Failure Analysis of a Hot Forged SAE 4140 Steel Kingpin
Abstract
The fifth wheel and kingpin connection system, a critical part of heavy vehicles, provides the link of tractor and trailer. The kingpin is usually manufactured by hot forging. A part manufactured by this process was assembled in a fifth wheel of an off-road truck and presented an early failure. The truck was used in a quarry until the kingpin failure, three months later. One process issue that can occur in hot forged products is a poor grain structure due to overheating, burning and cavitation. The Scanning Electron Microscopy (SEM) analysis of the failed part showed the presence of cavitation. However, the failure analysis results evinced that cavitation was not the main cause of the fracture, but a combination of wear, impact fatigue and overload.
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Introduction
The kingpin coupled to fifth wheel is extensively employed in heavy articulated vehicles to link the towing vehicle to the trailer. The kingpin is fastened in the trailer and it is coupled into the fifth wheel of a tractor. The horizontal load is transmitted from tractor to trailer through the kingpin positioned at the center of the fifth wheel. Hot forging from a steel bar is usually the adopted process to produce this part. Forged products may present the cavitation (grain boundary voids) phenomenon when the steel is preheated to a high temperature (usually > 1200°C) prior to hot working, or when a certain amount of non-homogeneity occurs in the chemical composition of the steel, or even due to problems with the die leading to a non-uniform deformation. Lubrication problems may also lead to grain burning. Thus, local melting can occur at the austenite grain boundaries as a result of segregation of P, S or C. According to ASM Metals Handbook, Vol.11 [1], “cavitation or ductile rupture is the more dominant mode of ductile fracture when working temperatures are higher than one-half the melting point of a given material”. Because of the too many parameters that may cause grain burning, it is not a very infrequent problem, and may affect significantly the service performance of the part.
A kingpin was processed by hot forging of a hot rolled 88,90mm round bar of SAE 4140 steel. The part was machined and thermal treated by quenching and tempering. The kingpin was assembled into a fifth wheel of an off-road truck. The truck was used in a quarry until the kingpin failure, after three months. The part fractured in two pieces that are shown in Fig. 1. No other kingpin from same lot was reported do fail since then. An analysis of the failed kingpin is performed in this paper.
Conclusion
Summarizing the facts, the truck was used in a quarry under severe operation conditions, riding on an uneven ground, which caused overloads on the fifth wheel coupling. The overloads were transmitted to the kingpin causing the external surface deformation which lead to a clearance formation. The random nature of the applied loads and changes in loading point due to maneuvering justify the irregular deformation pattern observed in the kingpin cross section. This scenario was sufficient to allow the occurrence of contact-fatigue and impact-fatigue mechanisms. Moreover, the finite element analysis performed here corroborates the occurrence of plastic (or ductile) deformation at the lateral surface of the king pin in the case of those severe conditions. The cavitation observed in the kingpin material was not the main cause of the failure but just contributed for the fracture propagation.