Heat Treatment and Nondestructive Testing: Finding Surface Cracks using Laser-Thermography

Thermography is a nondestructive technique that allows for a detection of a multitude of defects within a large variety of materials. For this purpose we need, on the one hand, an energy source giving rise to a heat flow within the work piece and, on the other hand, an infrared camera that we use in order to analyze the surface temperature distribution of the work piece in a spatial-temporally resolved manner. Novel approaches utilize lasers and hence allow for testing of tiny surface cracks. The present detection limit for the detection of cracks with submicron width and depths in highly-reflecting metal surfaces lies below that of the classical thermographic techniques and is actually within the range of fluorescent magnetic particle and penetration testing. Contrary to those techniques, neither commonly used facilities (casually environmental-polluting) nor surface preparation techniques are necessary, and the inspection is performed contactless and automated within distances ranging in meters. During heat treatment, particularly during case hardening using laser radiation, powerful energy sources are available generating intensive and dynamic temperature gradients. Consequently, the use of these facilities enables the possibility to implement thermographic test methods into the manufacturing process chain. The paper presents the latest developments of this promising laser-thermography technique and demonstrates why it has the potential to exceed magnetic particle and penetration testing methods respecting various inspection tasks.