Stainless Steel Types and Properties

Chromium is the key ingredient in stainless steel

Stainless steel mill finishes

Stainless steel is first rolled to size and thickness and then annealed. Standard mill finishes are applied directly by the rollers and/or later with abrasives.

No. 0 - Hot Rolled Annealed, thicker plates
No. 1 - Hot rolled, annealed and passivated
No, 2D - Cold rolled, annealed, pickled and passivated
No, 2B - Cold rolled, annealed, pickled and passivated, with an additional pass through polished rollers
No, 2BA - Bright Anealed (BA) same as above with highly polished rollers
No. 3 - Coarse abrasive finish
No. 4 - Fine abrasive finish
No. 6 - Matte finish
No. 7 - Reflective finish
No. 8 - Mirror finish

Adding more than 12% chromium, by weight, to certain iron-alloys results in the formation of a non-corrosive oxide, a "passive film," which serves as a protective barrier between the steel and its environment. This passive film keeps the steel shiny and will re-form if the steel is scratched. Adding various elements to the alloy result in different properties. Nickel is added for better corrosion resistance, ductility and weldability. Molybdenum is added to resist pitting. Carbon is added for higher strength and hardness. Aluminum and silicon are added for improved oxidation reststance.

Austenitic (non heat treatable) stainless steel grades

The austenitic grades, known as the 300 series are iron-nickel-chromium composites. They are non-magnetic in the annealed state and are non-heat treatable. They can be hardened by cold-working only. The retain their austenitic structure at all temperatures and are generally the most weldable stainless steels. Here are some of the more common types in the 300 series.

Type 301 - Highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than Type 304.
Type 302 - Same corrosion resistance as Type 304, with slightly higher strength due to additional carbon.
Type 303 - Easier machining version of 304 with the addition of sulfur and phosphorus. Also referred to as A1 in ISO 3506. Excellent for making heavier cuts in automatic machining operations.
Type 304 - The most common grade; the classic 18/8 stainless steel. Also referred to as A2 in ISO 3506. Low carbon content for good weldability. Good corrosion reststance. Easy to form.
Type 309 - Has better temperature resistance than Type 304.
Type 316 - The second most common grade after Type 304; Type 316L and Type 316LVP are ASTM F138 compliant Implant Grade Stainless Steels and are often used for food and surgical stainless steel purposes. This alloy has a higher nickel and molybdenum content for increased corrosion and pitting resistance. Excellent for chemical processing applications. Also known as marine grade stainless steel. Most watches that are made of stainless steel are made of this grade. 18/10 stainless often corresponds to this grade. Also referred to as "A4" in ISO 3506. Good weldability.
Type 321 - Similar to 304 but lower risk of weld decay due to addition of titanium. See also 347 with addition of niobium for desensitization during welding.

Martensitic (heat treatable) stainless steel grades

The Martensitic grades are strong, highly machinable, heat-treatable, and magnetic, but are not as corrosion reststant as the 300 series stainless steels. They are often used in bearings and cutlery. Here are some of the more common types.

Type 408 - Heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.
Type 409 - Cheapest type; used for automobile exhausts; ferritic (iron/chromium only). * Type 410 - Wear resistant, but less corrosion resistant.
Type 416 - Easy to machine due to additional sulfur.
Type 420 - Cutlery Grade martensitic; similar to the Brearley's original rustless steel. Also known as surgical steel. Excellent polishability.
Type 430 - Decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced temperature and corrosion resistance.
Type 440 - A higher grade of cutlery steel, with more carbon in it, which allows for much better edge retention when the steel is heat treated properly. It can be hardened to Rockwell 58 hardness, making it one of the hardest stainless steels. Also known as razor blade steel. Available in three grades 440A, 440B, 440C (more common) and 440F (free machinable).

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