5. Marking of Stainless Steel Fasteners

The marking of stainless screws and nuts must contain the steel group, strength class and the manufacturer’s identification mark.

Hexagonal screws and socket cap screws with hexagonal socket, as of an M5 nominal diameter, should be clearly marked according to the marking system. Where at all possible, the marking should be placed on the screw head.

1. Manufacturer’s identification mark
2. Steel grade
3. Property class

Nuts with a thread nominal diameter, as of 5 mm, should be clearly marked according to the following marking system. Marking on just one bearing surface is permitted and may only be used if recessed. Marking on the spanner flats is also possible.

1. Manufacturer’s identification mark
2. Steel grade
3. Property class

7. Thread Galling = Seizing of Stainless Steel

A few times each year, we receive calls from fastener suppliers who are dealing with complaints about stainless steel bolts and nuts. The issue raised by customers is that during installation, the bolts are twisting off or the bolt threads are seizing inside the nut. This happens even though all inspections show that the fasteners are within specification. The supplier is left frustrated because the parts appear acceptable, yet they still fail during use.

This problem is known as thread galling.

Thread galling is most common with stainless steel fasteners and other materials like aluminium, titanium and certain alloys. These materials form a protective oxide layer which gives them corrosion resistance. During tightening, pressure builds between the surfaces that are sliding against each other. The oxide layers can be damaged or wiped off and, as the metal surfaces meet, they can shear or lock together. This clogging and locking action increases adhesion. In severe cases, thread galling can lead to seizing, meaning the threads freeze together. If tightening continues, the bolt can twist off or the threads can be pulled out.

Fastenright gives three suggestions to help reduce the chances of thread galling in stainless steel fasteners:

Reducing the installation speed often helps or even solves the issue. Higher RPM generates more heat during tightening. More heat increases the risk of galling, so slowing the process down can make a clear difference.

Applying lubrication to the internal or external threads often prevents galling. Recommended lubricants include those containing molybdenum disulfide (moly), graphite, mica or talc. Some extreme-pressure waxes can also work well. Make sure the lubricant is suitable for the application, especially if the fasteners are used in food-related environments where certain lubricants may not be acceptable. Lubricants can be applied during installation or added beforehand as part of a batch process. Several companies supply anti-galling lubricants.

Choosing bolts and nuts made from different stainless steel grades also helps. The key is using materials with different hardness levels. For example, pairing grade 316 stainless with grade 304 stainless reduces the chance of galling compared to using two fasteners of the same grade. Different alloys work-harden differently, which lowers the risk of seizing.

If thread galling happens during installation, don’t panic. Try the suggestions above. One or a combination of them will usually solve the problem straight away.

8. Stainless Steel Fasteners in contact with other materials

May stainless steel fasteners be safely connected with other materials? The following sections discuss some examples of such combinations.

In the building industry stainless steel fasteners are frequently combined with aluminium alloys, because perfect appearance is maintained even after many years exposure to industrial atmosphere and the connections can easily be opened. These connections do not exhibit contact corrosion because the surface of the aluminium alloy is covered with an electrochemically insulating layer of aluminium oxide.

Combining stainless steel fasteners with normal steel necessitates perfect protection of the adjacent steel surfaces. A zinc plating for example has to be free of pinholes. The same applies to all other layers of surface protection. Also, where machine parts of cast iron are connected, whenever corrosion threatens the surface of the cast iron has to be protected.

Contact corrosion is not found when the surface of the fasteners is small compared to the surrounding surface. Additional protection is, however, recommended.

Stainless steel fasteners do not corrode as long as the timber is internally dry and only the surface becomes wet. Complete and extended soaking of the timber may cause crevice corrosion. Even under these conditions stainless steel fasteners endure many times longer than fasteners of non-alloyed steel.

As with timber, corrosion depends on the local conditions. The use of washers of polyamide or PVC may, under certain conditions, cause crevice corrosion. For example, in humid atmosphere or chlorinated water. These washers are therefore only recommended when exposure to humidity is infrequent. Plastic is easily deformed mechanically and therefore tends to creep even at room temperature.

9. Are Stainless Steel Fasteners Magnetic

All fasteners made from austenitic stainless steels are generally non-magnetic; a certain magnetisability may occur after cold processing.

Each material, including stainless steel, is labelled by its ability to be magnetisable. In all probability only vacuums will be fully non-magnetic. The gauge for the material permeability in a magnetic field is the magnetic permeability value µr for this material in relation to a vacuum. The material has a low magnetic permeability when µr is near equal to 1.

Examples: A2: µr ~ 1.8 / A4: µr ~ 1.015 / A4L: µr ~ 1.005 / AF1: µr ~ 5

10. Mechanical Properties and Strength of Stainless Steel Fasteners

Stainless steels divide into three groups of steel, austenitic, ferritic and martensitic. Austenitic steel is by far the commonest and offers the greatest scope for use. The steel groups and strength classes are designated by a four-digit sequence of letters and numbers as shown in the following example. DIN EN ISO 3506 governs screws and nuts made from stainless steel.

Example:

A2-80
A = austenitic steel
2 = type of alloy within group A
80 = tensile strength of at least 800 N/mm², cold work hardened

DIN ISO 3506 has summarised the recommended steel grades for fasteners. It is virtually only austenitic stainless steel A2 which is used here. On the other hand chrome nickel steels from steel group A4 tend to be used for very high corrosion requirements. The information below is based on screw connections made from austenitic steel in terms of mechanical strength values.

12. DIN and ISO standards – and what they mean

When components are standardised they are easier to work with because such components are interchangeable. For this to be possible, the fundamental characteristics of standard parts must be clearly defined by a central body and used by manufacturers and retailers.

DIN Standard

Issuer: Deutsches Institut für Normung (German Institute for Standardisation)
National German standard

DIN standards are issued for electrical components, organisational methods and fasteners. These standards remain common in Germany even though ISO adoption is increasing. DIN standards will stay in place for parts that have no ISO/EN equivalent or where standardisation is not needed.

ISO Standard

Issuer: ISO (International Organisation for Standardisation)
International standard

The term “ISO” comes from the Greek for “equal”. ISO standards apply worldwide and are suitable for global trade. Even though ISO usage is growing, the German DIN standard was a world leader for a long time.

ISO Standard

Issuer: ISO (International Organisation for Standardisation)
International standard

The term “ISO” comes from the Greek for “equal”.
ISO standards apply worldwide and are suitable for global trade.
Even though ISO usage is growing, the German DIN standard was a world leader for a long time.

EN Standard

Issuer: European Committee for Standardisation (CEN)
European standard

The EN standard was created to support equal preconditions for trade within Europe. Unlike ISO, EN standards only apply within the European Union. CEN works to align EN and ISO standards. In principle, ISO standards should be adopted unchanged as EN standards, retaining the same ISO number but starting with “EN ISO”. If this is not possible, separate EN standards are produced with their own numbering.