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CROSS Safety Report

Grades of weldable quality bolts

Report ID: 392 Published: 1 April 2014 Region: CROSS-UK

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Overview

A reporter says grade 8.8 bolts are not required to be manufactured as being weldable according to the relevant product standards e.g., ISO 898-1 and -2.

Key Learning Outcomes

For civil and structural design engineers:

  • In general, bolts and fasteners, whether grade 4.6 or Grade 8.8, are not supplied as weldable items in their respective product standards and should not be welded

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Further to previous reports on this topic, a reporter says grade 8.8 bolts are not required to be manufactured as being weldable according to the relevant product standards e.g. ISO 898-1 and -2.  In general, fasteners, whether grade 4.6 or Grade 8.8, are not supplied as weldable items in their respective product standards and should not be welded.

If, despite this, fasteners are welded it is necessary for someone with the relevant competence to review the actual material composition of the bolts as supplied to ensure that the weld procedures are suitable. Grade 8.8 bolts are likely to have a higher carbon equivalent value (or CEV as described in BS EN 1011-2 Annex C) than grade 4.6 bolts, and hence will be more prone to hydrogen induced cold cracking when welded.

If, despite this, fasteners are welded it is necessary for someone with the relevant competence to review the actual material composition of the bolts as supplied to ensure that the weld procedures are suitable.

However, the variation in carbon content between batches of the same product to the same standard, and even between bolts of the same batch, may be considerable. As a result, the material composition of each batch will need to be checked. It is also important that the supplier is informed as to the end purpose for the fasteners, so that they do not supply a ‘better’ (stronger) item instead such as Grade 8.8 instead of Grade 4.6, which is then less weldable.

In addition, continues the reporter, the use of non-destructive testing at a suitable time interval after welding to verify the absence of cracks is recommended. Where welded attachments are subject to fatigue, loaded or unloaded, more stringent quality control methods will be required, and fasteners or studs made from weldable steels of known composition should be considered.

This option will be a better solution in all cases anyway says the reporter. It is also interesting that on pointing this out, a sub-contractor checked through the material certificates and found out that some of the bolts specified as grade 8.8 were actually supplied as grade 4.6.

Expert Panel Comments

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Welding of the higher strength steels can lead to difficulties. The mechanical properties of 8.8 bolts are produced by heat treatment. Consequently, the application of heat during the welding process will change the mechanical properties to such an extent that the bolt will no longer conform to the property class 8.8. The rule is that heat treated bolts should never be welded.

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