Reply To: Hardening / Firing iron – problem
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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.