Toolbox Case Study 2: Conveyor Line King Pin Failure

By George M. Goodrich, Senior Metallurgical Engineer, Stork Climax Research Services

Introduction
A metallurgical failure analysis utilizes many tools to determine the cause for the failure of a component. These tools include:

1. Visual Examination
2. Chemical Analysis
3. Mechanical Properties
4. Hardness
5. Scanning Electron Microscope Analysis
6. Metallographic Analysis
7. Facts concerning the utilization of the component

Each of these tools provides information that can be used individually or in combination to determine the cause for the failure. The metallurgical aspects of the failure alone, however, are only part of the information required to solve a problem and prevent it from occurring in the future.

This document presents the second of seven case histories that have utilized the various tools to identify the cause of failure. In most instances, however, the solution to the problems that were created as the result of the failure related to practices that were instituted where the component was being utilized. In one instance, the practices involved inadequate quality control. In some instances, the practices involved utilizing techniques that were unacceptable in the application. In still other instances, no actual cause for the failure could be identified due to the practices that were employed prior to utilization of the component. These case histories are real case histories and represent examples of failures that are commonly experienced.

Case History #2 - Conveyor Line King Pin Failure

B A C K G R O U N D
The second case history concerned a king pin that had failed in service on an automotive assembly line. The king pin is shown in Photograph 1. The designated material for the king pin was not stipulated at the time of the investigation. However, it was determined that the king pin had been manufactured from AISI 4140 steel. The heat treatment was also not stipulated. However, the investigation revealed that it had been quenched and tempered to a hardness of 44-46 HRC with a through hardening heat treatment. Type 4140 alloy steel has a high degree of hardenability and is generally used in a through-hardened application that requires toughness. Although we believe that the stress rising effect of the quench crack was the root cause for the failure, the reduced toughness of the material contributed to the failure event.

D O C U M E N T A T I O N
The king pin exhibited evidence of fretting corrosion on the outside diameter as shown in Photograph 2. At the site of the failure, however, no evidence of the fretting corrosion was present. The fracture was transverse to the axis of the pin and appeared to be associated with either a radius or a thread as can be seen in Photograph 3.

The fractured surface visually exhibited four sites around the perimeter with a relatively smooth fracture appearance for a shallow depth. The sites are located in Photograph 4. The majority of the fractured surface, however, exhibited a gray granular appearance, indicative of a ductile overload as can be seen in Photograph 4.

Visual examination also revealed a longitudinal fracture. The step on the fractured surface at approximately the 10 o'clock position in Photograph 4 shows the location of the longitudinal fracture. This fracture was also associated with the oil hole in the center of the king pin segment as shown in Photograph 5.

Scanning electron microscope evaluation of the fractured surface revealed an intergranular fracture mode for the majority of the fracture. An example is shown in Photographs 6 and 7. In the smooth regions that were located at the four positions around the circumference, microvoid coalescence, indicative of a ductile overload, was identified. An example is shown in Photographs 8 and 9. The longitudinal fracture had an intergranular failure mode. This intergranular mode can be seen in Photograph 8 along the crack.

Quench cracks can exhibit intergranular fracturing characteristics. In this instance, it is believed that the longitudinal fracture on the king pin from the oil hole shown in Photograph 5 was a quench crack. This quench crack served as a site to concentrate stresses for a single event overload failure. Although the part was cracked before it went into service, the failure occurred at the juncture between this quench crack and the radius that was at the suspected origin.

C O N C L U S I O N S
The results of the king pin investigation revealed that the cause of the fracturing was a quench crack and indicated the potentially excessive brittle characteristics in the material. Although 45-46 HRC for AISI 4140 steel is an acceptable hardness for a wear application, the toughness of this material is compromised at this hardness level. We believe that the reduced toughness combined with the stress rising effect of the quench crack was the cause for the failure.

As a consequence of this failure investigation, it was recommended to the facility utilizing the king pin that other pins be evaluated for evidence of quench cracking. This quench crack type of event most likely was present in other king pins that had been treated in the same lot. Removal of other pins exhibiting evidence of quench cracks prevented unscheduled down time and catastrophic failure.

This document contains general information, no rights can be derived.

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