Movement Of S-7 After Heat Treatment

Terry Z · Feb 8, 2019

I have to make a small pressing fixture for bearing install. I need to hold some sizes +/-.002 and one +/-.0002. The .0002 I can surface grind to hit it as it is a depth of a pocket at the top of my fixture the rest are diameters. The part is 2.0 diameter X .75 tall. I need to make .900 +/-.001 pocket diameter and 1.247 +/- .002 diameter. Would it be best to turn after heat treating? And I need a thru hole of .376 for a line to line fit for a hard KO pin to guide my bearing.

metalmagpie · Feb 8, 2019

Good luck hard turning heat treated S7. I have to anneal first, not a pro though.

metalmagpie

1 Jaw Chuck (Chuck Evans) · Feb 8, 2019

Terry,

What is the final hardness after tempering?

I have turned RC 53 or so parts on the lathe with carbide tooling.

Very nice finish.

Chuck
Burbank, CA

Terry Z · Feb 9, 2019

Going to be 58 to 60 R I have turned heat treated S-7 before something I have not done much of. Some parts of this will be a P.I.T.A. to turn after heat treat so that's why I am asking how much it will move to try avoiding machining after heat treat. Thanks

Laurentian · Feb 9, 2019

foil wrap it, HT at low end of temp curve and air harden by slight air flow. temper it down as much as possible. I'd say go on small end of tolerance at lathe it mat very well grow bigger. hit or miss proposition until you have a few under your belt

Orange Vise · Feb 9, 2019

I don't think you'll have any problems as long as you machine the pockets on the large side. IME, pockets shrink in heat treatment.

S7 is less stable in heat treatment than A2 and D2, but probably sufficient given your relatively loose tolerances. With D2, I'm usually able to hold tenths.

Definitely send parts out to a reputable heat treater and make sure it's done in vacuum or inert atmosphere.

If you want to play it safe, finish on the lathe with CBN inserts.

DMF_TomB · Feb 9, 2019

how much does it change length and how much does it make bore id smaller ??
doesnt it expand outside dimensions bigger and make holes smaller ?? when heat treated

Bob E · Feb 9, 2019

Round S-7 seems more stable than flat or cubic S-7.

When I was doing alot of mold work, I usually only left .005" total on diameters for HT. This was for grind finish. We had a pretty good Hardinge, so turning 57-60 Rc was doable.

We heat-treated in house with foil and paper then cryo.

Shrinkage or growth was observed after HT mostly in long flat parts. With growth being along the grain.

TDegenhart · Feb 9, 2019

Can you grind the parts after HT?

Tom

DMF_TomB · Feb 10, 2019

many parts heat treated when originally machined tapped holes are done with oversized taps cause after heat treatment holes get smaller or out of round where screws wont go in holes.
.
after heat treated usually hss tap wont tap the holes even .001" bigger ifs its heat treated too hard. they do sell carbide taps but they are very expensive, shorter flute length and break easy.

Matt_Maguire · Feb 11, 2019

It grows, I have 2 references that match & both agree @ .001” growth per inch section. That’s a fair bit, & both strongly recommend double temper if oil quenched. This stuff would be pretty tough with charpy un-notched test values of 200ish ft/lbs with 400F double tempers.

The growth is in volume so holes may shrink some… Attached one reference.

Good luck,
Matt

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Movement Of S-7 After Heat Treatment

Terry Z

Cast Iron

metalmagpie

Titanium

1 Jaw Chuck (Chuck Evans)

Cast Iron

Terry Z

Cast Iron

Laurentian

Stainless

Orange Vise

Titanium

DMF_TomB

Diamond

Bob E

Hot Rolled

TDegenhart

Diamond

DMF_TomB

Diamond

Matt_Maguire

Stainless

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