Stainless Steel Metallurgy Question

loeman

loeman

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I'm curious about stainless steel as I know next to nothing about metallurgy. Correction, I know nothing. Does the original finish on a S&W stainless steel gun have a final finish? Here's what I mean. Does S&W do anything other than polish, brush or bead blast their final finishes?
 
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I would think not. Stainless is the same from the surface all the way through unless it's plated for some reason. I can't think of any reason that would be done


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There are different grades of stainless also; different quantities of the metals that make up the alloy. Which is usually steel with chromium.


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My thinking is that if it's shiny, unlikely to be passivated. A frosty finish seems the norm.
 
As we all know from the many threads regarding Flitz, Mother's Mag Polish, etc., someone always brings up that using these products will do something to the "top coating". Probably what Murphydog is speaking about in his post above. I'm just curious if there really is a "top coat". Some of us have probably had a stainless steel gun for nearly 50 years. I know I have, and darned if doesn't look just like it did 50 years ago, just like me! LOL. Is this due to what S&W may have applied or is it just good stainless steel through and through.
 
Most stainless guns are 410 or 416 alloy. With minimal care, these alloys will stay nice indefinitely. These are called "400 series" SS. They have enough carbon that they can rust if neglected. Any rusting on these will take place more slowly than on the 4140 carbon steel that most blued guns are made of.

The more rust resistant grades of "300 series" SS are ones like 303,304, etc., with 316 being the marine grade of SS. Even that one will corrode over time with salt water exposure. The 300 series is too soft to make guns out of, due to the lack of carbon, which gives the carbon steels more tensile strength.
 
Most, if not all, SS firearms are mfg from 400 series stainless, and it is magnetic. Passivation is an acid bath, but does not leave a film, it is to remove any loose ferrite particles from the surface. 400 SS is also heat treatable, 300 SS, (food grade), is not.

Have a blessed day,

Leon
 
I knew this forum and its members would come through with the answers. Thanks to all. Now I can say I know something about metallurgy.
 
ME NIETHER, BUT...

Chromedhearts said:
I would think not. Stainless is the same from the surface all the way through unless it's plated for some reason. I can't think of any reason that would be done


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Click to expand...

I did buy a pair of Sigs once, both stainless versions a 239 & 220 that had been nickel plated???
 
300 Series Rusting

The lower grade 300 series, Austenic stainless steels will surface rust. The most common grade 300 will acquire light brown surface rusting over time. Higher 300 series grades that contain stabilizing alloys, 316 (molybdenum) or 347 (titanium) will generally not rust in normal environments. As noted above, none of the 300 grades are hardenable by heat treating, so are generally not used in firearms parts.......generally. SS firearm parts that require any strength are 400 Martensitic series that are hardenable. Metallurgy is ever evolving, but one thing is certain. You don't really know the grade and temper of the part. It's a trade secret and should be. All SS grades are mostly iron. Rust is the red/brown iron oxide that you see. A light film of any good oil or protectant cannot hurt any SS firearm.
 
"The chromium [a component in the stainless alloy], in the presence of air (oxygen), forms a thin film of chromium oxide which covers the surface of the stainless steel. ... The stainless steel passivation process chemically removes these free irons and forms a passive oxide “film” layer which further improves corrosion resistance."​

It's the film of chromium oxide that imparts the desirable "extra" margin of rust resistance. And it's thin, only a few molecules thick.

It can be induced by applying a weak acidic solution and wait a few hours. Otherwise, it will form naturally over time from being exposed to the oxygen in the air. You couldn't stop it if you wanted to.

What is Passivation? How Does Stainless Passivation Work?
 
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The Stainless steel used for firearms is usually type 416 stainles but in some cases type 436 is used. ALL 400 series stainless steels are magnetic due to type of crystallization in the steel. Sorry but I just can't remember the specifics, steel alloys can either be Face Centered or Body Centered crystal shape and one is magnetic and the other is non magnetic.

