440C distortion

[nickjk]

I am designing a tubular shape detail out of 440c stainless that will be vacuum furnace heat treated to AMS 2759/5 spec’s using gas quench (Rock “C” 59-61). I am trying to determine an approximate value the O.D. and I.D. may distort and grow so I can determine which diameters will require a secondary grind operation and how much grind stock should be allowed. The research I have found shows .0001-.0002 in / in. This is very vague and I am not sure how it would apply in my case. Is the distortion on the O.D and does the /in relate to O.D. and what affect does the I.D. have?
One section of the detail has a 1.178 O.D. and 1.022 I.D.
Another section of the detail has a 1.415 O.D. and 1.022 I.D.

I asked the source that will be performing the heat treat today but was not able to get an approx value.
Any help would be greatly appreciated.

Thanks,
Nickjk

 

[mrfailure]

There should not be any "distortion" in terms of diameter. It looks like your data is accounting for thermal expansion (via coefficient of thermal expansion) during heat treatment. While the tube will grow slightly during treatment, it will contract back to original size upon quenching. The only way you might distort the tube is if it is constrained in some way during heating and there were enough growth of the tube relative to the constraint to plastically deform the tube during heating. This is unlikely though since the material constraining the tube would also thermally expand.

Aaron Tanzer

http://www.lehightesting.com

 

[redpicker]

Distorion is a strange phenomenon. It is very hard to predict with any accuracy. In general, both the OD and ID will grow during a quench and temper heat treatment, but this grwoth may not be uniform, causing the cross-section to go "egg-shaped" resulting in an apparent reduction in diameter. This possibility to go "egg-shapeed" is worse if the parts have a large length to diameter ratio and if they are not quenched in a vertical position.

This is an instance where experience is your best resource. If you have no access to anyone with the needed experience, I think I'd add 0.020" to the OD and ID for grind allowance and then check to see how much distoriton you get. If you can afford to scrap the start-up parts, you may consider only 0.010", but if that isn't enough, there isn't much to do except to scrap them (there isn't going to be a good way of salvaging the distorted parts).

rp

 

[Maui]

The transformation from an austenitic microstructure to a martensitic microstructure during heat treatment is accompanied by a corresponding change in volume. The parts should grow. How much they grow varies with the steel grade and the heat treatment parameters. Generally speaking, the volume increase is approximately 2.9%. Some good comparisons between heat treatment methods and the resulting hardness and microstructures for 440C are presented in the following paper. You may find this useful:

http://www.hayesheattreating.com/pdf/440C ASM Paper June 25.pdf

Distortion of the component during heat treatment is a separate issue, and usually occurs in the quenching operation. If the part is long and slender, or has varing thickness (especially where thin sections are directly adjacent to relatively thick sections), then heat will not be drawn out of the part in a completely uniform manner during the quench. The thinner sections will transform more quickly, resulting in transformation related stresses. This can lead to material movement, and may result in distortion or even cracking if the quench is severe enough. Parts that are gas quenched in a vacuum furnace rarely crack, but they certainly can distort for the reasons mentioned. In most cases an additional 0.010" - 0.020" grind stock allowance is usually sufficient. When more grind stock is included, the grinders usually complain rather vehemently. Best not to upset them unless it is deemed necessary.

Maui

http://www.EngineeringMetallurgy.com