Toolbox Case Study 1: Valve Spring 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 first 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 #1 - Valve Spring Failure

B A C K G R O U N D
A silicon-chromium-vanadium steel valve spring which failed during cycle testing was the subject of this first investigation. The spring that was submitted was one of several that had failed during the course of cycle testing and were from the same lot.

D O C U M E N T A T I O N
Photograph 1 shows the spring and the location of the failure. This failure was oriented transverse to the spring wire, approximately the third winding from one end. The fractured surface, shown in Photograph 2, exhibited a small region that was oriented at a 45° angle to the axis of the spring wire. This small region displayed evidence of a thumbnail pattern from fatigue. Fatigue would be the expected failure mode since the component was being cycle tested at the time of failure. The remaining portion of the fracture had a rough granular appearance, indicative of the last area to fail in a fatigue failure.

Metallographic analysis revealed that the spring had minor surface damage identified as adiabatic shear from excessive shot peening. Photograph 3 shows an example. The extent of this adiabatic shear, however, was less than 0.0005" deep and was not considered to be detrimental, based upon the previous experiences with a similar spring.

Chemical analysis revealed the spring wire did not have a composition indicative of the designated silicon-chromium-vanadium spring steel material. A comparison of the chemical analysis for the spring and a chemical analysis for the designated material is shown in table below:

C O N C L U S I O N S
The results of this investigation revealed that the spring company had previous experience which identified the fact that the spring was designed close to the maximum limits for the application. As a consequence, any deviation from the strength characteristics that would have been derived from the vanadium modification dramatically detracted from the life expectancy in cycle testing. The fact that this spring did not have the composition expected was the cause for the failure. The reason was the lack of sufficient fatigue strength to endure the loading for the application. The failure mode was fatigue. The site where the fatigue initiated most likely was adiabatic shear. However, in this instance, this spring would have failed by fatigue in any event and the adiabatic shear simply provided the most convenient site.

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

^{Photograph 1: Overall view of the valve spring submitted for analysis. The smaller spring at the top was inserted into the larger spring at the bottom. The larger spring broke at the third winding from the left.}

^{Photograph 2: Split-image scanning electron micrograph showing an area at the suspected origin on the failed valve spring that was examined with the scanning electron microscope. At the left the magnification is 18X at the right the magnification is 180X. The chevron markings at the right indicate that the valve fracture origin was at this site.}

^{Photograph 3: Photomicrograph showing a transverse cross section of the failed spring in close proximity to the fracture origin but a short distance away. This photographs shows evidence of shallow adiabatic shear that was less than 0.0005" deep.}

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