Evaluation Procedure for High Strength Steels for Offshore Structures Considering Strength Mis-Matching in Welds

本研究开发了一种基于焊接件设计方案的高强度钢焊接接头断裂性能评估方法。主要关注的是基体与焊缝金属强度不匹配对焊接接头断裂力学行为的影响。海上结构采用屈服点为420 ~ 500MPa等级的TMCP钢。在考虑强度错配效应的情况下，讨论了三点弯曲试验结果在宽板断裂性能评估中的可转移性。焊接金属的强度超配对热影响区断裂行为的影响取决于热影响区的韧性水平。在热影响区具有中等韧性的情况下，超匹配条件导致韧性裂纹向较软材料扩展的偏差。在不稳定断裂处产生的高临界CTOD并不一定意味着热影响区本身的实际韧性。另一方面，在脆化HAZ情况下，超匹配条件促进了HAZ内的起裂，降低了HAZ的断裂韧性。这是由于HAZ内塑性约束的提升。强度超匹配并不总是有利的，试样的几何形状也影响裂纹尖端区域的约束程度。深缺口弯曲试样最大程度地产生几何约束。深缺口弯曲试样的几何约束比中心缺口宽板受拉25%强度超配的约束更强烈。本研究提出了考虑约束效应的将韧性结果转移到宽板断裂评估的方法。提出了一种以焊缝变形能力为函数来确定焊缝断裂韧性要求的新方法。

This study developed a procedure for assessing fracture performance of welded joints of high strength steels in the light of design solution of weldments. Major concern was an influence of strength mis-matching between the base and weld metals on the fracture mechanical behavior of welded joints. TMCP steels for offshore structures with yield point 420 to 500MPa class were employed. The transferability of 3-point bend test results to fracture performance assessment of wide plates was discussed with attention to strength mis-matching effect.Strength overmatch of the weld metal gave rise to different effects on the fracture behavior of the heat affected zone (HAZ)depending on the toughness level of the HAZ.In the case that the HAZ had a moderate toughness, the overmatch condition resulted in deviation of ductile crack extension toward a softer material. The resultant high critical CTOD at unstable fracture does not necessarily mean the actual toughness of the HAZ itself. On the other hand, in the case of the embrittled HAZ,the overmatch condition promoted fracture initiation in the HAZ and lowers the fracture toughness of the HAZ.This was due to the elevation of plastic constraint in the HAZ.Strength overmatching is not always beneficial The specimen geometry also affects the degree of constraint in the crack tip region. The deep notch bend specimen produces the geometrical constraint to the largest extent. The geometrical constraint in the deep notch bend specimen was more intense than the constraint caused by 25% strength overmatching for the center notched wide plate subjected to tension. The present study developed the procedure to transfer toughness results to fracture assessment of wide plates with taking constraint effects into account. A new methodology for fracture toughness requirement of welds was proposed as a function of the required deformability of weldnents.