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Estimating A2 distortion during heat treat

snowman

Diamond
Joined
Jul 31, 2004
Location
Southeast Michigan
Estimating A2/M2 distortion during heat treat

I am machining two shafts, 2" diameter, 6" length. On each end of the shaft, it will be turned down to 1.5" diameter for 1" length. All corners will be rounded to reduce stress.

If the shafts go in to the furnace at 2" +/- .001, what kind of tolerances should I expect coming out? Both surfaces are bearing surfaces and the whole project will be turned between centers. Should I plan on having to grind in order to bring it back to my tolerances?

Also, the parts will be wrapped in heat treat foil. Do I have to take the parts out of the foil for the air quench, or is it better that they be left in to reduce surface oxidation?

Addendum...I just found that I can get a 24 lb drop of M2 for $50. Since I am always motivated by money, how much worse is M2 to work with? It is given as a 50% machineability in relation to W grade steels, but since I don't do a lot of tool steel work, I'm not overly sure what that means.
 
I don't think you'll be turning HT A-2. You can achieve about a 60Rc with that stuff. It's pretty stable in the oven. Grind to size. You can take it out of the foil when the visible red heat is gone. I use a Tempil stick to tell me the heat. I usually take it out far after the red heat is gone. Something like 4-500º. Don't let it get to room temp. You need to temper it for about two hours per inch thickness. When it reaches around 125-150º stick it in the tempering oven. Other and smarter ones will add their techniques.
 
I should add, I have an electric furnace with digital control that is fully capable of reaching all temperatures for both A2 and M2, as well as the tempering temperatures.
 
A2 set oven at 1750
M2 you will have to be 2000+
and the foil tends to deteriorate
Stick with the A2 if you can, less hassel
 
I'd stay away from the M2 unless you're very familiar with heat treatment of high speed steels. They're at least an order of magnitude more tricky to heat treat than normal tool steels, and, for shaft use I'd say the M2 lacks the necessary toughness if it is used at the normal HSS hardness levels.

Take a look at the hardness vs tempering temperature for A2 before you select your tempering temp. Hardness drops with increasing temperature and then begins to rise again before starting another drop. Tempering at the upper temperature will give you a tougher part at the same hardness if the hardness achievable at the higher temperature is sufficient for your use. Don't have my books handy, but IIRC it'll go to Rc56-58 at the upper tempering temperature which is (once again IIRC) around the 850*-950* F range.

If the shaft is going to run with needle bearings or roller bearings where the rolling elements run directly on the shaft (no inner race) then you'd definitely want to grind after HT, for both size and surface finish. If the bearings don't run directly on the shaft and you can get by with hardness in the Rc52 range, then you could use 4140 or 4150 with a couple tempering cycles in the 350*F range and still end up with a good combination of hardness and toughness.

Added: Laurentian makes a good point re the foil. I think 321 is standard and you have to use a foil made from 330-something stainless for wrapping HSS parts. I noticed a price a while back on the high temp foil, and while the standard stuff is far from cheap, the high temp stuff is on up there in heart attack range.
 
It's for a small rolling mill for pieces of silver/gold/copper/etc. Essentially I want hardened rollers, for no reason other than to protect the final polishing that I'll have to do to the shaft.

I can purchase an import mill for a couple hundred, but I'm not overly happy with the quality. The good mills jump up in price extremely quickly.

I think that even with the cost of getting surplus M2 dirt cheap (it's a piece that's 2 3/8" diameter about 20" long for $50), I'm still more comfortable with A2, just because of the lower temperatures, and less distortion during HT. I think that even with paying more for the raw material, I'll make the savings back in foil and time invested. My understanding is that M2 is a bit more difficult to grind, and since I don't do much grinding, I'm best going with the lower learning curve.

Plus, M2 you have to heat at a quick rate from it's critical temperature to it's hardening temperature, and I'm not sure the furnace I have is capable. I know I can HIT the temp, but the rate would be considerably slower.

As of right now, it's going to run on bronze bushings. Needle bearings would just unneccesarily increase the complexity.

Thanks for the advice...back to cleaning the garage!
 
I guess the original question still remains. Given A2, should I just finish turn it to the final diameter, or should I leave .010 to grind off? Or 0.005?

