I have two samples of steel, one that is annealed, and one that is not. Both samples are in the shapes of cylindrical pucks. In both samples, the steel was found to be slightly harder on the inside of the sample than on the outside, but the trend is slightly more noticeable in the un-annealed sample. Any ideas as to why this is? Thanks!
We could make a better guess if we knew what grade of steel you're talking about, and how it got to be "cylindrical pucks" (cast? machined from barstock? etc.).
However, if it's a common grade of heat-treatable steel, the most likely answers are these. The "annealed" puck that is harder inside was badly annealed. It should be within a couple of Brinell hardness points of the softest state of the steel (as low as 120 HB for low-carbon steel; much higher for some alloys, such as 269 HB max for 440 stainless). If not, it wasn't heated to a high enough temperature, or it wasn't held there long enough, or something wasn't done right in the cooling cycle. It would be interesting to know how the inside of the steel got hardened in the first place: cold-rolling or other work hardening? Or heat treatment?
As for the hardened one, it could be any of several things. The surface could be decarburized from being improperly annealed. That is, it was annealed without sufficient protection, or sufficient carbon potential, in the heat-treatment atmosphere.
Besides the grade of steel, it would help to know what the specific annealing treatment was. I assume you're using the term "annealed" as it's used in metalworking technology: heated above the transition ("critical") temperature and held there for a specific amount of time, and cooled at a specific rate. The "specifics" vary with steel grade and the thickness of the pucks. If it's a high-carbon steel, annealing also requires atmosphere control.
If you mean that somebody got it red with a torch and then walked away , that's another kettle of fish. Or another piece of wrecked steel.
-- Ed Huntress
Nicely said, Ed.
Harold
I was at a shop where in the back a pair of fellows were hardening the teeth of a large gear. Don't know if they made it but it would make sense - they were a machine shop after all. The gear was about 12 inches long and almost the same width, teeth were about an inch and a half or so from root to crest. One fellow had a torch and was getting each tooth red-orange, whereupon the other would douse it with water from a garden hose.
I hope that was a case-hardened gear. Otherwise, I hope that it wasn't a gear for a helicopter.
There are three basic ways to harden gears (through-harden; flame-harden; case-harden), and it's very tricky if the gear has to handle any load. It's really easy to wreck one, especially with flame-hardening. Flame-hardening, or localized induction-hardening, usually is done in a special fixture and semi-automated.
-- Ed Huntress
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