‘Stainless’ is a term coined early in the development of these steels for cutlery applications.
It was adopted as a generic name for these steels and now covers a wide range of steel types and grades for corrosion or oxidation resistant applications.
Stainless steels are iron alloys with a minimum of 10.5% chromium. Other alloying elements are added to enhance their structure and properties such as formability, strength and cryogenic toughness.
These include metals such as:
Chromium and Nickel:
Chromium is the element that makes stainless steel stainless. It is essential in forming the passive film. Other elements can influence the effectiveness of chromium in forming or maintaining the film, but no other element by itself can create the properties of stainless steel.
At about 10.5% chromium, a weak film is formed and will provide mild atmospheric protection. By increasing the chromium to 17-20%, which is typical in the type-300 series of austenitic stainless steels, the stability of the passive film is increased. Further increases in the chromium content will provide additional protection.
Nickel will stabilize the austenitic structure (the grain or crystal structure) of the stainless steel and enhance the mechanical properties and fabrication characteristics. A nickel content of 8-10% and above will decrease the tendency of the metal to crack due to stress corrosion. Nickel also promotes repassivation in case the film is damaged.
Manganese, in association with nickel, performs many of the functions attributed to nickel. It will also interact with the sulfur in stainless steel to form manganese sulfites, which increases the resistance to pitting corrosion. By substituting manganese for nickel, and then combining it with nitrogen, strength is also increased.
Molybdenum, in combination with chromium, is very effective in stabilizing the passive film in the presence of chlorides. It is effective in preventing crevice or pitting corrosion. Molybdenum, next to chromium, provides the largest increase in corrosion resistance in stainless steel. Edstrom Industries uses 316 stainless because it contains 2-3% molybdenum, which gives protection when chlorine is added to the water.
Carbon is used to increase strength. In the martensitic grade, the addition of carbon facilitates hardening through heat-treating.
Nitrogen is used to stabilize the austenitic structure of stainless steel, which enhances its resistance to pitting corrosion and strengthens the steel. Using nitrogen makes it possible to increase the molybdenum content up to 6%, which improves corrosion resistance in chloride environments.
Titanium and Miobium:
Titanium and Miobium are used to reduce the sensitization of stainless steel. When stainless steel is sensitized, intergranular corrosion can occur. This is caused by the precipitation of chrome carbides during the cooling phase when parts are welded. This depletes the weld area of chromium. Without the chromium, the passive film cannot form. Titanium and Niobium interact with carbon to form carbides, leaving the chromium in solution so a passive film can form.
Copper and Aluminum:
Copper and Aluminum, along with Titanium, can be added to stainless steel to precipitate its hardening. Hardening is achieved by soaking at a temperature of 900 to 1150F. These elements form a hard intermetallic microstructure during the soaking process at the elevated temperature.
Sulfur and Selenium:
Sulfur and Selenium are added to 304 stainless to make it machine freely. This becomes 303 or 303SE stainless steel, which is used by Edstrom Industries to make hog valves, nuts, and parts that are not exposed to drinking water.