ReHardening Your Plane Iron – Paul Sellers’ Blog

I will start this blog by saying I am a full-time woodworker and that necessitates much part-time metalworking for several good reasons. This extension from my last blog is to bring you up to date on taking charge of to-soon-dulling plane irons be they new or old and described as ‘vintage’ on eBay to give them some kind ofold-is-always-better’ feeling in order to sell something that is actually way past its sell-by date and pretty useless. Though a few Stanley plane irons were laminated, 99.9% of the blades can be hardened from cutting edge to top tip so you can salvage old blades and make a variety of cutting irons and tools from them for peanuts if you so choose. This article is about salvaging a few years of use from a well-used plane iron, one that was never hardened enough, one that has lost its temper through overheating during grinding and things like that. Economically it is still worth doing even though a replacement blade might cost only £10 or so. I would say time wise rehardening took me no more than five minutes max. I have hardened my tool steels with all kinds of vegetable oil including olive oil with no issues.

There is no set distance from the cutting edge to where the steel will be softer in a plane iron from what I can tell. The men and women in the moment hardening tool edges and blades might be distracted and it is likely that he and she had never used the tools they make in their lives anyway . . . no matter who they work for on which continent. Don’t expect too much. I have met makers at high-profile companies who never strayed more than a few inches from one task on one spot on one part of the tool-making production line five days a week. It’s surprising what you can get people to do for the right money and the least responsibility, etc. But then there are circumstances where many just have no choice too — they gotta pay the bills, take care of family and so on. I have had those pockets of time in my life too.

On many a plane, the cutting edge gradually reached close to the elongated cutout ending in the hole of the cutting iron. I usually pensioned the plane irons off and installed a new one because the steel isn’t hardened all the way through and so returns to the sharpening stones came a little too often. When this happens it is, of course, easier to buy in new but there is a simple and quick enough alternative and that is to harden the blade right there and then. The pros are that you will most likely get an extension to your plane iron and that’s always a bonus. I suspect that most of you will not need to go through this as the basic plane iron will last most part-time woodworkers a lifetime. I have gone through at the very least 12 plane irons because `I have been a full-time, six-days-a-week user for almost 60 years to date. I doubt that there are many alive that have done the same if any. So why the article? There is a good chance you will come to that point in your plane and then again you most likely will buy a secondhand plane that is well used and the plane iron needs remedial action.

To know if the cutting edge is hard enough simply take a metal file to it. I say metal file because you might just take a diamond hone of some kind which will indeed cut any metal at any time, that’s not what we are looking for. The cutting iron of planes is first hardened and then tempered to detention the stresses in the steel after quenching. To harden the steel the blade is held in a flame hot enough to turn the steel to an even redness level that when plunged into oil or water (or with some steel alloys just the surrounding air) the steel will harden to the hardness of say a file. After that, the blade is usually tempered although `i have many plane irons I have made that are not tempered and they have never had an issue. As I said, tempering the blade relieves any stresses in the steel caused by the quenching after the initial heating. It’s a no-brainer really, we achieve this by simply placing the blade in a regular oven at 205C for an hour and a half and then do it a second time after its cooled back down to room temperature. I put a nut roast in at the same time so as not to waste the heat. The maxim that if a file can scratch your material, then your material is softer than the file and if a file cannot scratch your material, then your material is harder than the file works. The file did not scratch my newly hardened cutting iron when I was done.

To check for hardness a decent flat file will slide off the cutting edge when offered to the bevel or the side edge of the cutting iron. It is more likely that you have found yourself returning to the stones more frequently though, as happened in my case on this particular plane.

My setup is simple. A blow torch capable of heating the steel cutting iron to cherry red or a non-magnetic state: I just go for colour. It has always worked for me without fail. Also, the advantage of a blowtorch over a barbecue pit is localised heating where I need it. The locking grips reliably hold the blade securely throughout the heating and quenching process. In the sugar bowl, I used some old olive oil as it doesn’t usually flair up. The Bernzomatic TS 4000 torch is one of my best buys. It reaches the higher temperatures needed for hardening tool steel and has a handy instant on/off mechanism with no matches needed as it’s Petzo ignition.

Heat the steel as evenly as possible to get an even hardness. This is not ready just yet but I am only a minute or so away from my cherry red. Make sure you look for colour in a shaded area as in full or strong light you might well be taking your steel to an unnecessary higher heat level.

Plunge and hold for 15 or so seconds and then allow the rest of the blade to cool before handling. The plunging will only cool the plunged area and not the main body of steel which will burn you or wherever you place it if combustible.

Discolouration and surface flaking from hardening and quenching are normal. It wipes off readily with a cloth or steel wool ready for clean up and sharpening with abrasive.

The file point on the side of the cutting iron shows the point where the blade changes from unhardened steel to hardened. The file just glances off without cutting anything more than the blacker oxidation on the last 3/4″. Just what we want.

The slight distortion into a minuscule hollow is not worth removing because it will disappear with subsequent sharpenings and you can see that the polished face does reach the cutting edge.

The next step is to temper the steel in the oven for 90 minutes. A domestic oven works perfectly well for this with a temperature of around 200-205C (400F for USA)

Repeat when the first heating has dissipated to room temperature, so two sessions in the oven. . .

. . . and don’t forget to capitalise on the heat by cooking a nut roast or similar.

The file passes over the surface of the bevel without scoring it at all.

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