Since beginning hand tool collecting 6 or 7 years ago, I have a few irons now that I suspect have been previously overheated while grinding and will not retain their edge. I was wondering if there were a practical way to restore these. I have little to no experience, a MAPP gas torch and a toaster oven.
The rudiments of hardness-tempering blades is basic, however to do it reliably and repeatedly requires some experience and knowledge. A big effort.
Having said that, most ‘modern’ irons for Stanley-type planes, stamped from a single piece, are available as spares, (new and SH), so that re-tempering is seldom cost effective.
If the culprit is softening due to over-heating on grinders, the good news is that the softened portion of iron is very often localised and seldom extends more than a fraction beyond the edge, usually confined to the corners. Beyond this lives good, hard steel.
A simple test is to run a good, sharp file along the suspected cutting edge; if it glides over without biting, the steel is good. If it bites and you can see file-marks, cold-grind the whole section carefully back to a point where the file does a gliding action.
The hard-tempered section of the blade usually extends up to a point close to the end of the screw-slot, so there should be plenty of meat on most modern-type blades.
Remember that old irons, usually from the 19th C, were thicker in section and as tool steel was expensive in those days, a thin fillet of relatively brittle, hard steel was forged onto a billet of softer iron to add strength. It is this thin back-layer that took the cutting edge, so keep this in mind when using the file: it WILL cut on the soft section – that’s normal – it’s the harder back that is important. The forged section of old blades is sometimes visible on the cleaned edge of the iron where the union of two steel types shows as a discreet line.
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