OPERATION 91 CASEHARDENING
Description of Operation--Packed in new,whole bone, heated to 750 deg. c. (1,382 deg. F.) and heated for 2 12 to 3 hr. Apparatus and Equipment: Used--Brown & Sharpe furnaces for crude oil 10 to 14 lb. air pressue: firebox 30 x 45 in.: cast-steel boxes hold 42 receivers: quenched in oil." page 80
This heat treatment process is called "Pack Carburizing", and is normally carried out at much greater temperatures, typically 1700 F and above. Carburizing is a time-temperature dependent process; the conditions stated above are just barely adequate to impart a few thousandths of case depth. It is hard to see how steel could be "burned" in such conditions. First of all, there is no oxidizing atmosphere present. Second, the temperature, 1385 F, is too low to cause excess oxidation if an oxidizing atmosphere were present.
It is interesting that the original HT process did not include a tempering operation. That means that the very thin carburized case consists mostly of untempered martensite, and probably, ferrite and retained austenite. This thin layer would be quite hard and brittle. More importantly, the bulk of the steel underlying the case would have low ductility as well.
There is a phenomenon called Temper Embrittlement which may account for some of the problems encountered. Typically, the work piece will exhibit lower notch toughness (increased brittleness) when heat treated at a low temperature. This can be avoided by increasing the case hardening temperature, and increasing the cooling rate. The oil quench used in the above HT process is relatively mild, and does not cool the interior of the work piece very rapidly.
Subsequent re-heat treatment can alleviate the two problems stated above. "The '03 Springfields" by Clark Campbell, p. 18, outlines the double heat treatment procedure: "The fully machined receiver was then surface carbuerized (sic)in bone at 1500 F and oil quenched. Quenching from this temperature (just above the 'critical') hardened the receiver all the way through - but served to 'freeze' the structure of the steel when its grain size was at a minimum. A second heating, this time to only 1300 F, softened the metal throughout but did not affect the grain size. Because a high carbon steel hardens at a slightly lower temperature than a low-carbon steel, quenching in oil from this particular temperature left the low-carbon core soft and tough while hardening the high-carbon 'skin'. A final 'draw' at 350 F took away a bit of the brittleness from the skin."
However, if the steel is truly burned, no heat treatment will suffice to render the material non brittle. This condition would most likely occur in the forging process where the material is over heated and over worked. If the steel forging blank is at maximum heat (it would forge easier), and is subjected to high deformation rates (to maximize production, perhaps), the mechanical thermal spike could actually cause melting at the grain boundaries. This is called "incipient melting" and it imparts extreme brittleness to the steel. When this happens, nothing can be done to correct the problem and the steel must be scrapped.
According to Hatcher's Notebook, embrittlement was not confined to receivers, but also to barrels. One incident, maybe more, of a barrel burst was attributed to steel "burnt" in the forging process.
