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    I'm with carbideBob, holes shrink!

    But where are you with the "heat treat" relative to the two graphs on this link

    Tool Steel | S7 Shock Resisting Tool Steel | S7 Steel

    Temperature dependent happiness. A lot like snow skiing ;-)

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    Quote Originally Posted by Terry Keeley View Post
    Press fitting a 0.200" pin in some S7 parts.

    Will the hole close or open during heat treat? During tempering specs says the material grows 0.1%. I've searched the forum and elsewhere and both answers are given.

    What say ye?
    what is the true position required?
    and what's the depth,

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    Quote Originally Posted by CalG View Post
    I'm with carbideBob, holes shrink!

    But where are you with the "heat treat" relative to the two graphs on this link

    Tool Steel | S7 Shock Resisting Tool Steel | S7 Steel

    Temperature dependent happiness. A lot like snow skiing ;-)

    I'm tempering at 400F so that chart says the part should grow slightly, about 0.03%, other suppliers quote 0.1% at that temp.

    I can confirm my parts did in fact grow, closer to the 0.1% tho. The 0.374" O/D grew by about 3 tenths and my 0.201" pins by a tenth or two.

    But the original question was with this growth will the hole close or open up? Seems to be different camps on this.

    My original thought was that it would open up and I was right (at least in this case), my 0.200" hole opened up a couple tenths.

    Makes sense to me, the material "expands" outwards so the hole opens...
    Last edited by Terry Keeley; 10-07-2020 at 05:21 PM.

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    Quote Originally Posted by Terry Keeley View Post
    Makes sense to me, the material "expands" outwards so the hole opens...
    If you do carburizing, like C-Bob says, it normally expands on the outside and shrinks on the bores.

    Normally.

    That's the tricky word, seems like every time I really depend on "normal" it bites me

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    Aahhh Terry:
    But it actually increases in volume, and when we measure a cube before and after HT it's "grown"
    Actually it's inflated like a pillowcase with air pumped into it.
    This is because the hardening process alters the position of the atoms in the structure of the crystal lattice with heat and then freezes them in that new position with the quench.

    The strain of them being in that new position is what confers the hardness.
    The new position of each atom in the altered lattice is spaced further apart so the structure occupies more volume.

    This is an awfully simplistic description, but the theory says outsides should grow and insides should shrink and flat surfaces should develop a bulge.
    Problem is, it's perverse...as Emanuel points out; just when you need to count on it , it fucks you over.
    The Machine Shop God is capricious and wants you to stay humble!

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    Quote Originally Posted by implmex View Post
    This is an awfully simplistic description, but the theory says outsides should grow and insides should shrink and flat surfaces should develop a bulge.

    I just can't wrap my head around how a hole could shrink after heat treat, I only see it growing. That is if the material grows overall. If it shrinks overall I could see how a hole would close up in that case.

    Like when you heat a bore to install a press fit bearing or shaft for example.

    Can anyone show me a reference on this subject?

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    The material grows into free space. Think all material stock expanding where it can.
    Hence the outside bigger and holes gets smaller.
    Bob

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    Default Controlling hole size

    Quote Originally Posted by Terry Keeley View Post
    Press fitting a 0.200" pin in some S7 parts.

    Will the hole close or open during heat treat? During tempering specs says the material grows 0.1%. I've searched the forum and elsewhere and both answers are given.

    What say ye?
    Whether the hole shrinks or grows after temper can be affected by the hardness the parts are drawn.
    Control size the by honing or lapping the hole to size. Machining the hole to size is not going to be a satisfactory process. Your trying to hold .0001 to .0002 tolerance with a reamer in the part before heat-treat changes the size that cannot be trusted to hold size.
    All the Best
    Roger 10/9/20

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    Quote Originally Posted by CarbideBob View Post
    The material grows into free space. Think all material stock expanding where it can.
    Hence the outside bigger and holes gets smaller.
    Bob
    In my case the hole got bigger tho, not smaller.

    You sure that "free space" ain't kinda like a "free lunch"?

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    Quote Originally Posted by Terry Keeley View Post
    In my case the hole got bigger tho, not smaller.

    You sure that "free space" ain't kinda like a "free lunch"?

    No free lunch. Now we get into the hole and stock around it.
    This is why I said above it sort of is a crap shoot and test parts must be done.
    For sure this happens and the reverse also happens.
    You will see hole bigger and hole smaller but not the same parts or HT source.
    There is often no right guess. Even the best will blow this wrong.
    I have no problem with you telling all others that I am not right as I have seen it go not the way I thought it should. ...and ....
    I can only post on my experience and have seen big and small.
    Bob

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    Quote Originally Posted by Terry Keeley View Post
    I just can't wrap my head around how a hole could shrink after heat treat, I only see it growing. That is if the material grows overall. If it shrinks overall I could see how a hole would close up in that case.

