Which materials have a tendency to develop reheat cracks?

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So what is reheat cracking? Which materials have a tendency to develop reheat cracks? 01 Definition: For certain high-strength steels and superalloys containing precipitation-hardening elements (including low-alloy high-strength steels, pearlitic heat-resistant steels, precipitation-hardened superalloys, and certain austenitic stainless steels), no cracks appear after welding; however, cracks develop during the heat treatment process. Such cracks are referred to as “stress-relief cracking”. Some welded structures operate under certain temperature conditions; for example, they may develop cracks when exposed to temperatures of 500–600°C for a long period of time. In engineering, the cracks that occur under the two aforementioned conditions (during stress relief 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 appear 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 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 occurrence 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 has a significant impact on the formation of reheat cracks. 2 Intragranular precipitation strengthening: Carbides and nitrides of elements such as chromium, molybdenum, vanadium, and niobium, as well as the precipitate phases in nickel-based alloys, dissolve due to heat during the initial welding process. They cannot fully precipitate during post-weld cooling. However, during subsequent reheating and heat treatment, these carbides, nitrides, and precipitate phases are redeposited within the grains, thereby strengthening them. At this point, the deformation resulting from stress relaxation is concentrated at the grain boundaries. When these grain boundaries lack sufficient plasticity, reheat cracks 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 cracks; the larger the grain size, the more likely reheat cracks 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 methods A high welding line energy causes coarsening of grains in the overheated zone. For certain steel grades sensitive to grain growth, the susceptibility to reheat cracking during submerged arc welding is greater than that during shielded metal 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 cracking, it is necessary to use a higher preheating temperature or combine it with post-heating for effectiveness. 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.