Grades of Stainless Steel used for Instruments & Valves

Why consider using stainless steel?
The instrument engineer will often specify stainless steel for field instrumentation wetted parts due to its mechanical strengths and resistance to corrosion e.g. when specifying control valve internals stainless steel offers good erosion resistance, or when specifying instrument enclosures stainless steel 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 Austenitic Group
This group is perhaps the most commonly encountered by instrument engineers and designers. The majority of pressure ports and process connections on industrial pressure sensors are made from some grade of stainless steel.
Austenitic stainless steels 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.
It is common to see a stainless steel grade with an "L" after the number e.g. grade 316L. The L denotes a lower carbon content than the standard grade. Although this makes the material a bit weaker it does offer an advantage when welded i.e. the lower carbon content helps minimise/eliminate carbide precipitation during the welding process. Therefore they can be used in the as-welded state without needing to be annealed. This makes them cheaper, so worth considering.

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
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. 316SS is often the default material for instrument tubing.

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

6 Moly (often written as 6Mo)
6-Moly, or as it is more properly known - UNS S31254 - with its high levels of chromium, molybdenum, and nitrogen, has excellent resistance to a wide range of chemicals and 6Mo alloys are especially suited to high chloride environments such as brackish water, seawater, pulp mill bleach plants and other high chloride process streams. It is not uncommon to find 6Mo instrument tubing and tube fittings (straight tube connectors, elbow, cross, or tee) specified in offshore applications.

 

 

Duplex Stainless Steels
The next most common types of stainless steel encountered by instrument engineers are the duplex and super duplex stainless steels. Although an expensive choice of material super duplex's excellent resistance against seawater, and high resistance to cracking lead to it being used in valve applications.
This relatively new group of stainless steels are designed to provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, and higher strength than standard austenitic stainless steels such as Type 304 or Type316.
Duplex Stainless Steels have 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 e.g. UR52N+, Zeron 100, 2507 or DP3W, whilst the most common 22%Cr grade, UNS S31803 has widely become known as 2205 regardless of its supplier.

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.

 

 

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.

L Grades
Most 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.

 

Further Reading

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