Welding defects in pressure vessels and knowledge of heat treatment

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I. Introduction Pressure vessels play a very important role in petrochemical production. Pressure vessels can serve as petrochemical equipment for reactions, energy exchange, separation, column operations, storage, transportation, and more. They pose a blast hazard, and their safe operation is directly related to both industrial production and personal safety. Therefore, the quality of pressure vessel products has always been **given great importance**. Over the past decade or so, the design, manufacturing, and management of pressure vessels in our country have been brought under legal oversight, resulting in a steady improvement in product quality. The welding quality is high and stable, with a beautiful and smooth weld surface. Welding has become a key process in the production of pressure vessels, and the quality of welding is a very important factor in ensuring the quality of these vessels. The quality of single welding is influenced by various factors: the welder’s skills, the chemical composition and mechanical properties of the steel, the welding materials, the welding processes and equipment, as well as the environment, all of which can affect the welding quality. To improve the quality of pressure vessel products, **production is permitted only upon obtaining a manufacturing license. For manufacturers that have obtained manufacturing licenses, welding procedures must be established before production is permitted; welders must hold relevant certificates to work, and control and management across all aspects of the quality assurance system need to be strengthened. The goal is to minimize potential quality issues and ensure the quality of pressure vessel products. With the rapid development of the petrochemical industry, pressure vessels are becoming larger in size and higher in strength, which imposes higher demands on their quality. This drives a continuous improvement in the welding techniques and processes used for such vessels. II. Welding defects: Cracking in weld joints. As we know, a weld joint is a region with non-uniform microstructure and non-uniform mechanical properties. During the welding process, near the fusion line of the weld joint, the temperature remains between the solid and liquid states. After cooling, the resulting microstructure is an overheated one characterized by coarse grains; both its chemical composition and microstructure are highly non-uniform. Consequently, its strength increases while its ductility decreases. Outside the fusion line lies the \"overheated zone,\" where the grains are coarse; Widmanstatten structure and sorbite often appear, resulting in a significant reduction in toughness. Outside the overheated zone is the “normalizing zone”; due to the recrystallization process caused by heating and cooling, fine, uniform ferrite and pearlite are formed. Further outward is the \"unstable recrystallization zone,\" where the heating temperature lies within the AC1-AC3 range. During heating in this zone, the pearlite and some of the ferrite in the steel transform into austenite with finer grains; however, some ferrite remains. Upon cooling, the austenite transforms into fine ferrite and pearlite, while the ferrite that did not dissolve into austenite remains unchanged, resulting in larger grains. This creates a structure with uniformly sized crystalline particles, while still retaining the band-like characteristics of the original structure. Since the crystallization and heat transfer in the fusion pool occur in exactly opposite directions due to the thermal influence, that is, the direction from the heat-affected zone to the fusion line to the weld is the direction of crystallization; crystallization occurs first at the fusion line, while it is slowest at the center of the pool. This causes the impurities in the molten pool to move from the weld line toward the center, resulting in slag inclusions being more likely to form in the center of the molten pool. Meanwhile, due to the faster cooling rate at the weld line, cracks are more likely to occur there. Corrosion cracking at welded joints can be caused by the hardenability of the steel, cold cracks resulting from hydrogen diffusion, reheat cracks, intergranular cracks, as well as welding defects arising from welding procedures and the skill level of the workers. Practice has shown that cracks pose the greatest threat to the quality of pressure vessel products. 1) Hot cracks occur due to segregation during the crystallization process of the weld pool. The segregated substances are mostly low-melting-point eutectics and impurities. During crystallization, a liquid interlayer remains; owing to their low melting points, these substances tend to crystallize and solidify last. After solidification, their strength is extremely low. When the welding tensile stress is high enough, the liquid interlayer is pulled apart or breaks shortly after solidification, resulting in cracks. 2) Cold cracks are cracks that occur during welding while cooling below the A3 temperature, or after cooling and subsequent heat preservation. The temperature at which cracks form is low; it occurs within the martensitic transformation range, namely below 200–300°C. Therefore, they are called cold cracks. Sometimes, cracks appear several hours or days after welding, or even after a long period of time; therefore, they are also known as delayed cracks. Its harm is even greater. Cold cracks often occur due to factors such as air intrusion during arc burning or the decomposition of flux components, which allows hydrogen to enter the molten pool and dissolve in the molten iron. At high temperatures, the molten iron can dissolve a large amount of hydrogen; however, its solubility decreases at lower temperatures. As a result, the hydrogen dissolved in the molten iron precipitates out, diffuses, and accumulates at defects within the steel, thereby increasing the local pressure and causing cracks to form in the steel. For this reason, cold cracks are also known as hydrogen-induced cracks. During rolling, severe lamellar non-metallic inclusions exist inside the steel, resulting in poor tensile plasticity in the thickness direction. High tensile and compressive stresses develop along the plate thickness, causing step-like laminar spalling. 