Nondestructive evaluation of creep-induced material degradation in low alloy ferritic steels

Material degradation by ccreep limits the lifetime of components used in high temperature applications such as piping systems in fossil power plants. The degradation is due to microstrucural changes in the dislocation and precipitation structure and, at a later stage, to the evolution of cavitation damage. Information on the actual microstructure state (particularly on the damage state) is necessary for reliable lifetime assessment of creep exposed components in order to exploit their full life capacity. This information should desirably be provided by NDT. Suitable techniques, however, are not yet available to date exceptfor the (quasi-) nondestructive in-field metallography. We have applied several NDT techniques (ultrasonics, micromagnetics) to speciments originating from uniaxial creep tests in order to detecct and to characterize microstructural changes caused by creep loading. The investigations were performed on low alloy ferritic steels (1Cr 1/2Mo, 1/2Cr 1/2Mo 1/4V), which had been tested under various creep conditions involving two different initial states of each material. The results show that the ultrasonic velocity is sensitive to creep cavitation. Under laboratory conditions, the detection limit with respect to the pore concentation is about 0.1 percent ( volume fraction) for longitudinal waves and about 0.2 percent for shear waves and Rayleigh waves. The results are in food agreement with density measurements and metallographic examinations. Micromagnetic parameters (coercivity, harmonic distortion factor) ore not significantly affected by creep cavitation. They prove to be, however, sensitive to microsturctural changes taking place during early creep until the beginning of secondary creep stage. In the paper, also details of the techniques used are described and aspects of practical application are discussed.