Trying to harden air hardening steel with a torch
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  1. #1
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    Default Trying to harden air hardening steel with a torch

    So I'm trying to make these small steel bushings that are tapered inside. They are 3/4"OD, 10mm ID by .625" long, pretty small.

    These are an extrusion nozzle for cold pressing seeds of various kinds, from an oil expeller.

    I made an initial set from W1, and of course, the customer is looking for longer life nozzles.

    Thought I would try A2 drill rod.

    Problem I'm having is that I am doing the heat treat 'on the bench' with a flamex torch. I'm guessing at the heat, but am going well past the orange color I would use for a W1. I cool the part with an air blast. Distortion is minimal. But, the parts don't seem 'file hard' like W1 would be (in a water quench). I try to keep the torch on it the whole duration to minimize exposure to oxygen. I would expect a similar amount of decarb as I would in treating W1 which exhibits very little, if any, soft skin.

    My question, which I can't really seem to find an answer to, is this: does the crystal transformation take place instantly upon reaching the 1900 degree temperature, or does it truly take a few minutes at a soaking temperature? I can probably have the whole operation done in a couple of minutes per nozzle, I'm just wondering if the material has not really transformed in that short of a time interval, hence the hardening is diminished?

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    Does A2 have superior wear resistance than W1? I would think that going to a harder, more wear-resistant steel (F2 or D3, if you can find them) would give you better results. At normal draws, W1 and A2 are about the same hardness (61/62 C).

    In any case, I think that its recommended that A2 be heat treated in neutral salt baths or controlled atmospheric furnaces. In fact, most of the wear-resistant steels (F2, D3) recommend the same thing.

    If you are doing torch HT, the w1 is less susceptible to decarburization than the more wear-resistant steels.

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    IME, A-2 has to soak for a while. its not what i'd go to for flame hardening . D-2 should be better than W-1 for wear,
    but like A-2, i wouldn't try to torch harden it . O-1 is my choice for flame hardening. a lot less picky about
    soaking time , in the oil quench you'll likely get better results.

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    I believe the time at temp is critical for success. I did some benchtop torch heating of A2 years ago. And the results were very poor with comically weak parts. I wasn't getting and "file" hardness with air cooling so i quenched in oil. They were nice and hard with a file test, but boy did they suck for strength.
    Multiple fails in my method, so I'd just wait a day and pay $80 to get em proper oven-cycled.

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    I believe the air hardening steels have a higher transformation temperature too, so I wonder if you're not getting them into the necessary heat range. My sheet (old) calls for cooling from 1800 F.

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    Stay with the W steel and have them nitrided.

    Tom

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    A2 definitely needs a minimum time at temperature, half an hour at 1800F. I've attempted what you are trying to do, but never with much success, I couldn't keep the heat even for that long. Also, A2 w/o tool wrap in raw atmosphere is bad to decarb/scale. Do you have a friend with an induction hardener? For small pieces those work like a charm.

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    Trying to perform metallurgical chemistry with a blow torch is not a good idea.

    A2 has to be slowly brought up to the forging temperature 1400F and soaked for about 20 minutes, then slowly brought to the hardening temperature of 1800F and then soaked again for about 20 minutes. Then it air cools.

    It needs to be fluxed and wrapped to prevent oxidation.

    You need a heat treat furnace to do it properly.

    Also, A6 is probably a better choice anyway for wear resistance because it has that high cro-moly content.

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    Ifd you don't have it and are going to do a lot of hardening, get Bill Bryson's book Heat Treatment, Selection, and Application fof Tool Steels.

    Or sign up with Carpenter steels and read their online literature.

    A torch is not going to be a useful method to make good parts, beyond O1 & W1/W2 steels for which a torch is actually pretty decent. Yes. most hardenable steels especially airhardening steels require fairly long soaks and may have several levels of soaking on the way up to transformation temp.