    Like when you heat a bore to install a press fit bearing or shaft for example.

    Can anyone show me a reference on this subject?
    Go back and read my reply.

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    Quote Originally Posted by CalG View Post
    I'm with carbideBob, holes shrink!

    But where are you with the "heat treat" relative to the two graphs on this link

    Tool Steel | S7 Shock Resisting Tool Steel | S7 Steel

    Temperature dependent happiness. A lot like snow skiing ;-)
    Don't always assume this condition, the heatreating process will distort geometry of your component. Large bores tend to grow in size as the machining surface fuzz gets burned away.

    Depend that bores will be larger an OD's will be smaller.

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    Quote Originally Posted by Rickyb View Post
    The answer is yes. It will grow or shrink...depending. Several factors here.

    Focusing on just the hole, think about its circumference. The material grows during quench and that means the circumference gets larger hence the hole gets bigger. Works every time if the material around it also grows.

    Since the entire part never quenches exactly at the same microsecond, some places will quench sooner, think solidify, before other sections make the same transformation. Bad thing and good things can happen because of this.

    Now go back to thinking about the hole. Suppose everything around it quenches first. Now comes the hole’s turn. The physics of the microstructure transformation forces the material to grow. It just has to. It can’t grow outward as everything around it is already solid. Only one place left. Grow into the hole. The hole gets smaller.
    So it can grow and get smaller...depending

    Quote Originally Posted by Rickyb View Post
    Go back and read my reply.

    The parts are just air cooled so the hole sees the same "quench" as the O/D, maybe that's why it opened up?

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    terry, its clear that the hole must grow if the part expands in a uniform way, like you say. but what really happens can apparently be different.

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    Quote Originally Posted by dian View Post
    terry, its clear that the hole must grow if the part expands in a uniform way, like you say. but what really happens can apparently be different.

    Like this?





    For my money when air quenching S7 I'll bet on the hole opening up...

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    This is not a simple process that you can apply this or that rule of thumb.

    It really doesn’t matter how it is quenched. Everything depends on the geometry And heat transfer. Whatever section is solidifying first, grows and solidifies first and affects everything around it.

    Air quenching just happens slower than water or oil but the process is the same. Holes always complicate quenching a part.

    Heat transfer is a combination of conduction radiation and convection. A hole doesn’t have anything to radiate the heat away to except itself. Convection in a hole depends on its orientation. Is it horizontal, vertical, or at an angle. The mass around the hole is conducting the internal temp from 360 Degrees Around the hole into the hole slowing its cooling and solidification. The hole cannot conduct the heat away to the air as fast as it is receiving Heat from its surroundings.

    It’s possible to computer model the transfer of heat, but not an easy matter. Anecdotal evidence of holes always shrink or always grow are just that, anecdotal. They are pretty much worthless in assessing apart with a different geometry.

    All I can advise it give critical thought to what cools first and whenever possible don’t make the hole until after heat treat.

    This might help the thought process a bit.

    An 8620 carburized part can only quench as deep as the carbon is infused into the surface. That entire layer grows At quench but the core does not. The core and case are in a battle. One grows the other does not but they are intimately attached. The core pulls back on the case holding back some of the expansion resulting in very high compressive residual stresses in the surface layer. The surface of the part is actually in compression. Machine away any area of the surface and you affect the equilibrium and the part will warp to bring stresses back into equilibrium. The surface of a Similar through hardened part will likely have tensile stresses because the core wants to grow after the surface has solidified. It pushes the surface outward as it quenches.

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  22. #57
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    Should have put my vote in. Looks to me like the hole in your part will egg shape longitudinally. Grow lengthwise and shrink radially. Don’t think anyone picked this one yet. Does the winner get a prize?

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    Quote Originally Posted by Terry Keeley View Post
    Well I guess a dozen parts could end up all different but on the one I just made the hole actually opened up slightly, I'd guess a couple tenths.

    I bored the hole initially so a 0.200" - gauge pin was a "tight" fit. I mean you had to have it lined up exactly right and then it took a bit of pressure to get it in.

    Now it slides right in, not sloppy at all but an easy "slip" fit.

    Go figure.
    Post # 35, the middle musta grown too...

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    Any hone, Sunnen included (I have a M1660 ? ) will follow what ever centerline is in the hole. However if the hole is out of round it will average out the differences and establish a new centerline. The size and length and amount to remove out of the hole in question any shift would not be a problem. One thing for sure it will definitely be round!

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    Got the new piece done, I'm glad I made the pin after heat treating the part with the hole. Got a good 1/2 thou press, went together nice.

    Now to see if this one holds up...




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