Powder Metal Hardening

[surfmase]

Hello,

Looking for some suggestions on hardening our powder metal parts. We have two types of parts: rollers from FC0205 and gears from FD0205. Both parts are compressed to relatively low densities then sintered. Sintered pieces are then machined and sent to heat treatment. I specify case hardness and depth on the drawing, but the machine shop contracts and communicates with heat treatment and I only get fuzzy information (language barrier, willingness, know-how, whatever). We have been finding cracked teeth in the receiving inspections, and I'm wondering if anyone has some insight into the heat treatment of these parts. (I suspect it is the heat treatment). As I understand they are carbonitriding at 850-900C for 2-4 hours then oil quenching without tempering. I have been reading some technical papers from GKN who supplies the powder material and this seems to be common practice, except most reports indicate a tempering around 205C after quenching. Spoke with someone at GKN and they said they would look into it, but haven't heard back since...

Thanks

[Hudson]

You do not indicate what type of machining is being done. Yes, tempering after heat treatment will reduce brittleness in PM parts but the cracks might be caused by the rapid quench.

Perhaps a sinter-hardened grade of material might help. When rapidly cooled after sintering it retains a martensitic structure.

[surfmase]

Thanks for your input. This is really quite a mess. I found out that my boss actually visited the heat treatment because we had some requests from a customer to increase the wear resistance. He didn't make any official changes, but I'm sure the heat treatment began playing with the recipe because shortly after we began to experience problems.

The thing is these parts have been used for over 10 years with no problems until now. They have their limits but work well for what they are. I had some old pieces checked by a lab and the result are that they were all through-hardened with a surface hardness depth of 3mm HG = 550 HV0.1. The parts are all lathed before hardening and some are ground after heat treatment. Through hardening is actually preferred because they alloys both have high hardenability. I expect sinter-hardening would increase my machining costs. I couldn't find any information from NGK or similar about through hardening. Really I'm just wondering how to specify this treatment.