Stainless steel is a generic term for a family of corrosion resistant alloy steels containing 10.5% or more chromium. All stainless steels have a high resistance to corrosion. This resistance to attack is due to the naturally occurring chromium-rich oxide film formed on the surface of the steel. Although extremely thin, this invisible, inert film is tightly adherent to the metal and extremely protective in a wide range of corrosive media. The film is rapidly self repairing in the presence of oxygen, and damage by abrasion, cutting or machining is quickly repaired.

All stainless steels have a high resistance to corrosion. Low alloyed grades resist corrosion in atmospheric conditions; highly alloyed grades can resist corrosion in most acids, alkaline solutions, and chloride bearing environments, even at elevated temperatures and pressures.

Some grades will resist scaling and maintain high strength at very high temperatures, while others show exceptional toughness at cryogenic temperatures.

The majority of stainless steels can be cut, welded, formed, machined and fabricated readily.

The cold work hardening properties of many stainless steels can be used in design to reduce material thickness and reduce weight and costs. Other stainless steels may be heat treated to make very high strength components.

Stainless steel is available in many surface finishes. It is easily and simply maintained resulting in a high quality, pleasing appearance.

The cleanability of stainless steel makes it the first choice in hospitals, kitchens, food and pharmaceutical processing facilities.

Stainless steel is a durable, low maintenance material and is often the least expensive choice in a life cycle cost comparison.

In addition to chromium, nickel, molybdenum, titanium, niobium and other elements may also be added to stainless steels in varying quantities to produce a range of stainless steel grades, each with different properties. There are a number of grades to chose from, but all stainless steels can be divided into five basic categories:

These are named according to the microstructure inherent in each steel group (a function of the primary alloying elements). Austenitic and ferritic grades account for approximately 95% of stainless steel applications.

When nickel is added to stainless steel in sufficient amounts the crystal structure changes to "austenite". The basic composition of austenitic stainless steels is 18% chromium and 8% nickel. Austenitic grades are the most commonly used stainless steels accounting for more than 70% of production (type 304 is the most commonly specified grade by far).

This group of alloys generally containing only chromium, with the balance mostly Fe. These alloys are somewhat less ductile than the austenitic types and again are not hardenable by heat treatment. These are plain chromium stainless steels with varying chromium content between 12 and 18%, but with low carbon content.

Basic properties:

Martensitic stainless steels were the first stainless steels commercially developed (as cutlery) and have relatively high carbon content (0.1 - 1.2%) compared to other stainless steels. They are plain chromium steels containing between 12 and 18% chromium.

These steels have been formulated so that they can be supplied in a solution treated condition, (in which they are machinable) and can be hardened, after fabrication, in a single low temperature "aging" process.

These are stainless steels containing relatively high chromium (between 18 and 28%) and moderate amounts of nickel (between 4.5 and 8%). The nickel content is insufficient to generate a fully austenitic structure and the resulting combination of ferritic and austenitic structures is called duplex. Most duplex steels contain molybdenum in a range of 2.5 - 4%.

Chloride Stress Corrosion Cracking (CSCC):

Austenitic stainless steels may be susceptible to chloride stress corrosion cracking (CSCC). The standard 304/304L and 316/316L grades are most susceptible. Increasing nickel content above 18 to 20% or the use of duplex, or ferritic stainless steels improves resistance to CSCC. High residual or applied stresses, temperatures above 65-71C (150-160F) and chlorides increase the likelihood of CSCC. Crevices and wet/dry locations such as liquid vapor interfaces and wet insulation are particularly likely to initiate CSCC in susceptible alloys. Initiation may occur in several weeks, in 1-2 years or after 7-10 years in service.

Methods of minimizing chloride stress corrosion cracking:

Examples: Fill or seal crevices, paint under insulation, keep tensile stresses below the yield strength, shot peen, provide galvanic or cathodic protection.

Examples: Alloy 330, 904L.

Examples: 439, 26Cr 1Mo, 18Cr 2Mo

Examples: 329, 2205.

Note! Stress relief treatments above 425C (800F) may sensitize stainless steel to intergranular corrosion.

All stainless steels have a much lower conductivity than that of carbon (mild) steel (plain chromium grades approximately 1/3 and austenitic grades approximately 1/4). This must be kept in mind for any operation which involves high temperature, e.g. effects during welding (control of heat input), longer times required for heating to attain a uniform temperature for hot working.

Plain chromium grades have an expansion coefficient similar to carbon (mild) steels, but that of the austenitic grades is about 11/2 times higher. The combination of high expansion and low thermal conductivity means that precautions must be taken to avoid adverse effects, e.g. during welding use low heat input, dissipate heat by use of copper backing bars and use adequate jigging. This factor must also be considered in components which use a mixture of materials, e.g. a heat exchanger with a mild steel shell and austenitic grade tubes.

Last Update: October 23, 1999

By: Serdar Z. Elgun