Choice of a method for assessing the depth of the tempered layer in a martensitic steel welded joint

Introduction. This review considers the choice of a nondestructive testing method for estimating the depth of the tempered martensitic layer in welded joints of martensitic steel pipes after high-temperature tempering. During heat treatment of welded joints, martensite does not always decompose through the entire weld thickness, especially in the root zone of thick-walled pipes, and averaged characteristics may conceal local areas of increased hardness and brittleness. Methods and Materials. Four groups of methods were compared: eddy current testing, magnetic Barkhausen noise analysis, ultrasonic methods and magnetic coercimetry. For each group, the depth from which the signal carries information about the structural-phase state and nanoscale features of the weld metal was assessed. Results. Eddy current and Barkhausen methods mainly characterize the surface and near-surface layer. Ultrasonic methods can probe deeper layers, but when the acoustic contrast between structures is weak, the tempered-layer boundary is difficult to identify from the backscattered signal, and the results are difficult to interpret. Coercimetry was chosen for field conditions. Interchangeable attached U-shaped electromagnets of different sizes magnetize the metal to different depths, which makes it possible to construct a conditional layer-by-layer magnetic profile. Discussion. It is proposed to select transducers, construct calibration curves on welded specimens, and check the transformed-layer thickness estimate by metallography, microhardness testing and, if necessary, X-ray diffraction. Conclusion. The resulting layer-by-layer profile is conditional. Depth estimation requires calibration for the specific steel grade and welding technology, with allowance for residual stresses and transducer orientation. Pipe inspection also requires allowance for lift-off, surface curvature and weld reinforcement.