M1A1 bolt heat treatment?

[newmexicocoonass]

Planning on trueing the sear notch on my M1A1 bolt as part of a tune-up. The bolt itself doesn't seem as hard as I thought it would be; file checking indicates 22-25r.

 

Anyone know what the metal is and what the heat-treat specs are? I would imagine they were through-hardened, but I'd like to learn more before I proceed. Thanks !!

 

Tommy

[TSMGguy]

Knowing that M1A1 bolts are US Ordnance Dwg. #6533921 maybe you can conduct some research and find the hardness?

Edited September 1, 2022 by TSMGguy

[reconbob]

I am away on vacation and so do not have access to my files and drawingsbut

 

An original M1 or M1A1 bolt is so hard a file will skid right off and not cut. Buried in Dougs papers I can probably find the alloy of steel used, but in the WW2 era the Ordnance Department did not use the Society of Automotive Engineers (SAE) steel numbering system. 4140 is an SAE steel. The steels called out on Ordnance drawings have a WD prefix which stands for War Department. I have been on the lookout for a conversion chart for SAE/WD and have not found one. But I think the reason for the WD steels is that they have tighter alloying specifications than the SAE steels. Maybe this is why there is no conversion chart.

 

A hardness of Rockwell C32 is nothing. If that value came from checking a Thompson bolt there is a mistake somewhere. If a bolt is only C32 it could be easily filed, hacksawed, etc. and would not hold up to the hardened sear.

 

Thompson bolts are probably RC 58-60 which puts them at the low end of case hardening.

 

The M1A1 bolts we (Phila Ord) make are machined from stress relieved 4140 which is hardened as high as we can go with 4140 which is around RC 54. We do not case harden the bolts to get up into the low RC 60s because we have not been able to do that without the round bolt head warping and bending. I have made and sold around 150 4140 M1A1 bolts and they seem to holding up just fine.

 

Getting back to the original question if your bolt is RC 22-25, its not hardened at all. I dont get it. The raw 4140 steel we make the receivers from is probably around RC 18-20 which is so soft the steel supplier gives the hardness on the Brinnel scale. (HB 180-220).

 

So if your bolt is that soft something is wrong. I guess you could keep using it but you should really find a correct bolt.

 

Thompsons are pretty indestructible. A few years back we had a post sample (07/02) that was a rental range gun and the guy sent it to be rebarreled because the barrel was shot out. I asked him how many rounds thru the gun and he said 50,000+. I was looking forward to seeing the effects of s well documented 50K rounds.

 

The gun got here and it was pretty dirty but when we cleaned it up it looked like it had 1000 rounds thru it - not 50,000. All of the parts in the trigger frame and the bolt had almost no wear.

 

Bob

[TD.]

I don't know a thing about the discussion, above. What caught my attention was the reference to bolt hardness by newmexicocoonass. Don't forget that Auto-Ordnance Corporation, West Hurley, New York manufactured or subcontracted for the manufacture of M1A1 bolts when their supply of USGI M1A1 bolts ran out. I have never seen one of these WH M1A1 bolts but have been told that the WH M1A1 bolts are not manufactured to USGI specifications and should be replaced. I would assume the WH M1A1 bolts are unmarked but that is just an assumption on my part.

 

Does any forum member have one of these WH M1A1 bolts?

[newmexicocoonass]

Some great information coming out here, thanks all. So the bolts are cased rather than through hard?

 

NMCA

[StrangeRanger]

That would depend on the steel used. Most high strength alloy steels like 4140 and 4340 are at their best when through hardened, low carbon alloy steels like 1117 are intended to be case hardened. Without knowing the composition of the WD steels, it's a guess. Given the application of high cyclic use and concentrated impact on the bolt head I'd just about bet the farm on through hardening and possibly a steel closer to 4340 (nickel-chrome-moly alloy) than to 4140 (chrome-moly alloy)

 

If someone with a British modified '28 bolt was willing to get the machined surface of the safety notch hardness tested that would answer the question. Since case hardening is only a few thousandth deep, if the machined area is soft then the bolts were case hardened.

[newmexicocoonass]

Sounds like I need to ID the bolt. If it is cased. I wouldn't want to cut through it.

