Why is Chromium Molybdenum steels used to build pressure vessels? What is special about the properties of Chromium Molybdenum steels that make them ideal for pressure vessels?

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The primary users of pressure vessels and piping are the chemical, petroleum, and electric power industries. The classification of pressure vessels regarding the material is based on the working environment and service temperature.

For ordinary-temperature service, the ultimate strength of steels remains relatively constant over the temperature range from -30 to 345, consequently the plain carbon steels are the most commercial.

For low-temperature service, to ensure safe performance, the steel must be resistant to the initiation and propagation of a crack under all service conditions. In thick sections, plain carbon steels produced according to fine-grain practice and normalized or quenched and tempered are used for service to -45 degrees centigrade. Low-carbon high nickel steels are used for service down to -195. Austenitic chromium-nickel steels, aluminum, and special copper and aluminum-base alloys have been found to be particularly suitable for applications close to absolute zero. Because austenitic steels have a face-centered cubic (FCC) crystal structure, they retain toughness to very low temperature.
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