Experimental Analysis of multiaxial residual stress states induced by laser surface hardening

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Subject of the proposed follow-up application is the development of a measuring and evaluation strategy for the non-destructive determination of triaxial residual stress depth distributions in the range of some tenth of a millimeter to some millimeters with neutron diffraction. This method is of essential significance in the case that the conventional approach for the determination of local residual stress depths gradients with diffraction methods, i.e. repeated X-ray stress analysis according to the sin²y-method in combination with a successive sub-layer removal, is accompanied with massive residual stress redistributions due to the materials removal. By this means erroneous residual stress distributions will be determined. The measuring and evaluation strategy will be developed and established for laser hardened surface states of steel SAE 4140. Laser hardening results in a local, complex residual stress distribution with compressive residual stresses in the process zone, which are balanced by rather high tensile residual stresses outside the process zone, while the transition from compression to tension shows a steep gradient. The method is based on through surface scans, for which a strategy will be developed to correct for spurious strains that result from the surface effects, which occur due to the nominal gauge volume defined by the primary and secondary apertures, which is only partially filled. These spurious strains, which are of the same magnitude as the elastic lattice strain due to the residual stresses, result in erroneous residual stresses. Within the first funding phase of the current project the surface effects were detailed studied experimentally and numerically for the neutron diffractometer Stress-Spec@FRM II. As a results instrument settings were determined for the minimization of the surface effects. Furthermore, the spurious strains that result from the surface effect can now be calculated through simulation of the diffraction experiment by means of Monte-Carlo simulations and by using an analytical model. As a consequence, the measuring data can be corrected accordingly. However, the application of the strategy to laser surface hardened material states is still problematic; since we could demonstrate that a second surface effect occurs at the interface between the laser hardened martensitic process zone and the ferritic base material. This effect will be experimentally and numerically studied in detail within the continuation of the successful project. Finally, a strategy for avoiding and for the numerical compensation of the surface effect for this interfacial zone will be developed. For this purpose, the analytical model for simulation of the neutron experiment will be extended accordingly. Moreover, the transformation of the method to further neutron experiments dedicated for residual stress analyses (e.g. Salsa@ILL) will be achieved.