I wanted to add this because Miller's point about temper when re-casing is important.
This is from a Table on "The Working of Steel" located within the Gutenberg Project, 1920ish Metalworking manual.
Techniques based on the technology of the time.
TABLE 23.—TEMPERING TEMPERATURES FOR STEELS
Temperature
for 1 hr. Color Temperature
for 8 min. Uses
Deg. F. Deg. C. Deg. F. Deg. C.
370 188 Faint yellow 460 238 Scrapers, brass-turning tools, reamers, taps, milling cutters, saw teeth.
390 199 Light straw 510 265 Twist drills, lathe tools, planer tools, finishing tools
410 210 Dark straw 560 293 Stone tools, hammer faces, chisels for hard work, boring cutters.
430 221 Brown 610 321 Trephining tools, stamps.
450 232 Purple 640 337 Cold chisels for ordinary work, carpenters' tools, picks, cold punches, shear blades, slicing tools, slotter tools.
490 254 Dark blue 660 343 Hot chisels, tools for hot work, springs.
510 265 Light blue 710 376 Springs, screw drivers.
It will be noted that two sets of temperatures are shown, one being specified for a time interval of 8 min. and the other for 1 hr. For the finest work the longer time is preferable, while for ordinary rough work 8 min. is sufficient, after the steel has reached the specified temperature.
So why post this?
Because we see annealing ocurring in 8 minutes or less. At quite low surface temperatures. Also, remembering that case is quite thin.
One technique I have read about, but have never seen performed, would be to electro plate the part with a heavy layer of copper. Shielding from plating areas that re-casing might be desired. Then briefly re-casing for a shallow case, and then quenching in a bath of effervescence. Copper then removed.
Anyone ever heard of that?
