[The Topher]

Hey I thought I would include a good tutorial for engineering students on the process. It has all of the the isothermal transformation graphs and explanations on how they work. In fact I learned back at Virginia Tech of a tutorial almost exactly like this. Ahhh how I miss it!

http://www.engr.ku.edu/~rhale/ae510/heattreat.pdf

[JoshEarl]

Russel,

I've heard this same thing, actually. I asked Mike about it once, and he said it didn't make a difference. Maybe he can chime in and clarify...

I can see both sides. On one hand, it makes sense that the steel would need to cool back down below Mf (the point where austenite ceases to convert to martensite), which if memory serves is around 200-250 degrees F. Cooling to room temperature would allow that to happen. The next cycle would temper the new martensite.

On the other hand, there is such a thing as "auto-tempered" martensite, which is an atypical heat treatment done by quenching the blade to 400 degrees F and holding it there for the amount of time you'd usually use in a tempering cycle. This prevents some of the stresses that normally occur during quenching, and I believe it results in a slightly tougher structure. It does require some more advanced equipment, otherwise it might be more widely practiced.

This might be one of those things that varies a bit based on the particular steel alloy under consideration. Mike's advice to me earlier might have been based on the simple steel I was using.

Mike?

Josh

Originally Posted by Russel Baldridge

There is a small difference between using one temper cycle versus two or three: any Retained Au,stenite is converted to Martensite during the cooling after the blade is heated to tempering temperatures, so the blade has some fresh Martensite along with the (more desirable) tempered Martensite after the first cycle. A second tempering cycle would temper the new Martensite and not alter the old stuff if you use the same temperature, but would make for a slightly better/more uniform microstructure. But the extent of this is based on the type of steel and effectiveness of the heat treating process.

There are even a few guys that recommend 2 or 3 two hour tempering cycles to be absolutely positive that the R.A. is converted and that the Martensite is tempered properly, but they mostly work with swords etc. So, for a razor, 2 or 3 one hour cycles wouldn't hurt and would be a slightly better procedure.

Otherwise, Great job!

[Russel Baldridge]

Yeah, I meant to clarify that it mainly applies (as far as I know) to a steel that has a tendency to have retained Austenite after heat treatment. I just wanted to make the point that it could be easily integrated into any H.T./tempering procedure as a type of safe guard just in case you're left with a little soft stuff after the quench.

(BTW, since we last traded info on smithing procedures I've picked up a digital pyrometer/controller from Auber Instruments. Definitely a helpful tool, much thanks for the suggestion of getting one.)

[Mike Blue]

I meant to get to this earlier but forgot.

At about eutectoid (which is a carbon content around 0.77-0.85% depending on which text you use for reference) all the available carbon will go into solution at the austenitizing temperature.

More carbon will essentially result in a supersaturated solution of austenite that when quenched, will have retained austenite because there is a finite amount of space between crystals to form martensite. The more alloying content, the more likely there will be retained austenite. One of my favorite exotic steels has, despite the best heat treatment possible, by the book etc., about 25% retained austenite and it's still hard and tough as the hubs of Hell. Retained austenite is more brittle/unstable that martensite so the fracture risk is higher/less tough.

For the most part, simple steels heat treated correctly will have little to no dangerous retention. More complex steels, it is something that will have to be accounted for. For the high alloy stuff, one method to reduce it is the liquid nitrogen cycling. But, even then you only reduce the amount of RA and may not necessarily eliminate it. Repeat tempering is definitely necessary afterward.

The phenomena of long term conversion of RA to martensite, as in, over years, has led to the belief that swords get more brittle over time. Likely because some austenite converted to untempered martensite and left a hard brittle spot to be found generations later.

As to the tempering cycle question. I have mostly always used a two hour temper. I think I experimented with three one hour cycles just so I'd know for myself what happened and frankly couldn't see any difference. But then I'm using simple steels and don't have much of a retained austenite problem to have to convert. Plus generally, the Ms, or martensite start temperature, is higher than the tempering temperature, so most austenite is well on its way to martensite before dropping down further to Mf or the temper bath.

Also, given that the conversion of austenite to martensite occurs at or near the speed of sound, the likely amount of retained austenite to convert is going to be really very small and far less time than the available hour or two of tempering.