Grades of Stainless Steel

Why consider using stainless steel?
The instrument engineer will often consider using stainless steel due to its mechanical strengths and resistance to corrosion e.g. when specifying valve internals SS offers good erosion resistance, or when specifying enclosures SS offers good corrosion resistance.

What makes a steel a "stainless steel"?
When an alloy of steel contains more than approximately 10 ½% Chromium it can be classified as a stainless steel. This is because Chromium has a high affinity for Oxygen and forms a stable Oxide film on the surface of the steel. This film is resistant to further chemical or physical change.
Stainless steels can be divided into four major groups, namely Martensitic, Ferritic, Austenitic and Duplex.

The Martensitic Group
The martensitic group of stainless steels contains a minimum of 12% Chrome and usually a maximum of 14% with Carbon in the range of 0.08 to 2.0%. Due to the high Carbon content of the steel it responds well to heat treatment to give various mechanical strengths, such as hardness. When heat treated this group of steels show a useful combination of corrosion resistance and mechanical properties that qualify them for a wide range of applications. These steels are all magnetic.

Type 410
A 13% Chrome, 0.15% Carbon alloy possessing good ductility and corrosion resistance. It can be easily forged and machined. Exhibits good cold working properties.

Type 416
Similar to Type 410 but has added Sulphur giving improved machinability. Usually supplied in bar form.

Type 431
A 17% Chrome, 2½% Nickel, 0.15% max Carbon stainless alloy. Has superior corrosion resistance to types 410 & 416 due to the Nickel. Usually supplied in bar form.

The Ferritic Group
This group contains a minimum of 17% Chrome and Carbon in the range 0.08 - 0.2%. The increase in Chromium gives increased corrosion resistance at high temperatures. However it can not be heat treated therefore its applications are limited. These steels are magnetic.

Type 430
A 17% Chrome, low alloy Ferritic steel. It has good corrosion resistant properties up to about 800°C. Usually on used in strip and sheet form due to its poor machinability.

The Austenitic Group
This group is perhaps the most popular amongst instrument engineers. It contains Chromium in the range 17 - 25% and Nickel in the range 8 - 20%, with various elements added in an effort to achieve desired properties. When fully annealed this group exhibits a useful range of mechanical and physical properties. These steels are normally non magnetic.

Type 304
An economic balance of alloying materials. Excellent corrosion resistance in unpolluted and freshwater environments, though not recommended for seawater.

Type 321
A variation of Type 304 with titanium added in proportion to the carbon content. This improves its high temperature properties.

Type 347,br>Very similar to Type 321 but uses Niobium instead of Titanium.

Type 316
The addition of 2 - 3% Molybdenum in this grade gives increased corrosion resistance in offshore environments, however it does pit when immersed in seawater. A nickel content of 12% maintains the austenitic structure.

Type 317
Similar to 316 but the 3 - 4% Molybdenum gives increased pitting resistance when immersed in cold seawater.

6 Moly
Or as it is more properly known - UNS S31254 - gives high resistance to sea water attack due to high levels of Chromium and Molybdenum.

L GradesMost austenitic grades can be provided as low carbon grades e.g. 316L where carbon is restricted to 0.03 to 0.035%. This reduces the tensile strength.

Duplex Stainless Steels
This relatively new group has a balance of Chromium, Nickel, Molybdenum and Nitrogen to give a near equal mix of austenite and ferrite. The result is a high strength, highly corrosion resistant material. Recommended extended use within temperature limits of -50 to +300 °C due to embrittlement. They are referred to by UNS numbers, or manufacturer's trade names.

UNS S31803
The most widely used grade of duplex and is typical of above description. Typical composition is 0.03% max Carbon, 22% Cr, 5.5% Ni, 3% Mo and 0.15% N.

UNS S32304
A low alloy duplex with similar corrosion properties to type 316, but with approximately double the tensile properties. Hence its primary use is in structures where mechanical strength is important. Typical composition is 0.03% max Carbon, 23% Cr, 4% Ni and 0.1% N.

UNS S32750
A super duplex exhibiting enhanced corrosion resistance and mechanical properties. Typical composition is 0.03% max Carbon, 25% Cr, 7% Ni, 4% Mo and 0.28% N.

Further Reading

For those who want to learn more about material selection and stainless steels in particular, then the following may be of interest: