Material Hard Spots: How to Detect and Handle Them
The Hard Spot Problem Hard spots are localized areas in the material where the hardness spikes well above the nominal range. A piece of 4140 that is 30 HRC overall can have a hard spot at 45 HRC. The drill hits that spot and the cutting edge chips. I have seen this most often in castings and forgings. A casting that cools unevenly develops hard zones where the cooling rate was different. A forging that was not properly stress-relieved after forming has unpredictable hardness distribution. ...