Meaning
Metallurgical degradation of low-alloy steels occurs when they are held in or cooled slowly through specific high-temperature ranges. This phenomenon, known as temper embrittlement exposure, leads to a loss of fracture toughness due to the segregation of impurity elements like phosphorus, tin, and antimony to the prior austenite grain boundaries. It is a major concern for heavy-wall pressure vessels and steam turbine rotors operating in high-temperature environments.
Degradation Mechanism
Impurities concentrate at grain boundaries when the steel is exposed to temperatures between three hundred and seventy-five and five hundred and seventy-five degrees Celsius. Prolonged temper embrittlement exposure reduces the cohesive strength of these boundaries, causing the fracture mode to shift from ductile dimple to intergranular cleavage. This shift is accompanied by a rise in the ductile-to-brittle transition temperature of the steel.
The rate of segregation depends on both the concentration of impurities and the alloy composition.
Mitigation Strategy
Steelmakers and alloy designers utilize chemical control and heat treatment protocols to minimize the risk of toughness loss. To reduce temper embrittlement exposure, the chemistry of the steel is optimized by keeping the levels of tramp elements as low as possible. Rapid cooling through the critical temperature zone during the tempering process also prevents the impurities from concentrating at the boundaries.
This cooling is supplemented by adding alloying elements like molybdenum, which retards the migration of phosphorus.
Materials Testing
Periodic evaluation of operating equipment is necessary to ensure that components remain safe for continued operation. Detecting temper embrittlement exposure involves running Charpy V-notch impact tests on samples that have been exposed to service temperatures or subjected to simulated step-cooling treatments. These tests measure the shift in the transition temperature to determine the degree of embrittlement.
This assessment is used to establish safe operating envelopes and determine the remaining life of the industrial equipment. If the transition temperature rises too close to the operating temperature, the vessel must be decommissioned or subjected to a thermal recovery treatment to restore its toughness.