    Once a furnace is needed and all the paraphernalia, A2 does not really make much sense as an improvement over 01. It is better, but at that point for the same effort, there are a bunch of steels that are "way" better. There are more factors than hardness that determine wear resistance. (to that point, W steels can be made and used at higher Rc hardness than A2)

    Here's a chart for hardening A2. For some steels (beyond A2)there are a range of narrowly defined temperatures at the transformation point that will significantly improve some characteristics at the expense of others including your choice to optimize hardness vs toughness (charpy values) vs wear resistance This is separate from the tempering temperatures. I don't think you will have this precise a discrimination capabilty with a torch and eyeballed color comparison.

    https://www.crucible.com/eselector/p.../airkoolt.html

    Scroll down this page to determining steels for wear resistance. Notice that A2 & D2 barely register on the chart, compared to steels optimized for that characteristic.

    https://www.crucible.com/eselector/g...eralpart1.html

    This is a steel I like a lot, though have not used it much recently. Not really ideal for your app, it is for mine and I am somewhat familiar with it. Notice on this one that in the hardening charts you can tailor the characteristics of the finished product by where (temperature) you choose to harden it from, in relatively small increments.

    https://www.crucible.com/eselector/p...e/cpm4hct.html

    Small difference in A2 vs O1 & other steel comparisons.

    https://www.crucible.com/eselector/p...&dchemtbl.html

    smt

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    I just made a casual piece yesterday from torch-heated A2. I got it up to non-magnetic and only held it for 30 secs or so. It hardened only to Rc 30 as tested on my calibrated hardness tester. For this part, that was ok and I was not surprised. Had I done the very same thing to O1, I. Would have expected 58 to 60 based on a large number (100's) of examples.

    I also like Bryson's book. But, it is also true that O1 is said to require moderately long soaks, but experience shows shorter times work pretty well for it but evidently not so well for A2. That was the first time I torch hardened A2. I have always furnace hardened with prescribed soak times and gotten excellent and predictable hardening.

    Denis

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    I don't think of CPM as falling into the back yards heat treating. Final soak of 2100-2200*F is beyond most non-professional heat treating as SS309 foil really doesn't go that high. Need either a atmosphere or Vacuum furnace.

    Tom

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    1800F is the upper limit for hardening A2. I have successfully torch hardened small parts to the low 60's. You should shoot more for 1775F. In a dimly lit room it will be the point where the color appears to transition from orange to yellow. If you can't decide whether it's orange or yellow, that's just about right. Hold it there for a minute or two if you can. Temper in an oven at 375-400F as soon as you can hold it in your hand to get a part that's really hard. Practice on a few scraps of similar size if you can.

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    I don't think of CPM as falling into the back yards heat treating. Final soak of 2100-2200*F is beyond most non-professional heat treating as SS309 foil really doesn't go that high. Need either a atmosphere or Vacuum furnace.
    My electric furnace was from a lab chip fab and has high temp wire elements; I use 309 foil which is rated for up to 2240F°. (2250F° sometimes spec'd depending on supplier) You may be thinking of standard 321 which is only good for 2000F°

    Tool Wrap - Precision Brand Products, Inc.

    CPM M4 fusion welded to mild steel shank.



    I was looking for toughness to avoid chippy edges, so quenched from 2100-2150F°. This was conveniently within the foil range and worked fine.

    Weld, immediately anneal= overnight ramp down 25°/hr, then heat treat.

    smt

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    Thanks for all the reports, especially those anecdotal efforts of those that tried the same thing and found it didn't work I will send these out for heat treatment (it is a big deal to do this up in my neck of the woods, they have to go for a 500 mile trip and take a week or two).

    It makes sense that if something is going to harden in air, that it has to have a pretty slow transformation rate when cooling, so likely the same when warming up. But it is hard to find this out in so many words.....kinda like when I was trying to figure out what sex was about 50 years ago....

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    something is going to harden in air... likely the same when warming up....kinda like when I was trying to figure out what sex was about 50 years ago....
    Of course! Metal processing = sex
    Only on a machinist site!


    smt

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    Default Not so fast! (But hotter)

    Hu,

    Don't give up on torch hardening A2.

    I am guilty of reporting poor results---only Rc 28-30---when torch hardening A2. But, based on Jim Lederer's success and suggestion that higher heat may be the answer, I went out to the shop this morning and cut a 1/4" thick coupon from 3/4" dia A2. I then heated to a higher, by eye, temp well past cherry red and into a bright yellow and then let it air quench. I then took three hardness readings on my Wilson tester. They averaged 58 with a three point spread hardest to softest.

