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.