Hello,
I am new to this site, but found this thread while looking things over. I actually wrote the tutorial for the Home Model Engine Machinist forum that was linked at the start of this topic. One thing I would caution about the Marlin thread (I was also on that forum many times in the past) is that the person who did this work was quenching the receivers at a very low temperatures, i.e., 1250 F. I do research in casehardening at the University of North Dakota and have done fairly extensive testing on this process over the last five years and the low temperature issue frankly disturbs me.
While there is going to be some difference in hardness, in my opinion, you should never quench below the critical temperature, say 1370 F (varies), if you want to actually harden the surface. Strangely enough the colors are oftentimes better at these temperature ranges under critical, but hardening is superficial if at all. This is not good if the original design specified a hardened surface. Of course, the reason that some like to do this is not only the pretty colors, but also the fact that the shock at these lower temperature ranges is considerably less than it would be if quenching took place above the critical (higher) temperature range. Consequently there is probably less chance of metal movement and thus reduced risk of damaging the receiver, or having to spend considerable amounts of time hard fitting components afterwards. Frankly I block everything I care about and expect to have to do some post fitting when moving parts are involved. I would not shy away from this task at the expense of a properly hardened part.
I rarely, if ever, caseharden parts much under 1425 degrees F and prefer to soak them in the 1450 range knowing full well that the pack will lose temperature between the time it leaves the furnace and can be delivered in the quench tank. Carbon absorbs faster at higher temperatures, but quenching above 1500 tends to produce very dark colors and above 1600 they pretty much dissappear altogether. This said, a lot of industrial carburization takes place in the 1600 plus range. I have soaked parts at higher temperatures and than dropped to low temperatures for the quench with some success however.
I have never had any problem obtaining very vibrant colors with my process and rarely do much more than wrap the parts in soft iron wire before placing them in the crucible. My standard mix is 50/50 wood/bone and either tap, or distilled water. We have used oil skims and various admixtures in the quench and also run air lines, but frankly, plain old cold tap water freshly poured does as good as anything. Filling a container shortly before the quench allows proper agitation which gets the oxygen content up to a point that you need and the wire creates disturbance enough to give you the spectacular patterns you are after. The wire also helps hold the charcoal mixture in contact with the workpiece for a longer period which also helps with the colors.
If anyone has any questions about the process I can be reached at:
ajohnson@business.und.edu.
Regards,
Alex Johnson