Zinc-plated B7 stud bolts and 2H nuts with iridescent finish

Zinc-plated B7 stud bolts with 2H nuts are a common combination in industrial bolted joints exposed to pressure and temperature. The components shown here belong to an EFS supply consisting of several sets, with metric sizes up to M30.

What immediately stands out, however, is not the stud bolt itself. It is the distinctive iridescent zinc finish, also commonly referred to as yellow or chromate zinc plating.

The stud bolt shown is M22 × 100, manufactured to ASTM A193 Grade B7 and paired with ASTM A194 Grade 2H nuts.

ASTM A193 covers several steel grades for bolting intended, among other applications, for pressure vessels, valves, flanges and fittings used in high-temperature or high-pressure service.

Why are B7 stud bolts paired with 2H nuts?

Grade B7 is a chromium-molybdenum alloy steel that is quenched and tempered to achieve high mechanical properties. Grade 2H nuts are made from heat-treated carbon steel and are suitable for use with high-strength bolting.

The B7/2H combination is intended to ensure that the nut can withstand the preload transmitted by the stud bolt without deforming or suffering thread failure.

The nut must therefore have mechanical properties appropriate for the stud bolt. Pairing a high-strength B7 stud with a nut that becomes the weak point of the joint would make little sense.

This is the main technical reason why ASTM A193 B7 stud bolts and ASTM A194 2H nuts are commonly used together in high-pressure and high-temperature bolted connections.

Zinc-plated B7 stud bolts: what does Fe/Zn 12 Type II mean?

The less conventional feature of this supply is the surface treatment.

The customer specified an iridescent electroplated zinc coating to ASTM B633 Fe/Zn 12 Type II.

ASTM B633 covers electrodeposited zinc coatings applied to iron and steel for corrosion protection. Among other requirements, it addresses coating thickness, adhesion, corrosion resistance and pre- and post-plating treatments.

The designation Fe/Zn 12 identifies a zinc coating on steel with a specified coating thickness class. Type II identifies the supplementary conversion treatment that produces the characteristic coloured finish.

When is an iridescent zinc finish used?

In this case, the iridescent appearance is not an aesthetic choice.

After zinc electrodeposition, a Type II conversion treatment is applied. This provides additional protection to the zinc layer and improves the corrosion resistance of the coated component.

Compared with a clear passivation, commonly associated with white zinc plating, an iridescent finish generally provides greater protection of the zinc coating in the presence of:

• humidity and condensation;
• atmospheres containing chlorides.

Performance is typically assessed by salt spray testing.

There is another important point to consider. ASTM B633 specifically addresses the risk of hydrogen embrittlement through pre- and post-coating treatments. The standard also notes that ASTM F1941/F1941M may be relevant for mechanical fasteners.

Specifying “zinc-plated” is not enough

For a technical supply, simply specifying a “zinc-plated B7 stud bolt” is not sufficiently precise.

At a minimum, the specification should define the material grade, the matching nut and the required surface treatment.

In this case, the designation ASTM B633 Fe/Zn 12 Type II identifies the required coating much more accurately than the generic commercial description “yellow zinc” or “iridescent zinc plating”.

EFS coordinates the supply and checks consistency between stud bolts, nuts, surface treatment and the documentation required for the order.

What sizes are zinc-plated B7 stud bolts available in?

The supply shown includes several metric sizes up to M30.

More generally, EFS can manage special fasteners with both metric and imperial threads, from smaller diameters such as M6 or 1/4″ up to M100 or 4″, depending on the drawing, specification and quantities required.

This is particularly relevant in international projects, where ASTM specifications and imperial dimensions may be used alongside metric components within the same plant.

Is there a European equivalent to ASTM A193 B7?

A European equivalent should not be identified automatically on the basis of material grade alone.

Chemical composition, mechanical properties, heat treatment, dimensions and the requirements of the applicable specification must all be considered.

A material commonly used for B7 applications is quenched and tempered 42CrMo4, material number 1.7225.

You can also consult the EFS conversion table for quenched and tempered steels.