Microstructural and corrosion characteristics of laser surface-melted plastics mold steels

Laser surface melting of plastics mold steels P21 (Fe-3% Ni-1.5% Mn-1% Al-0.3% Si-0.15% C) and 440C (Fe-17% Cr-1.1% C) was achieved by a 500 W CW Nd:YAG laser using different scanning speeds. The microstructure and the phases present in the laser surface-melted specimens were analysed by optical microscopy, scanning electron microscopy and X-ray diffractometry, respectively. The corrosion characteristics of the laser surface-melted specimens in 3.5% NaCl solution and in 1 M sulphuric acid at 23 °C were studied by potentiodynamic polarisation technique. X-ray diffraction spectra showed that laser surface-melted P21 and 440C contain martensite and austenite as the major phase, respectively. Laser surface-melted 440C exhibits passivity whereas laser surface-melted P21 does not. The corrosion resistance of laser surface-melted P21 in both corrosive media is improved as evidenced by a lower corrosion current density compared with that of the untreated specimens. The increase in corrosion resistance of laser surface-melted P21 is due to the dissolution of the intermetallic phase Ni3Al to form a homogeneous solid solution by rapid solidification. The corrosion resistance of laser surface-melted 440C in NaCl solution is also increased significantly, with the exhibition of a wide passive range and a low passive current density, but the improvement in sulphuric acid is less pronounced. The enhanced corrosion resistance of laser surface-melted 440C results from the combined effect of the refinement of carbide particles with increased C and Cr in solid solution, and the presence of retained austenite. The corrosion characteristics of all the laser surface-melted specimens are strongly dependent on the laser scanning speed, which in turn results in different microstructures.