Materials

Fastener material

The material of hot-dip galvanized fasteners should comply with the provisions of GB/T3098.1 and GB/T3098.2. In addition, when the total phosphorus and silicon content is between 0.03% and 0.13%, high temperature (530°C-650°C) galvanizing is recommended.

Note: The influence of hot-dip galvanizing temperature on product performance should be considered when selecting materials.

Surface condition

Before dipping into the zinc solution, the fastener surface should be clean and free of any contaminants. Otherwise, the galvanizing effect is affected.

Zinc

The zinc material used in this process shall comply with the provisions of GB/T13912.

Hot-dip galvanising procedures and precautions

Stress relief

For fasteners used in critical applications, stress relief may be required prior to pickling and hot dip galvanising.

Surface cleaning

Parts should be surface cleaned. During the surface cleaning process, hydrogen may be absorbed by the steel matrix during pickling. Hydrogen cannot be completely fused into the zinc solution and may therefore lead to hydrogen embrittlement fractures. Unless otherwise agreed, heat-treated or work-hardened parts with a hardness greater than or equal to 320 HV should be cleaned by corrosion-proof acid, alkaline or mechanical methods.The duration of immersion in the anti-corrosive acid depends on the condition and minimum duration that the receiving surface can accommodate. Note: A suitable anti-corrosive acid will reduce erosion and hydrogen adhesion to the steel.

Hot dip galvanising

Hot dip galvanising is generally carried out in a bath at a temperature of 455°C - 480°C. High temperature hot dip galvanising can be used to obtain a smooth and thin zinc layer. High temperature hot dip galvanising is generally carried out in baths at temperatures of 530°C - 560°C. The surface obtained is usually matt.

To avoid micro-cracks, bolts, screws and studs with specifications of M27 and above and performance class 10.9 are not suitable for high-temperature galvanising. Hot dip galvanising in baths at temperatures of 480°C to 530°C should be avoided.

Special technical requirements for nuts

Nut threads or other internally threaded parts should be tapped after hot dip galvanising, no double tapping is allowed.

Technical requirements and additional marks for thread tolerances

General Provisions

Two different methods and basic deviations are specified to accommodate the hot dipped galvanised layer (see standard for specific requirements):

The first method is to tap the nut with a 6AZ or 6AX (increased tapping size) tap after plating in order to meet the requirements for the fit of a bolt or screw with a preplated thread tolerance band position force g or h. Nuts with increased tapping size should be marked with the letters Z ( 6AZ class) or X (6AX class) after the performance class mark

The second method is to use a 6az (reduced thread size) bolt or screw before plating to meet the requirements for fitting a nut with a tapped thread tolerance zone positioned H or G after plating. The reduced thread size bolt and screw should be followed by the performance class mark, marked with the letter U

Nuts with increased tapping sizes (marked with a Z or X) should never be matched with bolts or screws with reduced thread sizes (marked with a U), as this combination increases the probability of thread release.

Thread interference can occur when hot-dipped galvanised nuts with tapped thread tolerance zones of H or G are fitted with hot-dipped galvanised bolts or screws with thread tolerance zones of g or h before plating.

Mechanical properties of nuts with increased tapping size and bolts, screws and studs with reduced thread size

For hot-dipped galvanised bolts, screws, studs and nuts greater than or equal to M12, the technical requirements of GB/T3098.1 or GB/T3098.2 should be met.

GB/T5267.3-2008 Appendix A specifies the guaranteed load and guaranteed stress of nuts for M8 and M10, as well as the minimum tensile load and guaranteed load of bolts, screws and studs.

Technical requirements for hot-dip galvanised layers

Appearance of hot dip galvanising

The surface of hot dip galvanised fasteners should be smooth, free from missed plating surfaces, dripping tumours, black spots, residual solvent slag, oxide inclusions and other defects that impair the intended performance of the part.

A lack of glossy appearance should not be a reason for rejecting the product.

Thickness of hot dip galvanised layer

The local thickness of the coating should be not less than 40μm and the average thickness of the coating batch should be not less than 50μm.

The measurement of the partial thickness of the coating should be measured in accordance with the position shown in 8.3 of GB/T5267.3-2008.

Adhesion of hot-dip galvanised layers (according to GB/T5267 . 3- 2008 Appendix E content)

To determine the adhesion of the zinc coating to the surface of the base metal, the zinc coating should be chipped or pried away by applying sufficient pressure with a hard knife tip. If the zinc layer is only delaminated or superficially flaking, the knife should be continued until the base metal is exposed.

Determination of zinc adhesion should not be carried out on edges or sharp corners (the point of lowest adhesion).

The use of slicing or chipping only removes a small part of the zinc layer without removing it from the metal matrix and should not be rejected.

Lubricants

To improve the tightening effectiveness of the installation, the threaded parts of nuts or bolts, screws and studs should be lubricated.

Technical requirements for contracting hot dip galvanising

When ordering hot-dip galvanised fasteners in accordance with the requirements of this part, the following information shall be provided to the supplier.

a) The standard number referencing GB/T5267.3 - 2008 and its plating mark;

b) The material of the parts, the number of manufacturing lots and the condition of the parts, such as heat treatment, hardness or other properties that may be affected during the hot-dip galvanising process;

c) whether a special coating thickness is required;

d) any additional tests required;

e) any additional treatment required, e.g. lubricant application, chromate treatment.