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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:
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Visual Examination
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Chemical Analysis
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Mechanical Properties
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Hardness
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Scanning Electron Microscope Analysis
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Metallographic Analysis
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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 third 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 #4 - Fuel Valve Component Failure
B A C K G R O U N D
The component in this case was less than 1" long. It was cylindrical in shape and exhibited the presence of a linear defect on the outside diameter as shown in Photograph 1. Further background information indicated that this component was from a fuel valve assembly that had failed within one week after installation. Interestingly, the component that was submitted, although it exhibited evidence of fracturing, was not instrumental in the fuel valve failure. We were requested to conduct an investigation that would identify the cause for the defect that was visually present on the outside surface even though this defect had not contributed to the failure of the fuel valve. The designated material for the component was Type 440 stainless steel and was confirmed. The heat treatment was not specified but the fuel valve had a quench and tempered structure with a 56-57 HRC hardness.
D O C U M E N T A T I O N
The component was cut in such a way that the fractured surface would be exposed. This surface is shown in Photograph 2. No visible evidence of a fracture origin was discernable. Scanning electron microscope analysis, as shown in Photographs 3 and 4, also failed to identify a fracture origin. The fracture mode, as shown in Photograph 4, was microvoid coalescence, characteristic of a single event ductile overload. Some discoloration was also associated with the fractured surface. This discoloration was similar to other surfaces such as the through hole shown in Photograph 1. Photograph 2, for example, shows the through hole, the fractured surface, and the cutting that was conducted to expose the fractured surface. This fracture discoloration indicated that the fracturing had been present at the time that the component had been heat treated.
C O N C L U S I O N S
The results of the investigation were not entirely conclusive. However, the opinion was that the fracture on this component existed prior to installation and that it existed prior to the heat treatment. It was also the conclusion of this investigation that the fracture on this component existed prior to final machining. The fact that the component exhibited microvoid coalescence as a fracture mode without indications of a fracture origin was the basis for this opinion that the fracture existed prior to machining. This type of fracturing ordinarily would display a fracture origin. Since the origin was not present, it was concluded that it had been removed from the surface during machining operations that involved the manufacture of the component.
The client was concerned that the application had caused the fracture. Since it was determined from this investigation that the fracturing existed prior to heat treatment and in fact, existed prior to final machining, the client was satisfied that they had not caused the crack and that it had existed in the part prior to machining. As a consequence, measures were taken at the clients operation to enhance inspection of incoming materials to prevent the occurrence of this type of defect in their final product.
This document contains general information, no rights can be derived. |
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Photograph 1
Photograph 2
Photograph 3
Photograph 4
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