Heat Treating Steel

[John_Mc]

In the Harbor Freight Tools That Don't Suck Thread, member 3Ts responded to a basic description by member 5030 of the heat treatment necessary to harden steel. He requested more information on heat treating. I spent most of my working life working for a company that buys steel rod, draws it to size (and sometimes flattens it), then heat treats it and sells it mostly to spring makers. I thought I'd share a bit of what I know about the process.

DISCLAIMER: I'm what we used to refer to at work as a "Butt-Metallurgist" (as in "Well, I'm not a metallurgist, but..." - a statement which was then followed by some metallurgical opinion which was probably best left to a real metallurgist.) This is just informational only. Use at your own risk.
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Stating with the basic post from member 5030 which gave a good overview of the process

If the steel cap is worth crap, harden it yourself. Heat it with a gas axe to dull red and quench it in oil. If it has sufficient carbon content it will case harden. After it cools, reheat to 600 degrees using the gas axe again and a non contact IR thermometer and allow it to cool on it's own. That will normalize it and take any stress out of it. Pretty simple to do. You don't need quenching oil either, motor oil will do in a metal coffee can.

Typically, a manufacturer will use low carbon steel (a.k.a. "mild" steel) unless it's for a part they or their customer intend to harden at some point in the future. If the the part in question is in fact mild steel, the process described will not harden it. In fact, it will likely make it softer. You need at least about a 40 or 50 carbon steel (0.40% or 0.50% carbon). (There are other elements which can affect the hardening process, but for simplicity's sake, I'm dealing with just carbon here.) The more carbon you have, the more time you have to get it quenched and get satisfactory hardening results. The limit where the steel is not hardenable is when the carbon content is so low that you have zero time (or so little time that you cannot practically cool the steel fast enough to form martensite, the structure needed for hardened steel). Personally, I find 40 carbon too low - you don't have the time to get a good quench on it.

The hardening process for carbon steel looks like this:

1. Heat the steel up high enough to form an Austenitic structure.
2. Then quench quickly to form Martensite. At this point, the steel is extremely hard, but also very brittle (you can shatter it with a good rap with a hammer, slamming it against a rock, or bending it). It's really not safe to use steel in this state for most applications. It chips easily and can send pieces flying like shrapnel. Even moderate flexing under load can break it.
3. Then reheat to relieve some of the stresses of quenching. It softens the material slightly from the "as-quenched" state. How high you reheat at this point affects the final properties. Colder = harder, but more brittle. The hotter you get it, the more it will soften the steel, but you also gain ductility: it's less likely to shatter or chip in use.

Depending on the size and allow of the piece, this will through-harden it, not just case-harden it. (bigger/thicker the piece, the more difficult it is to through-harden it - though this is also dependent on the actual alloy of steel you are using).

For a typical carbon steel (say .40 or .50 carbon) Austenite starts forming at about 1340˚F (725˚C) you don't get the full transformation until you hit about 1500˚F (815˚C). You want to be sure to get the steel fully transformed to Austenite before quenching. You can exceed 1500˚F to speed up the transformation process (we used to shoot for 1600˚F on our old heat treating furnaces which used molten lead as the heat transfer medium).

To harden, you need to form a structure called Martensite. This is formed by taking fully Austenitic steel and quenching very quickly down below a critical temperature, after which you have a bit more time to get it down to more-or-less ambient temperatures (i.e. below about 200˚F/100˚C)

You then "temper" the very brittle Martensitic structure by reheating the steel. Member 5030's suggestion of heating to 600˚F is a good place to start. In our manufacture of spring wire, we generally reheated to between 700-900˚F (370-480˚C). On rare occasions for a specialty item or some of the more unusual steel alloys, we'd go to 1000-1100˚F. For home heat-treating, I'd probably stay below 1000˚ (steel just starts to show a visible glow in daylight a little below 1000˚). IMO, if you hit 1000˚F, you've probably gone further than ideal, but the part is still harder than when you started (assuming you have a steel that is hardenable). While you don't want to hold the steel at this temperature for a ridiculous amount of time, you do want to keep it there long enough for the full piece to get to the desired temperature. If the piece was through hardened in the quench phase, you want it tempered all the way through (you generally don't want a tempered case around a brittle core).

