焊缝CTOD试验中的Pop-in效应及产生原因

Investigation on Pop-in phenomenon and its causes in CTOD test for weld metal

关键词:  CTOD  埋弧焊  Pop-in效应  局部脆化  断裂韧度

Key words:  crack tip opening displacement  submerged arc welding  Pop-in phenomenon  local brittleness  fracture toughness

摘要:

依据BS7448标准对海洋平台结构埋弧焊焊缝进行了CTOD测试,并评定试验中出现的Pop-in效应,计算CTOD的有效值.结果表明,当出现Pop-in效应时,断裂韧度CTOD的有效值δ_Pop比忽略Pop-in效应的CTOD值下降几倍到十几倍.研究发现Pop-in效应的产生是由于裂纹尖端出现局部脆化现象,如裂纹尖端局部出现夹渣缺陷、组织中出现沿晶网状析出物等;多层多道焊时其中一道焊缝出现粗大的侧板条铁素体和贝氏体,裂纹尖端恰好落在粗大脆性组织处且裂纹扩展方向与脆性组织生长方向一致时,易导致局部脆化产生Pop-in效应.

Abstract:

CTOD tests were carried out for submerged arc weld seam of offshore platform structures based on BS7448 standard,and the Pop-in phenomenon was assessed and CTOD values were calculated.The results show that when Pop-in phenomenon appears,the fracture toughness CTOD(δ_Pop) will decrease several to more than ten times than that without considering Pop-in phenomenon.The research shows that Pop-in phenomenon is mainly due to local brittleness in crack tip,which can be caused by the slag inclusion in local zone of crack tip or the precipitation net in the grain boundary appeared in microstructure of local crack tip.In addition,when there were massive FSP(ferrite side plate) and B(bainite) in one seam of multi-pass weld,local brittleness was easily to occur and lead to Pop-in phenomenon if the crack tip just located in the zone of brittle microstructure and its propagation direction was parallel to the growth direction of brittle microstructure.