Effect of boron on the sliding wear of directionally solidified high-chromium white irons

The effect of small amounts of boron (less than 195 ppm) added to a 16%Cr white iron were studied. Bars of 13 mm diameter were directionally solidified into hot ceramic moulds and heat was extracted from the bottom by a water cooled copper plate. The microstructure showed aligned eutectic carbides in the direction of the heat flow with a finer structure for the iron with the higher boron amount. The alloys were then heat treated at 950 C to destabilize the austenitic matrix; a stronger mainly martensitic matrix reinforced with secondary carbides resulted from such a treatment. The as-cast and -heat treated alloys were tested under dry sliding for wear behavior by using three different loads (50, 150 and 250 N) and a constant sliding speed of 0.7 m/s. Wear resistance resulted higher for: (i) the heat treated irons since the martensitic matrix reinforced with secondary carbides better supported the eutectic carbide phase against fracture, (ii) the boron added irons due to the finer structure, and (iii) the irons whose eutectic carbides were aligned parallel to the worn surface, when the carbides were aligned perpendicular to the contact surface, more carbide fracture was observed which contributed to destabilize the surface.