Development of Nondestructive Assessment Methodology for Residual Life of Martensitic Heat Resistant Steel

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High Cr ferritic heat resistant steel is widely applied to the thick section components of power plants to be used in creep regime. In this research, nondestructive residual life assessment of such high Cr steel with tempered martensitic lath structure was examined, and the results are summarized as follows :1.There are four factors that may control creep deformation and fracture of the high Cr ferritic heat resistant steels. They are dislocation(lath)structure, precipitates on grain- and sub-boundaries, fine particles within subgrains, and solute atoms such as W and Mo.Among these factors, the lath structure in the most important obstacle that determines yield stress of the steel. Its recovery is closely related to the progress of creep deformation and the subsequent failure. Therefore, this recovery of lath structure should be taken as the measure of life assessment.2.As a result of the recovery of martensitic lath structure, lath width increases with increasing creep strain. The relative increase in lath width Δλ/Δλ^* is linearly related to creep strain. Δλ is the change in lath width from the original value, and Δλ^* is the value of Δλ at rupture. The coefficient of linear relation does not change with creep condition or alloy composition.3.Based on the linear relation between lath width and creep strain, we can evaluate creep strain of a component of engineering plants. Creep curve under any creep condition can readily predict with the aid of creep database. Residual life can be assessed from the estimated creep strain and the creep curve predicated.4.The residual life assessment based on the measurement of lath width is applicable to rotor steel tempered at low temperature. However, in boiler steel(such as HCM12A)tempered at higher temperature, brittle fracture takes place by the coalescence of grain boundary cavities. Life assessment based on cavity growth should be employed in this case.