Annealing, Normalizing, and Quenching

Other higher-temperature postweld heat treatments are also available, but are very uncommon for pressure vessels welds. They would typically only be used when the metallurgical structure of the weld must be changed to more closely match that of the base metal. These uncommon heat treatments include:

• Annealing
• Normalizing
• Quenching

These higher-temperature heat treatments involve heating the material to a temperature above the phase transformation temperature for carbon and alloy steels, or above the recrystalization temperature for austenitic stainless steels. They therefore refine the crystalline structure of the material, and eliminate practically all residual stresses.

These higher-temperature heat treatments reduce the strength of the vessel materials to very low levels at the highest temperatures. Most vessels are not capable of even supporting their own weight. These heat treatments are therefore rarely performed on completed pressure vessels unless special supports are utilized. They are, however, common for pressure vessel plate and forgings, and for formed heads and shell course ring sections prior to final vessel fabrication.

The main difference between annealing, normalizing, and quenching is the cooling rate. For carbon and alloy steels, the faster the cooling rate, the harder and stronger the material.

• Annealing processes utilize a slow furnace cooling process to create very soft material.
• Normalizing processes use still air for cooling to produce material with normal “hot-formed” hardness and strength, but slightly improved impact properties. Normalizing is sometimes followed by a “tempering” heat treatment to further
improve the material’s impact properties.
• Quenching processes utilize water, oil, or air jets to rapidly cool the material to yield a hard and brittle, high-strength material. Quenching is usually followed by a “tempering” heat treatment to improve the material’s impact properties.

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