Dry sliding wear of low alloyed austempered ductile iron

Abstract

A multiple low alloyed ductile iron with 0.8 wt-%Ni and 0.25 wt-%Mo was austempered in single and two step processes at 300 and 400°C for 120 min. Specimens were used to study the effect of austempering conditions on the wear behaviour of this material. Sliding wear tests were carried out using a pin on disc apparatus, the tes tmaterials rubbing under dry atmospheric conditions against a surface of hardened steel (55 HRC) at speeds of 0.6, 0.7 and 1.0 m s-1 and normal loads of 15.82 and 22.84 N. Test durations were 30, 60, 90 and 120 min. Scanning electron microscopy was used to examine the worn surfaces of test specimens. It was found that two step austempered specimens exhibited wear resistance that was higher than that of specimens austempered at 400°C, and almost as high as that of specimens austempered at 300°C. These two step austempered specimens, moreover, gave the highest impact energy and showed the best combination of mechanical properties. During two step austempering, the first stage reaction (formation of ausferrite) was completed in the intercellular area before the undesired second stage reaction (precipitation of carbides) had started in the eutectic cells. The two step treatment resulted in a duplex structure: upper and lower bainitic ferrite without formation of carbides. This structure was responsible for the improvement of mechanical properties and the good wear resistance. The results show that a well balanced choice of smaller additions of multiple alloying elements can reduce the negative effects of segregation and resulting structural inhomogeneity. MST/5472