Quenching: you can expect some distortion during the heat treating process, so a precision fit part may no longer fit so well after the heat treat. It's possible to quench in water, but that has some adverse effects, including the potential for greater distortion of the part. Quenching in oil slows down the quench a bit, resulting in less distortion. True quench oils have very specific properties and may be customized to give certain quench rates. As 5030 mentioned, regular motor oil works. I do recommend having some means of extinguishing a fire handy. The flash point of quench oil is generally higher than that of motor oil. Either one could flare up. It's generally better not to use the bare minimum amount of oil needed to cover the part as that increases the chance of it flaring up. A larger amount provides some thermal mass to conduct the heat away from the part: less chance of flare-up and better chance of a good, even quench. Be smart: wear some eye/face protection and take other reasonable precautions.

 

[John_Mc]

This Metallurgy for Dummies page also provides more background. This TTT graph (Time, Temperature, and Transformation) came from that page (the bottom axis is time). THere are other graphs at the link which provide more detail.

the cooling rates A and B indicate two rapid cooling processes. In this case curve A will cause a higher distortion and a higher internal stresses than the cooling rate B. The end product of both cooling rates will be martensite. Cooling rate B is also known as the Critical Cooling Rate, which is represented by a cooling curve that is tangent to the nose of the TTT diagram. Critical Cooling Rate is defined as the lowest cooling rate which produces 100% Martensite while minimizing the internal stresses and distortions.

If you cool slower than "B" so the green curve crosses to the right into the "nose" of the red curve, you will no longer get pure Martensite. The resulting mixed structure will not be the most durable or predictable in terms of hardness, ductility, impact resistance, stress, etc.

The distance between the nose of that left-most red curve and the vertical axis is a matter of a couple seconds. The position and shape of that curve varies with the alloy of steel in question. The lower the Carbon (among other elements) the closer that nose of the curve is shifted toward Zero seconds, making achieving Martensitic structure impossible. Higher carbon gives you more time, and allows the option of a more "gentle" quench.

 

[John_Mc]

And a handy guide to telling the temperature of heated steel by its color. (I used to have one that showed the actual colors, but that link was broken, and at any rate, it would be distorted a bit depending on what type of monitor you were viewing it on, or how accurately your printer reproduced the colors if you printed it out.)

Colors of Heated Steel

A tip on getting a decent tempering temperature: as 5030 mentioned, a non-contact Infrared thermometer (also called an Optical Pyrometer, if you are searching for one on Amazon or eBay). Many of these will require calibration by setting the "emissivity" of the material you are measuring.

If you don't have the appropriate instrument to measure the temperature, steel just begins to glow at a temperature you can see in a dark room at about 750˚F. This would also be a pretty good tempering temperature (though have fun figuring out how to heat something with a torch in the dark without ruining your night vision). 885˚F is a visible at twilight. That temperature is getting on the high side for tempering, but if all of the heating and quenching up to this point were done properly, you will still have a part significantly harder that what you started with.

Personally, I'd get an optical pyrometer/ IR thermometer if I were going to be doing more than a part or two. They are not that expensive. The cheap ones have a narrower temperature range, and are not adjustable/calibratable. Calibration is made by adjusting the emissivity setting of the thermometer. Those with a fixed emissivity are not adjustable. A lot of the fixed emissivity units are set at .95. The emissivity of cold-rolled or oxidized steel is generally around .8-.9 (polished steel is much lower.) The further off the setting is from actual, the less accurate the rating. However, it will give a repeatable relative indication (for example, if the thermometer says 500˚ when the steel is actually 600˚, that same type of steel with the same surface condition will be at 600˚ if you measure it again later and the thermometer reads 500˚)

 

[Kyle_in_Tex]

Nice post. There is so much to learn about metallurgy. You can have a PhD and still not know the half of it.

Like your term "Butt-Metallurgist", made me laugh.

Always wondered what made spring steel, spring steel.....and then they discovered Nitinol.

The price of anvils can be crazy high. Is the manufacturing of them almost a "lost art", like canning?

 

[orezok]

Ah the memories. I was a metallurgy major at UTEP in the 60’s. After graduation I went to work for a steel fabricator. About a year later they closed down their steel fab division and transferred me to their construction division. I spent the next 40+ years in construction management and never used my metallurgy training again (at least directly). Funny how life flows. I’m not complaining as construction management provided me with a very very good life.

 

[hunt4570]

OUTSTANDING post, thank you!
The extent of my knowledge on this subject is from watching “ forged in fire “ so basically nothing. Nice to see/hear an explanation of the process.
Question for ya.. on the show I don’t see anyone re-heating to temper the metal, do you think they just don’t show it or are they leaving a step out?
Again, thanks for the write up!

