Fundamentals of Heat Treatment for Pressure Vessels

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I. Welding Heat Treatment: Sometimes, preheating, degassing, post-heating, and PWHT are collectively referred to as welding heat treatment. Preheating is intended to reduce the shrinkage stresses in the weld and the adjacent base material, to slow down the cooling rate of the workpiece in critical temperature ranges, to prevent excessive hardening of the workpiece, and to minimize softening in the weld and the heat-affected zone ； Slowing down the cooling rate of the workpiece as it passes through the 200°C temperature range gives hydrogen more time to diffuse from the weld area and the adjacent base material, thereby preventing the formation of hydrogen-induced cracks ； Remove pollution ； During preheating, it is best for the entire weld thickness to be heated evenly at the specified preheating temperature. Excessive local heating may cause material damage, so it should be avoided as much as possible. Post-weld heat treatment has temperature limits; according to GB/T30583, it is considered post-weld heat treatment only when the temperature for carbon steel and low-alloy steel exceeds 490°C, and for high-alloy steel, it exceeds 316°C. 480℃ and 315℃ are more suitable for the *habits of the Chinese people. II. Functions of post-weld heat treatment 1. Restoring material properties – When the degree of deformation from cold or warm forming exceeds a certain limit (as specified in clause 8.1 of GB/T150), the mechanical properties and corrosion resistance of the material decrease. If the heat treatment state of the material at the time it was supplied is altered during the forming process, heat treatment is necessary. 2. Stress-relief heat treatment: The post-welding heat treatment intended to eliminate welding stresses, and the heat treatment applied to cold spin-formed heads (when the degree of deformation does not exceed the limits specified in clause 8.1 of GB150.4-2011) to remove residual stresses from deformation. The purpose of such treatments is solely to eliminate stresses; they do not significantly alter the properties of the material, and therefore do not fall under the category of \"heat treatments aimed at improving or restoring the mechanical properties of the material.\" The temperature for stress-relief heat treatment is lower than the material’s recrystallization temperature. 3. Heat treatment to improve mechanical properties: Normalizing treatment is used to refine the grain structure and enhance toughness ； Another example is quenched and tempered plates: the design requirements specify that the heat-treated state of the material supplied should be the same as its state of use. However, during the manufacturing process of hot stamping, the temperature exceeds the original tempering temperature, resulting in a decline in mechanical properties. Improving these mechanical properties through heat treatment is referred to as \"heat treatment for improving mechanical properties of materials\". III. Cold working (cold plastic forming) and hot working: Cold working (cold plastic forming) and hot working (hot forming) are distinguished by the recrystallization temperature of the metal; processing above the recrystallization temperature is considered hot forming, while processing below that temperature is cold working.