A tale of poor materials quality control.
I needed a couple of samples of 304 for the development work I was doing, so I cut them from the threaded end of the bolt illustrated below. It’s nothing special, just a random sample from my large collection of old, glue-in, machine bolts extracted from crags around SEQ.
They were subjected to acid-digestion, and, at the end of this process, I noted that there was a perfectly flat, thin disc remaining, presumably because the reaction ran out of acid before it ran out of metal.
Ever curious, I put it under the microscope to reveal the image below.
Note the astonishing range of grain sizes. Normally I’d be expecting to see grains uniformly distributed from say 15um up to 25um, and as such they would appear tiny at this magnification.
The refinement of grain size and distribution is a centuries-old, metal working art that is manifestly absent in this modern product. Whatever happened here?
My guess is that the culprit is a process upstream of bar stock production. Perhaps the soak annealing process for the cast slab? In any event, what matters is the fact that bar stock with terrible grain uniformity has made its way through incoming goods inspection, and into production.
However, our woes don’t end with a criticism of grain refinement. An examination of the picture above reveals the presence of extensive slip-bands and twinning throughout the material as a result of the cold working processes it has been subjected to. So is it magnetic? See this post for chapter and verse on what I am talking about here.
A quick test with my crude magnetic balance showed the threaded section to be the most magnetic example of 304 that I have ever encountered. In the light of this discovery, I’m going to have to re-evaluate the calibration. For now, let’t say that it looks like the thread forming process has resulted in almost complete conversion of austenite to martensite.
Knowing what we now know about the role of martensite in promoting the degradation of stainless steel on high-sulphate sea cliffs, I would anticipate this material would last less than a year before becoming dangerously embrittled.
Based on what we see happening in the field, it is clear that the single most important point in the quality control system is that of ensuring that incoming bar stock is fit for purpose. Technically this is not so easy, and many manufacturers rely on the supplier of the bar stock to have the controls in place to ensure rogue steel does not enter the supply chain.
But, cost is king – downstream consequences don’t look so bad when the price is right. We will be fighting this demon for many years yet.