Stainless Steel Grades: Types, Properties & Common Uses

Stainless steel is an iron-chromium alloy that contains anywhere from 10 to 30% chromium which gives the metal high resistance to corrosion. Although there are many grades of stainless steel only a dozen or so are used with any regularity. For example, AISI Type 304 SS, having a chromium-nickel constituent and low carbon, is popular for its good corrosion resistance, cleanability, and formability, making it popular for many everyday items such as kitchen sinks. AISI Type 316 SS, containing the alloying element molybdenum, is even more resistant to chemical attack than Type 304, making it useful for exposure to seawater, brine, sulfuric acids, and other corrosives found in the industrial environment. This article briefly discusses some of the popular grades of stainless steel as well as the settings in which these grades excel.

The principal types of stainless steels include:

• Ferritic
• Martensitic
• Austenitic
• Duplex

Ferritic Stainless Steel

The addition of chromium (>17%) to a steel alloy stabilizes the ferritic phase of the alloy, making a material that is highly corrosion-resistant, if not exceptionally strong. It cannot be hardened through heat treatment but can be cold-worked to increase hardness. It is an inexpensive grade and is often used for kitchen equipment, architectural/ornamental applications, etc. where corrosion resistance, ductility, formability, and cost are important, and strength is not a concern.

Martensitic Stainless Steel

Adding carbon (up to 2%) to the chromium-iron alloy increases the alloy’s hardenability. Although unable to be hardened to the level of iron-carbon martensite, martensitic stainless steel can be sufficiently hardened to produce rust-resistant cutlery, surgical instruments, ball valves and seats, for example. Martensitic stainless steels tend to be used in specialty applications. AISI Type 410, for example, is used for making food-machine parts, pump shafts, etc., while Type 403 is used in high-heat applications such as turbines. Type 416 is considered free-machining and has the best machining characteristics of all the stainless steels; it is used for many turned SS parts. Martensitic stainless steel is magnetic and, with a high carbon content, difficult to weld.

Austenitic Stainless Steel

Adding nickel (8-20%) to the chromium-iron alloy produces a steel that is austenitic at room temperature, with a face-centered cubic structure that resists corrosion, and whose magnetic field is one of a soft magnet (ie, it can be magnetized in an electric field, but not permanently). These steels have relatively low carbon content, which makes them weldable. This group is the most commonly used of all the stainless steels, notably Type 302. The economical 304, sometimes called food-grade, is used for general-purpose corrosion-resistant applications where welding-related corrosion is of concern. The improved corrosion-resistant 316 is used for industrial applications and is considered the most corrosion-resistant of the austenitic stainless steels. An “L” after the grade indicates improved weldability under the harshest of welding conditions. Temperature resistance is increased by adding titanium, as in Type 321, a popular material in aerospace applications.

A relatively new grade of stainless steels, sometimes called PHSSs and carrying identifiers such as 15-5, 17-4, and 17-7 PH, are precipitation hardened. This special heat-treating process increases the metal’s resistance to stress corrosion cracking. Some of these PHSSs are austenitic, some are martensitic, and some fall somewhere in between. A-286 alloy was one of the first of the so-called superalloys.

Duplex Stainless Steel

Duplex steels have structures that combine both ferritic and austenitic phases, giving them almost twice the strength of austenitic varieties. With good corrosion resistance and weldability akin to that of austenitic stainless steel, they are used in a variety of special applications--on offshore platforms and in pressure vessels, for instance, where strength is imperative.

Stainless Steel Grades Summary

Table 1 below describes many of the AISI stainless steels, their strengths, and typical applications. Some of the steels with suffixes (L, S, etc.) have not been included, nor have many of the specialty PHSSs.

Summary

This article presented a brief summary of the principal types of common stainless steel and common grades. For more information on additional products, consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products.