Effects of cold working on pressure vessel materials

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The plastic deformation of anisotropic metallic materials and the subsequent heating have a significant impact on the microstructure and properties of these materials. When metal undergoes plastic deformation, not only does its shape change, but the grains within it are also stretched or flattened accordingly. When the deformation amount is large, the grains are stretched into a fibrous shape, and the grain boundaries become blurred. Plastic deformation also breaks grains into subgrains. During plastic deformation, due to the rotation of grains, when the deformation reaches a certain level (over 70%), the orientation of a particular axis in most of the grains tends to align with the direction of the external force. This phenomenon is known as deformation texture or preferential orientation. Deformation texture renders metals anisotropic; during deep drawing, it can lead to the formation of \"flanging\" phenomena, resulting in uneven edges and inconsistent thicknesses in the parts. Work hardening is the phenomenon in which, after a metal undergoes cold plastic deformation, as the amount of plastic deformation increases, the metal’s strength and hardness rise while its plasticity and toughness decrease. This is also known as strain strengthening. Due to work hardening, the deformed portion becomes hardened and stops deforming, while the undeformed portion begins to deform; therefore, without work hardening, metals cannot undergo uniform plastic deformation. Work hardening is one of the important methods for strengthening metals, especially important for those metals and alloys that cannot be strengthened through heat treatment. During hot forming, work hardening and recrystallization occur simultaneously; the work hardening is quickly offset by the softening resulting from recrystallization. As a result, the deformed material has a recrystallized structure, and thus no work hardening is observed. Residual internal stress: Internal stress refers to the stress that exists within a metal; it arises as a result of uneven deformation within the metal under the influence of external forces. When a metal undergoes plastic deformation, about 90% of the work done by the external force is dissipated as heat, while about 10% of the work is converted into internal stress that remains in the metal. Welding is a casting process that takes place over a small area; the large local temperature differences inevitably lead to residual internal stresses. The presence of internal stress reduces the corrosion resistance of metals and can easily cause deformation and cracking in compressed components during service. Therefore, after plastic deformation of metals and after welding of pressure vessels, annealing or post-weld heat treatment (low-temperature annealing) is necessary when required to eliminate or reduce internal stresses. Strain aging occurs in carbon steel and low-alloy steel that have undergone plastic deformation through cold working (such as cold rolling of tube sections or cold spinning of end caps). When these materials are left at room temperature for an extended period of time, or at higher temperatures for a certain duration, their strength and hardness increase while their plasticity and toughness decrease; this phenomenon is known as strain aging. The ratio of the difference in impact toughness of a metal material before and after strain aging to its impact toughness in the original state is known as strain aging sensitivity. The greater the strain amount in cold working, the more pronounced the strain aging becomes. Generally, significant strain aging occurs when the plastic deformation exceeds 3%. For example, a 24-mm-thick Q345R steel plate, after undergoing 5% cold plastic deformation and then being heated at 250°C for 1 hour, saw its impact absorption energy at -20°C drop from 36 J to 10 J, indicating significant strain aging. Steel that undergoes strain aging not only experiences a significant decrease in impact absorption energy, but also sees a substantial rise in the ductile-to-brittle transition temperature, exhibiting brittleness at room temperature. Therefore, once the cold forming deformation rate of the compressed component reaches the specified value, heat treatment to restore its properties (recrystallization annealing) is required. Furthermore, after the steel plates are stamped into various end caps, their thickness changes due to plastic deformation. For example, after a steel plate is stamped into a hemispherical head, the bottom becomes thinner while the edges become thicker. When designing pressure vessels, attention should be paid to such variations in thickness. The use of explosion-resistant stainless steel composite plates in pressure vessels is increasing gradually. The process of explosive machining metal composite sheets involves applying energy to the metal surface. Under the action of the high-speed explosion pulse, the composite material impacts the substrate at an inclined angle; in the state of metal jet formation, a serrated composite interface is created between the layered metal and the base metal, enabling atomic-level bonding. The base material, either carbon steel or low-alloy steel, after being processed by explosion machining undergoes a strain-hardening process that increases its tensile strength (with little change in yield strength), while reducing its plasticity parameters. The corrosion resistance of the clad stainless steel resulting from explosion machining is diminished as a result. Explosively clad stainless steel plates require leveling, trimming, or cutting, and are usually supplied after heat treatment. Heat treatment is beneficial for improving the mechanical properties of the substrate, but it reduces the corrosion resistance of the clad stainless steel. Furthermore, when the base material thickness of the explosive stainless steel clad plate used for pressure vessels exceeds a certain value, post-weld heat treatment is required to improve the properties of the welded joint, which inevitably weakens the corrosion resistance of the clad stainless steel. To alleviate the aforementioned problems, austenitic stainless steels with higher nickel content can be selected, and low-carbon or ultra-low-carbon austenitic stainless steels, or those stabilized with titanium or niobium, can be used as cladding materials.