Diffusion based processes (those resulting from the movement of carbon atoms) are driven by rates of nucleation (big word simple meaning- a place for the process to get a toe-hold and start) the more points of nucleation that greater the rate of diffusion, or at least the greater the amount of it going on. Pearlite is a product of diffusion. It needs points of nucleation to get a start. As mete mentioned (and then left me holding the bag
) the grain boundaries are points of high energy where nucleation can readily occur. Finer grains mean more grains, more grains mean more grain boundaries to separate them, this all equates to more areas for pearlite to grow. More areas for pearlite to nucleate will naturally push the nose on the TTT curve to the left, decreasing the time required to cool the steel below the point at which pearlite forms, lessening the chances of reaching maximum hardness.
Try this sometime, get a piece of 10xx (1080, 1084, 1095) that has good bevel ground on it and quench (full quench) it in a good oil, and then polish to see the temper line and where it is at. Normalize (or cycle) the blade a bit and then quench and notice where the temper (harden) line is now. If you are really good at temp. control and keep the heats in the proper range to refine keep refining those grains, you will be able to watch that martensite line move closer and closer to the edge, as the grain gets finer.
Fine grain is good for toughness to be sure, but everything in this business seems to be a compromise and a balance. Large grains promote deeper hardening but are not good for toughness for a couple of reasons. First, and simplest, fractures really like to travel along grain boundaries, the larger the grain, the straighter the course for the crack to take. Then you have embrittling precipitates in the grain boundaries that tend to be bolder in course grained structures. Finally, if you have large grains above .60%C the resulting martensite plates will be huge and will run into each other at high angles (like big ice sheets slamming into each other), creating really ugly micro fracturing that doesn't do a thing for the toughness of the steel
I know folks who have been involved with research involving the cutting ability if extremely fine grains (beyond what is typically achieved) and their results were disappointing. It would appear that edges could need a balance as well, although I would like to do some studies myself to see what could be going on there.
As for the desired grain size, that is hard to nail dead on without the equipment. I hope to have some appropriate microscope eyepieces for such work soon, but even then I wont be doing too much since it is a bit of work to prepare the specimens to properly rate grain size. One needs to prep and polish the specimen and then etch it correctly to get what you need under the microscope, and then you need to count how many grains occupy a given space under magnification. I believe it is around the average number of grains per square inch at 100X magnification. The ASTM chart typically ranges from 1 to 10 or so, in grain sizes with the higher number being finer grains. Mete will have to tell you anything about x-ray defraction since that is well beyond my means.
For most smiths the most common option is to break a treated piece and look at the fractured grain size. The rule of thumb for this could be if you can make out the individual grains well with the naked eye, they are probably too big
Fractured grain specimens are all but useless under a good microscope since the uneven surfaces make focusing nearly impossible.
Peter, I have limited experienced based information to offer on 5160, in my shop it is more like duke or earl, with other steels firmly reigning as kings or emperors, and the last lone bar wandered out of my shop about 5 years ago. It is, however no mystery that the steel is as popular with smiths as it is; it is very hard to make a bad blade out of it. Although the .6%C (often actually .55%) will limit the maximum hardness, due to the leftover ferrite, it will also keep the martensite entirely of the lathe variety (instead of plate). This makes the steel tougher by nature, and will even allow a few oops in the overheating and still not be as brittle as expected with a larger grain structure. The chromium allows it to harden deeper than one could expect from other steels of the same carbon level, like 1050 or 1060. All of this adds up to a steel that is very easy to forge, and somewhat self correcting as far as grain refinement (due to the chromium), it will harden up nicely with less chance of distortion from radical quenches, and will be pretty tough, even if the temps werent dead on. I have mostly fond memories of working the steel, (before I started mixing 2 steels in one blade), until the current reigning monarch of large blade steel in my shop, deposed it, and banished it from the kingdom.
