As described in API RP 571, temper embrittlement is the reduction in toughness due to a metallurgical change that can occur in some low alloy steels as a result of long term exposure in the temperature range of about 650°F to 1070°F (343°C to 577°C). The loss of toughness is not evident at operating temperatures; however, equipment that is temper embrittled may be susceptible to brittle fracture during start-up and shutdown. 2.25 Chromium 1 Molybdenum steel used in the petrochemical industry is known to be susceptible to temper embrittlement. Most guidance to prevent temper embrittlement is oriented to heavy wall hydroprocessing reactors.
In this work, a case history is presented where Fluidized Catalytic Cracking (FCC) components less than one inch in thickness exposed to temperatures in the 900°F to 1000°F (482°C to 538°C) range experienced temper embrittlement.
Metallurgical analysis that included chemical analysis, microstructure observation, and charpy impact testing at different temperatures before and after heat treatment helped to identify the embrittlement cause.
Several considerations to mitigate the risk in the short term including inspection, definition of minimum metal temperature to prevent brittle fracture, fitness for service, and modification of operational procedures, as well as long term considerations, including pipe component replacement, are described. Additional work regarding the selection of filler metals, welding procedure qualification tests that include step cooling tests, and learning that included bead sequence and heat input controls are also described.
Applicable API RP 934-A [Ref. 6] recommendations were incorporated into the project specification for this work, such as the consideration of chemical restrictions for this alloy, not only for heavy wall applications but also for thinner wall applications working in the temper embrittlement range.