Heat Treatment before vs after Machining 9

[huiying]

Hi,
I would like to know if anyone has experience regarding heat treatment before and after machining process.
From my knowledge, if heat treatment were to be done before machining process, special tool bit must be required because the material will be hardened therefore, typically, heat treatment is to be done after machining.
However, I'm not sure what is the effect of heat treatment on the tolerance if it is done after machining. Does anyone has any knowledge in this?
Is there any way to keep the tolerance the same before and after heat treatment?

Thanks & Regards.

 

[LORDLUCAN]

It all depends on the application and the size/shape of the component and what the tolerances are.

 

[Barry1961]

Parts with a tight tolerance are machined a little bit oversized then ground to size. The amount of warpage you get from heat treating varies with the steel and process. As a rule, less warpage is more money.

Barry1961

 

[mrainey]

I used to do a lot of aerospace machining of thin-walled 4130, heat treated up to Rc38-Rc45. Our standard practice was to leave .100 on all surfaces prior to heat treat. Warpage varied, but there were times when we were really glad we left so much stock. The material at that level of hardness is quite machinable, but you better not be in a hurry (200 - 300 SFM turning with carbide, 30 SFM drilling with HSS tools). We tried turning with ceramic, but the dull edge of the insert put too much pressure on the thin walls, resulting in chatter, roundness problems, and poor tool life.

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[huiying]

Thanks for the inputs.
Perhaps I should provide more info on my parts so that I could get more advice.

The material I'm using is Stainless Steel 17-4 and heat treatment to H900 and H1075. The tolerance I'm looking for is H7g6 for cylindrical interface. Is that realistic? Furthermore, if the size of the part is around 300mm to 500mm, how will the overall dimension varies? Is an overall dimensional tolerance of 0.1mm possible without grinding after heat treatment?

Thanks & Regards.

 

[LORDLUCAN]

For this type of fit I would say it's not possible.

 

[Nate101]

Again it depends on your setup and machine. The H900 condition is getting hard and would be tough to machine and hold this type of tolerance. Roughing and, finish anything your tolerances allow prior to heat treat would be essential (remember to calculate for heat treat shrinkage). I’d machine it, if it’s a rigid setup, but I’d expect some fall out. You’d have to weigh this against grinding. The H1075, I’d heat treat the stock first then machine it. 17-4 machines nice from the 1100-1025 range.

 

[huiying]

Is that to say that it is easier to machine 17-4 after H1075 then before? What kind of tooling do you usually use?

Is there a formula to calculate for heat shrinkage? How about warpage? I thought it is difficult to determine the kind of warpage that will be encountered?

mrainey mentioned that "... to leave .100 on all surfaces prior to heat treat. Warpage varied, but there were times when we were really glad we left so much stock...".
.100 is inches or mm? Also, how do I overcome clamping problem if I were to do grinding after clamping?

Thanks & Regards.

 

[unclesyd]

We routinely machined a tremendous number of precision parts from the PH alloys, 450,455,630, and others all in the heat treated condition H1000 to H1125. I cannot recall a PH material ever being machined in the solution annealed condition.

The more common parts were 6"x8" to 12" x 16". First 1/2" countersunk holes are drilled/machined on the periphery then the part is Blanchard Ground to the required thickness. At this point hundreds of 2.9mm or 3.2mm holes with L/D’s of 12 to 17 are drilled. The bottom of the hole is then coined to a 60° included angle. The coining resulted in slight bulge on the face which is lapped off using conventional abrasives to the required thickness generally .012+" over the coined area. At this point a hole of .009" dia. is either drilled, punched or EDM to the apex of the 60° cone. The hole is then broached and then the part is lapped to the required .0120" depth of the hole and a square edge on the .0090" dia hole.

These parts along with numerous other PH parts of various sizes and descriptions are being made everyday. Unfortunately not in the quantity they once were.

 

[mrainey]

We left .100 inches (2.54 millimeters) per surface. You might be able to get away with less, as we were dealing mainly with large-diameter, thin-walled parts, which tended to distort a good deal.

The hardened material was turned off. Didn't do any grinding, because the size and finish requirements didn't make it necessary.

The key to working with our parts after heat treat was to first chuck the distorted part in a manual chuck, using the least amount of clamping force possible. Some parts would have to be indicated in. We would then turn a perfectly round tooling diameter, with corresponding perpendicular face (light, cleanup passes only). This gave us something we could work with on the CNC lathes, with their less-sensitive power chucks, to do the actual finish turning.

Chucking a piece that was significantly out-of-round, then finishing other features on the part in that same setup, never worked well for us.


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[RebelBrill]

Republic Steel usde to publish a booklet with guidelines on machining, welding, forging, heat treating, etc for Precipitation Hardenable Stainless steels. The one I'm using was published in 1975.

They say "Normally, these grades are machined in the solution-treated condition (Condition A) to final dimensions because then, only the simple low temperature aging treatment is required to produce the desired combinations of mechanical properties."

They go on to give the following info on machinability
Condition H-900......20 ot 30 SFPM
Condition A..........70 to 80 SFPM
Condition H-1100.....80 to 90 SFPM
Condition H1150M.....100 to 130 SFPM

The booklet has lots of other good stuff. If you can get a copy, I'd recommend it.

Regards,
RebelBrill

 

[gbent]

With any heat treated part, the heat-treater is an important part of the equation. You will need to work with them to develop a procedure, and then make sure they follow it. There is a lot of art to heat treating when low distortion in needed.

 

[huiying]

Hi RebelBrill,
Thanks for the info. May I know if the title of the guide book that you mentioned? Any idea whether I can get it online?

Thanks & Regards.

 

[RebelBrill]

The reference that I cited is a 32 page booklet published by Republic Steel. The title is "Precipitation Hardenable Stainless Steels". It covers, in summary fashion, PH13-8 Mo, 15-5 PH, PH15-7 Mo, 17-4 PH, and 17-7 PH.

The copy I'm using was copyrighted in 1975 by Republic Steel corporation. The only other identifiers are on the outside bottom edge of the back cover. In the left hand corner of the cover it reads "ADV. 1958R3 10M 775E". In the right hand corner of the cover it reads "Printed in U.S.A.".

On the bottom left hand corner of page 32 is the following note
"For complete information on
these and other products,
contact Republic Steel Corporation
1441 Republic Building
P O Box 6778
Cleveland OH 44101"

The booklet was available free upon request when I got mine.

Good luck,
RebelBrill

 

[metman]

The following thread might be overkill for your application but likely relevant. It is concerning the same material 17-4 and tolerance control with the added complication of distortion control with respect to residual stress.

thread330-113147

the above thread is carried over to thread1135-113274 in the S.S. for engineers forum.

 

[surfdog]

17-4PH SST (cond 1075)is easliy machined with coated carbide tools. Get SFM (surface feet/minute) and chip load(ipt, inch/tooth) from tool supplier/manufacturer. We machine (mill) 17-4PH (cond 1075) all the time. It machines much nicer in this condition opposed to the annealed state (too soft). Use either a TICN coated tool or a TIALN tool. Good luck.