BTW, standard mild steels also exhibit this behavior but in the mild steels the shape of the crystal is related to the temperature. So you can make a mild steel non magnetic by simply heating it to the correct temperature but it will become magnetic again when it cools.

The 400 series steels contain a higher percentage of Iron in the alloy so they are not as corrosion resistant as the 300 series steels. However 400 series steels can be heat treated just like high carbon mild steels and type 416 is so similar to 4140 high carbon steel that it can be heat treated in the same batch as 4140 and the final products will have nearly identical hardness and strength. So, in some respects type 416 can be thought of as a corrosion resistant 4140.

Because 400 series stainless steels are somewhat prone to corrosion it's typical that this grade of stainless is "passivated" near the end of the final finishing process. What is done to "passivate" the steel is that it is dipped in a strong acid solution that actually leaches Iron out of the surface of the steel, typically to a depth measured in thousands of an inch. Naturally a longer soak acts to provide a bit more corrosion resistance but one "cost" of doing this is the surface can become more vulnerable to wear marks because that thin layer sort of resembles sponge on a microscopic level. So, Passivation is a bit of a balancing act. After passivation a light buffing with a felt or canvas wheel is usually done to bring a luster back to the surface.

I'm guessing here but I expect the finishing procedure at S&W probably goes something like this. First the brushed finish is applied using specialized "buffing" wheels. Next the parts are passivated by an acid bath followed by a rinse in a slightly alkali solution to neutralize any remaining acid and then a dip in a water bath. After that they will have a luster finishing operation that simply buffs the finish up without really doing any material removal.
 
Where's the nickel......

I would have there would been SOME nickel in it.

Chemistry Data : [top]

Carbon 0.15 max

Chromium 11.5 - 13.5

Iron Balance

Manganese 1 max

Phosphorus 0.04 max

Silicon 1 max

Sulphur 0.03 max
 
loeman said:
I'm curious about stainless steel as I know next to nothing about metallurgy. Correction, I know nothing. Does the original finish on a S&W stainless steel gun have a final finish? Here's what I mean. Does S&W do anything other than polish, brush or bead blast their final finishes?
Click to expand...

S&W finished Stainless Guns in different ways over the years and models. Some were bead blasted, some were just polished to a satin finish while there were a few finished to a high polish that almost looked like Nickel.

In general, they did not apply a finish over the Stainless Steel but I would bet over the many years they might have tried a black finish or two.
 
I think you will find that the "passivation" process is the final step in the actual making of the alloy (ingot, plate, sheet,etc) and the way it is sent to the gun companies, after the gun is actually built there is no further "passivation" taking place, just a brushed or matted finish done by the company building the gun.......being stainless is relatively soft, and further polishing, matting, ( 3M Scotchbrite pads) or any metal polishes Flitz, semichrome, Mothers, etc will enhance the final finish............
 
chief38 said:
S&W finished Stainless Guns in different ways over the years and models. Some were bead blasted, some were just polished to a satin finish while there were a few finished to a high polish that almost looked like Nickel.

In general, they did not apply a finish over the Stainless Steel but I would bet over the many years they might have tried a black finish or two.
Click to expand...
Back in the 1970s the Connecticut State Police carried Model 66 revolvers with a dark gray, almost black, finish because the brass thought the regular finish was too reflective.
Who wouldn't love to own one? Maybe someone on this forum has one?
 
chief38 said:
S&W finished Stainless Guns in different ways over the years and models. Some were bead blasted, some were just polished to a satin finish while there were a few finished to a high polish that almost looked like Nickel.

In general, they did not apply a finish over the Stainless Steel but I would bet over the many years they might have tried a black finish or two.
Click to expand...

Correct. They made a run of black finished 686s many years ago. I forget, but I think maybe they called the finish Black Magic or something like that.

In addition, S&W has applied the metal QPQ treatment called Melonite (referred to as Tenifer in Europe, but same process) to some stainless models including 3rd Gen Autos and some revolvers.

They may have had other black finishes on stainless models over the years, but those two come to mind.
 
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