Improving The Fatigue Performance Of Welded Stainless Steels

As with C-Mn steels, fillet welds in stainless steels can display low fatigue strengths. Therefore, possible ways of improving their fatigue performance were investigated, by choice of welding process or the application of a post-weld improvement technique. Three types of fillet welded specimen were made from three stainless steels, namely 10 mm thick 304L austenitic and higher strength S31803 duplex and 3 mm thick high-strength Cr-Mn austenitic steel. One aim was to show that the higher strength steels gained more benefit from an improvement technique. The basic test series were MAG welded, but TIG and powder plasma arc welds (PPAW) were included to investigate possible improved fatigue performance from these welding processes. In addition, four weld toe improvement techniques were applied, namely weld profile improvement by grinding or re-melting the weld toe with a TIG or plasma torch (TIG or plasma dressing) and the introduction of compressive residual stress by ultrasonic impact treatment (UIT). The specimens were fatigue tested axially at R = 0.1 or under high tensile mean stress conditions. Most tests were performed in air under constant amplitude loading, but toe ground and plasma dressed specimens were also tested in 3 % NaCl solution, while some TIG dressed and UIT specimens were tested under variable amplitude loading. Many testing conditions were selected specifically to investigate features of actual service operation that might reduce the benefit of an improvement technique. The TIG and PPA welds did not produce better fatigue performance than MAG welds. However, all the improvement techniques were beneficial, the improvement increasing with decrease in applied stress range. The basic level of improvement in fatigue strength in air for R = 0.1 was 30 %. However, this could be greater; especially the increase in fatigue limit, or less, depending on the method of application of the improvement technique, the operating conditions and to some extent the original weld profile. Thus, success with the re-melting techniques depended on the achievement of a generous weld toe radius, while UIT of a poor profile weld could leave flaws in the deformed weld toe material and actually reduce the fatigue performance. Grinding was the most tolerant technique, but it was less suitable for the 3 mm Cr-Mn austenitic steel because of the significant loss of thickness. The benefit of the weld profile improvement techniques was reduced in 3 % NaCl solution but not under high tensile mean stress or variable amplitude loading. However, UIT, which relies on the presence of compressive residual stress, was of little benefit under such conditions. There was no fundamental difference in the effect of an improvement technique when applied to the low and high-strength steels, except that the higher strength duplex and Cr-Mn austenitic steels could sustain stresses above the yield strength of Type 304L austenitic. However, ground or dressed duplex welds performed slightly better than the austenitic steel in 3 % NaCl solution.

The weld toe is a primary source of fatigue cracking because of the severity of the stress concentration it produces. Weld toe improvement can increase the fatigue strength of new structures significantly. It can also be used to repair or upgrade existing structures. However, in practice there have been wide variations in the actual improvements in fatigue strength achieved. Based on an extensive testing programme organised by the IIW, this report reviews the main methods for weld toe improvement to increase fatigue strength: burr grinding, TIG dressing and hammer and needle peening. The report provides specifications for the practical use of each method, including equipment, weld preparation and operation. It also offers guidance on inspection, quality control and training as well as assessments of fatigue strength and thickness effects possible with each technique. IIW recommendations on methods for improving the fatigue strength of welded joints will allow a more consistent use of these methods and more predictable increases in fatigue strength. Provides specifications for the practical use of each weld toe method, including equipment, weld preparation and operation Offers guidance on inspection, quality control and training, as well as assessments of fatigue strength and thickness effects possible with each technique This report will allow a more consistent use of these methods and more predictable increases in fatigue strength

Fatigue cracks in steel ships often occur at welded joints where stress concentrations due to the joint geometry are relatively high and the fatigue strength of the weld is reduced in comparison to that of the base metal. This becomes more critical in ships built of High Strength Steels (HSS) because the fatigue strength of steel in the a welded condition does not increase in proportion to the yield or tensile strength. In many cases, the fatigue performance of severely loaded details can be improved by employing good detail design practices, for example by upgrading the welded detail class to one having a higher fatigue strength. In some cases, however, there may be no better alternatives to the detail in question and modification of the detail may not be practicable. As an alternative to strengthening the structure at a considerable increase in costs, procedures which reduce the severity of the stress concentration at the weld, remove imperfections, and/or introduce local compressive stresses at the weld can be used for improvement of the fatigue life. Similarly, these fatigue improvement techniques can be applied as remedial measures to extend the fatigue life of critical welds that have failed prematurely and have been repaired. To date, weld fatigue life improvement techniques have been successfully applied in several industries. While there has been increasing interest in the application of fatigue life improvement techniques to ship structures, at present there is a lack of guidance on the use of such techniques for design, construction and repair. Hence the key elements of this project were to compile available data on fatigue life improvement techniques, assess the feasibility and practicality for their application to ship details, identify gaps in the technology, and finally to recommend design, construction and repair requirements.

Key articles from over 10 separate ASM publications are brought together as a practical reference on weld integrity crack prevention. This book thoroughly covers the essentials of weld solidification and cracking, weldability and material selection, process control and heat treatment, failure analysis, and fatigue and fracture mechanics weldments. Contents also include an appendix for quick reference of tabular data on weldability of alloys, process selection, recommended interpass and heat treatment temperatures, and qualification codes and standards.

The objective of the FATWELDHSS project was to study post-weld treatment techniques and their effect on the fatigue life of MAG welded attachments in High Strength Steel (HSS). Fatigue cracks in steel structures often occur at welded joints, where stress concentrations due to the joint geometry and tensile residual stresses are relatively high. Fatigue life improvement techniques, which rely on improving the stress field and/or the surface geometry around the welded joints, are generally known to be beneficial. Therefore, within the framework of this project, the following were examined: diode laser weld toe re-melting; High Frequency Mechanical Impact (HFMI) treatment; Low Transformation Temperature (LTT) filler wires Laser diode re-melting was used to improve the surface profile at the weld toe and thus reduce stress concentrations. HFMI treatment involving high frequency hammering of the weld toe is another technique that can produce a smooth weld toe profile but, more significantly, which also can introduce compressive residual stresses. Lastly, two new LTT filler wires were developed within the project as these can decrease or even remove tensile residual stresses resulting from weld zone shrinkage. An extensive fatigue testing programme was set up to establish the levels of improvement in the fatigue lives of the welded attachments achieved by application of the selected improvement techniques. Furthermore, two industrial demonstrators were selected that could show the project achievements in terms of facilitating the introduction of high strength steels by overcoming the limitations posed by the fatigue properties of the welded joints. In addition, modelling tools were developed to predict the residual stresses at the welded joint. Finally, practical guidelines were developed for enhancing the fatigue strength of HSS welded structures.