A2 air hardening technique?
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  1. #1
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    Over the weekend I fabricated a nice turning tool for my wood lathe. I bought a 1/4" x 2" x 7" piece of A2 tool steel. I milled a flat on some 1/2" mild steel rod (with my little Atlas shaper), and silver soldered the rod to the A2.

    I have ground the contour that I want and now I need to harden the tool.

    I've not been able to find any good description of the proper way to air harden A2 steel, if you are a home shop guy like me. I think the thing to do is to harden the end of the tool where it needs hold an edge, which is good because I don't have a furnace anyway.

    Here is what I thought I might try. Heat the cutting edge with my oxy-acetylene torch to the point where a magnet no longer sticks to it. Then cool off with compressed air and a blow gun. Or, should I just let it cool on its own? Or, do I want to slow down the cooling rate by wrapping it in some sort of thermal foil?

    I've searched the archives and not found an answer to this particular question.

    Thanks much.

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    Hopefully someone with more knowledge than I will respond, but, I have made several tools from A2 by heating to the critical temperature,(when a magnet won't stick), and alowing the tool to cool down naturally. This won't be the best, but it works!
    goog luck
    hms50

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    http://www.crucibleservice.com/index.cfm
    http://www.crucibleservice.com/esele.../airkoolt.html
    this is the link to Crucible tool steel. All you will ever want to know about hardening. Your immediate problem is that to get A2 hot enough to harden you will destroy your silver solder joint. A gentle fan is all that should be used on air hardeneing steel in thin sections unless specifically stated otherwise on the steel spec sheet.

    [This message has been edited by WILLEO6709 (edited 05-27-2003).]

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    I am no expert but there is one problem you might encounter, you will probably damage your silver soldered joint with the heat treatment temperature required.

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    Didn't mean to be repetitive but WILLEO6709
    was replying almost at the same time as me.
    What is it they say about great minds !

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    Since I only need to treat the business end of the tool, which is 6" away from the silver solder'ed shank, I'm hoping I can keep the solder from melting by clamping some steel stock in front of it to serve as a heat sink.

    If it does have to be re-soldered, it will not be a big deal.

    Looking at the crucible site, they have quite a process for the A2 steel.
    <b>
    Hardening

    Preheat: 1100/1250F (595/675C), equalize, 1350/1450F (775/790C), equalize.

    High Heat: 1750/1800F (995/895C), 30/45 minutes at temperature.

    Quench: Air, positive pressure vacuum to 150F (65C).

    Temper: 400/1000F (205/540C), hold 2 hours at temperature, air cool. Temper twice.
    </b>

    It looks like they want me to preheat to 1200 deg, do something called "equalize" at 1400 deg, heat to 1800 deg and hold it there for 30 - 45 minutes(!), quench with positive pressure vacuum (sounds like an oxymoron to me), then temper by bringing up to a lower temp (400-1000 is quite a range) and air cool twice. phew!

    First, I don't have a way to measure temperature (except a magnet stops sticking, color change, etc) second, I can't do things like hold at 1800 degrees for a 1/2 hour!

    Can I just heat the bugger up to cherry red and cool it off with compressed air? It may not be optimal, but I just need it to hold a decent edge.

    [This message has been edited by Lynn Kasdorf (edited 05-27-2003).]

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    Unless you have access to a vacuumm furnace or one with a controlled atmospere the usual recomendation for air hardening tool steels is to wrap them in stainless steel bags with charcoal and paper wrapped arount the part. The charcoal and paper act as a sink to convert any of the Oxegen in the bag or that may leak in so as to help prevent decarb of the surface. Very small furnaces that run on 110 or 120v are available. Another option might be if you know somebody with a pottery kiln. They'll get way over the tenp needed for A or D series steels.

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    I use a lot of a2 and d2 at work and heat treat it by the book. At home for wood lathe tools I heat it to a bright cherry red with a torch and throw it out in the dirt till cool. Then polish it up and draw it back to a straw color. It works every time.

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    If you were close by to me, I'd say bring it over and I'd send it to the heat treaters with my tooling in the next day or two.
    Old Dogs method should work fine for your tool. I can't quite do my A2 and S7 tooling that way. A double draw is important on my stuff as is a certificate to CMA.
    Les

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    Lynn,

    The directions given on the Crucible Steel web site will give you the best results. Heat treating is a critical element in industry, maybe a punch or die could last thousands of more cycles if heat treated properly.

    For home use, you'll probably be very happy with the results you get using home shop methods. I would hold it at temperature for as long as you can before letting it air cool.

    It will be really important for you to temper the tool after hardening to keep it from being too brittle. You can probably do that in a toaster oven.

    George

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    Lynn-

    I used to harden A2 with a torch on occasion. (Still do O-1 with a torch) But now for A2 and higher alloy steels I use a heat treat furnace with a programmable controller and it makes a difference. But the torch works ok on A2 after a fashion for personal use tools. The grain will probably be coarser than if done under more controlled conditions, there may be stress and surface decarb. But it will get hard. Old dog mentioned how to go about it, with the exception I would hang it up to cool rather than throw in the dirt which would slow it down some.

    Use the biggest torch tip you have or a rosebud and make the mix rich (carburizing)to keep from burning the steel. You want a pretty good excess acetylene central cone and starting to feather. If you keep the steel pretty well enveloped after it starts to glow, the excess acetylene will help scavenge atmospheric oxygen trying to combine with the carbon in the hot steel.

