How to prevent cold cracks

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The hazards of cold cracking: The damage caused by cold cracking is catastrophic. Apart from a very small number of pressure vessel accidents in the world that are caused by improper design or inappropriate material selection, the vast majority are due to brittle failure resulting from cracks. Characteristics of cold cracks: a. They occur at lower temperatures and some time after welding, which is why they are also known as delayed cracks. b. It mainly occurs in the heat-affected zone, but can also appear in the weld zone. c. Cold cracks may be intergranular cracking, transgranular cracking, or a combination of both. d. Failure of components caused by cold cracks is a typical case of brittle fracture. There are three factors required for the occurrence of cold cracking: 1. hydrogen concentration, 2. sensitive (brittle) microstructure, 3. constraining stress. In particular, in the heat-affected zone near the weld root and weld toe, these three conditions can be easily met; therefore, this is a region where cold cracks are prone to occur. Measures to prevent cold cracks: 1. Measures to reduce hydrogen content: use low-hydrogen welding materials, and thoroughly clean the area to be welded of oils, rust, scale, and other hydrogen-containing impurities prior to welding. 2. Measures to reduce hardness (hard phase structure): Use weldments containing a higher amount of alloy elements with low hardenability, and design the welding process to avoid rapid cooling ； Increase the preheating temperature, adopt post-heating measures, and ensure that the interlayer temperature is not lower than the preheating temperature; select appropriate welding parameters to prevent the formation of hardened structures in the weld. 3. Measures to reduce restraint stress: 1) Thoroughly clean the area to be welded of contaminants such as oil, rust, scale, and hydrogen-containing impurities prior to welding. Preheating before welding can also help reduce welding stresses, thereby decreasing the occurrence of cold cracks. 2) Perform dehydrogenation heat treatment promptly after welding. A process measure in which the entire weldment or a portion thereof is heated and held at 350–400°C to allow for slow cooling. 3) Pass sequence, joint shape design.