L51402 Composition Characterization of Hydrogen Cracking in Line Pipe Materials

A model has been established (based on the present results and on previous published data) to predict the effect of preheat, hydrogen and chemical composition on cracking in welds. This has been based on cracking occurring if the average hydrogen concentration remaining in the weld exceeds a critical value depending on the composition. Hence, the model is based on the diffusion of hydrogen. The response of the heat-affected zones of micro-alloy and low-alloy line pipe steels to changes in cooling rate can be represented by a hardening curve. The slope of the hardening curve is of fundamental importance in considering possible welding procedures to avoid cracking. The maximum hardness obtained in the HAZ from very fast cooling rates can best be indexed in terms of the steels carbon content. The critical cooling rate to produce a given hardness has been shown to relate to a carbon equivalent formula.

This study has investigated the effect of composition of a series of line-pipe steels on their cracking tendency using small-scale, self-restrained tests and their correlation with the C.E. formulae. The pipeline steels studied included conventional C-Mn, quenched-and-tempered, pearlite-reduced and MnMoNb types. The work was carried out in three phases. Phase I was an assessment of the hardening characteristics of the HAZ and its relationship to C.E. Phase 2 was a study of Y-groove restrained tests using high and low hydrogen processes, changing preheat and varying input levels. Phase 3 was a limited study of the effect of full-section weldments on HAZ properties.