Photothermal Depth Profiling in Hardened Steels: a new inverse approach based on Singular Value Decomposition

Quality control of the performance of mechanical components subjected to hardness processing is a topic of fundamental importance, both in the field of automotive and aerospace systems both for civil and military applications. The lack of cementation, the burns in the steels, and the decarburisations of the power gears, and the statoric and rotoric equipments may cause catastrophic failures with serious repercussions. The industry and the companies responsible for the hardening processes as well as for the quality control of the mechanical components are continuously seeking for improvements in the standard destructive tests performed by Vicker or Brinell durometer where one mechanical component is chosen for random testing. Since 1996 the use of IR systems based on photothermal radiometry for the non-destructive determination of the hardness profiles in steels has been deeply studied and discussed by several groups both in Europe in the framework of European Thematic Networks (BRRT-CT97-5032) [1], in North America [2], and more recently in Asia [3] as shown by the huge numbers of papers presented in the past ICPPP editions. In this paper we introduce a new PTR compact system, integrable with mechanized and robotic arms for industrial needs, which use a simple Ge lens for collecting the IR radiation from the sample to the detector. The inverse problem to reconstruct the diffusivity profile D(z) from the PTR signal in the frequency domain S(f) has been linearized and solved by the Singular Value Decomposition by using a new approach. The hardness depth profile HV(z) is eventually calculated thanks to the calibration curve hardness/diffusivity. Preliminary results on AISI9310 hardened steel gears show accurate hardness profile reconstructions in comparison with the hardness measurements by standard Vicker test