Causes and Preventive Measures of Welding Delayed Cracks

A large number of studies have proved that the hydrogen content of the welded joint, the hardening tendency of the steel and the restraint stress of the joint are the three major factors causing the welding delay crack, and the welding delay crack is caused by the joint action of these three factors.

Among them, hydrogen in the welded joint will make the metal hydrogen embrittlement and crack, and inevitably there are some micro defects (such as lattice defects, micropores, etc.). These micro defects can form a three-dimensional stress zone in the front of them under the action of constraint stress, which induces hydrogen protons to diffuse and gather in the three-dimensional stress zone. When the hydrogen concentration in the three-dimensional stress zone reaches a certain value, the hydrogen concentration in the three-dimensional stress zone will increase, The bond damage between metal atoms makes the original micro defects expand forward. The continuous expansion of micro defects results in the formation of microcracks. The hardening tendency of steel can determine the microstructure in steel. The greater the hardening tendency of steel, the easier it is to form sheet martensite. Sheet martensite can not only promote hydrogen embrittlement, but also provide a sensitive matrix for the propagation of microcracks, Therefore, whether it is hydrogen, hardened structure or enough restraint stress, it is necessary to produce delayed crack. The difference is that in some cases, it is one of the main factors.

The reason why delay crack has delay characteristics is related to the time required for hydrogen diffusion and accumulation. Hydrogen induced cracks, from latent, initiation, propagation to cracking, are completed by hydrogen diffusion and accumulation. Therefore, it takes time to produce cracks. The appearance of cracks has hysteresis, and some cracks can not pass the post weld flaw detection. In addition, the appearance of delayed cracks is usually sudden without obvious signs. If cracks appear after the equipment is put into operation, it will lay a major hidden danger for the safety of people's lives and property. In production, the following measures can be taken to prevent delayed cracks:

Select low hydrogen or ultra-low hydrogen welding materials, such as low hydrogen welding rod and low hydrogen flux, and adopt process measures to reduce hydrogen. For example, the welding rod and flux should be strictly dried before use, and should be taken as they are used. The welding wire and weldment should be carefully cleaned to remove oil stain and rust. Pre heating before welding or slow cooling after welding can avoid hardened structure and reduce welding stress, The stress concentration can be reduced by accelerating the diffusion and escape of hydrogen, improving the joint design, and timely post weld heat treatment can improve the joint structure or remove the welding residual stress.

Compared with the base metal, the weld in the pipeline has poor plasticity and toughness, and there is also section mutation, which is the damage area caused by stress concentration. Therefore, in the welding process, it should be noted that the fillet transition is not used for those that can use fillet transition, and the fillet transition is not used for those that can use smooth transition, and the section mutation should not be caused for those that can not cause section mutation, For example, the transition between the weld and the base metal should be smooth, the weld height should not be too high, the weld should not be lower than the base metal, the weld width should be consistent as far as possible, and the local weld should be polished to the same height and shape as the original weld after repair.

Basic knowledge about welded pipe