. PH 15-7 Mo stainless is used in applications requiring high strength and hardness, such as retain-ing rings, springs and aircraft bulkheads.

Duplex Stainless Steels
These alloys have a mixture of austenite and ferrite in their structure. They exhibit characteristics of both phases with higher strength and ductility. Nitrogen is added to second generation duplex alloys and provides strength and increased weldability.

Stainless steels are available in the form of

• plate
• sheet
• strip
• foil
• bar
• wire
• pipes
• tubes

Stainless steels are a iron-based alloy containing at between 10.5% to 30% Cr. Stainless steel achieve its stainless characteristic through the formation of an invisible and adherent chromium-rich oxide surface film.

Other alloying elements added to improve the characteristics of the stainless steel include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, nitrogen, sulphur, and selenium.

Carbon is normally in amounts from 0.03% to more than 1.0% in some martensitic grades.

Selection of stainless steels are in general based on

• corrosion resistance
• fabrication characteristics
• availability
• mechanical properties for specific temperature ranges
• product cost

Since stainless steel resists corrosion, maintains its strength at high temperatures, and is easily maintained, it is widely used in items such as automotive and food processing products, as well as medical and health equipment