Nondestructive Ultrasonic Evaluation of Ferritic CrW Steel upon Long-term Aging

The microstructural evolution of heat-resistant ferritic CrW steel during long-term aging was characterized by ultrasonic wave velocity measurement. Long-term aging was conducted at 700 & DEG;C for 4000 h. The ultrasonic wave velocity increased monotonically upon long-term aging. The ultrasonic wave velocity increased rapidly during the initial 1000 h of aging and moderately thereafter. The M23C6 carbide and Fe2W Laves phases underwent coarsening through the Ostwald ripening mechanism, causing a decrease in Cr and W solute atom concentration of the iron matrix during 4000-h aging. The dislocation density also decreased because of the heat treatment. The width of martensite lath increased owing to dislocation recovery at the lath boundaries. The ultrasonic wave velocity depends on the Young's modulus of the material, which may influence microstructural features such as dislocation substructure, precipitates, solute atom concentration, and martensite lath boundaries. This study utilizes ultrasonic wave velocity to quantify the damage level and microstructural changes during the long-term aging of ferritic CrW steel.