Evaluation Method for Fracture Performance of Offshore Structural Steel Welds

多道焊产生的焊接接头由于多次热循环而具有显著的显微组织变化。在熔合线附近经常出现断裂韧性显著降低的区域，称为局部脆性区（LBZ）。本文提出了一种确定海洋结构钢焊接接头可靠性的方法，并考虑了LBZ的重要性。对海洋结构用YP 420 MPa级TMCP钢多道焊缝进行了HAZ缺口CTOD试验，揭示了多层HAZ中的断裂起裂是由粗晶HAZ（CGHAZ）控制的，多层HAZ的断裂韧性下限是由粗晶HAZ（CGHAZ）控制的。焊道焊接是由粗晶热影响区的韧性决定的。粗晶热影响区的韧性主要受母材的化学成分、制造工艺等冶金性能及所经受的焊接热循环的控制。Si和Al含量的降低带来CGHAZ韧性的增加，并且低Al-B处理条件即使在低温下也表现出优异的CTOD性能。回火效果的后续焊道也导致CGHAZ韧性的改善，由于脆化的显微组织（MA成分）分解成细铁素体和铁素体。这些结果使我们能够保证海洋结构钢焊缝的良好韧性。此外，采用最弱连接模型，综合考虑了焊缝金属的不均匀性和断裂力学参数沿裂纹前沿沿着的变化，提出了由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.