Bob Dunlap--Metalurgy for Gunsmiths

Thanks very much for posting that link.
So much to learn...maybe just before I'm six feet under I'll feel comfortable referring to myself as a gunsmith...

Stainless (which he says isn't "real steel") is soft- as we all know. My primary experience with it has been mostly in marine applications over many years- where you learn that alloys referred to as "stainless"- that aren't 316L- aren't stainless, at all.

What strikes me as interesting, is that soft as it is- stainless, according to the manufacturers, is more resistant to throat erosion in barrel applications. So, my guess is that despite being softer, stainless is less subject to heat erosion- while being more subject to physical erosion- and by extension, throat erosion is mostly the result of heat rather than the actual burning powder being blasted through it (?). Make any sense?

Later he discusses how to determine the nature of the steel a part is made from. After grinding off any case hardening, he says to quench it, and "see if it got hard".

Is this able to be done, without four figure equipment for Rockwell or Brinell testing? Is there such a thing as files of varying hardnesses that could be used to try to etch the metal?
 
When I was in college in the early 80's, the "Metals and How to Weld Them", text book listed over thirty distinct types of AISI stainless steels. Not many are used in guns, but not all are soft...440-C is used in knife blades and it is pretty hard, as is the "AUS series" (Austenitic?) stainless as advertise as used some years ago by Kershaw.
 
There are spark tests, from grinding steel, which used to be printed in machinist books, about what sparks to look for.

The thing about a steel being case hardened, which you grind off, to see if it is hardenable, I would think, would defeat the purpose, since they generally only case hardened un-hardenable steel. Anything under 1016, I believe, wont harden, or they don't consider it worth it. It is easy to machine and form, but the carbon is very low. Steels like 1018, 1020, 1030-1035, and 1040-1045 are generally good choices for hardening.

The 300 series stainless doesn't have as much carbon in it, and has a high alloy comp. amount. The 400 and will produce a slight surface rust, and is hardenable. I'm pretty sure you can work harden 300 stainless though. I found that out by cutting a pump shaft off, in a bandsaw. It cut about 3/4 of the way, and that was it. A new blade didn't help. I had to finish cutting it with an abrasive chop saw, and that took a while.

edited to clarify from discussion below.
 
Last edited:
I clicked on the gear and went 2x talking speed. I already knew that about steel.

~ 15 years ago I bought his AGI Win 1897 video and it is very good and helped me build a shotgun out of a basket case I bought. Nice to see his face again.
 
Clark,

Yup, the short videos don't show much, but are educational for some. Really, I figure they're to draw you in to buy the longer versions, to be honest. I have the one he mentions, that does show him heating a S&W hammer in a crucible, with charcoal, to case harden it. It also explains hardening and tempering, etc..

One of the best investments I made, is buying the Society of Automotive Engineers (SAE) book, on steels. It tells each hardenable steel, what temperature to heat them to, and what to quench them in. Mine is from 1979, but that has all the gun steels in it.
 
It you don't already know what he is talking about, it will probably not make sense anyway. I have a piece of a receiver someone sent me before I moved and I have to find it and test it. There have been a lot of gunsmiths doing charcoal case hardening since Brownell's started to market their systems. I have seen examples of this one receiver color cased and I always thought that the receivers were 4140 (Or close). The only way I know how to color case is with water. I did it years ago with a carbon pack, but never used charcoal. Anyway, when I question using this receiver to color case I usually get an answer like "My Smith told me it is OK". When I ask the individual to question the gunsmith about what type of material it is, I never get an answer. Some people cut/weld receivers and then get them re-heat treated, but have no knowledge of the material the rod/wire is made of. I think material knowledge is a real problem in the gun fixit world. I will look for that receiver piece and post the results.
 
Really, the only need for annealing and re-hardening a cut and welded receiver, would be if it had been welded back around the locking lugs, where it needs to hold its heat treatment. The receiver sees recoil energy, that it dissipates into the stock, but if it's welded correctly, say if cut at mid-ways the frame, I doubt annealing for stress relief is really needed, except for the instance above, or the base metal is known to be brittle, though we all still have it done. Of course, a lot has to do with selecting the right rod or wire for the weld, and how it's welded. I use 70 type wire, or 7018 rod, which is 70,000 psi, and try to not put a lot of heat into the base metal, by alternating sides. I've not tried it, on cut receivers, but I wouldn't doubt it if would not hold up.

4140, shouldn't need case hardening, unless for looks, since it's tensile strength is really high, and it's hardenable; with something like 95,000-120,000 psi tensile, according if it is hot or cold rolled, and annealed or not. The yield is not quite half that, which I think is about 60,000 to 75,000 psi. With a 2:1 safety factor, you're talking about 30,000 to 35,000 psi, to design around. If you harden and temper it, it really shoots up. One might even get by with a 1.5:1 safety factor, as it is used a lot.
 
Quite a bit of that was covered at school. One test we learned from a book was to take the unknown metal (not a gun part) to a grinder and observe the color of the sparks. I learned from my neighbors (retired GM workers) that their guys at the plant did the same thing. This gave them a rough idea of what they had. If more information was needed, then the raw metal was sent upstairs to the metallurgy lab.
 
I don't even know if 4140 can be case hardened. That was my point. It can get like glass. It would be dangerous to water quench it, which is what is done when color caseing. Most guys never go out of the Mauser clan when they cut/weld. Mausers are junk metal to begin with, so welding will not really affect them if you stay away from critical areas and use a form of heat-sink. Same with Russian bolt guns. When you weld on a 4140 receiver it may get hot enough to anneal the lugs, depending on the form of welding. That is why I heat treat them again. I used to gas weld with 4140 rods and that gets the receiver seriously hot. I even welded up receiver rings this way. The result is: You get a complete receiver again and the blueing matches up way better.

