Determining and Avoiding HAC on Pipeline Repair

Abstract

Hydrogen Assisted Cracking (HAC) is influenced by the microstructure, deformation, and hydrogen concentration. Fusion welding changes all three influence factors. Tests have been made to solve the problem by FEM analysis. But practical conditions do incorporate many factors. Only very few of them can be steered directly. Therefore, a practical approach was chosen for identifying the critical situations. The first stage was determining the reaction of the construction on stresses which are induced by shrinkage of the weld zone after joining. A mockup was set up for this purpose. The intensity of restraint became very high for the last part of the bead, where the joint between the two workpieces became nearly complete. About 11kN/(mm*mm) were measured. Test welds were done on the mockup under this restraint. 50% of the welds showed large cracks. To become more flexible and less expensive, such high restrained conditions were set up in an IRC device (Instrumented Restraint Cracking). The workpieces are clamped in the IRC device into a stiff frame. The frame becomes closed by the test weld. The use of such a device gives the chance of exploring the different influence factors under similar restraint conditions as on the original setup, but much easier and with the record of shrinkage force and moments. The tests showed, that a high possibility for forming a crack is present, when no pre- and postweld heat treatment was done. Cracking took place in the root bead, especially when the first bead gave a connection to the other workpiece. The cracks were formed in the time range 15min to 3h. Under high restraint it becomes obvious that even low hydrogen levels offer a high chance of receiving HAC. Therefore, methods for avoiding critical conditions were looked on. Preheating reduces the cooling speed. Lower hardness can be received. Unfortunately, the preheating expands the workpieces and after cooling down higher stresses are present (influence from the restraint). Therefore, preheating gave more HAC. For avoiding high stresses during the critical times, a combined pre- and postweld heat treatment was done. Even with low temperatures HAC was avoided easily. The sequence of analysing the mechanical conditions, transfer them into a test device, and optimisation of the welding procedure gave the possibility to study the HAC behaviour. The knowledge of the mechanical behaviour offered the solution for avoiding cracking.