Failure Investigation of a Diesel Engine Piston Pin
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TECHNICAL ARTICLE—PEER-REVIEWED
Failure Investigation of a Diesel Engine Piston Pin Xiao-lei Xu • Zhi-wei Yu
Submitted: 6 October 2009 / in revised form: 10 March 2010 / Published online: 24 March 2010 Ó ASM International 2010
Abstract Transverse and longitudinal cracking took place on a diesel engine piston-pin used in a truck in service. The external circle and internal hole surfaces of piston-pin are specified to be carburized. The cracks originated from the internal hole surface and propagated, by fatigue, towards the external circle. The microstructure and microhardness profiles indicate that the serious decarburization occurred on the internal hole surface and that the hardening case was not present. The occurrence of the decarburization in the surface region of the internal hole decreases the fatigue strength to levels far below those for a carburized surface so failure initiated easily at the internal hole surface. To assure against similar failures in the future, make sure that the region around the internal hole obtains the specified depth of hardened layer.
Experimental Methods The chemical composition of the failed piston-pin material was analyzed by spectroscopy chemical analysis. Microstructure was observed by optical microscopy (OPM). Microhardness profiles, to estimate the depth of the carburized layer, from the surface to the interior in different regions were made by Vickers system with a load of 1000 g. The criterion for determining case depth described in the Chinese standard (GB 9450) [1] is the depth of material with hardness greater than HV550. The case depth on the external circle and the internal hole surfaces are specified as 0.8-1.2 mm and more than 0.5 mm, respectively, and the sum of case depth on the external circle and the internal hole surfaces is specified to be less than 2.5 mm.
Keywords Piston-pin Decarburization Carburizing process Fatigue fracture Failure analysis Results Introduction A diesel engine piston-pin used in a truck fractured in service. The accumulated service mileage was only about 750 km. The failed piston pin was made of 20Cr steel and the internal hole and the external circle surfaces of the piston-pin were specified to be carburized. The hardness of the external circle surface is specified as HRC57-64 and the core hardness as HRC28-43.
X. Xu (&) Z. Yu Electromechanics and Material Engineering College, Dalian Maritime University, Dalian 116026, P.R. China e-mail: [email protected]
The piston-pin fractured into two pieces transversely and the smaller piece was lost. The remainder of the fractured piston-pin is shown in Fig. 1. The macro-fracture features are shown in Fig. 2. From the fracture surface, longitudinal cracking was found on the piston-pin (labeled crack A and crack B in Fig. 2). Transverse fracture occurred at the contact location of the piston pin with the connecting-rod. The fracture surface was worn and significantly deformed, but the crack propagation marks along the circumference were observed. From the crack propagation marks, i
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