高張力鋼溶接部の水素に起因する破壊と破面形態の解析

抄録

The implant weld cold cracking test was carried out on the longitudinal rolling (X-), 45° direction of short transverse and short transverse (Z-) direction specimens prepared from 80kg/mm² class high strength steels, and the fracture morphologies of weld cold cracking and lamellar tearing were investigated by means of electron fractography.
The fracture morphology of weld cold cracking changed from dimple rupture (DR) to quasi-cleavage fracture of hydrogen embrittlement (QC_HE) and intergranular fracture (IG) with decreasing applied stress.
The fracture morphology of lamellar tearing was a terrace and wall like fracture at high applied stress levels and fast fracture parts, which include DR.+inclusion (MnS) and/or QC_HE+inclusion at the terrace part. But its morphology at low applied stress levels changes to QC_HE and IG without inclusion at the fracture surface, which is similar with the weld cold cracking observed at low applied stress levels.
On the basis of the results of internal friction, these fracture morphologies can be explained by the microscopic gathering of hydrogen at lath and grain boundaries which brings in the decrease of ductility.
One of the characteristic morphologies for hydrogen embrittlement is the quasi-cleavage fracture of hydrogen embrittlement (QC_HE) which involves sub-cracks along martensite lath boundary.