performance from medium carbon

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toddhill

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Sep 28, 2004
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Right now my passion is to recycle used steel, transforming junk into something valuable. I know that it is best to use new steel for knifemaking and maybe I'll be there some day. So I made a couple of knives from an edger blade, which I assume is the same steel as a mower blade. This isn't the highest in carbon. I clayed the blades and double quenched them in cool brine. I am not going to temper them because I figure they're about as hard now as a regular high carbon blade. I was wondering if my thoughts are on target. I edge tested one of the blades and the edge seemed very hard but didn't crack or chip. Could these blades be comparable in performance to a high carbon steel knife?
 
Tod, try to break one in a vise. If there is any carbon content At all, I think you will find them very brittle.
Many of the makers disagree with my feeling that recycling is a moral obligation. There is a great deal of validity to their argument because they make knives to sell to others.
I have to agree from that standpoint. OTOH for makers such as myself that make fro myself and friends, I like to recycle.

Now on to your question. There is too much emphasis put on hardness (IMHO).
Most of the older knives that we cherish as great performers, were on the soft side by todays standards. They cut like mad, needed to be resharpened often but were easy to touch up in the field with only a stone.

Use lower carbon if you want but don't try to make it something it is not. Temper it and accept it's limitations as well as it's virtues.

If you have to have a super steel, get some D2 planer blades and if you ever get it made into a knife, it will hold an edge quite well. You will need an industrial grinder to resharpen but it is hard....and free....and a PITA to work.
 
Thanks Peter, I appreciate your perspective and advice. Glad to hear from others using recycled steel. I'm not interested in getting a super hard edge that takes forever to sharpen. I want that perfect balance--something that will hold an edge for a decent while, and is fairly easy to resharpen. You don't think the edge test is a sufficient indicator of brittleness?
 
If it has more then around .6 % carbon it'll get as hard as high carbon but it's edge holding won't be as good. If it has over around .4% carbon it'll get pretty hard. Medium carbon steels as well suited to things like machetes, axes, tomahawks, swords, and other larger impact cutting tools that don't require as fine of an edge. In exchange for less edge holding you get more toughness.

You should still temper them however, the edge test depends alot on blade geometry besides checking the hardness/brittleness of the edge, a fat convex will perform amazingly at the edge test even if it'll break like glass in a vice. Even if you want to keep maximum hardness, just temper at a low temperature (often called a snap temper), to relieve the stress without effecting hardness much, around 325-350 would probably be a good starting point for experimenting.
 
When you drop one of those blades and it shatters (and it will) you will have what we refer to as a TEMPER TANTRUM! All hardened steel should be tempered.About 350 for your steel should work.A break test is a good idea on a sample piece of any recycled steel,before making it into a knife.Heat it to non-mag,quench it in oil (I don't like brine).Put it in a vise and break it.Look for fine grain.If it just bends,or has a coarse grain,save it for guards if you must recycle it,but not for blades.
 
Thanks for all the comments and advice. I had already clayed and quenched one of the blades in water. When I pulled it out and removed the clay there was a marked distinction between the two areas, but after etching it didn't show any hardening line. I may have etched it too long. I etched it in a 1:2.5 solution of ferric chloride for an hour. I tempered the blade for an hour at 375. I did some cutting with it, but it didn't hold an edge like I thought it should. That's when I figured since it was medium carbon, I needed to harden it more dramatically, hence the brine. After claying and double quenching in brine, the exposed edge has a mottled appearance that is in the steel. It looks seriously stressed. One of the blades took a slight curve. Since the upper half of the blade was clayed, I figured it would still be soft and might bend. I put the blade on the anvil and gently hammered on it. It straightened right out. There is a very slight wave on the edge but it wouldn't come out with hammering, indicating its hardness, but I'm pretty sure I can sand it out. The edge is quite thin and when I did an edge test, it flexed but didn't chip. Based on all this, I didn't think I should temper them. Todd
 
While you might get away with not tempering steels with lower carbon content [ .40% ] since it's a different type of martensite, it is always the best practice to temper and be sure.
 
Brine is slower than water.Oil is slower than brine.Make sure the steel gets to critical temperature before quenching it,too.Many poor hardenings are due to not being hot enough at quench.But ,don't overheat or you will burn the steel.
 
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