doublegunshop.com - home Forums General Discussion DIY Gunsmithing @ doublegunshop.com Case Coloring Metal Prep

OK,

Is the concept of a third stress relief heat after the annealing and coloring heats valid? If so, at what temp and for what duration? I've read instances of the metal being fairly brittle after the coloring heat and prone to flaking.

It's interesting that CS puts their temp at just a degree or two less than the 1333F critical temperature. Considering the wide range of temps used from other sources, some significantly higher.

A key to using the stainless steel wrap when annealing is to put a little piece of fine steel wool in with the part. That way the steel wool will burn and use up any air left in the wrapped package thus preventing any scale.

What a good idea! Does this apply to crucible use in the other heats as well?

Not really since the packing in charcoal in a crucible (tapping while packing) is meant to fill all the voids in and around the metal parts contained. So no need to have to put anything in it to burn up the air if it is packed right.

The steel wool makes sense. I'll need to try that. The charcoal off gases CO and CO2 and provides the oxygen free environment. Make sure you have good ventilation and a CO monitor is cheap insurance as well. The last time I annealed something with the door closed in the shop the CO alarm went nuts.

interesting stuff on the Temp. as well. Most things I have read call for a quench around 1450 F, I enjoyed Craig's post on the CSM video(which I need to track down) and it seems both CSM and ken's info come in at 1333 or so. Where did you come across this number Ken ?

is this the crucible?

http://www.brownells.com/gunsmith-tools-...24780-3746.aspx

1333F is the carburizing, or "critical" temp for steel. From what I've read, it's above that temp that the carbon absorption and consequential hardening occurs upon quenching. That's also the temp that needs to be attained/exceeded for annealing. I could be wrong, I've also seen it listed at 900C, about 1650F. But, I think 1333F is for low carbon steel.

The crucible in the piece was a commercial made one that looked cast, no welding, just a cover and everything else. Time is money, but I believe the Brownells crucible can be cobbled up pretty quick and easy, at least a fellow might be paying themselves a hundred bucks an hour to come up with something close.

Mine looks about the same. Instead of rings for the tongs, I just cut four v's off some angle iron with a chop saw and welded them on the sides, then the tong jaws go in between. For the cover, I decided to use a threaded pipe cap, and a grinder to clear out the threads for a slip fit. The only thing I'd do different that may not be a bad idea is some way to get the base off the kiln floor a little bit.

I've used stainless heat treat foil a bunch of times for other things. A little scrap of paper will consume any oxygen in there. A good consideration is to double wrap and burnish it down tight, but don't be surprised if some sharp corner pokes a hole in it.

I believe what happens somewhere around 133-something degrees is that steel looses magnetism. I believe that's the bare minimum change required for the steel to harden. Interesting thing, on the way up in temp, the point it becomes non magnetic is very precise, but on cooling back down, I know for myself that steel will stay non magnetic till quite a bit cooler and it will regain magnetism over a broad temp range.

I believe that is part of what's going on when there is an intentional delay between pulling the crucible from the oven and quenching. Maybe the 'art' part of it is the insulation of the crucible and charcoal on the slight cooling of the part, and the unevenness of the cooling of the part, just before it hits the quench.

I have case hardened steel for hammer or trigger sears or emergency homemade taps and dies using the old formula Kasenit, or even using plain table sugar to supply the surface carbon. But I have never done bone charcoal pack color case hardening, though I'd like to try it one day. The 1333 degree F temperature is the critical temperature for annealing steel. Hardening steel takes a higher temperature and involves a quench in air, oil, brine, or water, depending upon the steel alloy. Drawing the hardness is done at much lower temps. I have read of a wide range of temperatures that will work for carburizing or case hardening the surface of steel with time as an additional factor... lower temperatures naturally taking more time. Doug PA24 addressed this aspect earlier, and I do not know what effect time and temperature has on colors. Obviously, Doug has found a very nice combination of time, temperature, materials, and methods. I would assume that annealing steel in a pack of carbon rich charcoal or the like would not only prevent scale, but also add carbon to the surface of the steel, but would not result in colors or surface hardening without the quench.

The stainless steel foil does make a very good one-time use crucible, but as craigd notes, it is easy to puncture on a sharp edge. Speaking of sharp edges, the foil itself will cut you like a razor, so caution is required. Paper, sawdust, or the like placed inside with your parts will do a better job than steel wool of consuming any oxygen inside which would form scale because it will use up any free oxygen at a lower temperature. I've done just as well preventing scale when annealing parts or spring steel by using a capped section of pipe as a crucible with a very small vent hole drilled in one cap, and some wood or paper inside to consume the oxygen. A small plug of fire clay is used to seal the vent hole after the correct annealing temperature is reached. The large annealing furnaces at the steel mill where I did my electrical apprenticeship used inert gasses such as nitrogen and, believe it or not, hydrogen to purge out the oxygen and prevent scale. Our Salem furnaces used nothing more than dry sand to seal around the base of the furnace to keep oxygen from entering. We had a plant where we did catalytic cracking of liquid ammonia into nitrogen and hydrogen for the annealing departments.

Also, from what I've read, keeping the medium in close contact with the part is essential for good color coverage. The way the parts hit the quench is also important. The Marlin Forum thread shows extensive use of racks in some of the posts, with very nice results. On another site, I read where some students at a Gunsmithing school were wrapping parts with wire with good results as well. I'm going to be experimenting with those aspects, as well as with specific shielding to see what happens. One thing is certain, the Midway video that shows the parts and medium dropping loosely into the quench doesn't result in good color coverage.