This metal is hard!

[Kevin R. Cashen]

Aaaand...which condition(s) are best for HT response?

The finer the phase structures, the easier they will go into solution, and the greater the resulting hardening... unless too much is put into solution. And what may be true of one steel will not be the case with a another alloy with different chemistry. Sticking with the 10XX series, the condition that will go into solution the quickest would be that which you would get after just normalizing. For this you would want to get hot and waste no time quenching, especially with 1095. Next it would be a tie between coarse lamellar pearlite and fine spheroidal carbides- heat it up hold until heated evenly throughout and quench. Finally would be coarse spheroidal carbides. These are larger and very stable structures that will require higher temps or longer times to dissolve, a short soak at no greater than 1475°F could be in order.

For alloy steels things get much more complicated- alloying changes everything. Carbide former like Cr, will form VERY stable carbides in comparison and a very heavy spheroidal anneal is capable of what I call "carbon locking" the steel. Some of these steels are so completely annealed for free machining that they will not achieve more than a 63HRC without increasing temperatures high enough to break down those carbides. Whenever I see crazy temps being used, like 1550°F or greater for something like 80CrV2 or 52100, I know the steel is carbon locked, or being ruined in the heat treatment. The quickest and best cure for this condition is normalizing. One good heat to 1650°F followed by an air cool, and the next heat will easily give you maximum hardness.

 

[J. Doyle]

That is the beauty of it, it doesn't really matter. In this process, all the work is done at temperature, rate of cooling doesn't matter as much, and slower cooling only really adds up to more time at temperatures where spheroidizing may occur. With the other industrial, or isothermal method, cooling rate is much more critical because if it is not slow enough for the transformation to occur the carbon that is in solution will go lamellar. If you are just heating to 1300°F in a forge, cycling, rather than a hold, will not only probably be necessary, but it could be beneficial as the repeated cycles could result in more seed carbides for the spheroids to grow around.

If I don't have to do any serious machining and can do it with the quick 1300°F cycles I prefer to do so since the spheroids will be finer and more numerous. The larger the spheroids, the few there are, it is very soft but not as even of a microstructure.

That makes sense. Good to know. Thanks.

 

[tkroenlein]

The finer the phase structures, the easier they will go into solution, and the greater the resulting hardening... unless too much is put into solution. And what may be true of one steel will not be the case with a another alloy with different chemistry. Sticking with the 10XX series, the condition that will go into solution the quickest would be that which you would get after just normalizing. For this you would want to get hot and waste no time quenching, especially with 1095. Next it would be a tie between coarse lamellar pearlite and fine spheroidal carbides- heat it up hold until heated evenly throughout and quench. Finally would be coarse spheroidal carbides. These are larger and very stable structures that will require higher temps or longer times to dissolve, a short soak at no greater than 1475°F could be in order.

For alloy steels things get much more complicated- alloying changes everything. Carbide former like Cr, will form VERY stable carbides in comparison and a very heavy spheroidal anneal is capable of what I call "carbon locking" the steel. Some of these steels are so completely annealed for free machining that they will not achieve more than a 63HRC without increasing temperatures high enough to break down those carbides. Whenever I see crazy temps being used, like 1550°F or greater for something like 80CrV2 or 52100, I know the steel is carbon locked, or being ruined in the heat treatment. The quickest and best cure for this condition is normalizing. One good heat to 1650°F followed by an air cool, and the next heat will easily give you maximum hardness.

Much obliged, Mr. Cashen.

 

[J. Doyle]

That is the beauty of it, it doesn't really matter. In this process, all the work is done at temperature, rate of cooling doesn't matter as much, and slower cooling only really adds up to more time at temperatures where spheroidizing may occur. With the other industrial, or isothermal method, cooling rate is much more critical because if it is not slow enough for the transformation to occur the carbon that is in solution will go lamellar. If you are just heating to 1300°F in a forge, cycling, rather than a hold, will not only probably be necessary, but it could be beneficial as the repeated cycles could result in more seed carbides for the spheroids to grow around.

If I don't have to do any serious machining and can do it with the quick 1300°F cycles I prefer to do so since the spheroids will be finer and more numerous. The larger the spheroids, the few there are, it is very soft but not as even of a microstructure.

So I've been reading through this again and just want to clarify, what steps if any, come between the normalizing(1600+ f.) and spheroidizing(1300 f.)?

I'm assuming there would be some thermal cycling(1475 f.) in between?......speaking strictly for simple 10xx steels.

 

[Kevin R. Cashen]

Yes, many folks would also do some reduced heats after normalizing to refine grain size. One needs to remember that the more evenly distributed the carbon before the anneal the better the results. Also remember that diffusion is initiated, and driven, by points of higher energy within the matrix, so the less stored energy, the slower the process. For this reason I prefer to avoid pearlite before spheroidizing. Normalizing will result in pearlite in simpler steels. Some may all out quench the blade to accomplish this but I am not a fan of stressing a blade with full martensitic transformation until I am ready to harden. Instead I prefer to quench the normalized blade to around 800°F and then allow the formation of upper bainite. Just to be clear- upper bainite stinks out loud for just about any desirable property you could want in a blade, but what it does do is trap carbon in a very fine semi-solution throughout the matrix with ultra-fine carbide precipitates. On reheating for spheroidizing this results in very fine and evenly scattered spheroids, with the fine precipitates and points of higher energy from the bainitic transformation mechanisms acting as nucleation points for the carbides. This will result in better carbide conditioning and allow for easier proper solution when you harden. Quenching without the interrupt would also get you there, even more so, but would also introduce all kinds of excess strain energy into a system you are in the process of freeing from it.

 

[Kevin R. Cashen]

Here are some micrographs of annealed 1084 to illustrate what I am describing. It is not as pronounced with 1084 but other alloys, especially those with chromium, will really stick to the outlines of the previous carbide structures.