Improving the Fatigue Performance of Weld Joints by Weld toe reconditioning methods

The most onerous loading which affects the welded steel structure’s life is cyclic Loading. The Cyclic loading can trigger brittle-like fracture even as the local stress concentrations are lower than the yield limit, a crack can still initiate and propagate if enough cycles are applied.

Designers, Engineers, and researchers are always in pursuit of finding ways and methods to enhance the fatigue life of welded components.

This article is to cover weld toe reconditioning methods that may be used for enhancing the fatigue life of weld structures, particularly when the applied stress is normal to the axis of the weldments. Particularly, this article is all about the correction of weld toe and associated adverse features of weld toe, which demean the fatigue life of the welded structure.

Firstly, let’s discuss why we need weld toe reconditioning.

Prominently it is well known and proven fact that Poor toe blending is a stress raiser and creates hot spots of localized stress concentration. in addition, there is something else also, which is concealed at the weld toe.

?Research at The Welding Institute (TWI) Cambridge England in 1980 identified an acute line of microscopic slag intrusions along with the toes of the welds made by all arc processes except GTAW. However, all processes were found to produce some degree of undercut at the weld toe.

The empirical result is that all welds contain a pre-existing imperfection either microscopic undercut or slag intrusions or both.

There is no way of avoiding such kind of microscope flaws in the welds made using existing welding techniques and normal inspection/ NDT methods cannot detect microscopic discontinuities.

In any case, microscopic indications are produced at weld toes during welding. These flaws are inevitable and unavoidable, which demeans withstanding of the welds against cyclic loading.

?In plain material, fatigue life is spent on crack initiation and propagation.

In weldments, however, as afore-sated that microscopic discontinuities already exist. Therefore, the fatigue life of welds is spent solely on crack propagation along with residual tensile stresses at or near the yield point, is the essential reason why weldments can tolerate fewer cycles to fatigue failure than a similarly loaded plain material.

To deal with this problem, weld-toe reconditioning methods are utilized. The fatigue life of welds can be enhanced, by effectively executing weld toe reconditioning methods. In which the small pre-existing discontinuities are either removed or the sharp openings dulled, rounded, or blended.

The following techniques enhance fatigue life only from the point of view of failure from the weld toe.

1.???Hammer Peening.

2.???Toe Remelting.

3.???Toe Dressing with Burr Grinder (Also Known as Weld Toe Grinding).

Amongst them, the Toe Dressing with Burr Grinder is the most popular and effectively used Technique.

1) Hammer Peening:

Hammer peening is performed using steel hammer bits having Sami-spherical tips with diameters between 6 mm and 12 mm. In the peening process resulting indentation should be cantered on the weld toe so the metal on both sides (weld metal and parent metal) is indented and resulting in a smooth surface free from obvious individual blows.

2) Toe Re-melting by GTAW Welding Process:

The GTAW (TIG) dressing is an autogenous welding methodology that consists of remelting of weldment at the toe area to approximately 2 mm fusion penetration depth without using filler metal.

?The weldment subject to autogenous dressing should be free of rust, slag, and scale. The tip of the tungsten electrode should be kept sharp and clean.

?The tip of the electrode should be positioned horizontally 0.5 mm to 1.5 mm from the weld toe at the susceptible heat-affected zone. Optionally after autogenous dressing, the tempering pass can be applied

?3) Toe Dressing with Burr Grinder/ Weld toe Grinding

Prior to the commencement of weld toe grinding activities, Remedial cleaning with wire brushing or by electric rotating wire brush grinder should be performed in order to obtain a clean surface.

If any profile grinding is applicable for welds then it has to be subsequent to weld toe burr grinding. This is to ensure that the burr grinding is performed at the exact location of the weld toe.

A high-speed grinder with a tungsten carbide rotatory burr of 12mm diameter is recommended to use.?Which comply with the requirements of AWS D1.1, DNVGL-ST-0126, and NORSOK M-101.

The diameter of the burr shall be as large as geometrically possible to ensure the complete removal of the toes as well as a smooth transition at the start/stop grinding.

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The axis of the burr should be at approximately 45 ° to the main plate. The angle of the burr axis should be 45 ° of the direction of travel to ensure that the grinding marks are nearly perpendicular to the weld toe line (parallel to the direction of stress).

Grinding should extend below the plate surface.

The edges between the grinded profile and the brace/chord shall be rounded, i.e. no sharp edges are allowed.

If the weld toe grinding is not to be performed on the complete perimeter of the weld joint, then a smooth blended transition between the grinded profile and the non-grinded weld shall be ensured.

The depth of grinding shall be 0.5 mm below any visible undercut. However, the grinding depth is normally not to exceed 2 mm or 5% of wall thickness whichever is less.

The edges between the ground weld profile and the unground weld profile shall be rounded, i.e. no sharp edges are allowed.

Final grid marks should be kept small and always be normal to the weld toe if the main loading is normal to the weld toe.

Visual and NDT inspection e.g., MT or PT shall be carried out to prove that the ground surface is free any imperfection i.e. cracks, undercuts o or other discontinuities.

The final surface should show a smooth finish with no visible score marks.

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