Henry Granjon Prize Competition 2006 Co-Winner, Category A “Joining and fabrication technology” Models for Diode Laser Transformation Hardening of Steels

Abstract

Diode laser beam transformation hardened surfaces have been studied in order to develop models for estimating the hardness and depth of the transformed region. The surfaces of low alloy and martensitic stainless steels have been transformation hardened in inert and oxidising atmospheres to establish the dependence of processing parameters and oxidation on the absorptivity of steel surfaces. It was found that the absorptivity varied between 35 to 85%, depending on the oxide formation on the surface. A model for estimating the depth of the hardened region was developed. The model utilises data established by absorption experiments, and the variation of absorptivity with processing parameters is approximated in the model. Validation was carried out by measuring the hardened depth from transverse cross sections of processed samples and comparing the results to data obtained by the model. A good estimation of the hardened depth was achieved when the traverse rate was equal to or above 3.3 mm s−1. Hardening experiments were performed to 34 commercial steel grades. 7 steels were studied in detail to establish the phase transformations occurring during the hardening process. Based on these experiments, a model for calculating the surface hardness was developed. The model takes data from equilibrium diagrams. A kinetic term to express the rate of change during hardening was introduced to relate equilibrium data to the non-equilibrium process. A good correlation between the model and experimental results was achieved with all materials, excluding martensitic stainless steels. Developed models are applicable, within the established limits, for most commonly hardened steels and processing parameters. The presented model for calculating the hardened depth is not bound to the laser and beam type used in the experiments. Input data for estimating the absorptivity and heat flow is based on the beam-material interaction time and laser power on work piece surface. It can, thereby, easily be converted to be used with other diode laser sources. Correspondingly, input data for the hardness calculations is obtained from pyrometric measurement and is not, thereby, limited to the laser type, wavelength, beam profile or processing parameters.