Steel Types 101: Your Trusted Buying Guide

Steel is a malleable iron/carbon alloy containing from 0.10 to as much as 2% carbon, which determines the level to which it can be hardened. It is sometimes further alloyed with manganese, molybdenum, chromium, nickel, etc. to improve the ease with which it can be hardened and other characteristics such as corrosion resistance. Iron/carbon alloys having carbon in excess of 2% are called cast irons and are not malleable except in special forms known as malleable and ductile cast iron. As a metal, steel is characterized as being strong; fatigue-, impact-, and creep-resistant; electrically- and thermally-conductive; heavy; temperature-resistant; and fairly hard as compared with other materials such as wood, polymers, or ceramics. Steel is fabricated by removing impurities from pig iron in a furnace.

This article will briefly discuss some of the popular grades of steel as well as the settings in which these grades excel. The principal types of steels include:

• Plain Carbon Steel
• Alloy Steel
• Low Alloy Steel
• Stainless Steel

Plain Carbon Steel

Plain carbon steel contains no appreciable alloying element other than carbon itself, and, depending on the carbon content, is classified as low-, medium-, or high-carbon. Low-carbon steel (< 0.3% C) is used for making rivets, cold-drawn parts such as wire, stampings, etc. in the lower ranges and structural shapes, gears, cold-forged parts, and welded tube in the middle and upper ranges. Medium-carbon steel (0.3-0.5% C) is used for gears, shafts, connecting rods, seamless tubing, etc. and is sometimes called machinery steel. High-carbon steel (> 0.5% C) is used for springs, knives and handtools, taps and milling cutters, wire-drawing dies, etc. and is sometimes called tool or spring steel.

A simple AISI 4-digit numbering system designates plain carbon steel as 10xx, with the last digits approximating the percentage of carbon in the metal. A plain carbon mild steel designated as 1010 would contain 0.08-0.13% carbon, for example. A free-machining steel would be designated as 11xx.

To harden plain carbon steel, it must be cooled rapidly. This can lead to high residual stresses, distortion, diminished ductility, etc.

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Alloy Steel

While it is the carbon content of steel that determines the degree to which it can be hardened, certain alloying elements added to the steel can make heat treatment less traumatic, a benefit when it comes to reducing quenching distortion in complex, thin-walled parts, for example. The term hardenability refers to how deep a steel can be hardened, and alloy steels loosely fall into two camps around this measure: carburizing steel, which mostly hardens near the surface, and through-hardening steel, which can extend the hardening down into the metal’s core.

In the AISI numbering system, manganese steels are designated 13xx, nickel steels, 2xxx, nickel-chromium steels, 3xxx, molybdenum steels, 4xxx, and so on up to 9xxx for silicon-manganese steels.

Hardening of alloy steels can usually be done in oil for a slower quench than with water as required for plain carbon steels. This can reduce distortion and permit hardening to penetrate deeper into the material’s core.

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Low Alloy Steel

Sometimes called HSLA, or High-Strength Low-Alloy, steel, this metal offers improved strength over plain carbon steels and is used in settings where weight is a factor such as mobile equipment. It cold-forms well and is readily welded. It has better resistance to corrosion than plain steel, with good impact, fatigue, and abrasion resistance as well.

Other low alloy steels, with designations such as HY 80 and HY 90, are used for ship hulls and off-highway equipment. Still, other low alloy steels are available for specific conditions such as low-temperature toughness or to produce protective, weathering layers on decorative steel used for building facades.

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Stainless Steel

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.

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Summary

This article presented a brief summary of the common types of steel. 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.