    So, thank you, Jim, for your helpful remarks. Now I know that the key to torch hardening A2 is just getting it hotter than I would O1. Truth be told, I am a little embarrassed I did not think of that.... Could be that going just a bit hotter would have resulted in 60 to 62, but for my purposes, 58 to 60 is plenty for casually made parts.

    Denis

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    Quote Originally Posted by dgfoster View Post
    Hu,

    Don't give up on torch hardening A2.

    I am guilty of reporting poor results---only Rc 28-30---when torch hardening A2. But, based on Jim Lederer's success and suggestion that higher heat may be the answer, I went out to the shop this morning and cut a 1/4" thick coupon from 3/4" dia A2. I then heated to a higher, by eye, temp well past cherry red and into a bright yellow and then let it air quench. I then took three hardness readings on my Wilson tester. They averaged 58 with a three point spread hardest to softest.

    So, thank you, Jim, for your helpful remarks. Now I know that the key to torch hardening A2 is just getting it hotter than I would O1. Truth be told, I am a little embarrassed I did not think of that.... Could be that going just a bit hotter would have resulted in 60 to 62, but for my purposes, 58 to 60 is plenty for casually made parts.

    Denis
    Does a file skate over the edge of the surface or does it bite a little? That is what I found. It's got some hardness, but W1 water quenched is harder. For the short duration of heating, I don't think I'd get .002" of decarb especially since keeping the part in a slightly carburizing flame is supposed to protect it (somewhat).

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    Quote Originally Posted by HuFlungDung View Post
    Does a file skate over the edge of the surface or does it bite a little? That is what I found. It's got some hardness, but W1 water quenched is harder. For the short duration of heating, I don't think I'd get .002" of decarb especially since keeping the part in a slightly carburizing flame is supposed to protect it (somewhat).
    The good sharp file I used (right near the tang where I would not usually file) very definitely skates. 58 to sixty is pretty dang hard being close to the upper limit for most commonly used tool steels.

    Decarb is not really an issue for the brief time this steel is exposed to heat . If that were a concern, a proprietary protective coating can be used. For something that has to be free of any surface disturbance from the flame I use Brownell's non-scaling compound. A polished surface comes out an almost shiny slightly greyish matte if you use this and buffs right back to shiny with just a little 1000 grit si-carbide paper, Cratex, etc.

    Denis

    It is also possible to do some very rough testing of hardness using a hard center punch and a hammer. If you raise the hammer, say 8 inches, and let it fall onto the punch and do this on some soft 1018(Rc 8?), a hammer face (Rc 30 or so) and a ground parallel (Rc 60 even on my "import" ones) you will be able to observe obvious differences in indents.

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    There's another interesting aspect to "wear". You mention the application which is something of a chemical process. Many very abrasion-wear resistant substrates do not play well with certain chemical exposures particularly when the heat and concentration of something like extrusion occurs locally.

    My metanoia on that subject was as a late teenager when i learned that for industrial apps, carbide was not as good as HSS for woodworking pine (at the time). The reason was that the acids in the pine quickly eroded the binders under the heat of contact, and dulled carbide woodworking knives faster than M2. You could run a few thousand feet on carbide - and miles with HSS.

    I suspect that given what you imply will be a higher time & cost to properly HT A2; that the slight increase in benefits will not be economically efficient. Might be better to just make a pile of W1 nozzles cheaply and consider them consumables.

    My other approach to problems like this that include small parts is to just hard turn (if possible) &/or grind from the solid. A nozzle may be tricky, But I'd strongly consider just processing it from a round of M42 toolbit steel if you think abrasion is the issue. Or M2 as an easier first try if chemical issues might be involved. If you have EDM, even plunge/tap buster edm, it might be relatively convenient to process the internal features of a nozzle in some hard materials including carbide, and grind the OD.

    smt

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    Some thoughts.
    A red heat is probably around 1400-1500*F, depending on when you last had your eyes calibrated for color. For A2 the color should be orange to yellow. Grind .01-.02 from a surface before measuring. Make sure the parts set flat on the hardness tester. Get a copy of Bryson. If the part is uniform, quench in oil for max hardness.

    Tom


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