TIG Dressing Of Steel Weldments For Improved Fatigue Performance
Authors: Haagensen P.J. 
Published: October 2008

Remelting of the toe region of steel weldments using the Tungsten Inert Gas (TIG) method gives generally an increase in fatigue strength which is equal to or better than the improvement obtained by other methods, e.g. profile grinding. The TIG method is also less expensive and easier to perform than grinding. However, TIG dressing results in high hardness in regions near the weld, thus rendering the method unacceptable for some applications.

The paper describes a modified TIG dressing technique which gives hardness levels that are acceptable for offshore structures. Fatigue tests were conducted in air and sea water to study the effect of TIG dressing on the fatigue performance of three types of steels for offshore structures.

Fracture toughness tests were carried out to assess the resistance of TIG dressed specimens to brittle fracture. The TIG dressed specimens were found to have higher fracture toughness than the as welded specimens.

INTRODUCTION

It is generally recognized that the fatigue strength of steel weldments does not increase with the static strength of the base material, and most design codes for welded steel structures do not allow any higher working stresses for high strength steels than for lower strength steels. The use of high strength steels therefore has few advantages in dynamically loaded structure for which fatigue is a limiting factor.

The low fatigue strength of welded joints is caused by several factors; the most important ones being, firstly, the stress concentration caused by the geometry of the weld bead, and secondly, defects in the toe region consisting of crack-like non-metallic intrusions (1). Fatigue cracks thus start growing very early during service and the useful life of the structure consists mainly of the propagation of such cracks to some critical value at which static fracture takes place. The crack growth rate is largely independent on static strength (2); thus welded high strength steels cannot be expected to exceed the fatigue performance of. low strength steel, unless the weld is improved in such a way that the crack initiation stage occupies a large portion of the fatigue life. Several improvement techniques have been developed to improve the fatigue properties of welds; some of the more important ones are listed in Table 1.

The most effective methods are those that change the geometry of the weld, thereby reducing the stress concentration factor, i.e. methods 1 through 3. While grinding and peening are as effective as TIG dressing with respect to increased fatigue life, the latter technique offers some important advantages:–Lower costs. Studies (5) indicate that TIG dressing costs are only 10 to 30 per cent of the cost of grinding.–Better working conditions for the personnel involved in the fabrication process (much less noise and dust than grinding and peening).–Ease of inspection. Severe grinding may cause damage which is difficult to detect and excessive peening may cause cracks that are subsequently buried by continued peening. Faults in TIG dressing show up very clearly and defects are easily repaired.