Re-harden a plane iron

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Forum topic by Tim Dahn posted 01-20-2015 04:03 PM 1105 views 0 times favorited 7 replies Add to Favorites Watch
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Tim Dahn

1567 posts in 3593 days

01-20-2015 04:03 PM

Topic tags/keywords: resource question

I have a Sargent plane iron that is tapered, I was hoping to try it in a plane a due to it being thicker at the bevel end.

When I sharpened it today it felt a little soft “gummy” and I noticed the burr/wire edge was large, rough and just kept bending from one side to the other.

So my question is, Do you think the metal could it be re-hardened?

I may try to re-harden it but not sure if it would work.

Thanks for any input.

-- Good judgement comes from experience and experience comes from poor judgement.

7 replies so far

View ElChe's profile


630 posts in 1364 days

#1 posted 01-20-2015 05:06 PM

Metallurgy is your friend or perhaps your enemy. Without knowing the exact composition of the iron you may not be able to harden it appropriately. Do you oil v. water quench? What temperature to heat it to? Too many variables in my humble opinion. Take a look at Hock’s statements on his website about trying to harden plane irons. He does mention grinding the iron to see what type of spark it produces and then comparing it with a known iron to figure out the composition. Good luck.

-- Tom - Measure twice cut once. Then measure again. Curse. Fudge.

View jasondubose84's profile


2 posts in 1269 days

#2 posted 01-20-2015 05:36 PM

Hi Tim,
I’ve actually just been lurking till now, but wanted to chime in because I feel this is one of the only things I can contribute to right now!

I’m a metallurgist. Yes, you can re heat treat an old low alloy steel. but there are several tricks and things to consider. When heat treating a steel, you must heat it to a certain minimum temperature (austenitizing) for a while, then VERY quickly (often less than 2 seconds) quench it in oil or water. The result will be a very hard but brittle material that you will want to temper. Tempering will greatly increase the toughness of the blade without reducing the hardness much. Another thing to consider is you will almost certainly experience some distortion that you’ll have to lap out. But if your’e going to buy a new one anyway, why not give it a shot?!

As ElChe mentioned, it varies from alloy to alloy, but chances are you’ve got a low alloy steel at best. Austentizing occurs around 1600F for this type of steel, and should only be done in a furnace (no torches).

This can be done in oil or water. Water is faster, but you’ll need to agitate it to keep steam from insulating the surface. Oil will make your garage smell like a train exhaust. You need to go directly from the furnace to the water without hesitation.

a regular old knife temper is what you’re looking for. Higher tempering will result in softer (but tougher material). this can be done in your oven and should take an hour or two at most. maybe 300F will do.

slightly off topic but this is also a good time to mention that i’ve heard some people say hardening will occur when you grind your blade on a grinder then put it in water. That’s definitely not the case. A grinder will not get it hot enough to induce austenitization. but it likely will make it hot enough to temper it slightly more. food for thought!

View Tim Dahn's profile

Tim Dahn

1567 posts in 3593 days

#3 posted 01-20-2015 08:46 PM

Thanks, good info!

So if I am not sure what the alloy is, oil quench would be the safest (for the plane iron) way to go. I think I read that on Ron Hock’s blog.

-- Good judgement comes from experience and experience comes from poor judgement.

View stefang's profile


15881 posts in 3362 days

#4 posted 01-21-2015 01:29 PM

I tried hardening and tempering with limited success. I got some info on the new about determining the right heating temperature based on the color of the steel during heating. I probably didn’t do the tempering thoroughly enough and My results weren’t so great. However, it is a great skill to have if you can get it to work for you.

-- Mike, an American living in Norway.

View jasondubose84's profile


2 posts in 1269 days

#5 posted 01-21-2015 09:11 PM

That’s a safer bet, but you’re likely working with a lower alloyed material. lower alloyed materials have lower hardenability. Consequently, you need to quench even more quickly than you would, say an A-2 steel. If the oil quench doesn’t work, you can try it again with water or even brine.

after it’s cooled, run it to the pre-heated oven and you’re done!

View Redoak49's profile


3288 posts in 2016 days

#6 posted 01-21-2015 10:35 PM

Without knowing the steel grade, you are just guessing about heat treating. Quenching something too quickly or not tempering properly is asking for a piece of the steel to fly off like a bullet.

View Tim Dahn's profile

Tim Dahn

1567 posts in 3593 days

#7 posted 01-23-2015 10:01 AM

Well… I am going to try it, the iron was free so I have nothing to loose. After gathering up a few items I will probably wait for a nice warm afternoon this winter so I can do attempt this outside and not set anything on fire.

Thanks for the input everyone, I will let you know my results.

-- Good judgement comes from experience and experience comes from poor judgement.

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