A question for the metalurgists out there

[Kevin R. Cashen]

I suggest no firm number, and certainly no special significance of three, only that after you get within .5 HRC it is about as good as it is going to get (especially since the standard for Rockwell is - or + 1 point). The time would be entirely dependent on the equipment used, e.g. an oven may spend around 20 minutes of the overall hour just getting the steel up to temp. I use different equipment with different heating rates and conductivity so my times would be different than the next guys.

[Doug Lester]

I have good back to the books on this and it's enough to make your head spin. I guess that the main point is that the retained austinite is removed by multiple quenches. Or you could cryoquench to get the steel below the the Mf point, which is well velow zero for the steels that we use in knife making.

Depending on the temperatures and time used, the retained austinite can be coverted to ferrite and cementite, epsilon carbide ( a crystal made up of Fe2C and Fe3C structures), or untempered martinsite. I just wish that the texts stated under what conditions that these conversions took place in stead of saying in one place that products X are created and in another stating that products Y are created, or maybe even product Z.

I've also come to the conclusion that you're one sick puppy if you keep texts on metallurgy by your bed to do lite reading before turning out the lights for the night.

Doug Lester

[Thingmaker]

Originally Posted by Doug Lester

I've also come to the conclusion that you're one sick puppy if you keep texts on metallurgy by your bed to do lite reading before turning out the lights for the night.

HEY! I ain't no "puppy!"

[Doug Lester]

After doing a little light reading before bed I found that I had the answer all along, or at least a thumbnail sketch of one. On page 161 of Verhoeven's "Steel Metallurgy for the Non-Metallurgist" in the section on tool steels was a rather long paragraf on the conditioning of retained austinite. It restated that on tempering retained austinite decomposes into ferrite and carbides which is refered to as bainite. However, on hightly alloyed tool steels (and I assume any highly alloyed steel) this decomposition of the retained ausinite is not complete and it becomes conditioned (something that is not further defined). This conditioned austinite does decompose into untempered martinsite upon cooling. Multiple tempering cycles are used to assure that all of this conditioned austinite converts to martinsite and that the resulting martinsite is tempered.

Now Verhoeven doesn't say at which point a steel becomes highly alloyed so it leaves me to assume that there is a bit of a continuum here between where all retained austinite converts to bainite and where it all becomes conditioned. The thing to bear in mind is that this only explains what happens to retained austinite during tempering and that it can decompose under other conditions to untempered martinsite, which is undesirable. Also these microstructures are in addition to other microstructures formed in other heat treating processes which can leave a fully heat treated steel with a mixture of ferrited and carbides in different forms such as pearlite, bainite, or "free" ferrited or carbides dependent upon the heat treating process and the composition of the steel.

Doug Lester