    You need around 1750 (check the sheet on your particular steel) which is a bright light orange. It is hard to get and hold that temperature with a torch on a piece of any size. Just get it up there, and keep the torch playing until it's uniform about maybe 1"-2" back from the tip. then shut off the torch and hang it up to cool. As soon as you can hold the heated part moderately comfortably, get it in your kitchen oven at 400F, and leave it there for an hour. smt

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    Wow! great info here, folks.

    I just happen to have an old toaster oven that was rejected from the kitchen because it got too grungy, and I just KNEW I'd come up with a use for it in the shop.

    I guess my silver solder joint is gonna be toast after all this

    Seriously, i figured I'd clamp some big steel blocks just past the solder joint in hopes of drawing off most of the heat from the torch, but this seems pretty unlikely, if I'm supposed to keep the tip glowing as long as possible...

    Since the shank is already embedded in the nice black locust handle that I turned, i'll just unsolder the tool steel from the shank and do the heat treatment unencumbered.

    Once I get this figured out I have a bunch of tools I want to make. Fun stuff!

    BTW, I bought several 1/4" square tool blanks that I use in my shaper. These have "aaa" engraved in them, and they are apparently hardened already, as they hold an edge very well. Is this the case?

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    Lynn-

    with a torch, I would not hold it "too" long. Just at reaching the transition temp, moving the torch will chase shadows around in the steel, and it won't quite "hold" on color until enough heat is soaked up. as you move the torch, the temp will fall off ans so will the color. Then if you have enough heat to get there, it will stabilize, and soak up further excess btu's without increasing in color much. But there will be enough btu's beyond the minimum needed to reach the transition point that the temp won't drop behind it as the torch moves. You need to attain this point, hold for a minute or so to be sure, then shut down and let it cool. It the open air with a torch, any more will just continue to loose carbon from the steel.

    Yes, tool bits are already hardened. In fact a suggestion might be to consider making a shank to accept inserts that you cut off and grind out of high speed steel. Or silver solder on the hss, 1100+/- of the silver solder will not hurt the HSS for wood turning use. HSS is available from machinist supply houses in rectangular shapes and shortish lenghts 4" - 6" long, and in some interesting alloys besides M2. From woodworking tool suppliers (Charles G.G. Schmidt, e.g.) it is available in long bars (25" standard)of moulder steel and shaper tool bit stock usually in D2 and M2.

    Have fun and keep reporting back.
    smt

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    as for torch hardening, the critical temp of A2 is almost Yellow, a really bright orange, kinda like the Tennessee volunteers college colors."dull cherry red" = approximately 1200-1300. "bright cherry red" = approximately 1400-1500. Its hard to get a feel for colors unless you have worked around a heat treat oven..... zi'd probably keep the tip bright orange for about 2-3 minutes, then get it in front of a fan.

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    THE REQUIREMENT IS HOLD AT AUSTENITE TEMP. FOR ONE HOUR PER INCH OF MATERIAL, WHICH MEANS PRE HEAT IT TO 1250 THEN HEAT IT TO 1800 FOR 30 MIN. LET COOL DOWN (THIS IS THE LIFE AND DEATH OF THE PART)UNTIL YOU CAN TOUCH IT BUT IT IS STILL WARM LIKE 120 DEG. THEN YOU HAVE TO DRAW IT BACK AT 650 DEG. FOR 2 HOURS, COOL OFF AGAIN THE SAME WAY.THEN DRAW BACK AGAIN THE SAME WAY AS BEFORE, LET COOL THEN WALLAH SHINE IT UP AND DONE.
    JUST A LITTLE INFO.

  16. #16
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    On your next turning tool you may consider bandsawing a tang out of a larger piece of stock. Personally, I feel it would be a stronger, safer turning tool. If you study all handled tools; files,awls,gravers,screwdrivers,wood chisles, the metal portion of the tool includes the tang. To handle a tool, open a hole smaller than the tang, but the same shape ( small sqare tangs just go in round holes,like gravers or burens). Smaller tangs can be driven in cold, larger tangs can be heated a bit (not too hot, experience will tell) then inserted to burn a good seat then driven home. The skill involved is to get it straight , not too shallow or deep, and dont split it. Actually, I dont think your silver solder joint will flow, it's too far away. But be carefull using this tool, this is not how these tools are constructed, and there is a reason for that. Good luck, be safe, and have fun!

    While Im at it, I asked a guy to put a new handle on a large forging hammer for some heavy work I had to do, and as I watched from one of the many eyes in my head, I noticed he didn't know what he was doing. I don't want to insult anyones inteligence so if you allready know how, ignore the following. Shape the handle (carefully) to fit the contours of the lower taper (hammer heads have a double taper)making certain it is straight on all counts. Your looking for a very tight fit. You want to fit this so that once you drive the head on, it will be around a half inch from the shoulder. Next, band saw down, fore and aft a little past half way through the head. Now drive the head on by hitting the but end of the handle on the anvil, then trim flush with the head. Make up a pine wedge with a good taper on it (experience will tell) find your band sawing and drive that in as hard as you can, then trim that. Now make up one or two (depending on the size of the hammer) steel wedges, put some barbs on it, and drive them im crossways. Belt sand to finish. I use alot of hammers in my shop and some are big, some are quite small (1\2 oz.) many of them were aquired as just heads. A nice method for very small hammers, is to take a fairly thick wall piece of steel tube and grind a taper on the outside, dont band saw, just fit and drive. I hope this helps anyone thats interested, no one ever tought me this, I learned by studying many examples.


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