Hardening A2
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Thread: Hardening A2

  1. #1
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    I have a simple type of chisel I made out of some scrap A2 tool steel. I would like to know what would be the easiest way for me to "shop Harden" this tool? It is not a critical part, not worth sending off to heat treat.
    I know this is a "air hardening" tool steel, but don't quite understand the best and/or easiest way to do this with a oxy/acte torch.

    Thanks.

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    A2 takes some temp before you lay it down on the bench in still air. Something like 1750° F.

    Here is a little info.

    http://www.onlinemetals.com/alloycat.cfm?alloy=a2

    John

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    I have heat treated A-2 on a weekly basis for about 10 years. Although I had a furnace in which to do so. Your "chisel" will only get as hot as the torch, and I'm not sure what that temp is. As John stated, the temp needs to be 1725-1775 degrees F, in order to achieve full hardness.

    In your case, just heat it till it glows, and leave it cool at room temp. Should get pretty hard. You may have some scale as a result of not wrapping in heat treat foil, but it should easily sand or wire brush off.

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    That chisel will need to be tempered back, or it may shatter when struck..

    When I was a kid, I remember a neighbor that made some wood-splitting wedges out of D-2. He had left them at the same hardness as die parts. Each wedge shattered the first time he struck them. It was a miracle that no one was injured by the flying shrapnel..

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    And tempering temp can be as low as 350-400 degrees. Your stove could accomplish that. Leave it in there for a little over an hour, and shut it off. Take it out when oven and chisel have cooled.

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    Thanks for the info, kinda what I figured.

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    1750 degF is right for hardness, I recall the best temper for use as a tool for me was about 500 degF, gave reasonable hardness and incredible toughness.

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    Here is a rather lengthy discussion by Vance Burns I had in my files. Sorry but I don't have the original link. My apologies to Mr. Burns for posting this before obtaining his permission. I believe he is a member of the Home Metal Shop Club of Houston, Texas.

    Metallurgy Part 2 Practical Metallurgy for the Home Machinist
    By Vance Burns - HMSC Member
    A2 is an air-hardening steel providing justifiable cost benefits - nominal sizes require no quenching, thus avoiding thermal shock, and low quenching/hardening demands deliver superior results for beginners. In reality, the quenching process is still present, and the quenchant is ambient air. Within industry, liquid submersion of A2 is practiced; quenching requirements are influenced by mass - you will not quench your A2 bowling ball in still air. Your metals manufacturer will always shave the exact procedure available for a given material, contact them by phone or on the web. On average, we will heat a piece of A2 round, 2 x.75 in. to about 1400/1450F, the ramp up taking about 30 minutes. Then pour on the coal and bring it up to 1750F, soak for 20 minutes, then set it aside in still air to cool. Leave the piece alone and temper the operator with an adult beverage. You will see a hardness which will skate a file, about 64 Rockwell. Of course there is under and over heating questions, and while not impossible to ruin a piece, most air hardening steels are tolerant, but be safe, and be careful. An ounce of prevention will always be cheaper than tool steel. Look at the graphic at the left. optimally the sample must be below 1350F in under 6.5 minutes - depending on your mass, this will probably happen without intervention. During this time you can tweak the dimensions for up to 15 minutes, and then cool it off under an hour. This piece will be incredibly hard, yet very brittle. If your application justifies, and the cross sections permit, you might not temper your piece, or you might rethink and go with shock resistant steel, the air hard S series. Most samples will need to be softened (tempered) slightly. Tempering to 59 Rockwell requires about two hours at 600 degrees. Let it cool, then re-temper. Setting overnight in the deep-freeze, between temperings, does have some benefits. Some experimentation has shown the home deep-freeze can add demonstrable toughness at higher Rockwell. This is most evident in very thin cross sections, i.e. blades and cutters. A liquid nitrogen soak will provide measurable results, but until they start refilling liquid nitrogen dewars at Kroger, I’ll settle for the fridge. One item to note is machining hardenable steels; any of them will heat to hardenable temperatures under an aggressive cut. With even low mass, they will self quench, forming nuisance hard spots. Air hardening steels have greater potential, and require complete concentration.

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    A2 is a secondary hardening steel. If you look at the tempering temp/hardness curve, you'll see the hardness decreases with increasing temperature up to a point, and then begins to increase again. My HT info is at the shop, and I'm not, but IIRC the hardness for a 900* temper will be about the same as for a 500* temper. The difference is in toughness. Any secondary hardening steel will be a lot tougher if its tempered into the secondary hardening range than it will be if tempered at the lower temp which produces the same hardness.

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    I have a rebar roller I made out of a-2. Its 2" thick and 5"dia with a 2" hole in center. It has 38 teeth to grip the rebar for bending. I wand to harden it but All I have is a home made foundry furnace which uses propane and can reach 1900F ( copper melting point) How would you recommend I proceed.


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