Speaker
Description
Up to now, our residual stress and strain reference materials to develop steel health monitoring are autogenously welded steel samples introducing tensile and compressive residual stresses in the fusion zone and the heat affected zones, despite issues like phase transformations and repeatability process. However, both phase transformation and sample repeatability are important and enlarge the uncertainty in inter-laboratory comparison tests. For this reason, we have started using another method, according to which phase transformation and sample repeatability are much better, allowing safer inter-laboratory comparison tests. This method is the local induction heating using a 6 mm straight, water-cooled conductor, transmitting a high-frequency current, generating eddy currents and therefore heat along the steel. The heated steel is set on top of a ceramic base and two vices are loosely fixing the ceramic and the steel under heat. Due to the relatively low thermal conductivity of steels, a temperature profile is generated in the steel, perpendicular to the orientation of the straight conductor direction, which can be regarded as a heat-affected zone. The temperature distribution of the steel is monitored by an infrared camera. As soon as the maximum temperature reaches temperatures in the order of one-third of the melting temperature of the steel grade, thus allowing for no phase transformations, the two vices fix the ceramic-steel assembly and the whole assembly is inserted either in cold water or oil for quenching. This way, the temperature gradient is transferred into residual stresses, until the temperature in the steel is uniform.
