I had read that article, some time ago, and it addresses 3 issues discussed in this thread:
1. The "critical temp" of the case layer is 1319F (715C). This seems awfully precise given the variables but that's what the article says. In other words, if you don't go above this temp during carburization, the case will not harden (form martensite) when quenched.
2. You can quench below this temp, as low as 1100F (615C), and you will get a softer case but better colors at this temp. If you want a super hard surface, quenching needs to take place around 1300F.
3. You only get a few thou of case if you carburize at 1319F for an hour, and this makes no difference to the strength of the receiver at all, it's purely cosmetic. (Every other source says to go to 1650F for at least an hour if you want appreciable case thickness).
4. To get nice colors, you need to carburize no higher than 1319F or so. I was surprised to see the limit on carburization, as you don't get much at this temp. The need to keep the quench temp down seems to be the consensus to get colorsm even if at the expense of some hardness of the layer. FWIW, I dispute the limit on the carburization temp from personal experience. I have gotten very strong colors after carburization at 1500-1525F provided I came back to a lower equilibrium around 1300F before quenching. But I make sure the part is exposed to no air before quenching.
Did anyone witness Oscar's quenching? He refers to a method of keeping the charcoal in contact with the parts during quenching, but he never says what or how.
And does anyone have a link to, or care to post a copy of, Part 2? I don't think it's on the web anywhere, may be wrong on that.
