Effect of carbide orientation during single scratch test in directionally solidified and heat-treated high chromium cast irons

Abstract

In this work, a billet of eutectic high chromium cast iron, material commonly used in highly abrasive environments, such as in grinding and milling, was unidirectionally solidified. The resulting microstructure was composed of oriented M7C3 carbides and eutectic matrix. Specimens were cut parallel and transversally to the billet axis and split into two groups. Both were heat treated with different austenitization temperatures. The temperature for the first group was 1100 °C, and the value for the second was 950 °C, providing matrix instrumented indentation hardness values of 8.0 and 6.3 GPa, respectively. This procedure resulted in specimens with three carbide directions dispersed in two different matrices. Samples were evaluated by means of single scratch test with a cono-spherical diamond stylus under progressive and constant normal load conditions. To support the analysis, a non-contact 3D surface profiler and scanning electron microscopy (SEM) were used to characterize the scratched surfaces. Progressive load tests indicated a transition in wear micro-mechanisms which was mainly observed in the samples with the long axes of the carbides parallel to the wear surface and oriented orthogonal to the scratch direction and with matrix hardness of 6.3 GPa. On the other hand, constant load tests showed that carbides with the long axes parallel to the wear surface and oriented parallel to the scratch direction tend to produce deeper scratchers with higher ridges than in the other directions. Results also indicate that the hardness, despite being an important variable, does not entirely explain the behavior of the material under single scratch, since the orientation of the carbides with respect to the scratch direction must also be considered.