Low-chromium ferritic stainless steel that is subjected to a second laser-induced thermal cycle is susceptible to intergranular corrosion. Precipitation of carbides and nitrides depletes the adjacent regions of chromium. For this material, despite its low carbon content of 0.007%, laser transformation hardening has achieved a considerable increase in hardness. For certain thermal conditions during overlapping of tracks the above precipitation mechanism can take place. Numerical modelling was applied to analyse the laser parameters that could be critical for precipitation to take place. The results are compared to experiments that were screened with a standardised test method. Avoiding susceptibility to intergranular corrosion when laser transformation hardening was shown to be difficult during a second thermal cycle of an overlapping pass that passes a critical though narrow temperature range. Based on this, measures to mitigate the susceptibility was introduced, in the form of heating the material above melting temperature. When melting the material, even during the first pass, corrosion behaviour changes. It is shown that the risk of corrosion can be avoided during subsequent passes. Different thermal cycles are analysed to find limits for avoiding susceptibility to intergranular corrosion. By laser treating and mapping the critical thermal cycles, the material can be used in a wider range of applications.

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Jesper Sundqvist and Alexander F. H. Kaplan, "Analysis of inter-granular corrosion susceptibility stemming from laser-induced thermal cycles," Proc. SPIE 10911, High-Power Laser Materials Processing: Applications, Diagnostics, and Systems VIII, 1091107 (Presented at SPIE LASE: February 06, 2019; Published: 27 February 2019); https://doi.org/10.1117/12.2506912.