I’ve just spent a very enjoyable and instructive day with my friend Dave (who also happens to lay hedges) learning about the principles of heat treatment of tool steel. For those of you who have a sensitive disposition to non-wood posts – click away now!
I need hook tools for turning bowls. These can be bought, but sooner or later you need to make your own, and to do that you need initiation into the black art (or is that craft, or magic?)
Here are a few pictures of what we got up to in my workshop.
First we built a hearth or forge:
This is version 1. It’s built from firebricks which reflect the heat of the butane burner back onto the work without absorbing much, even better if the bricks are bright & clean (i.e. not partly blackened by having been part of my hand warmer over Winter!) as light colours reflect the heat better.
Then Dave showed me how to test the hardness of materials by a) filing and b) hitting with a hammer, which test a) hardness & b) brittleness. The silver steel Dave brought could very easily be filed and b) when hit with a hammer bent.
The we hardened it. This involves heating up to cherry red and then quenching (cooling quickly) in oil or water. Re-testing proved the steel was now a) almost impossible to file & b) broke when hammered so it was now brittle. This gives the hardness you might expect from a file for instance, very hard but very brittle. Not the right stuff for a wood turning tool, what is needed is hard and strong, i.e. flexible so that shocks will not easily break it. So the next process is tempering. In this case heating gently and observing the colour changes caused by oxidisation. First the work has to be polished so the colours can be more easily seen:
We sanded off the oil/oxide mess and then stropped it on my leather strop. Heated it to a purple/blue, quenched and tested. Now a) the file skidded across the surface of the treated section, but b) did not shatter or bend when hit with a hammer. Hey presto! Soft steel turned into the stuff that tools are made on.
Having explored the theory we put it into practice. We made a punch. Dave had brought along some thicker silver steel and ground a point on the end. The exercise was to harden this so it would mark steel (or in our case cast iron) and retain its sharp point. Same as above we heated it to cherry red:
Not quite there yet.
And quenched in oil (used engine oil in fact):
Then after checking it really was hard (file just skids) we tempered it.
Next up a bit of forging. I have three bill hooks I’m re-hafting for a client. Two are done, Yorkshire pattern sensible riveted jobs (see last post below). The last one is a Kentish pattern I believe, and has a short, tanged handle. This means the end of the blade is made into a soft long tail that goes right through the wooden handle and is then fastened in by peening over the end of the tang, over a washer, simple but effective. But first we rebuilt the hearth:
Now the torch is held fast in an improvised wooden vice so there’s no need to hold it.
Then we heated up the tang for forging. It needed to be a bit longer so it needed to be drawn out.
We also tempered a chisel as a cold chisel to make a 45 gallon oil drum into a small demonstration charcoal kiln:
And tempered it:
Then we had a go at the Bohemian bearded axe I bought on eBay a while ago. It has a soft edge that turns over (i.e, doesn’t hold its sharp edge) even when working green wood. The question was did the steel contain enough carbon to be hardened, or was it a lemon that I’d purchased that looked good, but would never work as an axe?
Here we go:
It took quite a while to get about half the edge cherry red. We’d reached the limit of temperature we could get with the torch. But on quenching (in water this time):
It turned out that we had a hard edge. All I need to do now is get a charcoal forge working for heating up big pieces like axe heads, and for forging. Watch this space.
Oh yes, and I finally levelled the chestnut bench before I set off for the woods, photo to come before I deliver it on Thursday.