Stainless steel material properties
Stainless steel fasteners obtain their properties from the chemical composition and subsequent manufacturing process. The level of corrosion resistance of a stainless steel is obtained by alloying the materials with chrome and nickel, as well as other elements.
With the material being heavily alloyed, cold working parts (cold drawing of wire followed by cold forming) does generally increase the strength of the end-product. As the amount of cold working will vary within the product, however the material properties will not be comparable to through hardened, quenched and tempered products.
Fasteners are mainly manufactured in four types of stainless steel, which are further divided into subgroups. Find out more below.
Austenitic Stainless Steel
Austenitic stainless steel is mainly classified as chromium-nickel or chromium-nickel-molybdenum austenitic grades. The mechanical properties and strength can be improved through work hardening.
The ductility of the material reduces when work hardened. The plasticity can be increased with the addition of copper. Through-hardening using quenching and tempering of austenitic stainless steels is not feasible, however materials can be heat treated after cold working to soften the material and increase its ductility.
|Chromium-nickel austenitic||A1*||Machining grade with a high sulphur content. It has therefore lower corrosion resistance||Not for use with non-oxidising acids and agents or in chloride rich environments|
|Most common grade of stainless steel for fasteners. Products are mainly cold formed but can be machined|
|A3*||Similar to A2, but with a better resistance to high temperatures|
|Highly improved corrosion resistance due to the addition of molybdenum. It is also known as an ‘acid proof steel’||This steel can be used in marine applications or chloride rich environments. There can be some restrictions in building and construction|
||Similar to A4, but with a better resistance to high temperatures|
|A8*||The high level of molybdenum provides a high level of corrosion resistance, including crevice corrosion, stress corrosion cracking and pitting|
*Austenitic grades, further information in ISO 3506-6
Ferritic Stainless Steel
Ferritic stainless steels are considered an economical compromise if a lower performance and corrosion resistance are permitted. Ferritic stainless steels have a limited cold working efficiency compared to their austenitic counterparts, but are magnetic, allowing it to be used for specific applications. Their use should also be avoided at temperatures below -20°C to avoid risk of failure due to low ductility and impact strength.
Martensitic Stainless Steel
Martensitic stainless steels can be hardened by quenching and tempering. Its performance increases with greater additions of carbon. Martensitic grades have generally a lower corrosion resistance than austenitic grades. At sub-zero temperatures, martensitic fasteners can show a poor impact strength and ductility. Martensitic products are always highly magnetic.
Duplex (austenitic-ferritic). Originally invented in the 1930’s, containing a mix of typically 40% ferrite and 60% austenite grains. Compared to austenitic grades, duplex steels show an improved relative resistance to stress corrosion or pitting resistance. Solution annealed duplex steels also show greater strength than austenitic stainless. This can be further work hardened, but at the cost of ductility.
|Grade||Type||Chemical composition (%)||Standards|
|A1||Austenitic||0.12||2||0.2||0.15 - 0.35||1||17 - 19||8 - 10||0.6||1.4305||303|
|A2||Austenitic||0.08||2||0.04||0.03||1||17 - 20||8 - 13||1.4301||304|
|A4||Austenitic||0.08||2||0.04||0.03||1||16 - 18.5||10 - 14||2 - 3||1.4401||316|
|410||Marstenitic||0.15||1||0.04||0.03||0.5||11.5 - 13||1.4006||410|
|430||Ferritic||0.12||1||0.04||0.03||1||16 - 18||0.75||430|
Corrosion resistance is not only defined by the chemistry, but also strongly influenced by the surface condition of the fasteners, such as passivation and surface roughness. Should corrosion happen depends on numerous factors. This can include the fastener itself, the mechanical stresses applied to the part, the environment including temperature and the potential galvanic reactions when connected to dissimilar materials.
Oxidising and chloride rich environments need additional considerations when using stainless steels, avoiding (chloride) stress corrosion cracking. Therefore, the use of stainless steels in construction and building is regulated and defined. Strong chlorides can cause pitting corrosion in the stainless steel.
Galvanic or bimetallic corrosion should be considered when using stainless steel fasteners in applications where it will be connected to dissimilar materials. This might create a flow of electrons from the noble material to the less noble, which becomes an anode, starting to corrode more quickly.
The speed of galvanic corrosion depends on multiple factors, including temperature, types of material used and the humidity/media in which the application is present. In certain cases, particulates from steel can be transplanted onto the surface of the stainless steel, causing the stainless parts to corrode. This can be due to galvanic reaction as well as contamination.
Exposure to higher temperatures can also lead to exaggerated corrosion. High temperatures can cause scale, leading to galvanic corrosion and even temporarily remove protective oxide layers, allowing corrosion to occur.
Due to the addition of alloying elements, such as chromium and nickel, the operating temperature range of stainless steels exceed these of generic carbon steels. Particularly the austenitic stainless materials can be used in sub-zero applications. Using stainless steels in higher temperature applications will likely have detrimental effects on corrosion resistance.
Recommended service temperature range as advised per ISO 3506-6:
|Type of Stainless Steel||Grades||Minimum||Maximum|
|Austenitic||A1, A2, A3, A4, A5, A8||-196°C||+300°C|
|Martensitic||C1, C3, C4||-40°C||+230°C|
|Duplex||D2, D4, D6, D8||-40°C||+280°C|
Often known as cold welding, it is one of the most common problems when tightening stainless steel fasteners. This can often result in seized bolts, broken fasteners, damaged threads, or generally weakened joints. Galling occurs when the protective oxide layers are removed from the thread surfaces due to the tightening, allowing the asperities (peaks on the surface) to weld together.
Design considerations to reduce galling:
- Lower installation speeds reduce the risk of galling by reducing the localised friction energy
- Lubricating or coating of stainless steel products reduces the chance of galling, providing an intermediate shear layer between the two surfaces. The use of certain lubricants or coatings can be restricted if used with potable water, in sensitive environments or in food preparation areas
- Harder materials have a lower tendency to galling than ductile or materials with low work hardening capacity. In certain conditions, austenitic stainless steels are prone to galling
- Therefore, choose mating materials with different hardness, ideally greater than 50 Brinell
- Surface conditions are a large factor in galling. Roughly machined surfaces tend to gall compared to cold rolled surfaces. Rolling does not only provide a smoother and more uniform surface, but it also work-hardens the material, increasing its hardness
- Using a course thread with sufficient clearance is preferred over a fine thread
- Keeping surface pressure to a minimum reduces the risk of galling. This can be achieved by ensuring suitable mating between the bolt and nut, applying suitable tolerances, therefore providing maximum contact area and minimum high localised contact pressure