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Monday, September 21, 2015

Steel: a Quick Overview

There are all kinds of stell for all kinds of purposes. Which can you use without a fairly fancy setup, or sending pieces out for heat-treatment?

Basic Steel 101
An alloy is a substance made of two or more elements mixed together. Steel, at base, is an alloy of iron with a tiny amount of carbon added. 0.1%, one tenth of one percent, changes iron to mild steel; the carbon content is low enough that, while it's stronger than iron, it can't be heat-treated to make it harder and/or tougher.

Increase that amount to about 0.4%, and you're into medium-carbon steel; it won't get very hard, but it will get harder than mild steel, and will be considerably tougher. Here you're getting into steel that can be used for some springs and other such pieces. Medium-carbon has a range between 0.4 to 0.7% -- the low end of that will harden enough to make some springs, and the high end is good for heavy chopping-type knives and tools.

At 0.8% and up, you're in high-carbon steel territory, generally running as high as 1.2% for the top end. Those tend to be specialized alloys that can require some fairly intricate heat-treatment to get the most out of them. Generally, the highest you'd want to work with would be ~1.0%

Common Steels and Their Uses
Rebar is a common use for mild steel, as well as just about anyplace the strength of steel is needed, but it doesn't have to be flexible*. The usual alloy for rebar and a lot of other mild steels is 1020; the alloying element is only carbon, the '20' meaning the amount (0.2%).

Medium-carbon steel is easily available as well. For instance, leaf and coil springs from car and truck suspension are generally an alloy called 5160. It contains the following:
  • Carbon 0.56 to 0.64
  • Manganese 0.75 to 1.00
  • Silicon 0.15 to 0.35
  • Chromium 0.70 to 0.90
  • The '60' means 0.6% carbon
Why all that? Adding those elements to the alloy along with carbon does two things: makes this steel very tough and good for springs, and one more very important thing in heat-treatment:
Generally speaking, the lower the carbon content, the faster the quench needs to be for the steel to fully harden. Stuff 0.5% or below has to be quenched in water or salt water, or it won't cool fast enough to make the changes in structure that make it hard (below 0.4%, it won't harden at all; at least not enough to tell). As the carbon content rises, however, the faster the quench, the more likely it is that it might crack in the quench. But if it doesn't cool fast ENOUGH, it won't fully harden. So, over time, it was discovered that if you add small amounts of the right stuff, it changes the reaction of the steel to the quench; 5160, for example, can be quenched in oil -- a much slower quench -- and still fully harden. This means that, especially in thin pieces like a knife blade, the thermal shock of putting red-hot metal into much cooler liquid is much less likely to cause it to crack.
Some high-carbon steel still has a pretty simple composition: 1090 or 1095 is basically iron with .90-.95% carbon content and is often used for files, rasps, wood chisels and other cutting tools. There's also W1 and W2 (the difference being one contains vanadium, the other doesn't); the 'W' stands for 'water-hardening', though for knife uses it'd be better to use oil.

Then you get into more complex alloys. For instance, one of the favorite steels of knifemakers for many years is O1, which has the following breakdown:
  • Carbon 0.85-1%
  • Chromium 0.4-0.6%
  • Manganese 1%
  • Nickel 0.3%
  • Silicon 0.5%
  • Vanadium 0.3%
Since I can't remember the specifics of what each adds to the mix, the short version is they allow it to be oil-quenched (in this case the 'O' means 'oil-hardening', the '1' nominally 1.0% carbon), the carbon content and other elements also add to wear resistance (which aids edge-holding ability in a cutting tool). This stuff will make blades that cut beautifully and hold an edge wonderfully, but heat-treatment is something the average guy can do without pulling hair out in frustration.

For a list of many knife steels, and their alloys, take a look here.

Then there's stuff like stainless and stain-resistant steels. It contains much more alloying elements, and heat-treatment gets very tricky**. Those greater amounts of some elements explains why so many stainless steels won't hold an edge very well: generally speaking, adding enough nickel and chromium to steel gets you stainless, but adding that much actually reduces the wear-resistance of the alloy. An example of this is 440 stainless steel: it won't rust unless you work at it, but won't hold an edge very well unless it has very good heat-treatment; and even then it still won't keep sharpness as well as a good carbon steel like O1. On the other hand, stain-resistant steels like D2 (for example) will rust if you don't take care of them, but they're much more resistant to it than a plain carbon steel, and with good heat-treatment can hold an edge quite well.

Get more into the subject and you'll find shock-resistant steel, an alloy designed to be able to absorb heavy shock or impact in use without cracking or breaking; high-speed steel, like drill bits, able to get hot enough to ruin the heat-treatment of standard steels while still staying hard and sharp; and lots of other alloys for different uses.

That's a basic look at steel.  Next, I'll talk in more detail about the subject of heat-treatment.

*'Flexible' meaning 'it will flex under stress, and return to original shape when the stress is removed', like springs. Rebar doesn't have to do that.

**What's 'tricky' heat-treatment? Higher temperatures for both quench and tempering, preferably in a controlled-atmosphere environment for the hardening, and for some you need to take it up to temperature in steps. Lots of these steels are air-hardening, which means when it's ready to quench you pull it out of the furnace and set it on a rack in the open air; the alloy will harden from cooling that way. Some, however, after cooling to ambient temperature, require a sub-zero (way sub-zero) quench to fully harden. And keeping something like liquid nitrogen around isn't something you generally do.

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