Hardening / Firing iron – problem
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I had a problem when heating some O1 tool steel to harden it, so the iron did not reach full temperature. I quenched the iron and let it cool. Before quenching, I found that only the 1/4″ near the edge was no longer strongly affected by a magnet. So, I was close when I had to abandon, but not quite all the way there.
When I return to this, can I just go right back to firing it to full temperature and ignore what’s been sone so far? I don’t need either anneal it or soften it first, do I?
11 July 2017 at 8:03 pm #313656I’m not an expert by any means but from what i know after a failed quench you want to normalise it a couple times to releave stress and prevent cracking.
Normalising means heating it up and letting it slowly cool to room temperature and repeat that 2 or 3 times. Propably best to google around a bit.I went ahead and fired this again before I saw your reply, just going directly to hardening the iron (cherry red, oil quench). I had flattened the back prior to hardening and then again before re-firing, and before tempering just to see if there was any warping. It was minimal. After tempering, again little was needed to flatten it.
So, I’m going to give this a whirl, but I’m still interested in replies, just in case I run into this again in the future.
I’m not knowledgeable enough to speak to all the risks of re-hardening un-tempered steel. Surely there was some elevated risk due to the potential buildup of stresses from the first quenching, and from what I’ve read, those stresses could result in weakness if the steel was placed under some load.
So you probably put the steel at risk when you re-flattened the back prior to your 2nd hardening procedure, but if it didn’t crack or otherwise break / warp, you “got away with it”. Also, if you went from room temp to blasting with a torch, instead of a proper pre-heat, the thermal shock can also cause the steel to crack or warp if there are pre-existing stresses, but again, you appear to have gotten away with it.
Having now re-hardened, properly quenched and finally properly tempered the steel, you’ve relieved any stresses, and so you should be gtg.
Did you hit it with a file, or spark test, or? What were the results?
The only spark test I know of is to tell carbon steel from high speed. In that regard it looks like carbon steel rather than HSS. Of course, I purchased carbon steel, so no surprise there.
The iron tempered to a pale yellow color. Tempering was at 350F for 60 minutes per the manufacturer’s table. The oven temperature isn’t calibrated, though, and I’d guess the color was a bit heavier than intended, so it may be tempered a little on the soft side, but not enough to concern me.
I fired the bottom 1/2 of the iron to cherry red with a torch (MAPP) as I was taught. A file skates off the bottom of the iron, but bites into the top portion, as expected.
I’m not aware of preheating prior to hardening. A plane maker taught me to harden O1 by going directly into the MAPP flame, getting to color in the middle of the blade, walking that down to the edge to minimize the time the edge is at temperature, then quenching. Then, go directly to tempering temperature in an oven and allow to air cool. The only twist here was the original screw up, which happened because I decided to try the barbecue method, which I’ll never do again. What pre-heating is supposed to be done, and how?
I suspect there are many ways to skin this cat…if you were taught by a planemaker you probably have better knowledge (or at least more appropriate knowledge to the topic) than I do — I was taught by a machinist who probably wouldn’t know a hollow from a round. That might be very important — what’s appropriate for the machinist’s needs from tool steels might not be important for the planemaker, and in this forum we’re interested in hardening tool steel just for woodworking, so any specific knowledge in that area is better, imo.
Anyways, I was told something like this:
Preheating, or slow heating, of tool steels provides two important benefits. First, most tool steels are sensitive to thermal shock. A sudden increase in temperature of 1500/2000°F may cause tool steels to crack. Second, tool steels undergo a change in density or volume when they transform from the as-supplied annealed microstructure to the high temperature structure, austenite. If this volume change occurs non-uniformly, it can cause unnecessary distortion of tools, especially where differences in section cause some parts of a tool to transform before other parts have reached the required temperature. Tool steels should be preheated to just below this critical transformation temperature, and then held long enough to allow the full cross-section to reach a uniform temperature. Once the entire part is equalized, further heating to the austenitizing temperature will allow the material to transform more uniformly causing less distortion to occur.
The “distortion” is both in the visible form — warping — and the invisible — sub-optimal crystalline structure. But again, this might be a machinist-level concern, where you’re making some part that needs to survive incredible stresses for all-day use in applications where extremely expensive machinery is at stake over years and years. For a (e.g.) molding plane iron, it might be below the threshold of perception for such an application, therefore not a concern.
Also, machinists might be called upon to make a 16″ thick tool steel part — night-and-day difference between that and planemakers, who probably have never seen a piece of tool steel thicker than an 1/8″ or so. So concerns about pre-heating and soaking and such might (I don’t know) simply not be significant in the planemakers world.
This is why I say your knowledge, which is entirely specific to the task, is likely better.
For completeness, I’ll also add that I was taught to go straight from the quench immediately into the tempering oven, look for color afterwards, and for most grades of tool steel, go into subsequent tempers. I have no idea if that would be appropriate for plane irons. Bottom line, the only way to know is to destructively test at least some of the steel after the process is complete, and look at the structure directly, but even then, there is the question of fitness for the purpose. Let’s suppose the plane iron for my panel-raiser was sub-optimally hardened…so what? Does that mean it won’t work perfectly for several lifetimes? To my knowledge, there is no science on this topic, so we can’t claim what is “necessary” or “required”, and a planemaker, who has a great deal of (admittedly anecdotal) evidence from first-hand experience, might well be the best judge of what’s the “best way”.
But it sounds as if you’re getting the desired results from your hardening and tempering process, so I just posted since you stated that you were still interested in replies. If that has changed, please let me know and I’ll stop replying.
@etmo Thanks for the details. Very helpful. I’ve yet to meet a machinist who was not exceptionally smart and deeply knowledgable of the trade. They seem not just to know machines, but also know metallurgy and a gaggle of other things. So, my bet is that what you were taught is dead on. It agrees with what I was taught, but preheating was never mentioned to me. You probably nailed it, though: If you’re heating a 1/8″ thick steel just an inch wide and an inch long, that’s a different kettle of fish vs. a big hunk of metal. Also, remember how I was taught to bring the center band of the blade to color, then walk the color to the cutting edge? With such a small amount of metal, I’ll bet this makes the entire iron reach just below Tc especially since you don’t just fire one spot on the iron, but walk the heat around. Finally, two torches are used, one on each side. Bottom line- I’ll bet all the things you were taught are probably happening here, but now I understand better. Many thanks.
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