A Strong Family Tree: An Introduction to Stainless Steel Grades and Types

In life, there are many elements that can make a person stronger: hard work, good nutrition, a supportive family. Likewise, in metallurgy, it is the addition of alloying elements that turns a metal into something really useful. This is certainly true of stainless steel, where the addition of chromium and carbon to iron takes a strong metal and makes it more resistant to corrosion. As discussed in a previous article (From Cutlery to Space Shuttles: The History of Stainless Steel) it was the experimentation in the late 19th century with the levels of chromium (at least 10%) and carbon (less than 0.2%) that brought about today's impressive array of stainless steel grades. There are over 100 grades of stainless steel, but all of them fit into five different types, or branches of the family tree:

Austenitic steel: Austenitic steel is a chromium-nickel alloy that contains at least 16% chromium and 6% nickel (the basic grade 304 is referred to as "18/8", meaning 18% chromium, 8% nickel). Molybdenum is added to some grade for increased corrosion resistance. Austenitic steel is non-magnetic.

There are two series of alloys that fall under this category, the 200 Series (common alloys 201, 202, 203, 204 & 205) and the 300 Series (common alloys 302, 302, 303, 304, 305, 308, 309, 310, 314, 316, 317, 321, 330, 347, 384).

Ferritic steel: These are plain chromium steels with a chromium content in the range of 12-18% and whose structure consists largely of ferrite. Ferritic steel is magnetic.

Common alloys are 405, 409, 429, 430, 434, 436, 442, 446.

Martensitic steel: Like ferritic grades, martensitic steel has chromium as the only major alloy, in the range of 11% to 17%. However, carbon is added in amounts from 0.10% to 0.65, giving it very different characteristics. Martensitic steel is magnetic.

Common alloys are 405, 409, 429, 430, 434, 436, 442, 446.

Precipitation hardening steel: Chromium-nickel steels that are given very high tensile strengths by precipitation hardening.

Common alloys are 13-8, 15-5, 15-7, 17-4, 17-7.

Duplex: A mixture of austenitic (chromium-nickel stainless) and ferritic (plain chromium stainless) structures. The combination was originated to offer more strength than either of those stainless steels.

Common alloys are 329, 2205, 2304, 2507, 3RE60.

Ocean State Stainless offers fasteners in a full line of domestic stainless steel, Grade 8 and more, including:

*ASTM Standard Specification for Alloy-Steel and Stainless Steel Bolting Materials for High Temperature or High Pressure Service and Other Special Purpose Applications.

More articles from OSS:

From Cutlery to Space Shuttles: The History of Stainless Steel and Superalloys
The History of the U.S. Fastener Industry
An Introduction to Superalloys
The ABC's of Nuts and Bolts

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