Toolbox Case Study 3: Coal Chisel Hammer 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 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 #3- Coal Chisel Hammer Failure

B A C K G R O U N D
This third case history concerned a coal chisel hammer which had failed in service at a power plant. The hammer was approximately 15" long x 1-1/2" wide. The service life was not stipulated. The designated material was also not stipulated but chemical analysis conducted during the investigation identified the material as AIS] 4150 steel. The heat treatment was also not stipulated but the hammer had a hardness of 34 Rockwell C, indicative of a normalized 4150 steel. The tensile strength was 155,300 psi; the yield strength was 97,600 psi; percent elongation was 16 and percent reduction of area was 51.

D O C U M E N T A T I O N
The fracture on the hammer was at approximately mid-length as shown in Photograph 1. At the site of the fracture, a surface discontinuity was distinctly visible as indicated in Photograph 2. The fractured surface visually exhibited evidence of chevron markings emanating from one corner as shown in Photograph 3. This corner had two significant features. First, the corner was relatively sub-surface as shown in Photograph 4. Secondly, this sub-surface region represented the surface of the defect that is shown in Photograph 2. The chevron pattern emanated from a relatively smooth region on the fractured surface that was at the conjunction between the sub-surface defect and the outside corner. This aspect is shown in Photograph 4. This smooth fractured surface area had fatigue striations as determined with the scanning electron microscope. The chevron region had cleavage-type fracturing, indicative of a single event overload in a material that had limited ductility.

Metallographic analysis revealed that the surface discontinuity shown in Photograph 2 existed in the cross section to a depth of approximately 1/3 the thickness as shown in Photograph 5. Photograph 6 further revealed that this sub-surface defect had decarburization associated with the two surfaces and that the contour of the two surfaces did not match. Since the contour of the two surfaces did not match, the sub-surface separation was not due to fracturing. The observed conditions were considered indicative of a lap that occurred during the forging operation when the coal chisel hammer was formed.

C O N C L U S I O N S
The results of this investigation indicated that the coal chisel hammer had fractured as the result of fatigue that initiated at an internal defect. A catastrophic failure also had occurred for more than 90% of the cross section after the fatigue had propagated a short distance. The defect that caused this failure was identified as a forging lap.

The user of this component was advised that forging laps can be the consequence of die geometry, forging sequence, and/or starting material quality. They were also advised that since this defect could be the consequence of the original manufacturing processing, hammers of similar configuration should be evaluated for evidence of a forging lap.

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

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Photograph 2
Photograph 3
Photograph 4
Photograph 5
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