Brittle Cracking of Flanks of a Big Gear
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TECHNICAL ARTICLE—PEER-REVIEWED
Brittle Cracking of Flanks of a Big Gear Xiao-lei Xu • Zhi-wei Yu
Submitted: 11 April 2012 / in revised form: 22 June 2012 / Published online: 5 September 2012 Ó ASM International 2012
Abstract Several teeth of a big helical gear were found cracked following coarse-grinding teeth after the gear was carburized. Visual observation shows that cracks are present only on the flanks at one side having an acute angle to the gear end. The cracks propagated from the flanks toward the gear end. SEM observation on the fracture surfaces reveals that intergranular and quasi-cleavage brittle fracture is the cracking mechanism of the flanks. The cracked flanks at the ‘‘acute angle’’ side have a weak ability to support the grinding stress induced by grinding tooth operation, which is mainly responsible for cracking of the flanks. The thick case-harden depth and weak inter-crystalline strength nature of the carburized layer facilitate the initiation and propagation of crack. Unsuitable grinding operations probably aggravate cracking of the flanks. Keywords Gear Brittle cracking Grinding tooth Failure analysis
Introduction Several teeth of a big gear were found cracked following grinding the flanks. The cracked gear is made of 20CrMnMo steel. The addendum circle diameter is 1,538 mm, the flank width is 380 mm. The main steps involved in the fabrication of the gear and the related technical specification are given in Table 1.
X. Xu (&) Z. Yu Electromechanics and Material Engineering College, Dalian Maritime University, Dalian 116026, People’s Republic of China e-mail: [email protected]; [email protected]
Experiment Methods The chemical composition of the failed gear material was determined by spectroscopy chemical analysis. The microstructure at various locations was observed by optical microscope (OPM). Micro-hardness profiles from the surface to the interior were made to determine the depth of the carburized layer in various regions. A 1,000-g load and Vickers indentor were used for micro-hardness analysis. The criterion for determining case depth described in Chinese standard GB [1] is the depth of material with hardness greater than HV550. The crack surfaces were observed visually and using scanning electron microscopy (SEM).
Observation Results The failed gear is shown in Fig. 1. Visual observation revealed that several gear teeth cracked and cracking occurred on the flanks on the same side of the tooth (marked in Fig. 1). It was noted that flanks of the gear teeth were ground and the roots of the teeth were not ground. Three adjacent cracked teeth were cut from the gear for detailed visual examination (marked in Fig. 1). The cracks are slightly curved and the present location of the crack; the crack orientation and the crack length of the three cracked teeth are almost identical (marked in Fig. 2). One end of crack is about 7 mm from the tooth tip and another end is about 9–10 mm from the gear end, and the crack is about 16–17 mm in length. The grinding marks on the flan
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