When complete joint penetration groove welds are made with backing bars, sometimes I am asked to remove them after welding and sometimes I am told they can be left. Why is this?

A weld backing bar is used at the root of a welded joint to support the molten weld metal and thus facilitating the achievement of a complete joint penetration weld. A steel backing bar may be positioned prior to completion of the groove weld with discontinuous/continuous fillet or tack welds.

In some instances, we are required to remove the backing bar and in others we are not. The backing bar can be considered undesirable and is removed for a variety of reasons including: 

1) Fatigue Life Enhancement

The fatigue life of a structural or mechanical component subjected to cyclic loading is reduced by any discontinuity in its geometry. Any surface notch or discontinuity in the structural geometry can act as a stress concentration (or stress riser) and reduce the fatigue life of structures subjected to cyclic loading (considered in Dynamically Loaded Structures in CSA W59, Section 12 [1]). By removing the backing bar and grinding the weld flush, the stress concentration produced by the backing bar is removed. The stress concentration produced by the backing bar and its positioning welds may also impair the resistance of the structural connection to impact loading. The decision to have backing bars removed, based on fatigue life concerns, is made by the design Engineer considering the applied loading, structural geometry and orientation of the weld relative to the loading.

The effect of backing bars on the fatigue performance of groove welds was demonstrated experimentally and described using finite element modelling [2]. The backing bar can reduce the service life of a structural component by introducing an unnecessary stress concentration and result in a fatigue crack such as the one shown in the figure below. Cracks can initiate at the backing bar or at the discontinuous fillet or tack welds used to position them prior to completing the groove weld.

Groove Weld

Since not all structures are subjected to significant cyclic loading, the stress concentration posed by a backing bar may not be significant for structures that are designed to support static (constant) loads. For this reason, removal of a backing bar for statically loaded structures may not be required.

2) Improved Inspection

When the steel backing bar is removed, often by air carbon arc gouging the root of the weld may be restored by rewelding. When the backing bar is removed, the weld root may be visually inspected to ensure that there is not lack of fusion or other weld faults.

If the backing bar is left in place, the use of ultrasonic inspection of the weld root is difficult because of reflector generated by the joint between the backing bar and the joined members.

A reduction in the quality of inspection may result in weld faults being left in the weld as it enters service and this can reduce the service (fatigue) life of the welded connection.

3) Improved Corrosion Resistance

The geometry of the backing bar and the crevice between the backing bar the joined components can be problematic with respect to the prevention of corrosion. If the structure is to be painted/coated after welding, the backing bar may cause groove weld with a backing bar may result in a geometry that is difficult to coat or leads to early coating failure and thus promotes corrosion. If the component is not coated the crevice created by the backing can support an environment that promotes corrosion. The potential for corrosion and rate is determined by the environment and the materials involved. By removing a backing bar, a welded connection can be better coated and the potential for localized corrosion at the weld reduced.

Let’s face it, aesthetics may also be a concern. Staining of the coating at cracks in the coating at a backing bar may be concern for some structures.

References

[1] CSA Group, “CSA W59 - 2018 - WELDED STEEL CONSTRUCTION (METAL ARC WELDING)

[2] BAKER, K. A., KULAK, G. L., “Fatigue Strength of a Groove Weld on Steel Backing”, Department of Civil Engineering, University of Alberta, published in Canadian Journal of Civil Engineering, Vol. 11, 1984

Aaron Dinovitzer MASc., MBA, P.Eng.
Vice President – Capability & Strategy Manager – Asset Performance Services
BMT Canada Ltd.


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