File check: Cut a small piece of an old high-power gun barrel off, heat it up cherry red, and throw it in OIL. Check with a file. It should be difficult to get the file to even bite on it.

Do the same thing with a piece of cold rolled flat steel (Hardware store). It will file about the same both before and after you HT it. Once you feel the difference, you will know.

Check out Brownell's HT page. I have not looked in a while, but they used to have some interesting stuff there. I used Kasenite many times to to spot harden low carbon steel I had built back up with weld.
 
The 300 series stainless doesn't have any iron in it, I believe, but they call it steel.
Click to expand...

Where do ideas like this come from?
Cheap and nasty 304 stainless of the 18-8 cutlery family is minimum 65% iron.
 
Jim, for grade 303:

<0.15% C, 17-19% Cr, 8-10% Ni, <2% Mn, <1% Si, <0.2% P, >0.15% S

remainder iron

304:

<0.08% C, 17.5-20% Cr, 8-11% Ni, <2% Mn, <1% Si, <0.045% P, <0.03% S

remainder iron

404:

Iron, Fe Balance
Chromium, Cr 11-12.5
Nickel, Ni 1.25-2
Manganese, Mn 1
Silicon, Si 0.5
Carbon, C 0.05
Phosphorous, P 0.03
Sulfur, S 0.03

http://www.azom.com/article.aspx?ArticleID=964


http://www.azom.com/article.aspx?ArticleID=965


http://www.azom.com/article.aspx?ArticleID=8284

Edited to correct the compositions, that Jim caught, where I bungled it up.
 
Last edited:
You are reading the chart wrong, that is less than .15% Carbon.
Add up all the other alloy constituents and see what is left. That is iron.

The 300s are nonmagnetic because they lack enough carbon to form magnetic domains, there is plenty of iron in there.
 
Jim, you're right, as I used those charts when I was listening to what the video said about stainless, (not being able to call it steel), and they seemed to agree, when I first commented. The first chart has an S for silicon at the end, but no space, so I didn't notice it being there. It makes it look like the Iron, at the front is lower, but now I see the comma.

They didn't have the 300 compositions listed the same as the 404, in a regular chart. If they had, it would have cleared it up.

I fixed the charts above, so nobody would get confused the way I did.

I guess Bob got it wrong, at 1:18 in it, when he described the stainless series, by him saying you couldn't call it steel. I always took his word for it, and never looked. That much iron, and its a steel alloy.
 
Last edited:
I buy steel.
If I let the labels fall off, it is like losing the label on a can of soup.
I just lost 90% of the value.

I got a ~100 pounds of 4140 that had purple paint on it to show what it was.
There is no standard for what color is what steel. Each little shop has a different color code.

4140 is black
http://www.metal-stock.com/site/misc data/Metal Stock Colors Codes.html

4140 is blue
http://www.southerntoolsteel.com/color-codes.html


So when I die, who ever gets the purple steel, they may have to do some tests.
Maybe that Dunlap's video will get them going.
 
When I worked at RPM, making pump parts, all the scrap, the excess drops from the saw pretty much, was saved and color coded with spray paint, then stored in sections along one wall. The color was internal, or what RPM said it was.

Instead of one color, it was three colors, in three lines. The combination told what it was, from a list.
 
I found that piece of receiver that I was looking for. I heated a small piece with a torch and threw it in motor oil (Had no quench oil). Using a file, I could tell it got harder. I heated another piece and threw it in water. This piece was way harder. I no longer have access to a Rockwell tester, but if I had to guess, I would say it is in the 4100 series somewhere. Whatever it is, it has a high carbon content. This was a piece of a '99 Savage receiver. I have seen photos of these guns with color cased receivers. Maybe cyanide. I never did cyanide caseing and am not sure of the process. Anybody here have hands on experience with that? It sure seems dangerous color caseing high carbon steel with the old carbon pack/water quench method.

Another receiver I have machined on that "Feels" like 4140 is the 1936 MAS. There are others around.
 
I read about cyanide hardening, but forgot about it, as I didn't intend on doing it. There's too many dangers to it, plus a small shop might not be able to buy it.

What I remember is that you heat the part in molten salts. However, what would you do with any fumes?

From About.com's section on metals:

"A process of introducing carbon and nitrogen into the surface of steel by heating it to a suitable temperature in a molten bath of sodium cyanide, or a mixture of sodium and potassium cyanide, diluted with sodium carbonate and quenching in oil or water. This process is used where a thin case and high hardness are required."

One might try bone charcoal on 4140 scrap, and see what it does.
 
It will do nothing good. There is a chance it will go up near 60RC. 4140 is oil hardening steel. Even if you could use oil with a carbon pack (you cannot), wouldn't drawing back the hardness ruin the color? I still think it is a dangerous conversion.
I think it was back in the late 70's I was at a gun show with a buddy and he was looking at a table with M-14 rifles. We talked to the guy and they were de-mils that they welded back together and assembled with surplus parts. The dealer assured us they were re-heat treated. I asked what kind of welding rod was used and he just did a deer in the headlights look. That is a lot of slamming on that rifle.
There are a lot of shakey things done in the gun world and I used to see it all the time. How many times have you been at an auction and seen a Russian bolt gun converted to 30-06? How about a receiver with more than 6 base screw holes (One with part of a tap still in), or as in the video, globs of brazing rod?
 
You must log in or register to reply here.
Back
Top