Case Study 3: Improving Manufacturing Quality – What’s the Root Cause?

A client makes high reliability steel components that were failing an inspection test at the last step of a complicated, multi-step fabrication process.  The test was used to identify the presence of defects over a certain size.  The failed samples had to be thrown away, resulting in a loss of $500 per sample.  About 100 parts had been thrown away and several batches of parts were on hold, waiting for disposition.  It was assumed that the quality of the incoming steel bars was at fault.  A great deal of time and money was spent trying to get the supplier to improve the steel quality.  This was getting them nowhere.

How did the materials engineering perspective help solve this problem?

All the factors that influence the quality of a fabricated component must be considered.  In this case the factors that influenced the presence of defects were: 1) the raw material, 2) the heat treatment process used near the end of the manufacturing process, 3) the machining processes used before and after the heat treatment, and 4) the inspection test.

A review of the steel specifications revealed conflicting requirements for the defects allowed in the incoming material.  This allowed a supplier to provide material that did not satisfy the most stringent of the specifications.  However, a deeper investigation revealed that the specifications were developed many years ago and that steel quality had improved significantly since then. 

Next, the inspection test method and its capabilities were reviewed.  The test was supposed to detect defects at and below the component’s surface.  Test samples with known levels of defects were made and evaluated.  The penetration depth of the test was found to be minimal.  Any defects detected were on the surface, leading to two conclusions.  First, if there were defects in the raw material, they would not have been detected.  Second, surface defects could be removed by touch-up machining.

Finally, a metallurgical examination of the microstructure of a failed sample revealed that the sample was heat treated improperly, making the material susceptible to cracking during machining.  Cracks were one of the defects in question.  The heat treating specification did not indicate the required microstructure after heat treatment.  The component specification was revised to include the microstructure requirements.

Busted Myths

Intuition was not enough to determine the root cause of the problem.  It was necessary to understand the impact of all aspects of the manufacturing process on the steel’s properties and level of defects.  Furthermore, understanding the effects of the different processes on the steel was straightforward, and didn’t turn into a research project.