Macropitting in Microsegregated Induction-Hardened Gear Tooth
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CASE HISTORY—PEER-REVIEWED
Macropitting in Microsegregated Induction-Hardened Gear Tooth Nicholas Bantz
Submitted: 30 January 2014 / Published online: 14 March 2014 Ó ASM International 2014
Abstract A sample from a large gear was submitted for investigation into the cause of apparent macropitting observed around the pitch line on the loaded face of one of the teeth. A banded (light and dark etching) martensitic structure was observed, which varied significantly in hardness. Cracking was observed to be isolated to the casehardened surface. The macropitting observed appeared to be consistent with contact fatigue. The nonhomogeneous microstructure observed may have been particularly susceptible to fatigue initiation and propagation. The microstructure observed appeared to be due to a lack of homogenous diffusion of carbon during the austenization of the part from the annealed condition.
Experimental Procedure The chemical composition of the base material was determined by optical emission spectroscopy (OES) per Fed Test Standard 151B, Method 112.2. The part was initially examined visually and from 10 to 30x with a stereoscope. Full profiles of the gear teeth were taken, polished, macroetched, and examined. In addition, cross sections from the case-hardened surfaces were taken, mounted, and polished in accordance with ASTM E 3. The mounted cross sections were examined with an optical microscope. Hardness measurements were performed in accordance with ASTM E 18 and ASTM E 384.
Keywords Contact fatigue Induction hardening Heat treated Results and Discussion Introduction A sample from a large gear was submitted for investigation into the cause of apparent macropitting observed around the pitch line on the loaded face of one of the teeth. No other information was provided as to the condition of the rest of the part and the remaining gear teeth.
N. Bantz (&) Metals Engineering & Testing Laboratories – Engineering, 2040 West Quail Avenue, Phoenix, AZ 85027, USA e-mail: [email protected]
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The base material chemistry was consistent with 4140 Steel (UNS G41400) (Table 1). The loaded face around the pitch line of one of the gear teeth displayed an extensive amount of macropitting (Fig. 1). Some wear was also observed adjacent to this pitting. The other gear tooth of the provided sample did not display the same extensive macropitting. This gear surface did show some wear at the pitch line on the loaded face though. A profile through each of the teeth was taken. These profiles were polished and etched with a Nital solution (Fig. 2). The etched profiles revealed case-hardened surfaces that appeared to be only on the flank and root of the gears. These case-hardened surfaces appeared to be from a gap-by-gap induction-hardening process. Macroscopically, a segregated microstructure was observed throughout, consisting of dark and light bands.
J Fail. Anal. and Preven. (2014) 14:282–285
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The general microstructure consisted of tempered martensite in the core and untempered martensite in the case along with what appe
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