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Heat treating d2 getting cracks

eqshop9689

Plastic
Joined
Aug 31, 2017
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
 
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?
 
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
 
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?
 
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?



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

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.

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@Winklershop any photos maybe of the surface cracking you are describing experiencing you'd like to post ?
 
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.
 
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"
 
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.
 
I've been asked to quote a job in D2, but have no experience of heat treatment, other than chisels in a forge !
The job is a round collet, 3.5" dia, 3/4" thick, with a bore of 0.375" Close tolerance. The piece is then cut into 4 segments, then heat treated. It's used to hold the tube on a hydraulic tube bender.
I'm good with turning, and I thought I'd ream the bore, then cut segments on a band saw.
I have access to an electric furnace, but no vacuum, or controlled atmosphere.
So, from what I've read:
- wrap in SS foil
- heat to 1400 f and soak for 1 hour or so.
- heat to 1800 f and soak for 1 hour.
- remove from oven, leave in foil to cool in air, with some movement, down to 150 f.
- temper - not clear on the best approach.
I'm thinking to clean the job up after heat treating, media blast, and call it good.
If I have to grind the segments after heat treatment it'll need a fixture plate made for the grinder.
I'll discuss this approach with customer beforehand, to make sure he's good.
What do I need for tempering please ?
Bob
 
Overland, I'd recommend not cutting the pre-HT piece completely apart. Leave tabs or about 0.05 uncut material in the slots where the pieces are to come apart. This will help prevent unwanted out-of-roundness developing during HT. You can then grind out the tabs or remaining material.
Your HT oven will not cool down quickly enough to be useful for tempering. You will need another oven, ideally pre-heated to 400F, 500F or whatever you decide on for temper. In my shop I use an old 24" wall oven racked below the HT oven. Don't use a cheap toaster oven, as they don't control heat at all well. If you have a fancy "true convection" oven in the break room, you might give that a try.
 
You don't HAVE to have a vacuum furnace to deal with oxidation (though it's nice). Putting parts in bags helps, but they will still scale due to the air trapped in the bag. Bag them with a little square or cardboard inside. As the temp rises, the cardboard combusts and burns up the O2. Parts come out of the bags with a lovely dull grey color and no scale.

I think the others are on track as far as tempering goes. Heat treating can be a funny business, but it helps to think of it as the initial soak is going to make the part as hard and brittle as can be. Tempering will bring the hardness back down to where you need it and help with cracks. IMO, a hardness tester is a necessary accessory for an oven. We've had parts that didn't anneal right for one reason or another, and you can re-anneal them before moving on to other operations like grinding, but you have to be able to measure how hard they come out, even if it's just a set of hardness files. It's the same as measuring your parts when they come out of the mill or lathe. You don't want to assume anything.

Also, while air hardened steels are pretty versatile, the reason they have A2, D2, W1, O1, etc. is because each were developed to work in different situations, depending on part geometry, usage, target hardness, etc. We used to use M1 for a part we make and were always having issues with it. Switching to D2 solved all of it and produced a longer lasting part.
 








 
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