Welding defects—A detailed explanation of reheat cracks

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01 Definition: In some high-strength steels and superalloys that contain precipitation strengthening elements (including low-alloy high-strength steels, pearlitic heat-resistant steels, precipitation strengthening superalloys, and certain austenitic stainless steels), no cracks appear after welding, but cracks do occur during heat treatment; such cracks are referred to as “stress-relief treatment cracks”. Some welded structures operate under certain temperature conditions; cracks can also occur when they are in use at temperatures ranging from 500 to 600°C for extended periods of time. In engineering, the cracks that occur under the above two conditions (during the stress-relief process and during service) are commonly referred to as \"reheating cracks\". 02 Main characteristics: 1) They all occur in the coarse-grained areas of the welding heat-affected zone and present as intergranular cracks. 2) Before stress relief treatment, the welding area had high residual stresses along with stress concentrations of varying degrees. 3) There is a most sensitive temperature range in which reheat cracks occur; this range is related to the reheat temperature and reheat time, and varies depending on the material. 4) Only metal materials containing certain precipitation strengthening elements are susceptible to reheat cracking; carbon steel and metal materials strengthened by solid solution generally do not develop reheat cracks. 03 Mechanism and causes of reheat cracking: Observations made using scanning electron microscopy and high-temperature metallographic microscopy confirm that reheat cracking occurs as a result of preferential sliding at the grain boundaries, which leads to the nucleation of micro-cracks in the metal material or alloy. In other words, during post-weld heat treatment, the grain boundaries of the material become weakened, while the interior of the grains becomes strengthened. However, there have always been two different views regarding the mechanism of reheat cracking; one holds that grain boundary weakening is the main cause ； Another theory holds that intragranular strengthening is its main cause. These are the existing weak chemical theory of grain boundary impurity accumulation and the strong secondary theory within the grain. 04 Several explanations for the formation of reheat cracks 1. The weakening effect of impurity accumulation at grain boundaries. Tests on low-alloy high-strength steels that develop reheat cracks have shown that impurities in the steel accumulate at the grain boundaries, leading to their embrittlement; this phenomenon plays an important role in the occurrence of reheat cracks. 2. Strengthening by intragranular precipitation: The carbides and nitrides of elements such as chromium, molybdenum, vanadium, and niobium, as well as the precipitate phases in nickel-based alloys, dissolve in the metal due to heating during the initial welding process. During cooling after welding, they cannot fully precipitate out. During subsequent reheating treatments, these carbides, nitrides, and precipitate phases precipitate within the grains, thereby enhancing the strength of those grains. At this point, the deformation resulting from stress relaxation is concentrated at the grain boundaries; when the grain boundaries lack sufficient plasticity, reheat cracks can occur. 3 Creep fracture theory: During the reheating process, creep occurs alongside stress relaxation. 05 Factors Affecting Reheat Cracking and Their Prevention The main factors affecting reheat cracking are the chemical composition of the steel grade (which directly affects the plasticity of the coarse-grained areas) and the residual stresses in the welded area (especially at stress concentration points). 1. Metallurgical factors 1) The influence of chemical composition on reheat cracking varies depending on the type of steel, and can be evaluated using criteria for reheat crack sensitivity. 2) The grain size of steel has a significant impact on reheat cracking; the larger the grain size, the more likely reheat cracking will occur. 3) The effects of different parts of the weld joint and notch effects on reheat cracking also vary. 2. Welding process factors 1) Influence of welding method: High welding heat input can cause the grains in the overheated zone to become larger; for certain steel grades that are sensitive to grain growth, the susceptibility to reheat cracking is greater during submerged arc welding compared to manual arc welding. However, for some steel grades with a higher tendency to harden, GMAW exhibits a greater tendency to reheat cracking compared to SAW. 2) Effects of preheating and post-heating: To prevent reheat cracks, it is necessary to use a higher preheating temperature or combine it with post-heating for effective results. 3. Use welding materials with low matching properties. 4. Reduce residual stresses and avoid stress concentration. Materials prone to reheat cracking include 15MnVR, 15MnNbR, 18MnMoNbR, 13MnMoNbR, 07MnCrMoVR, 07MnNiMoVDR, as well as Japan’s CF-62 series of steels. In summary, since reheat cracks do not occur during the welding process but rather during heat treatment or operation, they are somewhat concealed, which makes accidents caused by such cracks unpredictable and can lead to greater losses. Therefore, it is necessary to take into account the possibility of reheat cracking at all stages of the pressure vessel’s preliminary design, manufacturing, and testing, in order to select appropriate solutions that prevent the occurrence of reheat cracking. During the manufacturing process, it is possible to completely eliminate and prevent the formation of reheat cracks by adopting appropriate methods.