Induction Infrared Thermography for Non-destructive Evaluation of Welding-Induced Sensitization in Stainless Steels

For medium- and high-carbon stainless steels, welding can induce sensitization in the heat affected zone (HAZ) adjacent to the weld, where chromium carbide (Cr23C6) precipitates out of the bulk metal and accumulates at the grain boundaries. This chromium depletion and precipitate accumulation at grain boundaries makes the alloy susceptible to intergranular corrosion, environmentally assisted cracking, and broader fracture. In this paper, we investigate the efficacy of using induction infrared thermography (IIRT) as a non-destructive method for detecting welding-induced sensitization, using the AISI 440C steel as an example. We start by presenting a laboratory experiment to demonstrate this approach, using a radio frequency function generator, an induction wand, and a FLIR SC8203 infrared camera. We use traditional metallography techniques and scanning electron microscopy (SEM) to identify the location of any sensitized regions and characterize the corresponding microstructure. The IIRT experimental results reveal a distinguishable heat signature, with higher temperature observed within the sensitized regions. Next, we present a computational study to simulate the IIRT experiment and investigate the underlying physics. We adopt a three-dimensional thermo-electro-magnetic model including Fourier's law of heat conduction and Maxwell's equations for predicting the electromagnetic field caused by a sinusoidal excitation current flowing through the induction coil. We solve the system of governing equations using the commercial solver COMSOL Multiphysics. To numerically investigate the possible causes of the disproportionate heating within the sensitized regions, we realistically vary the values of electrical resistivity and magnetic permeability therein. The simulated results indicate that the heat signature observed in the laboratory experiment may result from the increase of both electrical resistivity and magnetic permeability in the HAZ. The possible physical relationships between sensitization and the variation of these two properties are discussed.