Mo' Rockwell
Okay...back!
Since this subject comes up from time to time, I thought it might make for an interesting discussion.
As anybody who has thought it through knows, the hardness of any given part is a compromise between durability and longevity. Too soft, and the part wears out fast. Too hard and it can be brittle. These hardness requirements are set forth by the engineers and designers to insure the
best of both, as far as it can possibly be achieved....Compromise. The
fly in the ointment in any design. So we have it that any part which must wear well AND resist impact stresses must be hard AND tough. In other words...How hard is TOO hard?
Another point that rarely condsidered by any except students of engineering and design, is that whenever two parts are in contact with each other under pressure, that the softer part will wear faster than the harder part. So...it stands to reason that unless a hammer and sear are
very close to the same hardness, one will wear while the other will only be burnished. Exact matches are possible, but would require testing every part until a perfect mate could be found. Not acceptable, since Rockwell testing very often destroys the part. So, we accept another compromise.
We spot test a lot or group of parts to insure as nearly as we can that all will fall within an acceptable range, and go with that.
This leads to case-hardening. Case-hardening makes the surface of the steel harder than the material under the skin. It's normally done to something on the order of 1018 "Cold-Rolled" steel that doesn't harden
by heating to a pre-set temperature above the lower critical line, and quenching. This temperature is determined by how hard the part needs to be, as well as the method of quenching the part. There are oil-hardening steels...water-hardening steels...and air-hardening steels.
Quenching in a fluid shocks the steel...depending on how cold the fluid is.
Quenching a red-hot workpiece in ice water can actually cause the steel to fracture. Oil is gentler on the steel...and the hardening process can change with the number of parts that are quenched in it. The more parts that go into the fluid, the warmer it becomes...and the "softer" the resulting part will be. As long as the hardness isn't above or below the blueprint specs, it's acceptable in the design compromise. So...the temperature of the steel AND the quench must be controlled.
But back to Case-hardened steel.
The hard "case", or surface of the steel can be file-hard, while leaving the
steel below the surface soft and malleable...which serves to create a material that's hard as Hell's Hinges, but not brittle. Here, the temperature is even more critical because the depth of the case is critical. Too shallow, and the hard case will wear through earlier than it should, and once the soft underbelly is exposed to friction, the part will fail quickly.
Case-hardening worked very well a century ago, when steel alloys were limited pretty much to wrought iron and cold-rolled steel...and it still does today, although case hardening a part is slower and much more tedious than heating and quenching. The part is usually sealed in a container with
a carbon bearing substance...bone meal is one such...and heated slowly
in a furnace. The temperature is held for a set period of time, depending on how deep the case needs to be...and brought back to ambient temperature by simply turning off the furnace and waiting for it to cool by
convection. Small, or thin parts can be hardened more quickly, but the depth of the case can't be as closely controlled outside of the furnace...and even then, there's a fine line between a perfectly case hardened part and
a part that'll break like glass. To demonstrate, put on a pair of safety goggles and some heavy clothing and welding gloves, clamp a 6-inch
mill file in a vise, and see how far it'll bend before it snaps.
The problem with case-hardening is that rockwell testing of the part often gives a false reading because the diamond point used to test the part can go deeper than the casing...OR...it compresses the softer material underneath, which also produces a false reading. Testing case-hardened parts require a different method, and even that is often "iffy".
Tool steels have a high enough carbon content to harden without using an outside source. They harden throughout the whole part...the consistency of which depends greatly on the quality of the steel...or as to how many impurities it has in it. The fewer the impurities, the more consistent the hardening throughout the steel. The better the control of the smelting and alloying process, the more consistent it is too. Think of it like mixing cake batter. If you don't get it blended thoroughly, you get pockets of flour
in the cake.
To the machinists and toolmakers out there: Can you imagine lockwork
made of HSS? (High-Speed Steel) How 'bout THAT for some long-lasting lockwork!! Expensive to be sure...but one set would wear out 50 guns...
NOTE: HSS is the stuff that conventional drill bits are made of...
just FYI.
Cheers all!
Tuner
