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Best choice of steel for color case hardening?

Posted: Mon Sep 04, 2017 10:51 pm
by Rwilliams
I have made many parts out of different grades of steel in the past but never with color case hardening in their future. With many choices of steel to work with on this future project, I would like to make the best choice that will respond well with good color when completed.

So far I have considered regular 1018 CRS, 12L14, and Starrett low carbon free machining #498 ground flat stock. Which of these materials would yield the best color case hardening when properly prepared?


Re: Best choice of steel for color case hardening?

Posted: Fri Oct 27, 2017 6:54 pm
by N.R. Davis
I heard 1018 works fine. Any way you can do some Test Pieces?

Re: Best choice of steel for color case hardening?

Posted: Sun Oct 29, 2017 5:26 pm
by Rwilliams
With over 200 views and no responses until recently, I began to wonder if color case hardening was a well kept secret.

Sorry, I do not have a way to do test pieces. Without a good temperature controlled furnace, quality heat treatment is out of my league.

I do have options for the variety of steel and was wondering what knowledge might be out there in regards to something other than regular 1018 steel.

Re: Best choice of steel for color case hardening?

Posted: Sun Oct 29, 2017 8:50 pm
by GlennW
Try contacting Turnbull.

He knows a thing or three about color case hardening.

Re: Best choice of steel for color case hardening?

Posted: Mon Oct 30, 2017 7:33 pm
by WesHowe
I am not an expert on case hardening, by any means. One thing I know is that 1018 is harder to get a good finish on than something like 12L14 or 4041. I would think that maybe anything you are color case hardening might also have good looks as an objective (as opposed to just case hardening it with some Cherry Red and a torch).

I have blued 1018 and I can tell you that all the machining marks are still visible afterward.

Re: Best choice of steel for color case hardening?

Posted: Thu Jun 21, 2018 4:34 pm
by tetramachine
Case hardening refers to adding carbon to the outer surface of steel, which allows it to be harder than normal. So it takes low carbon steel which would not be hardenable, and causes the areas that are now higher in carbon to get hard.

To induce carbon into the surface, the work is heated to it's Curie point, stuck into a carbon Rich bath(many kinds) until cool. Now it's surface is very hard, too hard to be used without cracking, it is now brittle hard. So it needs to be tempered. The work is now an ugly black color. It is polished, and the art of color comes into the work. There are compounds that brushed on leave surfaces tinted. Now the work is heated again, to tempering temperature 400-900 F. Let to cool on air is when the colors come out. They are caused by the reaction with oxygen and surface coating.

Re: Best choice of steel for color case hardening?

Posted: Sun Dec 16, 2018 4:16 am
by WolframMalukker
What type of steel colors best is irrelevant.

What kind of steel you use, should be based on what kind of stresses it will be required to endure. The heat treatment HAS to be based on what you require it to do after final hardening and tempering.

Color case hardening is done the same as any other old-school case hardening-in a sealed container with the part, bone charcoal, wood charcoal, leather charcoal/powder, etc. Everyone seems to use a different mix. Personally I use powdered bone charcoal and powdered hardwood charcoal. I use two different methods depending on what end result I need.

The part I'm casing will be backed in a 80/20 mix of hardwood to bone, then the can is sealed with a 1/32" hole for a vent. The whole pack is into the furnace for 6 hours, (that's for my particular furnace and pack size) at the temperature that the steel I'm using (AISI 8620) requires. For me, that's 1700F. 8620 is an oil-hardening steel, and I carburize at 1700F for 6 hours, take the whole pack out and let it cool, blocking the 1/32" vent hole with a block of steel. Once cooled to room temperature, I remove from the box, wrap in stainless foil (I leave all the carbon stuff stuck to it) and and reheat to 1550F for 1 hour per inch thickness, then it comes out of the furnace, out of the foil, and into the quench oil. This provides me with a hard, tough core, with fine grain and a somewhat coarser grained, very high hardness case. The colors are there as soon as it comes out of the foil, and goes into the quench tank. The part will be unmachinably hard-you will have to grind it if you need to cut it. There is NO TEMPERING STEP.

For a firearm receiver, I generally don't need maximum strength, so I heat in the pack to 1700, allow the pack to come to room temp. I'll unpack it, wrap in the stainless foil and heat to 1550 for 1 hour per inch, strip it out of the foil and quench. Then, re-wrap in stainless foil with a sliver of wood or bone, heat to 1350 for 1 hour per inch, and strip it out of the foil and quench. THERE IS NO TEMPERING STEP. As before, the colors are there as soon as it comes out. No painting, polishing, or anything else needed or wanted.

In order to utilize the differential carbon content to the advantage in the second process, you have to heat the core to the critical temperature of the core, and quench from that point. Then, you will heat to the critical temperature of the case, which is lower due to the higher carbon content, and quench from that point. Now, both steels (you have created a single part consisting of two different compositions of steel) have been hardened to their optimal strengths-A somewhat soft, tough, but very impact-resistant core, and a case hardness that will defeat a file.

Re: Best choice of steel for color case hardening?

Posted: Thu Dec 20, 2018 8:17 pm
by mihit
I have recently been contemplating setting this up. The method I was going to use involved the pack-carburising (bonechar and charcoal, sealed box), but once out of the kiln/furnace, immediately it's dumped into a tank of water with air being bubbled through.

I have been given to undertand that the differential cooling gave resultant colours. I don't get to choose the steel that comes in, it's all gun metal.

The piece would then need annealing/tempering.

Re: Best choice of steel for color case hardening?

Posted: Fri Dec 21, 2018 5:25 pm
by WolframMalukker
Unfortunately, without knowing the alloy you can not be assured of any colors. One part may work, the next may be flat grey, the third might come out with a huge splotch of blue and a single gold speck. It will be almost impossible to nail down any kind of hardness as well.

The colors are due to the thickness of the oxide layer on the surface, and the composition of those oxides. Keep in mind your part will need to be fully polished BEFORE you harden it, because the colors polish off in *ten-thousandths* of an inch of depth.

Re: Best choice of steel for color case hardening?

Posted: Sat Apr 27, 2019 2:15 pm
by David2011
I was a student and later an instructor at a community college gunsmithing course. Some of the students were highly skilled and built some beautiful rifles that had color case hardened receivers. The method they used after prepping the metal was to make blocks to support tangs and other thin parts that could/would warp. These blocks had tapped holes to attach the receiver's thin parts. The metal to be treated was placed in a crucible of welded steel along with the charcoal, bone charcoal, leather bits and any other ingredients. The crucible was sealed with clay; no vent hole. The crucible was put in the oven to heat soak at 1400° F. During this time air from a compressor was run through the water in a 55 gallon barrel to oxygenate it. When the crucible was removed from the oven after about 2-1/2 hours it was taken immediately to the barrel of water with tongs and the tongs were rapped on the edge of the barrel, dumping the hot contents into the water. There was a basket with a screen liner in the bottom of the barrel to retrieve the parts and charcoal. No further treating was done other than to keep the parts oiled as they aged for a few days.

Re: Best choice of steel for color case hardening?

Posted: Thu Nov 21, 2019 2:46 pm
by Herb Kephart
Years ago, an "Old Timer" told me to use chicken bone, broken into the smallest pieces possible. Worked rather well, but as time went on I learned that bone is loaded with Phosphors, which makes steel brittle. The brittleness would only be on the surface, but that's where cracks start. Is all this not worth worrying about--or completely BS?