Metallurgy

More Metallurgy Information

 

Stainless Steel

 

A stainless steel is a steel alloy containing a minimum of 11% chromium to give some resistance to atmospheric corrosion. A higher amount is required to give resistance to strong oxidizing agents, such as nitric acid. Nickel is added to give resistance to strong reducing agents, such as hydrochloric acid.

Categories of Stainless Steel

The stainless steels of primary interest to home workshops are classified into three general categories. In order of importance, particularly for welding applications, these categories are: austenitic, ferritic, martensitic. The characteristics of each of these categories are described in the following table.

 

Some Important Properties of Steel
Type Composition Description Examples
Austenitic "18-8" Chromium 18% and Nickel 8% is the base composition for the austenitic steels. The standard grades of these steels all contain at least 2% manganese. The carbon content of austenitic steels is typically in the range of 0.08 - 0.15%, chromium content a minimum of 16%, and nickel typically at least 7%. In austenitic steels, the iron does not transform to ferrite on air cooling, but remains as austenite (gamma iron). Austenitic steels are:
  • Non-hardenable by heating and quenching; they are hardened by cold working only.
  • Work harden easily and can be work hardened almost to the level of martensitic stainless.
  • Non-magnetic when annealed or soft. Somewhat magnetic when work hardened.
  • Difficult to machine. Some are classified as "free machining," but this expression is used relative to other austenitic steels, not in an absolute sense.
  • Easily welded.
  • 302: The standard 18-8 austenitic stainless steel, with 0.15% carbon, from which the others are derived.
  • 303: Classified as "free machining" compared to other 300 series materials, but difficult to weld.
  • 304 and 304L: Low carbon 18-8 chromium-nickel stainless steels, preferred to 302 for welding applications.
  • 316: In addition to chromium (16 - 18%) and nickel (10 - 14%), this alloy contains molybdenum (2 - 3%) and has higher corrosion resistance than types 302 and 304.

Of the three general categories of stainless steel, austenitic stainless steels are the most widely used.

Ferritic 430, the basic ferritic grade, contains ~17% chromium. The typical composition of ferritic steels is:
  • Carbon: 0 - 0.12%
  • Chromium: > 15%
  • Nickel: 0% in the standard grades; non-standard grades often contain a small amount.

In ferritic steels, the austenitic iron (gamma iron) transforms to ferritic iron (alpha iron) on air cooling. The ferritic stainless steels are:
  • Magnetic.
  • Non-hardenable by heat treatment (a few can be slightly hardened).
  • Less ductile than the austenitic stainless steels.
430: The standard ferritic stainless steel, from which the others are derived. 430 contains:
  • Carbon: 0.12% (max.)
  • Chromium: 16 - 18%
  • Nickel: 0%
  • Manganese: 1%
  • Silicon: 1%

Ferritic steels are essentially soft iron containing chromium. The most well-known application of ferritic steels is as the "chrome" for automobile trim. Other uses include pots and pans.

Martensitic The typical composition of martensitic steels is:
  • Carbon: > 0.15%
  • Chromium: 12.0 - 14.0%
  • Nickel: 0%
Martensitic steels undergo the same austenite to martensite transformation that takes place in plain carbon steels. Martensitic steels are:
  • Magnetic.
  • Heat treatable: they can be hardened by quenching and tempering.
  • Generally somewhat difficult to machine. The exception is 416, which has a rating of 97%.
  • Weldable. The exception is 416, which gives brittle welds that have a tendency to crack.
410: The standard martensitic stainless steel, from which the others are derived. 410 contains:
  • Carbon: 0.15%
  • Chromium: 11.5 - 13.5%
  • Nickel: 0%
  • Manganese: 1.00% (max.)
  • Silicon: 1.00% (max.)

440-type alloys contain 16 - 18% chromium, at least 0.75% carbon, and 0.75% molybdenum, and provide maximum hardness. Typical applications of martensitic steels include cutlery, springs, and mechanical parts subject to high stress environments, such as turbine baldes. 440C is used in gun manufacture.