German plastic injection molding machine shaft forging lap

20-foot forged shaft, fatigue crack after ~18 months in service originating from a 1-inch × 1/4-inch forging lap. Used to illustrate that forging laps are diagnostic: oxidized surface from 2,000°F formation in air gives them away. German manufacturer initially denied.

How common are they? They're not very common. I actually can only think in my career of one forging lap defect that caused a problem. Well, I take that back, I can think of two. One was — the Germans had forged a 20-foot-long shaft for a plastic injection molding machine, and it had been machined and everything, went into service, and about a year and a half later had a big fatigue crack. It had a forging lap that was about an inch long and about a quarter inch deep, and that's where the fatigue crack started. I mean, you get a flaw that big in something that's rotating. Of course the Germans said it couldn't have been ours — except you always know it's a forging lap because the whole lap surface is oxidized. It was formed at 2,000 degrees Fahrenheit in the air, and so it's full of oxide and decarburization if it's steel. It's unmistakable. So eventually they had to own up to that.