Numerical Modelling of Cold Cracking Initiation and Propagation in S 1100 QL Steel Root Welds

Although the phenomenon of hydrogen assisted cold cracking (HACC) and respective avoidance procedures have extensively been investigated in the seventies and eighties, the reasons for recent failures are still a lack of knowledge about the basic hydrogen effects on steel microstructures and, in particular, a lack of welding procedure specifications and standards accounting directly and consistently for cold cracking avoidance in modern high strength structural steels with yield strengths of up to 1 100 MPa. In previous several contributions the consequences of various heat treatment procedures targeted at HACC avoidance have been shown, as for instance their effects on stress-strain build up and on hydrogen diffusion in high strength steel welds. But, a principal interaction of three local influences on hydrogen assisted cold cracking, i.e. local microstructure; local mechanical load and local hydrogen content have not yet been studied in detail for these materials. For this, a numerical model for HACC has been developed, accounting particularly for crack-initiation and crack-propagation criteria, like the hydrogen redistribution during the process of cracking. The numerical model has been used to investigate HACC in such materials, i.e. in the weld microstructures of an S 1100 QL steel, under severe restraint and various hydrogen levels. The results were achieved by in depth thermal and structural finite element simulations combined with numerical hydrogen diffusion modelling. By such procedure, HACC in single-layer welded plates with thickness of 20.0 mm at realistic restraints has been studied. As a particular result, it turned out that the crack-initiation location is typically in the centre of the weld metal (WM), where only a single crack is initiated at hydrogen contents of up to 10.0 Nml/100 g Fe. But, it was evidently shown by such analyses that the crack-initiation location is shifted into the HAZ and that multiple cracking occurs at higher hydrogen contents of up to 15.0 Nml/100 g Fe.