17-4PH Condition A Heat Treating to H900 problem

[BMEgrn]

We recently heat treated a number of 17-4PH Cond A prototype parts in house to my H900 specification. The parts are 0.125" diameter and 6.25" long. They ran approximately 600 shafts of similar size range at the same time

The guys stacked the shafts in a pile and ran the program for 1 hour at 900*F to get to H900. Since our hardness tester can't test these small diameter parts we threw in a slug on top and checked the hardness before and after heat treatment.

The Slug went from 28HRC to 43HRC. However, I am having some concerns that some of the parts I tested arent hardened. These fail at only a slightly higher torque than the Cond A parts.

I also had 20 initial prototypes (same dimensions) heat treated the same way, but without anything else so they were spread out more. The 20 parts were done a couple of weeks ago and they are certainly harder than these are. However, I don't think they remembered to throw a slug in with the earlier prototypes.

I'm looking at the loading at the moment and curious about if they overloaded it since the shafts stack rather neatly.

I'm not an expert in heat treating at all, but am I on the right track?

 

[Metalguy]

Either they got to 900 for an hour or they didn't. In any event, I think you can simply reheat them to 900 for a full hour--try a couple of shafts and see if they pass your torque test.

Hopefully these shafts do not get wet in service.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[BMEgrn]

I'm going to do a batch of 20 reheating to 900 to see what the torque test results are.

They actually do get wet in a saline solution, but they are only used for a few seconds with torques in the inch-lb. level then discarded. So cracking hasn't been a problem at this time.

For a bigger batch would it make a difference to increase the warm up time? or should I look at the temperature gauging for a failure?

 

[Metalguy]

Can you bury a thermocouple in the middle of the stack? If not, you may have to increase the warmup time.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[BMEgrn]

All I've got for temp readout is two sensors at the top of the oven

 

[Metalguy]

Cheapest thing to do is buy some Tempil sticks or pellets. They melt at precise temps. I like the pellets--hope they still make them.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein