Ed Harris on Metallurgy
Friday, March 16, 2012 Filed in: General gun stuff, Shooting industry
(Editor’s Note: Ed’s back with an incredible article on firearm metallurgy! This originated as a reply to an email from a “DG”. Ed gives some phenomenal information on the metals used at his employer, Sturm Ruger, to build their guns. I think you’ll find it very interesting, if a little complex!)
DG: A toolmaker friend wants to know what types of metal are used in a revolver. Having read your posts, I figured you would probably have the answers. Please feel free to be as technical as necessary...(Editor's Note: remember, folks, he asked for it!)
EH: At Ruger chrome-moly revolver frames are typically 4140LS blended at the mill to specific (and proprietary) chemistry to give the desired structures in the cast parts. Mostly this involves holding the sulphur within very stringent limits which are lower than those used by other manufacturers, and having additional restrictive requirements to eliminate silicates or phosphorous to the extent that they are below the detection limit by x-ray diffraction. There are some other elements which are manipulated to get specific properties related to the casting process which I am not at liberty to discuss, but suffice to say the investment casting process varies depending upon whether you are working with CM (chrome moly) or SS (stainless steel.)
The stainless is vacuum melted and poured under controlled atmosphere, such as in argon or nitrogen, whereas the CM can be poured in ambient air, though oxidation protection is provided by pouring a powdered antioxidant over the open mould sinks after the sprue is full.
All of the steel used at Ruger is ordered in 100-ton heat lots and produced by a continuous casting process which ensures uniformity in the billets produced. The billets are then cropped, and rolled per Ruger's specs.
Cast parts generally incorporate about 50% virgin material, and 50% remelt scrap which results from Ruger's own operations. Scrap is kept separate by machining line and is tagged by heat lot and type of material so heat lot integrity can be maintained as long as they are running that batch. A sample of every lot of material cast in the foundry is sent to the lab for analysis, generally 4 times per shift.
The cast parts are visually inspected, annealed, straightened, then gaged, sorted and either x-ray or ultrasonically tested. Rough machining is done in the annealed state. Finish machining is done after final heat treatment.
Barrels and cylinders are not machined from castings, but are produced from bar stock or forgings, depending upon the gun model. Barrels and cylinders are generally heat treated to Rc35 Min at Ruger, whereas other makes are typically 20-24. Ruger frames are generally Rc 28-35, whereas a lot of S&W frames used in the Model 10 and similar guns won't even register on the C scale, but may be around 80-90 on the B scale.
The stainless material used for revolver frames and cylinders is a 410 series, whereas barrel stock is a modified 415. Lockwork is a 300 series stainless in both blued and stainless versions. Critical parts like barrels and cylinders are 100% Magnafluxed using the wet method with circular continuous magnetization.
After final assembly proofing is done with standard military HPT or SAAMI specification proof cartridges, one per chamber. I might note that some other makers do not proof all six chambers of a revolver, but try to cut corners on the proofing. If all six chambers are not proofed the cylinder is not equally stressed and you may not detect flaws such as secondary piping, or nonmetallic inclusions or laminations which might occur in the melt shop at the steel mill because the fellow cropping the billets was having a "bad hair day".
We set up our steel specs and receiving inspection on barrel and cylinder steel to pretty much eliminate that type of problem by specifying ingot position, and requiring on-line ultrasonic and x-ray testing of the bars, which were also bumper straightened and checked with eddy current for flaws before the mill length bars were loaded onto the trailer.
When we received a shipment we'd take samples, cutting the ends off of a specified number of bars, based on a statistical sampling plan, and run them into the lab to verify the structures and chemistries against the mill cert. We'd send the driver off to a local hotel for a steak and a shower on us while it was going on so he wouldn't be as unhappy if we rejected the batch and told him to take it back (which we did a few times when I was there).
When I was there only two mills, Timken and SKF, were able to consistently produce 4140LS to our specs for cylinder blanks and Mini 14 receivers and bolts. This material is almost identical to Navy-nuclear pressure vessel grade material, and exceeds normal gun-barrel quality. Similarly, the stainless was vacuum melted, argon-oxygen decarburized and ladle refined similar to a Navy-nuclear or aerospace bearing grade of material.
Most of the other makers buy standard AISI grades in gun barrel quality, typically 1137 for shotgun, blackpowder and .22 rimfire barrels and 4140 for centerfire barrels. Most stainless target rifle barrels are made of 415 or 416 series stainless, but both the re-sulphurized CM and the free machining SS (which produce "mirror finish quality") have sulphur or selenium additives to improve machinability. If the distribution of these elements is nonuniform, the clumped inclusions can form stress risers which impair ultimate strength. For this reason they cannot be used in applications such as M14 or M1A barrels which have complex exterior machining which might produce stress risers. Nor can they be used in hammer forging of barrels which will undergo significant reduction and elongation. Generally, steels used for cylinder blanks or for hammer forge barrel applications cannot exceed 0.006% max. S or Se.
We spent a lot of time and money at Ruger developing tooling, coolants and processes which would permit machining to good interior finishes with materials giving the maximum ultimate strength and ductility. We had our own vacuum heat treating facilities in-house for stainless, and gas furnaces for CM.
Some types of stainless, such as used for Mini-14 firing pins and barrels and Redhawk revolver cylinders, would get a nonconventional cryogenic stress relief rather than the usual low temperature (1045-1050 deg F) "bake" to normalize. This, combined with the particular chemistry we used, resulted in firing pins which were file hard but which you could bend into a pretzel shape without any cracks, and barrels you could elevate to cook off temperature with 180 rounds of full auto fire then set up a bullet-in-bore obstruction and fire a proof load in the hot barrel without it bursting. Try THAT with an M16!
We converted entirely to synthetic coolants, such as Trimsol 6-8% concentrate in distilled water while I was there and got all the chlorinated paraffins out of the shop entirely. We ran hourly refractometer readings on the coolant used in the CNC machining centers and had thermocouples at the machining stations to monitor the incoming coolant temperature and the exit coolant entering the scavenger pumps, and fed the used coolant through filtration, centrifuges and heat exchanging equipment before putting it back into the pipeline. We also set up our own water treatment and recycling plant to purify city water to remove the chlorine, because we could not use it to mix machine coolant. This also permitted us to recycle machine coolant water and dispose as hazardous wastes.