权益分类	功能权益	普通用户	{{item.name}}会员
{{category.name}}	{{benefitItem.name}}

Quantification of transformation plasticity effects in steel welds

钢焊缝相变塑性效应的量化

基本信息

批准号：
EP/H048294/1
负责人：
John Anthony Francis
金额：
$ 10.86万
依托单位：
The Open University
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2010
资助国家：
英国
起止时间：
2010 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FH048294%2F1
关键词：
Quantification transformation plasticity effects steel

项目摘要

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.

人们早就知道，熔焊会产生大量的残余应力，这些应力通常对所连接部件的完整性和性能有害。这种应力是由高度局部化的热作用造成的，这反过来又导致局部热收缩应变，这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。但是，如果部件较大或壁厚，则在组装后通常不可能进行PWHT操作。因此，高水平的有害拉伸残余应力通常存在于焊缝附近。在过去的几年里，一个令人兴奋的研究领域出现了，基于利用钢中发生的固态相变的可能性，以减轻焊接过程中产生的残余应力。这些转变，或原子排列的变化，有相关的应变，根据转变机制和温度，可以设计来补偿焊接冷却时产生的热收缩应变。通过这种方式，设计具有精心设计的转变温度的智能焊缝填充金属可以大大减少焊缝中产生的残余应力，从而激发了基于预防而不是治疗的焊接新理念的发展。然而，实现这种低转换温度（LTT）技术的潜力仍然存在一些重大障碍。首先，为了优化钢的转变温度，事先预测转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优化转化温度的钢的能力，同时还要满足其他重要的材料性能要求，例如坚韧或耐腐蚀。在这项工作中，目的是量化两种转变塑性机制（即格林伍德-约翰逊转变塑性和变体选择）在焊接热循环中对钢的转变应变的影响程度。在相变过程中，当硬的或强的子相的生长引起较软的母相的塑性流动（变形）时，就产生了格林伍德-约翰逊相变塑性。同时，当在转变过程中机械应力的存在有利于某些晶体取向的形成而不是其他晶体取向时，就会发生变异选择，导致依赖于材料内部方向的转变应变。在量化这些机制的贡献时，将建立一个框架，将这两种转化塑性机制纳入焊接的有限元模型。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，来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测量方法（如膨胀测量法）进行验证。这项研究将有助于开发具有改进焊接后性能的新钢材，也将提高我们评估现有焊接钢结构的剩余寿命和可能性能的能力。