FEATURE ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2023 47 Because bearings operate at elevated temperatures and because bearing life depends on hardness, the hardness of the bearing material at operating temperature is significant. Data on short-term hot hardness[4] indicate that there is a significant difference between bearing components of low-alloy steels, such as AISI 52100 (100Cr6 or WN 1.3505), and those of high-speed tool steels, such as M50 (80MoCrV42-16 or WN 1.3551), in the ability to maintain their hardnesses as temperature increases. Most bearing alloys fall within the low-alloy or highspeed tool steel categories. However, a heat treatment that is optimal for a material in a tool is not necessarily optimal for the same material when used as a bearing component. Unfortunately, most heat treatments for these bearing materials are based on the heat treatments that have been developed for the materials as tools. CASE STUDY: IMPROPER HEAT TREATMENT OF OUTER RING RACEWAY Description of the Problem. A large bearing from a radar antenna was replaced because of deformation, surface cracking, and spalling on the raceway of the outer ring. Figure 2a shows a sectional view of the bearing. Data Collection. Specifications required that the rings be made of AISI 4140 (42CrMo4 or WN 1.7225) steel. The raceway surfaces were to be flame hardened to 55 HRC minimum and 50 HRC at 3.17 mm below the surface. Other surfaces of the rings were to have a minimum hardness of 24 + 4 HRC. The bearing load-carrying capac- ities at n = 1.67 revolutions per minute were 7130 kN in the radial direction and 4590 kN in the axial direction. Analysis. To check the material and heat treatment, inner- and outer-ring samples 150 to 200 mm long were sent to the laboratory for examination. Examination revealed that the raceway of the outer ring was damaged by deformation, surface cracking, and spalling (Fig. 2b). The raceway of the inner ring exhibited little or no damage. Chemical Composition. Wet chemical analysis of the material in the inner and outer rings, done to determine if the metal composition was the one expected, showed it to be an AISI 4140 (EN 42CrMo4 or WN 1.7225) steel. Molybdenum contents were 0.26% in the inner ring and 0.31% in the outer ring, both slightly above the specified range of 0.15 to 0.25%. However, this deviation is not significant and would not affect strength properties or hardenability. Hardness Testing. To determine if the components exhibit the right mechanical properties, a cross-sectional specimen approximately 9.50 mm thick was taken through the center of each sample; after cutting, both surfaces of each specimen were ground smooth. Hardness of the raceway of the outer ring was found to be from 29.8 to 11.7 HRC. A horizontal traverse through the center of the ring showed a hardness of 26.1 to 18.7 HRC. A vertical traverse ~41 mm from the outer surface showed a hardness of 25.2 to 18.9 HRC. These low hardness values for the outer-ring raceway indicated that it had not been properly flame hardened. The hardness of the top and outer surfaces of the rings was within specifications. The hardness traverse of the inner-ring raceway showed a hardness of 46.8 to 54.8 HRC, which was below the specified minimum hardness of 55 HRC. Microstructure Examination. The specimens were etched in 3% nital to differentiate between hardened and unhardened areas. The inner ring showed a well-developed hardened case at the raceway; the outer ring did not. Metallographic examination of an etched specimen showed that the structure of the inner ring adjacent to the raceway was a mixture of tempered martensite and some ferrite. This suggested that heat treatment was insufficient to make the structure completely Fig. 2 — Large-diameter 4140 steel slewing ring used in a radar antenna that failed because of improper heat treatment of the outer-ring raceway. (a) Configuration and dimensions. (b) Fractograph showing typical damage on outer-ring raceway. (c) Micrograph of section through metal in outer ring adjacent to raceway showing ferrite, scattered patches of pearlite, and tempered martensite. (d) Micrograph of section through outer ring at raceway showing grains elongated by metal movement (raceway surface is at top). (a) (b) (d) (c) 11
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