Questions

Given that the AVERAGE hardness meets the requirement minimum, are the actuating pins acceptable for use as-is? How do others in the industry handle this issue?

Answers

Testing should always be based on adherence to specification and a combination of reasonable engineering practice and past experience. This will define the minimum that should be done with respect to testing and quality control. 

Looking at the test results on each of the coupons and the referee test performed, the concern appears to be with the values at 57.9 HRC, which is 0.1 below the lower specification limit. This type of result can be addressed in the following manner. If one looks at the accuracy of the test block used for verification testing (which in this case is a good “referee”), one finds at this hardness range that the accuracy is ± 0.5 HRC point. It is also important to remember that the test block itself exhibits non-uniformity in hardness both within a block and from block to block (Fig. 1). 

The value of the test block also represents the variation in hardness test results that can be expected at a given hardness range. Thus, a value of 57.9 HRC is really a range of hardness from 57.5 HRC to 58.4 HRC (based on this one individual reading). This is why the average is considered to be an appropriate method of determining if parts are in compliance. As a result, the pins are acceptable for use as-is.

It is worth noting that there is danger in taking too few hardness measurements (in this case only 1 per coupon, rather than at least 3) and trying to use a limited data set to determine acceptance criteria. Some people use the “Student t-test” to determine if the difference between mean values for two or more specimens is statistically valid – even when this is not a requirement. Others look at a sample size of N=30 or N=60 readings to determine pass/fail from a statistical standpoint.

Finally, with readings so close to the minimum of the specification it would be prudent to review the material chemistry, heat-treatment practice and the accuracy of the control instrumentation and make adjustments so as to bring average hardness values into the center of the range. 

References

1.  George Vander Voort, Vander Voort Consulting, private correspondence
2.  Yamamoto, Takashi, Masayuki Yamamoto and Kensuke Miyahara, “Accuracy of Standard Blocks for Hardness and Uncertainty of Hardness,” XIX IMEKO Work Congress, Lisbon, Portugal 2009
3.  Low, Samuel R., Rockwell Hardness Measurement of Metallic Materials, NIST Special Publication 960-5, 2001
4.  E. Lysaght and A. DeBellis, Hardness Testing Handbook, American Chain and cable Company, 1969
5.  H. E. Boyer (ed.), Hardness Testing, ASM International, Metals Park, OH, 1987