The issue of temper brittleness in reactors

Temper brittleness is the phenomenon of reduced toughness in certain low-alloy steels that are operated over a long period at temperatures ranging from 343°C to 593°C, due to metallurgical changes. This change is reflected in the increase in the ductile-to-brittle transition temperature of the material during impact testing. Since 2.25Cr-1Mo or 2.25Cr-1Mo-V are materials highly sensitive to temper brittleness, and the operating temperature of hydrogenation reactors falls within the range at which such steels develop temper brittleness (343°C to 593°C), temper brittleness fractures can occur over time as a result of long-term operation. Although the reduction in material toughness at operating temperatures is not significant, the equipment may suffer from brittle fracture due to temper brittleness during start-up and shutdown phases. Ranked from lowest to highest sensitivity to temper brittleness are: 3Cr-1Mo, 2.25Cr-1Mo. 2.25Cr-1Mo produced before 1972 is particularly sensitive, as are some high-strength low-alloy steels. C-0.5Mo and 1.25Cr-0.5Mo are not sensitive to temper embrittlement, but other high-temperature failure mechanisms can cause microstructural changes that result in alterations in high-temperature toughness. In other words, temper brittleness is a degradation of the material that occurs over time as the equipment is in use. This issue need not be a concern when the equipment is first put into operation; however, after it has been running for some time, certain precautions must be taken during shutdowns for maintenance and when restarting the equipment. For example, during shutdown, the pressure should be reduced first before the temperature is lowered, while during startup, the temperature should be increased first before the pressure is raised. The specific parameters required depend on the characteristics of the reactor in question.