Recent advances in friction-stir welding : Process, weldment structure and properties

Many of the degradation mechanisms relevant to power plant components can be exacerbated by stresses that reside within the material. Good design or structural integrity assessments require therefore, an accounting of residual stresses, which often are introduced during welding. To do this it is necessary to characterise the stresses, but this may not be possible in thick components using non-destructive methods. These difficulties, and a paucity of relevant engineering data, have led to an increasing emphasis on the development and validation of suitable modelling tools. Advances are prominent in the estimation of welding residual stresses in austenitic stainless steels. The progress has been less convincing in the case of ferritic alloys, largely due to the complexities associated with the solid state phase transformations that occur in multipass welding. We review here the metallurgical issues that arise in ferritic steel welds, relate these to the difficulties in calculating residual stresses,...

Welding Residual Stresses in Ferritic Power Plant Steels

Steels can be designed to exploit the coordinated motion of atoms during the bainite and martensite reactions In this way, it has been possible to make exceptionally strong and tough alloys in bulk form, and at an affordable price Crystal sizes can be reduced to between 20 and 180 nm, by annealing heterogeneously deformed martensite, or by forming bainite under conditions where there is no atomic mobility Engineering failures are dominated by fatigue, the effects of which are exacerbated by welding and the associated residual stresses Alloys can now be designed such that the deformation caused by transformation eliminates residual stresses, with extraordinary improvements in the fatigue life of constrained assemblies of metal These important achievements are described here, within a framework which relates the atomic mechanism to the engineering scale

Developments in martensitic and bainitic steels: role of the shape deformation

About the fatigue crack propagation threshold of metals as a design criterion – A review

An engineering model of fatigue crack growth under variable amplitude loading

Superior fatigue crack growth properties in newly developed weld metal

The fatigue strength of developed box welds was improved about two times of the conventional box welds. The improved welds used the low transformation temperature welding material which contains 10% Cr and 10% Ni. The expansion of the developed welding material was 0.55% due to the transformation from austenite to martensite. This expansion induced the compressive residual stress around welds. On low stress range condition, the stress ratio effect by this compressive residual stress avoids the fatigue crack initiation at the weld toe. On high stress range condition, the fatigue crack initiated from weld toe and the fretting was observed on the fractured surface. While in the conventional welds, no trace of fretting was observed.

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Fatigue strength improvement of box‐welded joints using low transformation temperature welding material

Unique fatigue threshold and growth properties of welded joints in a tensile residual stress field

Fatigue strength evaluation of welded joints containing high tensile residual stresses

The potential improvement in fatigue performance of lap welded joints in 2 mm thick 540MPa and 780MPa class steels from the introduction of compressive residual stress by welding with low transformation temperature (LTT) welding wire was investigated. Comparative fatigue tests in bending at a stress ratio of R=0 on specimens made with conventional and LTT welds confirmed that the fatigue performance of such thin specimens was improved, particularly in the long-life regime. Furthermore, the benefit was greatest in the high-strength steel, the improvement in fatigue strength at 107 cycles being 1.4 times for 540MPa steel and 1.6 times for 780MPa steel.

Yoshio Maeda

Papers

Superior fatigue crack growth properties in newly developed weld metal

Fatigue strength improvement of box‐welded joints using low transformation temperature welding material

Unique fatigue threshold and growth properties of welded joints in a tensile residual stress field

Fatigue strength evaluation of welded joints containing high tensile residual stresses

Fatigue Strength Improvement of Lap Welded Joints by Low Transformation Temperature Welding Wire — Superior Improvement with Strength of Steel