Mechanisms of Liquation Cracking in Weld Heat-Affected Zone of High Strength Steel

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Mechanism of grain-boundary liquation cracking in weld heat-affected zone of 3.5% Ni-1% Cr-0.3% Mo (HY130 type) HT 100 high strength steel was investigated using a high temperature microscope and an electron probe X-ray microanalyser. The main findings obtained in this study are as follows : 1) In the tested high strength steel, grain-boundary liquation was obviously observed at the temperature of about 1355℃∿1360℃ on rapid heating (heating rate about 270℃/sec). This temperature was fairly lower than the bulk solidus temperature (aoubt 1440℃) on rapid heating. 2) Some unetched white-looking grain boundaries were often observed in the heat-affected zone of the specimen rapidly heated to the temperature of above 1360℃, and it was concluded that this whilte-looking grain boundary was the evidence of liquation. 3) Arrays of MnS precipitates were often found in the white-looking grain boundary and by direct observation with a high temperature microscope it was confirmed that the precipitates were formed below about 1000℃ on rapid cooling from the peak temperature. 4) It may be presumed that the potent factor causing grain-boundary liquation occurring in the heat-affected zone of the tested high strength steel is the eutectic reaction between Cr and Ni elements swept up to grain boundary during rapid heating rather than the sulfides, such as MnS.