Contribution to comparison of methods for determining welding procedures for the avoidance of hydrogen cracking

A comparison of methods for determining welding procedures for the avoidance of hydrogen cracking during fabrication was made by Hart et al. for the International Institute of Welding (IIW Doc. IX-1602-90). Five published methods dealing with primary cracking parameters were used to assess safe welding conditions for two C-Mn steels. This paper contributes to the work by adding one more method to this comparison. This method, denoted as the VTT/OU method, was jointly developed and published in 1984 by the Technical Research Centre of Finland (VTT) and Osaka University, Japan (OU). The VTT/OU method described briefly in the paper is based on the comparison of two values of the stress field parameter at the crack initiation area. The required value of field parameter is assessed by the restraint of the structure and the joint geometry. The attainable level of field parameter is determined by the steel composition, weld thermal cycle and diffusible hydrogen level. Safe welding conditions were determined by the VTT/OU method corresponding to that in the paper of Hart et al. in terms of minimum arc energy without preheat, and minimum preheat for three arc energies: 1, 2 and 3 kJ/mm. As parameters, the hydrogen level, joint type (butt and fillet) and plate thickness were varied. The VTT/OU method was shown to take a somewhat different stand from the others on the importance of crack-controlling factors. The joint geometry, especially with butt joints, is regarded as an essential factor affecting the level of precautions necessary. A symmetric double-V groove was chosen for calculations by the VTT/OU method, as the joint geometry was not specified in IIW Doc. IX-1602-90. Examples are, however, given on the other joint geometries. Owing to the sensitivity of necessary precautions to these geometrical factors, the level of requirements set by the VTT/OU method can vary considerably. The range of variation is of the same order of magnitude as those differences Hart et al. showed to exist between the five methods considered in IIW Doc. IX-1602-90. The VTT/OU method considers the use of low hydrogen electrodes and hydrogen removal by preheating above 100 °C more effective, than reducing HAZ hardness by increasing the arc energy in the practical range of welding. Therefore, especially with thicker plates (50 mm) the VTT/OU method is more conservative than others for min. arc energies at room temperature, especially with higher hydrogen levels. For the same reason, however, minimum preheat levels at constant arc energies are not in excess of those of other methods. For the fillet welds, the VTT/OU method is found to agree very well with the CTS test results given in Hart's paper for both steels under consideration. It should, however, be noted that due to the statistical nature of many parameters involved, the VTT/OU method always applies a safety factor. For this reason the VTT/OU method recommends for both steels a 0.5 kJ/mm higher arc energy than does the TWI method for fillets corresponding to the CTS tests. According to the present calculations, the TWI method predicts arc energy values that correspond to a 50 % risk of cracking in CTS tests. Further R & D work in international co-operation is suggested for improving the consistency of the methods for predicting safety against hydrogen cracking. The applicability of these methods for new low-carbon steels, such as TMCP steels with a low impurity content, also deserves increasing attention in future studies.