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Two Minute Course in Stainless Steel
Courtesy of the Specialty Steel Industry of North America
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This Benefit Added On: February 4, 2001
There are four basic categories of stainless steel: austenitic, martensitic, ferritic and precipitation hardening.
Austenitic is a chromium, nickel and manganese alloy and is identified as 200 and 300 series, according to Kennametal. Grade 304 is the most common, known for great ductility and can have a tensile strength of 80,000 psi or greater. As with most stainless steels, tools for machining it need a high positive rake, sometimes as high as 30 degrees and preferably a chipbreaker. Austenitics also need a high chip load and an adequate lubricant flow to carry off the heat. The metal's low thermal conductivity can cause chip build up at the tool tip.
Machining austenitics generates tough stringy chips. Shallow cuts are difficult because of the metal's tendency to work harden.
Ferritic. the 400 series, has chromium as the primary alloy. It has a higher machinability than austenitic but a lower ductility than 200 and 300 steels. It has slightly less tensile strength than the austenitics and needs a less positive tool rake. The metal is also less prone to work hardening during machining.
Martensitic is also a 400 series stainless, but with less chromium and more carbon than the ferritic steels. It needs heat treating to increase the hardness. In fact, it depends on heat treating for hardness and strength. According to Kennametal, tensile strength can be as high as 300,000 psi
and it is free machining in it's non-heat treated condition. Ductility falls upon heat treating, whereupon machining may require a negative tool rake.
Both ferrtic and martensitic machining generates brittle, stringy chips and work hardening may be a problem.
Precipitation hardening is a small group of steels with high chromium and nickel content. These metals harden by aging and have characteristics close to martensitic steels, particularly the machinability. Tensile strength might be around 160,000 psi.
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