3) Reheat cracks: Some steels containing alloying elements such as Cr, Mo, V, and B do not develop cracks after welding. During stress relief treatment, or after long-term use at a certain temperature, cracks occur along the grain boundaries in the heat-affected zone; these are known as reheat cracks, or simply SR cracks. Reheating cracks occur because the supersaturated and dissolved carbides (mainly those of Cr, Mo, and V) formed during the first heating process precipitate again upon reheating, resulting in grain strengthening. This leads to the concentration of slip strain at the original austenite grain boundaries; reheating cracks appear when the plasticity of these grain boundaries is insufficient to withstand the strain generated by the relaxation stress. This type of steel has a sensitivity zone around 600°C. Sensitivity decreases above 650°C. 4) Methods to prevent crack formation: To prevent cracks from forming, it is possible to limit the S and P contents in the steel and welding materials: by adjusting the chemical composition of the steel ； Refine weld grain structure ； Increase the alkalinity of the welding material ； Improve segregation ； Control welding specifications ； Increase the weld coefficient, use multi-layer multi-pass welding, and employ low wire energy ； The casting breaks the arc, reducing arc pits. Low-hydrogen alkaline electrodes can also be used; these electrodes must be thoroughly dried and taken as needed ； Select appropriate welding specifications ； Remove hydrogen immediately after welding ； Improve the quality of steel and reduce laminar inclusions in steel ； Financial aspects of various process measures to reduce welding stress. Reduce residual stress and stress concentration ； Slow cooling in the preheating machine, post-weld heat treatment. When applied properly, these methods can all help improve welding quality and prevent defects. As for issues such as incomplete penetration, lack of fusion, slag inclusions, pores, surface defects on the weld surface like undercutting, and weld dimensions, these can all be detected through non-destructive testing. By identifying the location of these defects and employing appropriate and effective repair methods while following proper procedures, it is possible to eliminate weld defects and ensure the internal quality of the product. III. Post-weld heat treatment: Post-weld heat treatment can eliminate residual stresses and prevent deformation; in other words, it can relieve welding-induced residual stresses and stabilize dimensions and shape. Post-weld heat treatment can also improve the properties of the base material and the structural components in the welded area: specifically, it can soften the heat-affected zone, increase the ductility of the weld metal, enhance fracture toughness, remove harmful elements such as hydrogen, improve corrosion resistance, and boost creep resistance as well as fatigue strength. However, an inappropriate choice of post-weld heat treatment process can actually reduce the performance of the welded joint. Therefore, post-weld heat treatment becomes an important step in vessel manufacturing. The most widely used heat treatments for welded joints after welding are high-temperature tempering, normalizing, and solution treatment. High-temperature tempering can address the adverse effects on the quality of pressure vessels caused by welding and deformation. 1. Post-weld heat treatment can relieve welding residual stresses. As the heat treatment temperature increases and the holding time lengthens, the residual stresses in the welded area decrease accordingly; when the temperature rises above 550°C, these residual stresses can be considered to be completely eliminated. However, the effect of holding time is less pronounced than that of increased temperature. 2. Softening of the hardened zone in the heat-affected zone of the weld joint: As the residual stresses are reduced, tempering improves the microstructure, enhancing plasticity and toughness; as a result, the hardenability decreases, leading to the softening of the hardened zone in the weld joint. 3. Reduction of hydrogen in welded joints: During heat treatment, the temperature of the welded joint rises, which increases the diffusion rate of hydrogen allowing it to escape outward. Generally, heating to below 300°C for 2–4 hours is sufficient to achieve hydrogen removal; moreover, when heated to 550–650°C, complete hydrogen removal is achieved. 4. Effect on the tensile strength of the weld metal: Post-weld heat treatment affects the tensile strength of the weld metal, and this is related to the heat treatment temperature and the holding time. The higher the heat treatment temperature and the longer the holding time, the lower the tensile strength of the weld metal at room temperature. Additionally, the higher the alloy content and the greater the carbon equivalent, the greater the rate of strength reduction. 5. Effect on the impact toughness of weld metal: Excessive heat treatment causes a decrease in impact values for any type of steel. For Cr-Mo, Cr-Mo-V, and the vast majority of pearlitic maraging steels, appropriate post-weld heat treatment can improve impact toughness. For some high-strength steels, the impact value decreases after heat treatment. For carbon steel and Mn-Nb-Ni steel, the impact value remains essentially unchanged after post-weld heat treatment. 6. Effect on the width of the decarburized layer: The higher the heat treatment temperature and the longer the holding time, the greater the width of the decarburized layer. This is because unequal element concentrations occur during the formation of carbides, leading to carbon diffusion – carbon diffuses toward the side with lower concentration, thus forming a decarburized layer; this phenomenon is particularly severe in joints made of different steel types. Tempering involves heating the welded part to 500–650°C, during which carbides further aggregate, resulting in a microstructure that is a mixture of ferrite and fine cementite – known as tempered sorbite. This process is referred to as high-temperature tempering, and the resulting structure possesses excellent strength, elasticity, plasticity, and toughness. Normalizing involves heating the welded piece to 30–50°C above Ac or Acm, holding it at that temperature, and then removing it from the furnace to cool in air. Purpose: to improve the structure and refine the grains. Single normalizing cannot eliminate the residual stresses after welding. Solution heat treatment involves heating the steel to 920–1150°C and then cooling it rapidly, causing carbides or brittle phases to precipitate at the grain boundaries of the austenitic weld joint at temperatures between 450–850°C; these phases are subsequently remelted back into the austenite, thereby fixing them in place and resulting in a uniform solid solution. Thereby eliminating intergranular corrosion. It also improves the corrosion resistance and mechanical properties of the welded joint, and eliminates work hardening. Solid solution heat treatment should involve uniform heating of the entire area, without using local heating methods. To achieve the desired post-weld heat treatment results, it is essential to conduct thorough research; selecting an appropriate post-weld heat treatment process is very important. IV. The process parameters for post-weld heat treatment should be determined based on the following factors: a) the temperature inside the furnace when the welded parts are inserted into it; b) the upper and lower limits of temperature during heating; c) the upper and lower limits of heating rate; d) the upper and lower limits of holding time; e) the upper and lower limits of cooling rate; f) the temperature upon removal from the furnace; g) the temperature difference across various parts of the welded parts during the heating process; h) the temperature difference across various parts of the welded parts during the holding period; i) the atmosphere inside the furnace. It can be seen that there are many factors that affect heat treatment, and a cautious approach should be taken when making choices; selections should not be made arbitrarily.