This is a letter from Dr. Gaddy to Frank Brownell regarding Case Coloring temperatures. Very informative.

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Regards
Ken

Thanks for sharing this. I order from Brownell's from time to time, mainly because they are in Montezuma Iowa, and I served in the USMC-- anyway, sounds like the late Dr. Gaddy was a bit tripping over his ego when he chided the "Head Fred" at Brownell's for their failure to give him the credit he felt was due him.. Not in the "Don Vito Corleone playbook"..

Sure, but I think he was correct in pointing out the pitfalls of the Brownell-recommended temperatures. Especially since he had performed the actual analysis to determine the lack of Martensitic hardening present in Vintage doubles. This was also independently confirmed to me by the owner of an industrial heat treatment firm, who recommended that I allow the pack to cool to lower than critical temp before quenching. This falls in line with Gaddy's recommendations.

I'll be confirming this myself once a nagging hand injury heals. For those of you out there who have sent me sets, this is the reason for the delay. I don't want to take the chance on dropping anything as well as the safety considerations when performing high temperature processes.

Regards
Ken

Nice find Ken. Thanks for sharing it.

Thank you for posting that. I have had arguments with other gunsmiths who stated it was absolutely impossible to achieve a case hardened surface if the parts were quenched below critical temp.

Having done experiments myself, I know this to be untrue. I have quenched with temps down to 1150 F (after holding in the 1400 range like Mr. Gaddy recommend) and have had parts that stood up to very heavy filing without being able to break through the case. And more importantly, very minimal to no warping.

It is nice to read Mr. Gaddy's very detailed description of the science behind why that is true. I will commit it to memory.

FWIW, I considered myself a good friend of Oscar's. I probably averaged a visit per month and we did hunt together (with Dolly).

What I didn't do was ask him much about the color case process, mainly because I thought I had enough to do and the process was his to distribute not mine.

He did show me how to Niter (nitre) blue one day and it stuck in my mind like glue. I firmly believe that he was correct, it is an oxide. I do have anecdotal evidence to back this up. I had niter blued a trigger guard and quenched it in what I now believe was water that was to cold and the niter blue flaked of just like some of the old Winchester blue.

I'm going to post a picture of a Fox that I did many years ago that Oscar color cased for me. Its hard to describe the colors but they are lovely and transparent - best I can do for a description.

Oscar did indeed have an ego - don't we all! He was, however a professor of electrical engineering and computer engineering at the University of Illinois when he retired and was at the top of his game when it came to firearm metal finishes at the time. He also had a wonderful but dry sense of humor.

As the carbon content of steel increases its critical temp becomes lower. I was a machinist, not a metallurgist, but I highly suspect what Dr Gaddy was doing was reaching the higher critical temp for the low carbon steel & then as it absorbed carbon dropped the temp to reflect the lower critical temp of the now higher carbon content. I believe it is necessary to be above the critical temp for hardening to take place, & also some minimum amount of carbon must be present. I do not know what the absolute minimum is but know that regular heat treating is not generally done on stells from around 1020 & down. These steel are normally carburized & hardened (Case Hardened).
The carbon content in the case on carburized steel can become quite high, so would have a lower critical temp than most carbon steel alloys.

I can also agree with His comments on Nitre bluing. And Brownells still incorrectly describes this process and how to do it.

I use their salts, and i heat them to much higher temps than they suggest with excellent results. It does not destroy the salts as they suggest.

Miller, it could be possible that the thin case isn't hardened in a heat treating sense, but might skate a file. A thin oxide layer, not related to case colors, can be hard on cutting tool edges, and some folks like to remove it first before machining a piece. In any event, an abrasion resistant surface may meet the goal of wear resistance of the working parts of a gun.

I think if temps are actually 1200* to 1250*, that'll likely be less than critical temp for any steel. I think Doc Gaddy's drop and hold at a lower temp keeps some predictability in the process. The other strategy of letting the crucible cool for five, or whatever time, minutes before quenching the contents seems more related to the 'art' of case coloring.

