doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Decalscence/Recalescence temperatures for Actions?

1400 degrees is what Ed's torch is set at. That's how attains those beautiful ere'decent colors.

We have figured that out Pineknot, but the case layer does have enough carbon in it to make the temperature important. What nobody seems to know is how much carbon content is typically found in the case, and thus the min quench temp to ensure hardening. There is also a strong correlation between case depth and temp/time. So if one is casehardening fresh metal, a higher temp and longer soak are necessary to get the carbon absorption in the first place; if recoloring, less time and a lower temp should be OK provided one starts above the recalescence point of the case layer...which nobody seems to know because we don't know the carbon content of the case. It seems that 1450F is a safe quench temp but a lower temp may work (and is desirable if it does to get better colors and less distortion) if you are coming off a higher temp. How low nobody seems to know. A charcoal fire can be hot or cool depending on how much air is forced through it. So knowing that this was once done on an open charcoal fire doesn't tell us much about how to do it today in a proper furnace.

FWIW I successfully recolored some parts from three different guns this evening...ramped up to 1525-1550F, held it there for 40 mins, then came down to 1300-1325F, held it there for 15 mins, then immediately quenched in water with some KNO3 in it. Parts have hard surface and strong color. If anything too dark and too much brown but that's probably a function of the charcoal mix. I used bone:vegetable charcoal in the ratio of 1:2. Not sure how much carbon was left from the original case hardening but I am presuming a lot of it.

Here is the real deal on carbon absorption:

Casehardening: The box must be packed full with charcoal then sealed. The steel must be clean (scale free) to start. 1425°F is the minimum recommended case hardening temperature and it takes a lot less time at the maximum of 1625°F.

Carbonnitriding is done by using a liquid salt bath (cynaide was commonly used) and the time/temperature ratio is supposed to be more or less the same for case hardening by carbon packing and gas methods. The results are the same.

Carbonnitriding starting with low carbon steel (SAE 1008):

1 hour @ 1425 to 1450°F results in a case .004" deep.
2 hours @ 1425 to 1450°F results in a case .006" deep.
3 hours @ 1425 to 1450°F results in a case .009" deep.
4 hours @ 1425 to 1450°F results in a case .011" deep.

1 hour @ 1600 to 1625°F results in a case .015" deep.
2 hours @ 1600 to 1625°F results in a case .021" deep.
3 hours @ 1600 to 1625°F results in a case .026" deep.
4 hours @ 1600 to 1625°F results in a case .030" deep.

Heat Treaters Guide, 1982, ASM, p.25 chart (referenceing Metals Handbook 8th ed., Vol 2, ASM.

So to get only 20 thou of case on virgin mild steel (or case-hardened steel that has been filed or polished), you need 2 hours at 1600F and that's too hot to quench from and still get color, hence the need to come down to a lower temp (precisely what temp is what we are trying to ascertain) and stabilize before we quench.

I just finished to delete dents and stratches from receiver for my DR project, there were some minor case color traces in some hidden places...
I have to say case hardering do not ad any significant aditional hardness to the receiver I believe, the only reason of case coloring-hardering - to protect steel surface from small dents and stretches and that's all.
You could make it, case-coloring, at low temperature (it will be smarter) or at high temperature. Why smarter?
IF the receiver steel is something just a little bit higher than mild steel in % of carbon content, you risk to turn steel to fragile composition and without the following steel normalizing or tempering.
Broken re-case coloring receivers, which we see from time to time is result of wrong case coloring mid carbon content steel at high temperatures OR many times re-case coloring, when low carbon steel turns to something else fragile and large granular.

Kids don't try this at home.

Geno I think you are right...20-30 thousandths of hard case is not going to make any appreciable difference to the tensile strength of a bar of steel 1" thick. It does prevent wear on moving parts, protect hammers from deformation, prevent scratches on cosmetic surfaces, and reduce the propensity of the steel to corrode. It also looks pretty and cuts down on reflection..a practical consideration on a waterfowl gun or a rifle.

Low carbon steels are prone to growth in grain size when heated much above 1600F from what I have read. So if one were to recolor a receiver repeatedly at high temps, grain size would grow making the receiver more brittle.

doublegun, if you haven't seen this before, it is Oscar's way Part 1.

http://doublegunshop.com/doublegunjournal_v7i4_9.htm

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.