Design News for Mechanical and Design Engineers

A very popular brown meat sauce is named after a central Massachusetts city with "shire" appended. Other producers could not use the name, but may try to imitate the sauce, which is mostly water, vinegar, and salt with some added flavorings. Some years ago a major food products company was producing such a sauce in large quantities. The sauce was being pumped under pressure through a stainless steel pipe approximately 6 inches in diameter. The temperature was 240F, well above the normal boiling point of water.

A 22-year-old male was on duty in the plant, mainly keeping an eye on things. Suddenly, a stream of superheated sauce burst from the piping, catching the unsuspecting young man squarely on his gluteus maximus. Although the burns were not serious enough to cause maiming, the young man's social life was certainly curtailed for a while, depending, of course, on his preferences in entertainment. Quite reasonably, he started legal action: his attorney hired me as consulting metallurgist.

Sections of the pipe were flanged at the ends and connected by a hinged clamp that held a polymer gasket in place. One of these clamps had fractured, which allowed the sauce to spurt out. I was given a report written by a consulting engineer, who concluded that the clamp failed by fatigue. This conclusion was not supported by any of the microscopy routinely needed in identifying fatigue failures. The consultant's letterhead listed him as an expert in Civil, Mechanical, and Electrical Engineering. He also apparently considered himself qualified to analyze what was obviously a Metallurgical Engineering case.

My primary investigative techniques were chemical analysis, hardness testing, optical microscopy, and scanning electron microscopy, all standard tools in a case of this sort. The clamp contained 18% chromium and 8% nickel, so was the very common type 304 or 18-8 stainless steel. The steel was in the cold worked condition, which gave it added strength. Scanning electron microscopy of the fracture surface showed an intergranular attack characteristic of sensitized stainless steel. There was no sign whatsoever of the striations that would indicate the fatigue failure proposed by the earlier mentioned consultant engineer.

Chromium in concentrations above 12% renders steel stainless. The steel in the clamp contained more than enough chromium, but also contained nearly 0.1% residual carbon from the iron and steel making processes. Slow cooling through the red heat range allowed chromium carbides to nucleate heterogeneously on the grain boundaries. The adjacent regions in the grains were depleted of Cr too far below the 12% threshold and were no longer as corrosion resistant. A highly corrosive solution cut through the sensitized boundaries like a hot knife through butter.

Even sensitized stainless steel is adequate for many mildly corrosive environments, including households. The acidic, salty sauce was clearly too aggressive an environment for the sensitized steel. Such attacks occur all too frequently, often with far worse consequences than a burned posterior.

Heating the steel sheet (prior to cutting and forming) to bright red heat followed by an air cool would have removed the sensitization. This heat treatment would have cost very little, but was not done. Apparently, it was more cost effective to pay for the occasional failure than to do a minor heat treatment.