Quantification of transformation plasticity effects in steel welds

众所周知，熔焊会产生很大程度的残余应力，这些应力通常对已连接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造成的，这反过来又导致局部热收缩应变，这些应变与远离焊缝的材料不相容。减少焊接残余应力的传统策略包括对感兴趣的项目进行焊后热处理(PWHT)程序，即在指定的持续时间内将其加热到较高的温度。然而，如果部件很大或厚壁，一旦组装好，通常就不可能进行PWHT操作。在过去的几年里，出现了一个令人兴奋的研究领域，这是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，可以对这些应变进行工程设计，以补偿焊接冷却时产生的热收缩应变。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显著降低焊缝中产生的残余应力，从而激励基于预防而不是治疗的新焊接理念的发展。然而，要实现这种低转变温度(LTT)技术的潜力，仍然存在一些重大障碍。首先，为了优化钢的相变温度，能够预先预测相变应变的大小是至关重要的。其他重要的挑战包括设计具有最佳相变温度的钢的能力，同时满足其他重要的材料性能要求，如韧性或耐腐蚀性。在这项工作中，目的是量化两种相变塑性机制(即Greenwood-Johnson相变塑性和变种选择)在焊接热循环期间对钢的相变应变的影响程度。在相变过程中，当较硬或较强的子相的生长引起较软母相的塑性流动(变形)时，就会产生Greenwood-Johnson相变塑性。同时，当在相变过程中机械应力的存在有利于某些晶体取向的形成时，就会发生变体选择，从而导致取决于材料内部方向的相变应变。在量化每一种机制的贡献时，将建立一个框架，将两种转变塑性的机制纳入焊接有限元模型。在这项工作中，将应用最先进的衍射技术，使用中子和高能X射线，研究钢在固态相变过程中行为的一些复杂方面。用这些技术获得的结果将与更传统的方法(如膨胀法)的测量结果进行验证。这项研究将有助于开发具有更好焊接后性能的新型钢，它还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.