American Machinery's Handbook. The Critical temperature at which pearlite is transformed to Austenite as it is being heated is also called the decalescence point. The temp at which it turns back to pearlite upon cooling is called the recalescence point. The recalescence point may be, depending upon the exact alloy, from 85° to 215° F Lower" than the decalescence point.
"These critical points have a direct relation to the hardening of steel. Unless a temperature sufficient to reach the decalescence point is obtained, so that pearlite is changed into austenite "No Hardening Action Can take Place and unless the steel is cooled suddenly before it reaches the recalescence point, thus preventing the changing back again from Austenite to Pearlite no hardening can take place"".
I put in over 35 years as a machinist, with some 25 of tham as a modelmaker where I wore out a vast number of files. I never had a file "Skate" on a part that was not Glass Hard.
Personally I would want no shotgun frame of mine quenched at a temperature lower than that recalescence point, regardless of who did it.
The Decalescence point seems to bottom out at about 1325°F with .85% carbon. I have not looked up the Recalescence points but I highly suspect Dr Gaddy was staying barely above them.1325°F - even the low limit of 85°F would equal to 1240°F. There may be just a bit more room to play with than that.

It's easy to demonstrate what you're describing at home. If you check a piece of steel with a magnetic when it's heating up, there will be a point when it suddenly becomes non magnetic. If you remove the heat, the piece will stay non magnetic while it noticeably cools and will slowly regain magnetism. It'll still be in decalescence much cooler than it took to put it there.

The interesting part of Doc Gaddy's comment was that he didn't catch the temp of the steel coming down, he brought it down to below 'critical', and then held it for an hour. The other interesting tidbit is he said that case hardened original receivers tested to have pearlite in the thin case layer, not martensite which would be the form it would be in if it were hardened steel.

I think file 'skating' and 'glass hard' is a bit subjective. When I ever do it, I'll only try it where there's no chance of showing. Just because the file doesn't 'bite' into the steel doesn't mean the finish won't get scratched up. I think case coloring is sort of playing games with poor heat treating practices in exchange for appearance and fit. I think that's Doc Gaddy's point. Only thoughts is all.

We should also remember that back when L.C. Smith, Parker and Ithaca (among others) started, 1020 mild steel with lower carbon content was the "state of the art" for forging receivers on drop board molds-- and "double fitting" was std. for gun making then- not until WRA, a pioneer in metallurgy working with USSteel, developed the M21 and the fitting of heat treated AISI 4140 nickel chrome moly alloyed medium carbon steels- as heat treated, eliminating a costly and somewhat in-accurate surface hardening technique- Dr. Gaddy was a very well informed man, and his detailed analysis and printed results are 'gospel"--

It's been too many years since Oscar shared here. He was spot on, in his letter to Mr. Brownell regarding temperatures. He often warned folks he'd help that the information was not correct from Brownell's. Drop a line to Mr. Schilling who's helping many European Manufacturers now if you're not convinced.

Geography for the Running Fox, and all.
Early morning 27 July this year, I drove down into Montezuma to "open up" Brownells. There they were....GONE.
An open "corner store" told me they had moved up onto I-80 closer to Des Moines.
So Brownells is now located in Grinnell Iowa - but NOT several klicks north up into the town proper (ask me how I know), but rather almost just right off I-80 exit.
Watch for the sign - which should BY NOW (one would hope) be raised and visible from the Interstate.
And the "new" Brownells, is NOT the dinky little order office where you ordered your parts on a "fill it in yourself on the pre-printed form" and then the staff went off into the warehouse and found your selected items for you. The new Grinnell location is a mega state of the art warehouse with a co-located large retail store. In the retail store, sit at the computer bank, follow the bouncing ball on the computer screen, submit your order, wander around for 10 or 15 minutes right in the big store, or sit down and read the interesting magazines in the commodious waiting room, and then your "picked order" from the co-located warehouse appears, pay the lady, and get back on I-80.