On the Significance of Transformation Plasticity and Its Contribution to Residual Stress Development in Welds

The performance of power plant components is often limited by detrimental residual stresses. Approximately ten years ago, however, the potential to engineer weld residual stresses in welds through the choice of a low-transformation-temperature (LTT) filler metal was first reported by Ohta, Wang and their co-workers. These researchers designed and applied LTT welding electrodes and demonstrated that these produced corresponding improvements in the resistance to fatigue crack growth in welds. Unfortunately though, their welding alloys lacked toughness. More recently, it has been demonstrated that it is possible to design LTT filler metals that will have adequate toughness and, at the same time, produce welds with improved residual stress distributions. Importantly, this establishes the scope for the development of novel weld filler metals.

While the development of LTT welding alloys is gathering momentum internationally, it has become increasingly apparent that, in order to optimise residual stresses and the performance of LTT welding alloys, it is necessary to better understand and quantify the strains that arise as a consequence of the phase transformations. In this respect, two transformation plasticity effects are believed to play an important role in determining stress. The first arises when the growth of a daughter phase induces plastic flow in the parent phase, and it is referred to as Greenwood-Johnson transformation plasticity. The second arises when a macroscopic stress in the material induces anisotropy in the transformation strain (even without plastic flow), and is referred to as variant selection. In this paper, the author will report on recent work which aims to assess the extent to which each of these mechanisms is active under representative welding thermal cycles in some power plant steels. The results of a program of thermo-mechanical simulation tests and texture measurements will be presented and discussed.