Mysterious holes

"We carry wide-spiral wire-link belts in a triple construction in our product range. These belts are used where small parts are input as bulk material and passed through different temperature ranges. In the case in question, the temperature range was just over 800 °C," says Edgar Kischel, head of wire belt sales, who has been working for HEIN, LEHMANN for around two decades."

One day, a long-time HEIN, LEHMANN customer contacted Mr. Kischel with a problem concerning his individual design of a wide-spiral wire link belt: The customer reported finding isolated holes in the belt – and assumed material defects."The holes were signs of embrittlement. These form in a so-called carburization process due to extreme heat. The material absorbs carbon due to the heat, which usually makes it fragile after a few months, at which point it needs to be replaced. The curious thing here, however, was that the carburization was happening in a much shorter time and occurred only occasionally. The holes didn't form on the whole belt width, but only in some places, which also showed no regularity."

The cause could not be identified right away. If it was a normal carburization process, why did it not affect the whole belt? Mr. Kischel was faced with a puzzle that needed to be solved. So, parts of the belt were removed and examined metallurgically in our laboratory.

"The result amazed us, because in the affected areas there was actually extremely substantial carburization. We were able to rule out material defects on our part, as the rest of the belt was not affected. But we needed to find out what caused it. We wanted to learn from this phenomenon."

This was followed by meetings between the operator, furnace maker and experts from HEIN, LEHMANN. All sorts of scenarios were discussed, all parameters checked – but no errors or changed settings were found.

"It was like staring at a hole in the wall. The hole was there, clearly, and everyone could see it. But we couldn't figure out who made it, and how."

But life moves in mysterious ways, and chance played a big role in solving the puzzle. After all the facts had been checked and all possible scenarios rejected, all those involved decided to meet again on site.

"When we got together, we saw one of the employees responsible for the facility throw his sandwich wrapper onto the conveyor belt. It immediately burnt up and, poof, it was gone. And we all suddenly realized what had been going on."

The reason was less the wrapper itself, but the residues of fat or margarine stuck to it. When exposed to extremely high heat, wide-spiral wire link belts need to be kept absolutely clean and free of grease – to last longer. If there is grease on the belt, the combustion causes explosive carburization, which prematurely destroys the material.

"That was in line with the phenomenon, of course – and also why the holes did not have a fixed or recurring pattern. After all, the employee had been throwing his sandwich wrapper on the belt at different times. Later, he admitted that he'd also tossed the entire sandwich on the belt if he didn't like it."

The puzzle had been solved. That random observation on site did more than tons of experience and investigations could have accomplished.

"We were all relieved to have found the answer. But the whole effort around identifying the cause was not in vain, because we learned important lessons – for example, that you can never rule out anything. There are always factors that are not on the radar right now, so you really need to take a much broader view of things."

Even today, the 'sandwich incident' is passed on among colleagues – especially when there are problems with isolated holes forming.

"We at HEIN, LEHMANN benefit from our decades of experience. And that doesn't just consist of technical know-how and practical knowledge. It's also stories like these that remind us to think one step ahead at any given time. That's one of the big advantages we bring to our daily work and projects."

Do you have big plans and need a partner who is really knowledgeable? Then contact us – we will gladly advise you!