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Someone asked me this question on another gun board:

I've heard, from several sources, that sometime between WWII and the introduction of the Series 70, Colt greatly improved the metallurgical qualities of their pistols. Not sure if they started using better steel, better heat treatment technology or both, but supposedly those newer Colts were considerably stronger than the ones they replaced.Do you happen to know when this change took place?
Can anybody answer this question?
 

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I know the pistols of the early '50s came to be known as 'hard-slides' because of the improved heat treatment, but I don't know the details.
 

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Prior to the end of WW2 the slides on Colt pistols were only oil-quenched or flame spot-hardened in critical areas. Technology didn't exist at the time to harden a slide along its length without warpage. Remington Rand (one of the M1911A1 contractors) perfected Austempering, but they didn't use it on their military contract guns. Colt did pick up the technique however immediately after WW2. The first few thousand commercial pistols assembled after the war's end used up existing GI slides, but once they began making new commercial slides from scratch (around 1948 or so) they used the Austempering method, and ever since then the slides have held up very well. Just ask any gunsmith who has to try milling a Colt slide for dovetail sights.

Frames were not hardened, nor are they today either to speak of. It must work out fine because the old GI frames being used by the MEUSOC boys have been holding up to literally hundreds of thousands of rounds.
 

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The original frames were 1035 or 1127 steel heat treated prior to heat treatment according to Kuhnhausen II pg 69, then commercial frames "were" 4130 or 4140 Chromoly (I don't know when they started that, but chromoly would seem preferable to mild/medium carbon steel to me.). Pg 81 says "Late ordnance specified slide mat'l is gun quality 4140 or 8650 steel". I also read that both frames and slides were made soft during the war, and flame or induction hardened slide lugs sounds like a likely compromise.

Oh yeah, they mentioned that frames were hardened before machining, 22-24 RC (Pretty soft, but not dead soft by a ways) at some early point, which abrogates warping problems. I would love to go back in time and watch the process.
 

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Interesting, I'll have to dig out my Kuhnhausen books. I had been told before that the frames weren't hardened, but if they were made 22-24RC then the information may have simply been that they weren't hardened much.
 

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I have a book with reproductions of the Army drawings for the 1911A1 pistol. The receiver drawings a pretty hard to read being reduced in size from a "D" or "E" size down to about 11x14. The notes on the drawing say the receiver material is to be 1035, Spec QQ-S-631, 1137, Spec QQ-S-637, or another type of steel that I can't read. There are no notes concerning hardening or heat treating the receiver. The original date for the drawing is Jan 15, 1936, though the reproduction is not good enough to read any of the revisions or the revision dates.

The slide drawings are interesting. They show that the slide material should be 8650 or 4140 (Gun Quality) steel with Austenite grain size of 5 or smaller. The locations to be hardend are the breech face (RC33 - 46), the lead corner of the dust cover curve to the bottom of the slide rail (RC40-44), the slide stop notch (RC40-44), the take down notch (RC40-44), and the thumb safety notch (RC40-44). The interesting thing is the detail drawings have origination dates of March 11, 1960, but one of the revision notes looks like it is dated 1947, so it appears this drawing was a replacement drawing, as one other slide drawing has May 1, 1928, as the original date.

Another thing that is interesting about the drawings is the tolerances, at least the ones I can read. In most cases they are quite respectable if not what might be considered snug. In most cases critical dimensions only have a single direction tolerance rather than a +/- tolerance. Plus there are a few places that I have seen so far that have tolerances that are noted in ten thousandths (1/10,000) of an inch. So I now no longer believe any of the things I have heard about the Army loosening the tolerances of the pistol to make it easier to interchange parts from different manufacturers. I believe the opposite happened, the tolerances got tighter.
 
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