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Anomalous defects on nitrided parts

January 11, 2012

Q. We have alloy steel rods which were subjected to ion nitriding. Base material and nitrided layer properties result to be complying.
Most of the batch items highlight plenty of defects (macrosopic and well visible at the naked eye) spread on several surfaces, both nitrided and not nitrided (only ground) surfaces.
Defects are a sort of swallow and large pits of very regular shape: we see a nearly rectangular material loss when observed in sections, with a a high width-to-depth ratio, and they are full of iron oxides.
Could it be some anomalous corrosion phenomenon occurring during nitriding? The only further step is lapping (barrel finishing).

Andrea Vecchi
R&D - Carpi (MO), Italy

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January 16, 2012

Could be that stresses caused by grinding are overcoming matrix strength of intruded surfaces. You might try stress relieving after grinding but before noticing. Something like 2-4 hrs. at 192 C.

Kent Backus
Fort Worth, Texas, USA