Hardening Steel With Heat

We have touched on the topic of the effects of heat on steel in the past. These little reminders are directed at those of us who are not accomplished thermal treaters of steel. And, that is based on the assumption that you wouldn’t be reading this article if you are already proficient in hardening steel. NOT INSTRUCTIONAL! “Random thoughts on hardening steel” in his article are strictly for information purposes only.

To a large part the carbon content of steel determines its hardenability. More carbon/higher hardness. Low carbon (below .26), virtually means no hardenability. Although some people will argue that, and that’s fine, it makes for a nice bet in a bar. A very general guide for a novice is; “through-hardening” alloy or carbon steel yields a maximum hardness equal to carbon times one hundred, i.e. .30 carbon = 30RC, .40 carbon = 40RC, etc. For general reference, most pre-hardened steels offered to the public, will be roughly 30RC. At that hardness, they have good “toughness”, which is a combination of hardness and strength. Surface hardness (skin hardness, flame hardness), is somewhere around ten points over the above shown numbers, so, you can get approximately a 50RC surface hardness on a piece of 4140. It’s actually a little harder but that will give you a ballpark idea.

Through hardened steel has many advantages. One potential disadvantage might be that it could be a bit brittle. Surface hardening is generally preferred when the application may have impact or flex. In that case you would want to retain some core “flex” (ductility) in the steel you are working with. In general terms; strength goes up when hardness goes up. Too hard and brittleness (potential for abrupt fracture) becomes a factor.

The speed at which you cool the steel from the hardening temperature will affect the resultant hardness. Like baking bread or deserts; think ingredients, time and temperature. Get them worked out correctly and you have a pleasant result.

Other elements added to steel (inside the steel) or added to the furnace as a gas, will also affect the overall hardenability. There is no substitute for knowing the grade of the steel and its current hardness.

ALL IS NOT LOST! If at first you don’t succeed, try, try, again. Steel is generally very forgiving. You can repeat the thermal treatment over and over without damaging the material provided you have not exceeded the hardening temperature. If you end up with a lot of surface decarb (the powdery grey skin and scale that forms on the surface of the heated steel due to the carbon burning off), you will have to remove that so that it doesn’t act as a “heat sink” and inhibit subsequent hardening attempts. You will also have to research the thermal treatment procedure of “annealing”, as that may be required prior to your subsequent hardening attempts.

It will be necessary to remove the decarb anyway if you intend to accurately determine the hardness of the steel. The old rule used to be; “Grind to bright metal.” Decarb can still be present in bright metal. So, grind to remove all decarb. It is a safe bet, with almost any carbon or alloy steel; if you are getting a Rockwell “C” hardness reading under 22RC, you are probably still in decarb, since most carbon and alloy steels will pick up some hardness following a thermal procedure unless you just happened to accidentally perform a  perfect anneal.

-Howard Thomas, August 6th 2019


Surface Hardening vs. Through Hardening

In the world of heavy industrial maintenance steel, whether you call it Case Hardening, or, Surface Hardening, or, Skin Hardening, it is all the same thing. This is a localized method of hardening employed to develop a wear resistant surface while maintaining a somewhat ductile (shock resistant) core. With production items, such as gear teeth, this may be very fine tuned, sophisticated, accurately measurable. In maintenance, “one-off” items, it can be somewhat erratic and capable of surprise. If you are contemplating increasing the surface hardness of a piece of steel, please recognize that increasing the hardness, especially localized hardness, is also increasing the brittleness which subsequently increases the chances of facture. Wear appropriate safety gear. Any surprises may not be very forgiving. 

IN GENERAL, the two types of hardening are self-explanatory. A through hardened piece of steel is pretty much the same relative hardness from surface to core. Most common prehardened steels, carbon or alloy, are often shipped at a hardness of approx. 30RC. As the cross sections get larger, the hardness will “drop-off to core”. That is, as you get closer to the center of the mass, the hardness may drop a few points. Those are still considered to be Through Hardened. Surface hardened levels, typically those used in hydraulic applications, and precision automation rail applications, will be supplied with a very thin hardened surface “skin”, at about 60RC, with a great drop off in hardness toward core.

The surface hardened material provides great resistance to sliding abrasive wear while resisting bending and torque. The through hardened alloy or carbon material provides a good balance of toughness (a combination of wear, impact, and gouging resistance). The through hardened material makes no pretense to be particularly ductile. In fact, as through hardness increases, the potential for general fracture also increases.

Caution should be exercised when attempting to surface harden small cross sections. Even though your intent and processing method may be aimed at surface hardening, small cross sections cool rapidly. The rapid cooling may actually result in a through hardened condition with potentially dangerous brittle hardness.

-Howard Thomas, Nov 8th 2018