Used Equipment

How can I eliminate internal cracks when nitriding 15-5 ph stainless steel. I need case hardness of 90?

I am gas nitriding 15-5 PH stainless steel to get a case depth of .0035 inch over a flat geometry. The part is heated to 1000 deg F in a chamber of 70% ammonia for 3 hours then the ammonia is reduced to 30% and baked for 8 more hours.
Problem: after the nitride there are small hairline cracks within the nitride case. The cracks extend longitudinally in the case. I have tried varying the cool rate but that does not eliminate the cracks. I can’t change the 15-5 material.
Are there any suggestions to eliminate the cracks, such as different ammonia ratio or temperature?

considering the size of the part, you may be simply have too much of a differential in material properties between the case and the bulk such that upon cooling you will get cracking, regardless of rate. (what hardness scale were you referring to when you wanted a hardness of 90?)

Perhaps a lower temperature during nitriding may help, but I doubt it since you can only drop it another 100°F or so and still get good nitriding. Might ion nitriding be an option?

two suggestions would be to try posting this at finishing(dot)com or contact a licensed metallurgist to help consult on this. I would suggest mee-inc(dot)com. They have experience solving nitrided case problems in PH steels.

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