Laser surface melting of martensitic stainless steel 440C for enhancing corrosion resistance

Surface modification of martensitic stainless steel 440C was achieved by laser surface melting, using a 2-kW continuous wave Nd:YAG laser. The corrosion characteristics of laser-melted specimens fabricated under different processing parameters in 3.5% NaCl solution at 23 degreesC were studied by potentiodynamic polarization technique. The pitting corrosion resistance of all laser-melted specimens was significantly improved, as evidenced by a shift of the pitting potential in the noble direction, a wider passive range, and a lower passive current density. The pitting potential of the laser-melted specimen fabricated under a power density of 4.2 kW/cm(2) and a scanning speed of-25 mm/s (specimen P12-440C-25) was increased to 200 mV (SCE), which was much higher than that of annealed and conventionally heat-treated 440C The pitting corrosion characteristics of the laser-melted specimens were strongly dependent on the processing conditions which resulted in different microstructures. The enhanced pitting resistance resulted from the combined effect of dissolution or refinement of carbide particles and the presence of retained austenite. By using different processing parameters, it was found that the pitting resistance of the laser-melted specimen P12-440C-25 was the highest by virtue of a high volume fraction of retained austenite and a small amount of precipitated carbide.