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