440C heat treat distortion

[nickjk]

I am making tubular sleeves from 440c stainless.
The sleeves call for a Vacuum Furnace Heat Treat AMS 2759/5 Gas Quench
(Hardness Rock “C” 59-61)
I have made four batches of these sleeves and up till now distortion after heat treat has been very constant.
The first four batches the part shrunk close to .002” in axial length. The diameters also shrunk but the amount was dependent on the cross section design. The details with a uniform cross section the diameter shrinkage was minimal (.0000-.0002). On details were the cross section was not uniform the diameters shrunk almost .001.
Anyway, on the last batch of parts I had heat treated, which was a new design, the parts actually grew close to .002” in axial length. They also grew in diameter.
I asked the heat-treater if he had done anything different during the process.
If I understood him correctly he said the hardness was not high enough on the sample piece so he had to do something different with the nitrogen quench but was within the AMS 2759/5 standard. I know he double tempered the parts but I thought that was part of the standard.
I am a little confused; I thought by calling out AMS 2759/5 Gas Quench I would have consistency in the heat treat process. Am I wrong and what can I do to provide consistency?
I also noticed during the last batch that the parts went out of round more than the 1st 4 batches.
Any help would greatly be appreciated.
Nickjk

 

[redpicker]

I don't know exactly what your heat treater did, but he admittedly changed his process. I suspect he is correct, that the changes he made are allowed per the standard, but they did change the result (both the hardness and the observed distortion). He may have run the parts twice to correct the low hardness situation, which could have a noticeable effect.

Heat treat distortion is a funny thing. I think everyone who has had to deal with it has had your experience; the first few times you monitor the distortion, you get predictable results. The very next run, you get the opposite from what you had observed before.

About all you can do is make sure you have a consistent starting material. To hold the tolerances you are quoting, you may have to pre-harden and stress relieve prior to machining. This will result in a much more consistent starting microstructure than you would get with just a mill anneal, which is what I expect you would get without specifying anything different. You will probably have to get your heat treater to see how he can better control his process to reduce the amount of distortion. If distortion control is more important than final hardness, you may consider having him discuss out-of-tolerance hardness conditions with you before changing the process.

Good Luck

rp

 

[unclesyd]

The suggestion posted above is good one. Though we do not harden the parts we stress relieve, rough machine and stress relieve prior to HT. Our parts are sleeves 3" to 4" Id x 6" to 8*' long, All parts are jig ground as final machining.
Our parts were used on split-case pumps. We got away from 440C and went to Liquid Nitrided 17/4 PH running against Nitronic 60.

 

[mfgenggear]

nick

RP & uncle are both very good recommendations, I do both when necessary, in addition, look at the heat treat process, if you have a good relationship with the Heat Treat facility. do the following.

#1 ) request a fixed process planning. discuss each operation with the heat treater.
have a picture taken of the set up
#2) use graphite plate with holes to secure the parts during heat treat.
discuss this with your heat treater.
#3) source inspect the facility & the heat treater while your parts are being set up
#5 take measurements before & after heat treat to make sure the parts moving & it's not
machining issues.
# 6) part of #1 , learn what heat treat options are available, vacuum with high pressure gas quench.


Mfgenggear