FEATURE ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2023 48 austenitic before quenching; therefore, the full hardening capability of the steel was not attained. The microstructure of the material away from the hardened surface was a mixture of finely divided pearlite and ferrite, which resulted in the metal being relatively soft. The microstructure of the material in the outer ring adjacent to the raceway (Fig. 2c) was a mixture of white ferrite, scattered patches of pearlite, and martensite, which showed that the steel had been improperly austenitized, producing very low hardness. Shape, Distortions, and Morphological Analysis. Displacement of metal on the outer raceway is shown in Fig. 2d. The grain structure had been elongated, indicating metal movement. Rolled-out and embedded metal particles were also found in the surface. Countertesting. To confirm that the most probable root cause comes from the process itself and not from the given steel used, hardenability of the metal in the outer ring was checked by heating a specimen 25 by 25 by 6.4 mm along one edge with an oxyacetylene torch to 870° to 900°C, then quenching in water. Hardness was 57 to 60 HRC in the heat treated area and 23 HRC in the untreated area. Conclusions. Failure of the raceway surface of the outer ring of the bearing was the result of incomplete austenitization. The steel in the inner-ring raceway had been hardened to slightly below the specified hardness of 55 HRC minimum, but the outer-ring raceway had a maximum hardness of only 29.8 HRC. The raceway surface in the outer ring was not properly heated by the flame-hardening process; therefore, subsequent quenching and tempering operations, if any, would have had virtually no effect on hardness. Simulation. It is now also possible to simulate this type of case study, where the material has insufficient hardness and/or hardness depth to sustain the given applied external loads. Figure 3 shows the typical results of a parametric analysis to determine the hardness and hardness depth of the raceway material, using the variation of the maximum Hertzian pressure (pHmax) or minimum static safety factor (S0min) versus the raceway material hardness and hardness depth. In this particular case study, one can easily see that at the very low measured hardness (29.8 HRC), the case depth should be greater than 3.3 to 3.5 mm under the applied external loads. Furthermore, at a lower hardness of 25.6 HRC, the applied load cannot be sustained without yielding damage. Prevention Measures. To avoid these failure types for large-scale parts be sure to choose the most appropriate heat treatment process and that the steel family used is dedicated to those types of heat treatments and exhibits good results. Always remember that superficial treatments only give a hard case layer, but the core remains soft. Thus, if necessary, check for or request information about the minimum hardnesses (surface and core) as well as the hardness depth (case-hardness depth/surface-hardness depth). Contact or Hertzian stresses deploy themselves within the surface depth, with a maximum value below the surface. Thus, two parameters must be Fig. 3 — Evolution of minimum static safety factor, S0min, versus raceway material hardness and hardness depth. In this case, the hardness depth should be greater than 3.3 to 3.5 mm at 29.8 HRC. CHD, case-hardness depth; SHD, surface-hardness depth. Extracts from Bearinx Parameter Analysis. 12
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