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The chemical composition of stainless steel makes it hard and durable. steel wrench image by Warren Millar from Fotolia.com Stainless steel is broken up into a variety of types by the Society of Automotive Engineers (SAE). These classifications are based on the steel's chemical composition. Stainless steel 420 is composed of 0.37 percent carbon, 13 percent chromium, 0.4 percent manganese and 0.4 percent silicon. Its properties depend on the temperatures at which the metal is hardened and tempered.
Machinability
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Machinability is a numerical value established by the American Iron and Steel Institute (AISI). It is a number attributed to all metals based on an arbitrary comparison to one percent carbon tool steel, which is rated 100. The value is assigned based on the average normal cutting speed, surface finish and tool life of the metal. A machinability rating of 80 is assigned to 420 stainless steel.
Hardness
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The best way to evaluate the hardness of a metal is with the Brinnell hardness test. In this test, a machine presses a 10mm hardened steel ball on the substance. The resulting depression in the metal is measured and the values are plugged into an equation. The resulting number is a representation of the metal's hardness. Depending on the temperature at which the metal is tempered, 420 stainless steel's Brinnell hardness is between 514 and 302. Lower numbers represent greater hardness, in this case achieved by increasing the tempering temperature. The drawback to this, however, is making the metal more brittle.
Tensile Strength
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Tensile strength is a measure of the amount of force it would take to break a metal. The tensile strength of 420 stainless steel ranges depending on the tempering temperature. As opposed to hardness, when the tempering temperature is lowered, the tensile strength increases. At a 400 degrees Fahrenheit, 420 stainless steel's tensile strength is 252,000 pounds per square inch (psi). At 1300 degrees, this value drops to 147,000 psi.
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