The objective of the project was to evaluate the wear mechanism of selected high alloy tool steels, 440C, 440XH, and M62, fabricated by powder metallurgy (PM) techniques via the hot isostatic press (HIP) process, and compare these results to two conventionally produced wrought alloys, 52100 and M50. Samples from the five candidate metals were characterized microstructurally via optical and scanning microscopy methods. A micro-pit wear testing machine was purchased and used to simulate bearing loads in a wind turbine gear box The candidate samples were subsequently heat treated in accordance with industrial protocols and machined into wear test specimens suitable for the micro-pit wear testing machine. Wear testing produced surfaces that were subsequently analyzed for wear and damage using both optical microscopy and scanning electron microscopy methods.
Optimized rolling-sliding experiments with a relatively small slip ratio (5%) and higher load (650N) exhibited signs of abrasive wear on all specimens, but not all specimens had the same degree of micro-pitting damage. The conventional wrought 52100 steel had the most severe micro-pitting, consisting of distorted craters, many of which were connected by micro-cracks. Cross-sectional examination showed multiple micro-cracks extending into the alloy microstructure from the same pit.
The depth of the cracks was several times the depth of the micro-pit. Type 440XH had the least amount of micro-pitting damage under similar test conditions. Micro-pits on 440HX were more localized and the micro-cracks associated with them tended to avoid carbides during propagation. Abrasive processes were observed on all the test specimens and were produced by debris from micopitting.
The combination of micro-pitting in conjunction with abrasive processes could work together to worsen the surface damage, diminishing bearing service life. The results suggest that there is a potential advantage of PM consolidated high alloy tool steels, such as 440XH produced via hot isostatic pressing to better resist micro-pitting when compared to other alloy types and wrought processing methods.