Stainless Steel 316
Often referred to as "Marine Grade" stainless steel it has very similar physical properties to 304 and can be used in fabrication in much the same way. However, by including molybdenum in its formulation it has increased corrosion resistance particularly against "pitting corrosion". The major cause of pitting corrosion is the presence of chlorides, hence the need for enhanced resistance in the marine environment.
Stainless steel 316 is an austenitic steel with excellent welding and forming characteristics. It is the second most commonly used stainless steel and represents around 20% of the entire world production of stainless steel.
The key element that differentiates it from most other stainless steels is the inclusion of molybdenum, between 2% and 3%, which enhances its resistance corrosion in general and pitting corrosion in particular.
It is malleable and ductile and has good weldability. Its austenitic structure allows it to be deep drawn without intermediate annealing. It is also unnecessary to anneal it following welding thin sections.
However, the inclusion of molybdenum may have some adverse effects on its formability.
It is widely used in:
- Chemical processing and storage equipment.
- Refinery equipment
- Medical devices
- Marine environments, especially those with chlorides present
In its annealed state it is virtually non-magnetic even when cold worked, unlike 304 which can be significantly attracted to a magnet after cold working. This may make it more suitable in some applications.
The range of properties of 316 make it second only to 304 in quantity used and it is employed in many industries.
It is a good choice for architectural applications in coastal areas where there may be a significant amount of salt in the atmosphere however it is more expensive than 304 and is unnecessary away from the coast. Onboard seagoing vessels it is the natural choice for steel fittings whether on-deck or below.
It should also be considered for use in cold climates where significant amounts of de-icing salt is used on the roads. In these environments the level of salt in the atmosphere can be much higher than that found in coastal areas.
The combination of greater corrosion resistance and good formability make it a good choice food and beverage processing equipment; hot water systems; and plant for chemical, petrochemical, mineral processing if chlorides are likely to be present. It should, however, be borne in mind that it is not as easily machined as other austenitic steels.
The combination of greater corrosion resistance and good formability make it a good choice food and beverage processing equipment; hot water systems; and plant for chemical, petrochemical, mineral processing if chlorides are likely to be present.
It should, however, be borne in mind that it is not as easily machined as other austenitic steels.
Corrosion Resistance
The major benefit of 316 is its increased corrosion resistance. In addition to its resistance to chlorides it is also highly resistant to sulfuric, and sulfurous acids, acetic acid, as well as many industrial chemicals and solvents. These types of corrosive process chemicals are used to make a wide variety of products including inks, textiles, photographic chemicals, paper, textiles, rubber, and bleaches.
You should be aware that there are limitations to 316's corrosion resistance and if it is being used in hostile environments care should be taken. It is generally safe to use in the offshore industry in northern European waters this is regarded as the maximum temperature at which it is safe to use for prolonged exposure to seawater.
The UK HSE has produced an excellent paper on the selection of stainless steels for the offshore industry. Click here.
Heat Resistance
316 resists oxidation well in intermittent exposure to temperatures up to 870oC and in continuous exposure to temperatures to 925oC.
It is not recommended for use in the 425oC to 860oC range if resistance to aqueous corrosion is required.
If there is a danger of intergranular corrosion 316L, the lower carbon form, is more resistant to carbide precipitation and should be considered.
If there is a danger of intergranular corrosion 316L, the lower carbon form, is more resistant to carbide precipitation and should be considered. 316H provides greater strength at higher temperatures and is often used in high-pressure applications above 500oC. If there is a need to provide increased resistance to chlorides you should consider 316Ti. The addition of titanium results in the forming of titanium carbides rather than chromium carbide resulting in better resistance to intergranular corrosion.
The major drawback of 316 over 304 stainless steel is the increased cost - generally about 20% to 25% higher. However, when considering cost the "whole-life" cost should be calculated and the increased corrosion resistance of 316 may result in it producing a considerable saving.
Stress Corrosion Cracking
Austenitic stainless steels can be subject to stress corrosion cracking but 316 is generally more resistant particularly at ambient temperatures. However, there have been some curious examples of stress corrosion cracking that are worth bearing in mind.
In continuously high humidity environments in the presence of halides stress corrosion cracking at ambient temperatures has been noted. at least one swimming pool roof has collapsed as a consequence.
It should be emphasised the the production of a good surface finish and good welding techniques do much to reduce the vulnerability of all forms of corrosion.
| Chemical Element | % Present |
|---|---|
| Carbon (C) | 0.0-0.07 |
| Chromium (Cr) | 16.50-18.50 |
| Manganese (Mn) | 0.0-2.0 |
| Silicon (Si) | 0.0-1.0 |
| Phosphorous (P) | 0.0-0.05 |
| Sulphur (S) | 0.0-0.02 |
| Nickel (Ni) | 10.00-13.00 |
| Molybdenum (Mo) | 2.00-2.50 |
| Iron (Fe) | Balance |
| Physical | Value |
|---|---|
| Density | 88.00 g/cm3 |
| Melting Point | 1400 °C |
| Thermal Expansion | 15.9 x 10-6/K |
| Modulus of Elasticity | 193 GPa |
| Thermal Conductivity | 16.3 W/m.K |
| Electrical Resistivity | 0.74 x10^-6 Ω .m |
| Mechanical | Value |
|---|---|
| Proof Stress | 200 Min MPa |
| Tensile Strength | 500 to 700 MPa |
| Elongation A50 mm | 40 Min % |
| Hardness Brinell | 215 Max HB |
316 is available in both High and Low carbon forms. There is also a Titanium (Ti) that has the addition of 0.4% to 0.7% TI
- Tube
- Pipe
- Fittings
- Flanges
- Special Sections
- Sheet
- Plate
- Flat Bar
- Round Bar
- Hollow Bar
- I Beam
- U Channel