And where does one buy 2 1/8" A2 for a reasonable price? It seems that once you get above 2", the price goes up my orders of magnitude. I understand that the weight increases, but holy crap! I don't need ground drill rod, just annealed A2. (I really want to stay away from oil hardening steels too, the amount of oil I would need and the resultant fire is a little extreme for my residential shop...don't need the fire department showing up)
 
I guess the original question still remains. Given A2, should I just finish turn it to the final diameter, or should I leave .010 to grind off? Or 0.005?

And where does one buy 2 1/8" A2 for a reasonable price? It seems that once you get above 2", the price goes up my orders of magnitude. I understand that the weight increases, but holy crap! I don't need ground drill rod, just annealed A2. (I really want to stay away from oil hardening steels too, the amount of oil I would need and the resultant fire is a little extreme for my residential shop...don't need the fire department showing up)

Because these rolls are going to be highly polished, if you can grind them after heat treating, I would. If that is not practical, when you wrap them in foil, include some carbonous material (I use activated charcoal) to consume the small amount of oxygen present. Also prevents decarb.

Size change: according to my reference, "Heat Treatment, Selection, and Application of Tool Steels", Bryson, the dimensional change is minimum at a tempering temperature of 700F and is .00045"/inch. He doesn't say if this is shrinkage or increase.

He also recommends preheating at 1200F for 10-15 minutes for dimensional stability before austenizing. The quench should be ok in the foil, that's the way I have always done it. As stated earlier, it is important to begin the temper when the steel has cooled to 150F.

You may want to have the heat treating done professionally, the cost isn't all that great and it will come back exactly as you want. Cost around here is about $100.

MSC lists A2 and the cost difference between 2" and 2 1/4" is almost exactly the difference in the volume, 26%.

Tom
 
I would strongly encourage you to check Cincinnati Tool Steel. Not the fastest necessarily but I just bought twenty inches of 2.5" A2 decarb free at a very good price from them. I had shopped around.
Denis
 
Do you mind me asking what you paid?

I got a quote from Alro for 55" of 2" A2 (a drop) at approximately $155. Much better than I had found elsewhere.
 
You may want to have the heat treating done professionally, the cost isn't all that great and it will come back exactly as you want. Cost around here is about $100.

Cost is similar in this area, but since I have the equipment, I'd rather do it myself and save the money. I have a feeling I'll get the grinding farmed out since I've never actually used my toolpost grinder, my dad just picked it up at a flea market for me and I don't even know if the bearings are good.

What would be a fair price on the grinding? Grinding was always part of a whole quote when I used to get stuff made, never something that was quoted individually. Basically just the bearing surface and the 4" length on both between centers...don't need them to be precise, just cleaned up and concentric so they are easier to polish.
 
Even if your tool post grinder turns out to be good,here are a few tips: First,let it run for 1/2 hour before grinding. This gets the bearings warmed up and they will grind smoother.

Run your lathe FAST and backwards. That seems counter intuitive,but it is the best thing to do.

Cover the lathe with plastic. Cloth will let the dust right through. Carefully fold the plastic without spilling any dust AT ALL on your lathe. Then,wipe the lathe down anyway.

Don't forget to dress the tool post grinder's wheel first. I hope you have the diamond tip that comes with it. You usually clamp the diamond dresser in the lathe,and run the tool post grinder past it until the wheel is dressed. Often the diamond is clamped around a piece of steel bar clamped in the chuck.
 
Snowman,
Use what you have as long as it is a single point dresser and do not dress the wheel until the grinder is completely warmed up. Light passes with the dresser at the same setting until it no longer cuts.

Scott
 
Do you mind me asking what you paid?

I got a quote from Alro for 55" of 2" A2 (a drop) at approximately $155. Much better than I had found elsewhere.

I pulled out the packing slip and it was 72 dollars for 20" plus shipping. Your Alro price sounds very close in price per pound. Price from MSC or MCM if they had the size were much greater but they may have been quoting a finished surface bar (which I did not need) vs decarb-free which I bought.

I really like using A2. I too have used Bryson's book as a guide for successfully hardening and tempering it. Pretty easy stuff to machine and HT.

Denis
 
I had considered making a heavy duty rolling mill a while back. Using (senior moment) those 4"long hardened steel spacers from horizontal milling arbors. However,I turned one's end,and the hardening was only about 1/16" deep,which made me think it might cave in under heavy pressure.
 








 
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