Characterisation and corrosion performance of laser-melted 3CR12 steel

Dual-phase ferritic-martensitic 3CR12 steel, exhibiting an excellent corrosion resistance under ambient conditions, is a potential substitute for FeCrNi stainless steels in many applications. However, the steel suffers pitting corrosion in Cl−-containing environments, usually associated with the presence of non-metallic inclusions. The present work investigates the possibility of improvement of the pitting corrosion performance of hot-rolled and annealed 3CR12 steel using a laser surface melting (LSM) technique. The laser processing was carried out using a 3 kW CW Nd:YAG laser with 10 and 13 mm line beam profiles. Pitting resistance in de-aerated 3.5% NaCl solution was substantially increased by LSM. The degree of improvement is associated with the formation of ferrite and reduction of martensite in the melt region, the extent of which depends on the processing parameters. Observation of pitting morphologies of the laser-melted surfaces showed that corrosion was initiated at the ferrite grain boundaries, where the martensite or possibly impurities were located, compared with more random pitting in the banded microstructure of the initial steel.