Stainless Steel – Base Metals

Stainless steels were accidentally discovered in the 1800s when cannon barrels were being cast. They noticed that the castings from a certain ore didn’t rust. They had found a load of ore that was high in chromium. Steel is classified as a stainless steel when its chromium content exceeds 10.5%. At this level a thin, invisible oxide layer forms on the surface of the base metal. This adherent oxide layer effectively stops the oxidation (rusting) process. This protective film is self-healing in the presence of oxygen (20.9 %of the make-up of air is oxygen). Carbon steels, of course, form a loose, flaky oxide that we call rust. As this rust flakes off, more of the base metal is exposed to the atmosphere, which allows the corrosion process to continue. Stainless steels are defined as steel alloys where the chromium content ranges from 10.5% to 30%.

There are five distinct types of stainless steel. Each is iron-based with alloying additions designed to modify specific characteristics. The major grades are as follows

Ferritic

Austenitic

Martensitic

Precipitation Hardening

Duplex

Ferritic Stainless Steel

Ferritic stainless steel contains from 10.5 to 30% chromium, is low in carbon, with some alloys containing major amounts of molybdenum, columbium and titanium. These alloys are typically used where corrosion is not severe, such as in automobile exhaust systems, cookware, architectural applications and automotive trim. Ferritic stainless steels are magnetic at room temperatures due to their body centered cubic crystal microstructure.

Austenitic Stainless Steel

Austenitic stainless steels contain from 16% to 26% chromium, up to 35% nickel, and have very low carbon content. Some of these steels are also alloyed with a minor amount of molybdenum, columbium and titanium. Austenitic stainless steels are used where corrosion can be severe. They are easily weldable. All alloys of this type are non-magnetic due to their face centered cubic structure at room temperature.

Austenitic grades include the 200 and 300 series of stainless steels. The 304 and 316 grades are very commonly used in welded fabrications. Over 80% of today’s stainless steel welding applications are done with these types of grades.

Martensitic Stainless Steel

Martensitic alloys contain from 12% to17% chromium, up to 4% nickel and.1% to 1.0% carbon. Some alloys will also have minor additions of molybdenum, vanadium, columbium, aluminum and copper. These alloys are used where high mechanical strength, hardness and corrosion resistance are required. They are not easily weldable.

Precipitation Hardening Stainless Steel

Precipitation hardening alloys contain between 11% and 18% chromium, 3% and 27% nickel and low carbon content. Some of the alloys will also have minor additions of molybdenum, vanadium, columbium, aluminum and copper and boron.

Duplex Stainless Steel

Duplex stainless alloys have 18% to 28% chromium, 2.5% to 7.5% nickel and low carbon contents. Some of the alloys will also have additions of nitrogen, molybdenum and copper. Duplex alloys have a ferritic and austenitic make-up. They offer the high strength properties of the ferritic stainlesses combined with some of the corrosion properties of the austenitic stainlesses.