 

[John_Mc]

OUTSTANDING post, thank you!
The extent of my knowledge on this subject is from watching “ forged in fire “ so basically nothing. Nice to see/hear an explanation of the process.
Question for ya.. on the show I don’t see anyone re-heating to temper the metal, do you think they just don’t show it or are they leaving a step out?
Again, thanks for the write up!

I've never watched the show, so can't say. If they are making martensite, I can't imagine stopping after quenching. Unless they were working with some strange alloy I've never dealt with: the result would be a knife so hard it would be extremely difficult to sharpen, and so brittle you'd risk shattering it if you dropped it on a tile floor.

There is another method of hardening steel which ends up with a Bainitic structure. We didn't do that for our wire. It's just not economical for a continuous process like heat treating strands of wire. Some people do use the process when heat treating parts. Since we didn't use the process, I'm not as familiar with it, but it involves heating up the steel to austenitize it (as with Martensitic heat treating), then, instead of quenching rapidly down to something close to room temperature, you cool rapidly to a higher temperature than for martensite (I don't recall the temperature - maybe 600-700˚F ??) then hold it at that temperature for a period of time until the bainite forms. Then you just allow it to cool to room temperature. The result does have good wear properties, less notch sensitivity than Martensite (a nick in the surface doesn't tend to cause as much of a problem in Bainite as it does on Martensite), and tends to have less distortin from the heat treating process. I sort of doubt this is what they are doing. If you see them quenching in oil and bringing it well down in temperature (below a couple hundred degrees) then they are certainly not making bainite. I mention it because it does not involve "reheating" the steel.

Since we have a real metallurgist on here, perhaps Orezok has some thoughts on what they are doing (if in fact they are not reheating after quenching, and not just failing to show us that step).

 

[aczlan]

I've never watched the show, so can't say. If they are making martensite, I can't imagine stopping after quenching. Unless they were working with some strange alloy I've never dealt with: the result would be a knife so hard it would be extremely difficult to sharpen, and so brittle you'd risk shattering it if you dropped it on a tile floor.

There is another method of hardening steel which ends up with a Bainitic structure. We didn't do that for our wire. It's just not economical for a continuous process like heat treating strands of wire. Some people do use the process when heat treating parts. Since we didn't use the process, I'm not as familiar with it, but it involves heating up the steel to austenitize it (as with Martensitic heat treating), then, instead of quenching rapidly down to something close to room temperature, you cool rapidly to a higher temperature than for martensite (I don't recall the temperature - maybe 600-700˚F ??) then hold it at that temperature for a period of time until the bainite forms. Then you just allow it to cool to room temperature. The result does have good wear properties, less notch sensitivity than Martensite (a nick in the surface doesn't tend to cause as much of a problem in Bainite as it does on Martensite), and tends to have less distortin from the heat treating process. I sort of doubt this is what they are doing. If you see them quenching in oil and bringing it well down in temperature (below a couple hundred degrees) then they are certainly not making bainite. I mention it because it does not involve "reheating" the steel.

Since we have a real metallurgist on here, perhaps Orezok has some thoughts on what they are doing (if in fact they are not reheating after quenching, and not just failing to show us that step).

They are not repeating after quenching, they're on a time limit in the first round to get a rough shape outlined and quenched, then after that time is over the oil quench containers are gone.

If they want to quench after that in the second round, they have to heat up with a torch instead of a forge and cool it off with water.

Then in the second round after the first person is eliminated, the other 3 have to finish off their blank and put a handle on it.

The loser from the second round goes home and the other two go back to their home forges to make whatever the cutting implement of the week is, but even there they rarely if ever show them reheat after the first quenching.
In fact the judges usually have something to say about someone who tries to quench a second time about it changing the crystalline structure and making it more brittle, but that may be related to heating it back up to the initial temperature that you use the first time, not heating up to 600 or so degrees like was discussed in this thread.

It's an interesting show to watch, there is significantly less drama than most "reality" shows.

Aaron Z

 

[John_Mc]

The time sensitive parts from the point of view of the heat testing are the initial heat up followed by the quick quench. You can let the part sit around for days after that before heating it back up to temper the blade.

The initial heat up and quench can be dramatic (relatively speaking): glowing steel followed by a dunk in something cold. The reheat is boring: just warm it up, probably well below the temperature where the steel glows. Maybe they don't show it because it doesn't make for good TV?

 

[Gordon Gould]

Very interesting John. Thanks for taking to time to explain. I have a question on mild steel if that is OK. If you crudely heat a piece to dull red or maybe even orangey/yellow with your torch so that it bends/deforms easily is there any difference in the end product properties if it is left to air cool or is quenched in water?

gg