Heat treating d2 getting cracks
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    Default Heat treating d2 getting cracks

    We heat treated some d2 tool steel in house instead of out sourcing. We don't have a vacuum kiln, but tried wrapping them in the ss foil. Scale was a little worse than I am used to. When I surface ground these blocks, was getting what look like small surface cracks. Any ideas? Thanks

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    Did you quench it too fast?

    It's very sensitive to overheating when hardening.

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    I talked to a local metallurgical place and they recommended soak 2 hours at 1400 then ramp up to 1850 2 hours, remove and air quench with a fan fast no temper. I had the parts wrapped in ss foil so it took a little bit to un wrap them, but I did that in front of the fan.

    I am thinking about adding an argon purge line to my kiln.

    Where is a good source for heat treating info?

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    Welcome to the learning curve. For delicate and critical parts I would recommend that you return to having them outsourced.

    Get a copy of "Heat Treatment, Selection, and Application of TOOL STEELS", William Bryson, Modern Machine Shop Publications. Not too theoretical, lot of practical information.

    Argon purge will buy you nothing. This has been proposed and tried before. Either get the proper equipment or accept what you get.

    Tom

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    The no temper part makes me say hmmmmmmmmmmmm.

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    I heat treats tons of D2 at work and never have any issues with cracking. Parts from 2.5'' diameter up to 8''+ typically with a hole of some size through the middle. My procedure is to wrap in foil, ramp to 1750F, soak at that temp for 2 hrs, pull out and remove foil in still air, separate parts to ensure proper cooling, then temper for 2 hours at 500F after they have cooled completely from the quench. If you get a good seal on the heat treat foil, scaling is pretty minimal.

    How large are your parts? 123 blocks by chance?

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    Quote Originally Posted by jspivey View Post
    I heat treats tons of D2 at work and never have any issues with cracking. Parts from 2.5'' diameter up to 8''+ typically with a hole of some size through the middle. My procedure is to wrap in foil, ramp to 1750F, soak at that temp for 2 hrs, pull out and remove foil in still air, separate parts to ensure proper cooling, then temper for 2 hours at 500F after they have cooled completely from the quench. If you get a good seal on the heat treat foil, scaling is pretty minimal.

    How large are your parts? 123 blocks by chance?


    Quote Originally Posted by jspivey View Post
    pull out and remove foil in still air
    Not to be too "Captain" obvious - if surface cracking is a problem for OP then maybe the "Still air" - no fan - would be a better way to go.

    Perhaps obviously the surface is hard, more brittle but contracts too fast, cools too fast compared to the core of the material that is still retaining a lot of heat / thermally expanded compared to the cooler contracted and stressed surface.


    and

    Quote Originally Posted by jspivey View Post
    How large are your parts? 123 blocks by chance?
    And like ^^^ The geometry and size and shape / surface area to volume ratio on different parts will make a difference. + composition of material from skin to core.

    __________________________________________________ __________________________________________________ ______________________________

    @Winklershop any photos maybe of the surface cracking you are describing experiencing you'd like to post ?

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    When I surface ground these blocks, was getting what look like small surface cracks. Any ideas? Thanks[/QUOTE]

    How often do you dress the SG wheel? Dull grinder wheel will cause heat checks in a workpiece.

    Ed.

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    Temper the parts. Untempered D-2 will probably be too brittle to be of much use anyway. Unless you're making a letter opener? And you especially need to be extremely careful if you are grinding untempered (or high tempered) heat treated steel. Too heavy a grind or as mentioned a dull, glazed or loaded wheel can cause those cracks in an instant. Even if they are too small to see, they can cause early part failure in use. If you are getting any burn marks at all you are in the danger zone.

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    D2 is one of those tool steels that has a hump in the hardness- tempering temperature curve at 520 deg F. If your goal is maximum hardness the part should be tempered at that temperature.

    There are other recommendations:

    Tempering should begin when the work piece cools to 150 deg F rather than waiting for it to reach room temperature,

    There should be two or three tempering cycles to convert the retained austenite to martensite,

    Your current process creates a superficial tempering cycle using a grinding wheel as the heat source, This is a effective way of cracking a tool steel that is considered to be resistant to cracking.

    Reference ASM Handbook "Heat Treating"

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    as has been said: temper is a must. 200°c minimum, usually 2x 2h, immediately while still warm. they often temper for 5h or more to relieve internal stressed depending on part geometry.

    that "hardness hump" is a good idea, but depends on austenising temp. and other history of the steel. there is also a toughness dip around that temp. (depending on how its measured). however tempering higer than 400°c gets rid of a large portion of the retained austenite, which might be the problem. it converts to untemperd martensite and expands during grinding which together with thermal effects is creating the cracks.

    also i would try holding at 1020°c for just 30 min to get less retained austenite and prevent grain growth.

    most steel manuf. have info on heat treat d2 online.

    Need Help Grinding D-2 Steel

    edit: even without visible cracks or burn the surface can be damaged. a way to check would be to lightly etch with nital. darker/brownish areas are an indication of overheating.

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