Monitoring the Process Quality of Press Hardened Steels (PHS) with Non-destructive Testing (NDT) Methods

Abstract

An overview about new monitoring concepts of nondestructive testing (NDT) methods for inspection of PHS (press hardened steel) process quality control will be given. Inline testing systems based on electromagnetic electromagnetic acoustic transducer (EMAT) techniques (see Figure1-a) could be used to inspect the raw material (decoiled strip or sheets), semi-finished and final products for defects like cracks or inclusions. The EMAT technique is well known for inline- and in-process inspection of materials which are electrically conductive and / or ferromagnetic. If such defect detection methods are combined with so-called micromagnetic methods (see Figure1-b), it is possible to simultaneously determine isotropic and anisotropic material properties, like tensile and yield strength, vertical and planar anisotropy as well as residual stress. During stamping the monitoring of structure-borne sound with tool integrated sensors is a possibility to recognize the crack formation in the part. Stamped parts can be inspected for typical drawing defects, like cracks or neckings with ultrasonic NDT techniques and thermography as well. On the other hand the assembly process is increasingly important for the PHS. In case of Resistance Spot Welding (RSW) the strength of the welds is mainly defined by size and quality of the weld nuggets. Fraunhofer IZFP has developed an Ultrasonic Welding Monitor (USWM) allowing real-time observation of the RSW process with US transducers integrated in the welding gun. Using this NDT system the size of the welding can be determined during its formation. Moreover welds can be distinguished from so-called “Kissing Bonds”. RSW generates a local tempering and hence a weakening in PHS. Weld spot size as well as hardness profile in the welded and heat-affected zone and further more a quantification of the corresponding layer thicknesses can be determined with the micro-magnetic 3MA technique. It was shown, that a sufficient correlation between destructive and nondestructive measurement values can be achieved. Manual or automated testing systems have become common tools for nondestructive testing of mechanical properties, material irregularities and joining quality in PHS. Especially the in-line monitoring of appropriate NDT data (shown in Figure2) enables an optimization of the PHS production process itself via feed-forward and –backward control not only for steel producer but furthermore for component supplier and original equipment manufacturers (OEM). As a consequence less waste and efficient lightweight constructions with high specific strengths and a high product quality, reliability and efficiency can be assured