Comprehensive Evaluation of Aging Behavior in M250 Grade Maraging Steel Using Nondestructive Parameters

In order to non destructively characterize the material (microstructural and mechanical) properties of M250 grade (UNS K93120) maraging steel, a study was designed to understand the influence of complex microstructural changes (precipitation of two intermetallics and austenitic reversion) of this steel on the mechanical properties and on three nondestructive testing (NDT) parameters. The NDT parameters (positron lifetime, magnetic Barkhausen emission peak root mean square voltage and ultrasonic velocity) and mechanical properties (yield strength and impact toughness) were measured on solution annealed (1093 K for 1 h) samples aged for various durations (755 K for 0.25 to 100 h). In the present study, an attempt has been made to compare the sensitivity and ability of the three NDT parameters in characterizing the developed microstructures. The decrease in discontinuity density due to discontinuity annihilation during initial aging and the increase in discontinuity density due to intermetallic precipitation upon longer aging could be revealed by positron lifetime. The study further showed that the precipitation of intermetallics during initial aging can be revealed by the ultrasonic velocity, and the austenitic reversion obtained upon longer aging can be revealed by the magnetic Barkhausen emission. However, none of the selected NDT parameters is found suitable for unambiguous identification of the technological regime (that is, optimum material properties achieved upon aging for 3 to 10 h). This paper demonstrates the applicability of a combination of two NDT parameters for qualifying the aging treatment corresponding to the technological regime in maraging steel.