Nondestructive Evaluation of Material Degradation in 2.25Cr-1Mo Steel by Electrochemical Technique

2.25Cr-1Mo steel is one of heat resistant materials used most widely in fossil power plants as a header and a boiler tube. The most important phenomenon in material deterioration of 2.25Cr-1Mo steel during a long term operation is carbide coarsening. Carbide coarsening causes such changes in mechanical properties as softening or reduction both of creep strength and of toughness. In this paper, a new nondestructive method for detection and evaluation of these material degarda-tions in 2.25Cr-1Mo steel by means of an electrochemical technique is described. The main results obtained are as follows. (1) A degree of material degradation can be evaluation by the peak value of current density (ΔI_P) obtained by the anodic polarization measurement. Since Δl_pcorresponds to the rate of selective dissolution of coarse carbides M₆C, the amount of M₆C carbides can be evaluated quantitatively by ΔI_P. (2) Ah shows a good correlation with ΔFATT. Hence, the toughness reduction can be nondestructively estimated by ΔI_Pmeasurement. (3) ΔI_pcan be regarded as a representative parameter showing the decrease in Mo concentration in grains caused by M₆C (Mo-rich) carbide coarsening. ΔI_Pshows a good correlation with hardness change ΔHv. Based on this interrelation, the degree of softening can be nondestructively estimated. (4) The L. M. P. value of materials used in actual plants can be estimated by ΔI_Pmeasurement. Hence, the actual operational temperature can be estimated from the operation period with high accuracy.