O-1 vs W-1 (2025 Update)

I made some replacement pipe cutter wheels from EN24T (4340 a low alloy steel common in the UK), similar to 4140, and treated them like carbon steel for quench and temper and when tested they crumbled as still brittle. When I looked up the heat treatment for the alloy online, took awhile, the quench may have been similar but the tempering for what I was aiming at was up around 500C + and held for a longer period than basic carbon steel so the temper temperature for the alloy was some 300C

- 350C higher than a basic carbon steel for the same hardness. Does make me wonder when I watch FIF how they manage with unknown steels as the heat treatment requirements can vary widely depending on alloy composition.

So does that pretty much call the whole show "just a show?" Their "tests" subject the knives to some pretty harsh stuff - the Marines' KA-BAR replicas chopping away at a rifle barrel and still holding their edge (though one got a pretty good chunk busted out) - how could that and the other nasties they do work if the heat treat is all wrong?

Again, I wish they'd spend more time on the craft and less on the show. Some of these guys are really talented, but it gets lost in the high drama they are attempting to create.

You may have gotten enough of the picture from the other posts, and I don't want to add confusion, but maybe a different description will flesh it out.

"Water hardening" and "oil hardening" are just names that refer to the necessary quench rates of different steels. It doesn't mean that you have to quench it in water or oil. In fact, you may need to quench thick oil-hardening in water, and thin water-hardening in oil.

The terms refer to the *quench rate* required for a *typical thickness* of each steel, to convert it to the hard, martensitic phase. A thin piece of W-1, like many knife blades, will be quenched with adequate speed in oil. Preferably, you don't quench it any faster than necessary, but you have to quench it quickly enough to get a complete, or near-complete, conversion of the austenetic phase to a martensitic (hard) phase. Plain carbon steels (W-1) require a faster rate than oil-hardening alloys, which, in turn, require a faster rate than A-1 air-hardening. The higher-alloy, slower-quenching steels don't get any harder. W-1 will get as hard as the others. But it may not harden as deep, if it's a thick piece.

You want a complete conversion with *maximum safety*. That means you want to minimize the chance of cracking. The thicker steel is, the more likely it is to crack from an excessive quench rate, both from differential thermal shrinkage and from something I'll explain below. Once you've reached the necessary quench rate, quenching it faster won't make it any harder -- or not enough harder to be worth the risk.

The hardness of the finished piece depends on the percentage of the steel that was converted to maretnsite. We'll put tempering aside for now -- that confuses the picture a bit. But be aware that the different steel phases (ferrite, austenite, martensite and the mixtures, like pearlite) have different densities. When you convert ferrite to austenite by heating it above its critical temperature, it expands from the heat. When you quench it, it shrinks. But, say, you've quenched a thick piece of W-1 in water, and the inner part of the piece doesn't quench quickly enough to get a high conversion to martensite. That's common. So now you have a martensitic outer, say

Hmm. I'm getting a little windy. <g> Steel is very complicated. I studied it for years when I was materials editor at _American Machinist_. It kept me busy. But you don't need all of that detail to get the baisic idea. If you need more explanation, let me know.

Keep in mind that recommended tempering temperatures often are based on *expected uses* of the steel. If it's expected to be used for a straight razor, the tempering temperature will be low. If 4140, it's expected to be used in a structural application and it will need to be less brittle. If it's 5160, it's expected that it will be used for a spring, and it will need to be tough and even somewhat ductile at high stress levels.

And so on. d8-)

The 5160 froe is intended to be driven by a wooden club (beetle) and thus could be tempered hard and possibly brittle. The instructor suggested a temperature that a toaster oven can reach, though I completed an old heat treating oven project to do it.