Quote:
Originally Posted by Bruce Day
I suppose same spec barrels can differ by batch and have the same constituents.
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You're right, Bruce, but the key word is 'spec'.
Industry specifications for all alloys of steels, and stainless steels give ranges for each off the roughly 8 principle elements, other than Fe (iron), which makes up the remainder. They are Carbon (C) Silicon (Si) Manganese (Mn) Chrome (Cr) Nickel (Ni) and Molybdenum (Mo). Phosphorous and Sulfur, always reported, are kept as low as possible and are in the .015% range.
Carbon ranges for the Cr-Mo, Cr-Ni-Mo, and, the later developed Cr-Mo-V (vanadium) go up from lows of .12% to over .40% as strength requirements dictate. In most all low alloy steels, Fe ranges from 94 to 97%, so regardless the alloys name, it's still a lot of iron. What does iron do?
It rusts.
In it's natural state, Carbon doesn't mix well (go into solution) but as controls increase in heat treatment, carbon is more finely dispersed, though always visible at grain boundaries when photo-micrographed. As critical as the temepatures in the heat treat cycles, the method and rate of cooling from those temperatures is equally important. The more quickly the steel is cooled, the better the carbon is trapped in solution, and not migrated dback to the grain boundaries.
Bruce, your assumption that grain structure plays a major part in the finish appearance of cold rust bluing is correct. While the iron content seems to vary very little, maybe 3%, it too plays a significant part in the cold
rust process. Also, as Cr, Ni and Mo play a big part in corrosion resistance (rust is corrosion) they have an inhibiting factor in the cold rust process. In the end though, Iron (Fe) wins. (Iron looses when Cr, Ni and Mo gang up on it when those three elements make up up to 30% of stainless steel)
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