Growth of 4140 during Heat Treat to Rc 45

[luthster]

I had some prototype parts machined from annealed 4140 bar through hardened to Rc 42-47 using austempering process. The OD is approximately 3.00 inches and grew about .005" after heat treat. I did not expect this much growth and when I talked to the heat treat source he was surprised by the amount of growth also.

1) Is this amount of growth expected?
2) What would be the tightest reccomended tolerance after heat treating for this OD?
3) If I need +/-.001 inches for the final assembly will I have to machine after heat treating?

 

[mfgenggear]

http://en.wikipedia.org/wiki/Austempering

#1) not that much usually .0003-.0005 per inch to my experience
depending on the configuration & type of heat treat.
my parts are mar tempered with is very similar.
#2 & 3) finish machine after heat treat for any critical dimensions

the expected expanding will vary with the heat treat & material chemistry.
for +/- .001 dimension you could do several test runs to obtain a average dimension value.
record before & after heat treat to make sure it's not machining errors. if this was not done then the
above results may not be from heat treating.

Mfgenggear

 

[redpicker]

Holding ± 0.001 on a 3 inch OD is a bit tight for machining tolerance, let alone after heat treatment. If you have to have this tight of a tolerance, I don't see any way around machining after heat treatment.

There are some things you can do to reduce the amount of growth you experience, however. Starting with Q&T stock (or at least normalized) instead of annealed with help, with perhaps a stress relief operation prior to final machining. If there are large changes in cross section, preheating and stabilizing the temperature at 1300F prior to raising the temperature to austenitizing temperature will also help. The Asutempering process is a good choice for minimizing distortion.

Consider what your machining tolerance before heat treatment must be to hold ± 0.001" after heat treatment. In all cases, it will be less than ± 0.001 since you never get the same distortion twice. If you have to have a ± 0.001" tolerance in the final part, I think you'll be better off performing final machining after heat treatment. The reasoning is that if the distortion is not what you expect, you aren't put in the position of needing to accept out-or-tolerance parts to meet cost and/or delivery concerns. That is always a bad spot to find yourself and it is much better to avoid it if you can. Of course, if you don't really need the ± 0.001" tolerance, then you may be able to live with the distortion that results from heat treatment.

rp

 

[TVP]

You will definitely need to machine after HT if you need to maintain a tolerance of ± 0.001 inches on a dimension (diameter, length, etc.) that is larger than ~ 0.5 inches, has any changes in cross-section, and deviates at all from a simple shape (small shaft, sphere).