 

NMCA

[reconbob]

I did not mean to convey that Thompson bolts are case hardened. What I meant was that at RC60 the hardness is in the low end of parts that are case hardened.

Buried in Dougs papers/archives is heat treating data - I will have to see if I can find it,

I would say that the bolts are hardened throughout, not surface hardened,

Back when all this was happening - WW1 and shortly after - alloy steels were in their infancy. When you made a part for a gun, it was either raw steel, or case hardened.

As an example the WW1 Springfield started out with what they called a single heat treated receiver which was pretty much case hardened and was brittle. In 1918 the developed what they called a double heat treated receiver which had a hard surface for wear resistance but a tougher core for strength. It was only a couple of years later that AO was developing the Thompson so this was all happening when alloy steels, their mechanical properties, and ability to be heat treated were new and not yet fully understood.

 

Bob

[StrangeRanger]

I can't put my hands on Hatcher's Notebook at the moment but he listed the alloys used in 03A3s. IIRC the bolt and receiver were 4140 and 4340 but I don't remember which was which. Given that nickel increases toughness and impact resistance but is a bit harder to machine, I would guess that the bolt was the nickel part

[newmexicocoonass]

Bob - understood. I do color case hardening from time to time and my cut off point is WW1 and later - there's a lot of steels that I can't identify or predict how they will respond to real bone and charcoal color case hardening.

 

I didn't think truing the sear notch would hurt, but when in doubt, ask. Appreciate all the great info !!!

[mnshooter]

I would assume the WH M1A1 bolts are unmarked but that is just an assumption on my part.

 

Does any forum member have one of these WH M1A1 bolts?

 

Though I no longer own it, West Hurley "True M1" (Serial No. in the low 700 range), had an unmarked bright M1A1 style bolt.

Edited September 4, 2022 by mnshooter

[TSMGguy]

Does any forum member have one of these WH M1A1 bolts?

 

Yes. I retained the unmarked WH bolt as well as the cast WH parts when PK sorted through my WH M1. He replaced all of the WH parts with NOS USGI M1 TSMG parts with the exceptions of the frame and receiver. It was noticed at the time that the WH M1A1 bolts were longer than original USGI bolts. Can't find the thread now.

 

Small Arms Review Story here:

 

THOSE WEST HURLEY THOMPSON PARTS - Small Arms Review

Edited September 6, 2022 by TSMGguy

[TD.]

TSMGguy,

That is one of my favorite stories!

 

PM sent.

[1gewehr]

Here are pictures of the original WH bolt from my M1A1.I do not know how many rounds the gun had through it before i got it. But, this should clearly show why the WH M1A1 bolts should be replaced with a good GI bolt.

[Doug Quaid]

Steel is one of the materials that has a crystal structure. The mechanical properties are changed by adding energy to the structure and also adding impurities, such as carbon and alloying materials.

Steel is extremely complicated mechanically and the way it is used is also complicated.

Also, all through the 1900s, steel formulation and use was in rapid development.

So it's impossible to assume what metal was used for what purpose at what time and how it was "worked" or hardened.

For example, AFAIK, the Garand was designed to use a high nickel alloy steel. But as soon as the war started, they switched to low alloy steel. Because nickel is a strategic metal and was needed for other stuff.

One thing that you do know is general stuff like what kind of metal is used in firearms generally. You do know that in general, steel used in firearms has very good "toughness", the ability to absorb energy without fracturing. Like the steel they use in crankshafts.

And you know that hardening steel causes it to become more brittle. Which is why it is frequently case hardened instead of through-hardened. Case hardening increases the surface hardness without causing distortion or embrittlement of the part.

The thing to do would be send a bolt to a lab and let them test it. Unless someone can find the material specs.

[giantpanda4]

Generally, on the drawing (especially for military applications) and in the revision block there would be information on what material and how it was processed. I do not have a set of drawings but I suspect they are there....

 

So no need to keep guessing if someone can find the information on the drawing!

[TD.]

PM sent to 1gewehr

[reconbob]

I have many original WW2 Ordnance drawings and the heat treatment is not on the drawings. Sometimes the steel - WD is listed - but thats it. The heat treatment and hardening seems to be on separate sheets or charts.

 

Bob

[giantpanda4]

Then I stand correct - thanks Bob