Evaluation Method for Fracture Performance of Offshore Structural Steel Welds

由于多次热循环，多道次焊接接头的显微组织发生了明显的变化。熔合线附近往往出现断裂韧性显著降低的区域，称为局部脆性区（LBZ）。本文研究了一种考虑LBZ意义的海上结构钢焊接接头可靠性确定方法。对海上结构用YP 420 MPa级TMCP钢多道焊缝进行HAZ缺口CTOD试验，结果表明多层HAZ的起裂受粗晶HAZ （chaz）控制，多道焊缝的下限断裂韧性由chaz的韧性决定。ghaz韧性主要受基体的化学成分、制作工艺等冶金性能和焊缝热循环的影响。Si和Al含量的降低增加了chaz的韧性，低Al- b处理条件即使在低温下也表现出优异的CTOD性能。由于脆性组织（ma成分）分解为细小的铁素体和渗碳体，后续焊接道次的回火效果也导致了CGHAZ韧性的提高。这些结果使我们能够保证海上结构钢焊缝的良好韧性。在此基础上，采用综合考虑金相非均质性和断裂力学参数沿裂纹前沿变化的最薄弱环节模型，提出了基于CTOD试验结果确定焊接接头断裂概率的方法。

Welded joints produced by multi-pass welding have a marked variation in microstructure due to multiple thermal cycles. Frequently regions with considerably reduced fracture toughness, which are called local brittle zones (LBZ), appear in the vicinity of fusion line. In this study a method for determining the reliability of welded joints of offshore structural steels has been developed with taking the significance of LBZ into account.HAZ-notched CTOD test on multi-pass welds of TMCP steel, YP 420 MPa class for offshore structural use, revealed that fracture initiation in multi-layered HAZ is controlled by the coarse grained HAZ (CGHAZ) and that a lower bound fracture toughness of multi-pass welds is determined by the toughness of CGHAZ. The CGHAZ toughness is mainly controlled by the metallurgical properties, such as chemical composition and making process, of base steel plate and weld thermal cycles subjected. Decrease in Si and Al content brings about increasing CGHAZ toughness and a low Al-B-treated condition exhibits excellent CTOD performance even at low temperatures. Tempering effect by the succeeding weld passes also leads to the improvement in the CGHAZ toughness, due to the decomposition of the embrittled microstructure (MA-constituent) into the fine ferrite and cementite. These results enable us to guarantee a sound toughness of offshore structural steel welds. Moreover, by using the weakest link model considering both the metallurgical heterogeneity and the variation of fracture mechanics parameter along the crack front, the procedure was proposed for determining the fracture probability of welded joints from the CTOD test results.