The proof loading is just part of our QC procedure after manufacture. All mandrels are proof loaded, and then we decided to send this batch out for nitriding.
We spec our tempering temperature to be above 593°C. We don't spec a tempering temperature and time however, we specify a set of...
I received some more information on the nitriding process we used:
-Nitriding bath @ 570°C for 1.5-2.5 hours
-Oxidizing bath immediately after nitriding (350-400°C) for 20 minutes
-Water cool (near ambient) before bead blasting
-Oxidizing bath and water cool again
Our nitriding outfit says this...
-Hardness before nitriding was around 38 Rockwell C. After nitriding, it was about 58 Rockwell C.
-The nitriding outfit insists sufficient surface cleaning was done.
-The solvents they used shouldn't have caused any harm to the material.
All of the mandrels were proof loaded before they were sent out for nitriding, and no issues were encountered. We have hundreds of these mandrels out in service right now without nitriding and this type of failure has not occurred on any of them. I suppose we aren't 100% sure that the cracks...
We've recently run into a problem with surface cracks occuring in nitrided parts both during the nitriding process, and in during proof loading of the parts. Of the 8 parts which we sent out for liquid nitriding (the ARCOR process), 1 came back with cracks on the surface and another 4 deceloped...
