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Nitriding nodular cast iron

Nitriding nodular cast iron

Nitriding nodular cast iron

Hello guys,
Has anyone had problems on nitriding cast iron, regarding hardness of the layer?
We currently gas nitride some parts made of GGG-50 to a hardness of 650HV. In some cases the entire lot of pieces comes with low hardness (~350-450HV).
Our supplier says (we don't nitride in house) that the nitriding parameters and procedure is the same when the parts archieve the hardness and when they don't, and that the it must be a problem with the material. Analyzing the material we can't find anything unusual in the microstructure, and we are always having this fight.
This material and specification were already defined once I joined the company and I have some thoughts on what is going on. I'd like do share with you for some guidance.
1) I believe 650 HV is too high for a GGG-50 white layer, right? But then, we do get that hardness most more than least of the times. What could be an achievable target hardness for this material?
2) The thickness of the white layer is around 5-7µm. The indentor must be puncturing the white layer when we measure the hardness (our lighter load is 300g) and we are getting the hardness of the diffusion layer. How to run from that? A large white layer may be too brittle and break, and the graphite gets in the way as well.
3) Suggestions on how to measure a hardness profile of the diffusion layer when the material is heterogeneous like GGG-50?

Looking foward on some thoughts, especially on item 1).
Thank you in advance.

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