Quantification of transformation plasticity effects in steel welds

长期以来已知的是，熔焊产生相当大水平的残余应力，并且这些应力通常对已经接合的部件的完整性和性能有害。这种应力是由高度局部化的热施加引起的，这又导致局部化的热收缩应变，该热收缩应变与远离焊缝的材料不相容。用于减少焊接残余应力的常规策略将涉及使感兴趣的物品经受焊后热处理（PWHT）程序，由此将其加热到高温持续指定的持续时间。然而，如果部件较大或厚壁，一旦组装，PWHT操作通常是不可能的。在过去的几年里，一个令人兴奋的研究领域已经出现，基于利用钢中发生的固态相变来减轻焊接过程中产生的残余应力的可能性。这些转变或原子排列的变化具有相关的应变，根据转变机制和温度，可以设计这些应变以补偿随着焊接冷却而产生的热收缩应变。通过这种方式，具有精心设计的相变温度的智能焊接填充金属的设计可以导致焊接中产生的残余应力的显著降低，从而激发了基于预防而不是治疗的新焊接理念的发展。然而，在实现这种低转变温度（LTT）技术的潜力方面仍然存在一些重大障碍。首先，为了优化钢的相变温度，至关重要的是可以预先预测相变应变的大小。其他重要的挑战包括设计具有优化相变温度的钢的能力，同时也满足其他重要的材料性能要求，如坚韧或耐腐蚀性。在这项工作中，目的是量化的程度，两种机制的相变塑性（即格林伍德-约翰逊相变塑性和变体选择）有助于在焊接热循环过程中钢的相变应变。在相变过程中，当硬或强子相的生长在较软的母相中引起塑性流动（变形）时，格林伍德-约翰逊变换塑性出现。同时，当在转变期间机械应力的存在有利于形成某些晶体取向而不是其他晶体取向时，会发生变体选择，从而导致取决于材料内方向的转变应变。在量化这些机制的贡献，将建立一个框架，包括两种机制的转换塑性焊接有限元models.In这项工作的国家的最先进的衍射技术将被应用，使用中子和高能X射线，调查一些复杂的方面的行为钢在固态相变。这些技术所获得的结果将通过更传统的方法（如荧光光度法）进行测量来验证。这项研究将有助于开发焊接后性能得到改善的新钢材，也将提高我们评估现有焊接钢结构剩余寿命和可能性能的能力。

It has long been known that fusion welding generates substantial levels of residual stress, and that these stresses are generally detrimental to the integrity and performance of the components that have been joined. Such stresses result from the highly localised application of heat, which in turn leads to localised thermal contraction strains that are incompatible with material further away from the weld. A conventional strategy for reducing weld residual stresses would involve subjecting the item of interest to a post-weld heat treatment (PWHT) procedure, whereby it would be heated to an elevated temperature for a specified duration. However, if components are large or thick-walled, a PWHT operation is often not possible once they are assembled. As a consequence, high levels of detrimental tensile residual stresses often reside in the vicinity of welds.In the past few years an exciting area of research has emerged, based on the possibility of exploiting the solid-state phase transformations that occur in steels in order to mitigate the residual stresses that arise during welding. These transformations, or changes in the arrangement of atoms, have associated strains which, depending on the transformation mechanism and temperature, can be engineered to compensate for the thermal contraction strains that arise as a weld cools. In this way the design of smart weld filler metals with carefully engineered transformation temperatures could lead to dramatic reductions in the residual stresses that arise in welds, thus inspiring the development of a new philosophy for welding, based on prevention rather than cure . However, there are still some significant obstacles to the potential of this low-transformation-temperature (LTT) technology being realised. Firstly, in order to optimise the transformation temperature of a steel, it is vital that the magnitude of the transformation strains can be predicted beforehand. Other important challenges include the ability to design steels that have optimised transformation temperatures while also meeting other important material property requirements such as being tough or resistant to corrosion.In this work, the aim is to quantify the extent to which two mechanisms of transformation plasticity (i.e. Greenwood-Johnson transformation plasticity and variant selection) contribute to transformation strains in steels during welding thermal cycles. Greenwood-Johnson transformation plasticity arises, during a phase transformation, when the growth of a hard or strong daughter phase induces plastic flow (deformation) in the softer parent phase. Meanwhile, variant selection occurs when the presence of mechanical stress during a transformation favours the formation of some crystal orientations over others, leading to a transformation strain that is dependent on direction within the material. In quantifying the contribution of each of these mechanisms, a framework will be established for the inclusion of both mechanisms for transformation plasticity in to finite element models for welding.In this work state-of-the-art diffraction techniques will be applied, using neutrons and high energy X-rays, to investigate some complex aspects of the behaviour of steels during a solid-state phase transformation. The results that are obtained with these techniques will be validated against measurements made by more conventional means, such as dilatometry. This research will assist in the development of new steels that have improved performance after welding, and it will also improve our ability to assess the remaining life and likely performance of existing welded steel structures.