OFCC University - Metallurgy 102 (External Coatings)

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OFCC University - Metallurgy 102 (External Coatings)

Postby Morne » Mon Dec 22, 2008 11:30 pm

The next topic prompt from Mr. Glock:

2. Coatings: External (outside the barrel..bluing, parkerizing, etc)


There are several categories of external coatings. Some are designed to work against wear and others against corrosion. LOTS OF CLAIMS get made - I'll try to dispel some of them:

Corrosion Resistance Coatings have performance often measured in terms of "hours of salt spray". This is a farce. While the ASTM B-117 salt spray test does give some indication of corrosion resistance it has limitations. For instance, if you replaced the lab-controlled salt solution with actual seawater you get substantially different results due to the microbes present. Do not take the "Salt Spray Resistance" numbers and extrapolate a head-to-head comparison for real world corrosion resistance. Just like everything else that never leaves a laboratory the salt spray cabinet has no idea what is really out there. 8)

Phosphating - either Manganese or Zinc (Zinc being better) - AKA Parkerizing
Long a classic, this is a reasonably good corrosion resistant coating. The bare steel parts are immersed in a phosphoric acid solution (hence the name) and the surface gets a very small amount of the phosphate compound deposited onto it. Due to the bubbling (that's hydrogen coming off) the coating is porous. This porosity is desirable for either future paint application or impregnating with something lubricious (like oil). Because it is so flexible (corrosion resistant, basis for paint, basis for oilling) it is a popular choice to this day.

Black Oxide - AKA Bluing
Another timeless firearm finish - this has less corrosion resistance than Parkerizing. For those who wonder - there is no chemical difference between "bluing" and "black oxide". Processes exist for both "hot" and "cold" application with the "hot" generating a thicker and overall better coating. Unlike the Phosphating acidic process Black Oxide is applied via a basic process (remember your acids & bases?). Essentially, you are oxidizing the iron atoms in a controlled fashion (unlike rust, which is uncontrolled). Doing this results in VERY LITTLE measurable coating thickness. While the black oxide layer won't flake off like red rust it will still permit oxygen to diffuse through it. Thus, it only slows down corrosion - it does not stop it. Cosmetically this coating can be quite attractive but that has more to do with how much polishing is done beforehand than any chemistry in the tank. This process only works on steels (not aluminum/titanium/polymers). While you could do it to a stainless steel there are better ways to protect them.

Passivation of Stainless Steel
Protecting stainless steel is easy. You clean it really well, grit blast it and drop it in a heated nitric acid solution. Unlike the Black Oxide process which goes after the iron the Passivation oxidizes the chromium atoms in stainless steel. The resulting extremely thin layer (no dimensional change is noticed) of chromium oxide is a true barrier to further oxygen diffusion. Thus, passivated stainless steel can be expected to survive a long time in ambient atmospheric conditions without corroding at all. To some degree the stainless steel would do this on its own when exposed to oxygen - the trick is getting a uniform layer before any iron atoms oxidize and form a pore in the surface. Thus, a passivation leyer is partially self-healing, though a good scratch/gouge might later prove to be a point of attack. Obviously, the more chromium content in the stainless the better this works but there MUST be at least 12% free chromium available or it won't work at all.

Paintable coatings
There are many paintable coatings available for today's firearms. Everything from el-cheapo Krylon to pricy-DuraCoat has been used. The single most critical thing with any of them is SURFACE PREPARATION. With good surface prep (usually cleaning/degreasing and grit blasting) the coating will adhere. Since these coatings are merely barriers to oxygen diffusion (and buffer layers for wear) they have to remain in place to work. If they flake off they won't work. Thus, a properly prepped surface with Krylon over it is better than an improperly prepped surface with DuraCoat. All the latest and greatest whiz-bang organic chemistry on earth won't save you if your coating isn't on your gun. And yes - some of the better paints have interesting corrosion resistant chemicals embedded in them like strontium chromate. You pay the big bucks you get the good stuff. Still, since this coating has some real thickness to it you have to consider the dimensional tolerances of parts especially in dynamic interfaces (like frame/slide rails).

Anodizing
This is what you do to protect aluminum from corroding. Just because aluminum "can't rust" doesn't mean that it can't oxidize. In point of fact, both aluminum and titanium are actually MORE ACTIVE than iron and thus want to oxidize (corrode) even more quickly than iron. There are a couple of kinds of anodizing to know - regular and hard. Regular anodizing is a non-porous corrosion resistant barrier layer, about .0002-.0004" thick, formed of aluminum oxide. This is a GREAT prep for painting. Hard anodizing is much thicker, running about .0015-.004" thick, and usually gets impregnated with something like Teflon afterwards to both improve its wear properties and to seal up those pores. Since it is porous in nature it is NOT as good in corrosion resistance as regular anodizing. Still, a few thousandths of an inch of aluminum oxide (the stuff the grit in sandpaper is made of) can really resist wear. Both regular and hard anodizing is done in an acidic bath (chromic, sulfuric and boric are all popular) with the aid of an applied electrical current. Just like Black Oxide you are oxidizing the aluminum atoms in the surface in a controlled fashion. However, since electricity is forcing the process forward you build up a thicker layer. When calculating the dimensional change for anodizing it should be noted that half of the coating thickness is grown INTO the part and half OUTWARD. Thus, a .002" hard anodize will only close up a hole's diameter by .002" (rather than .004" on the diameter or .002" per side/radius like with a plating).

Wear Resistance Coatings often like to brag about their "hardness". This is misleading. While hardness is a factor in wear resistance many others things play a part (coefficient of friction, surface finish, material of the other side of the wear couple, bearing strength, lubrication, etc...). Wear is a phenomenon that you can't simply throw a coating at and solve the problem. In many cases you just have to accept that wear will inevitably occur and pick the part to get worn out (the sacrificial part in the wear couple). You can perhaps extend the time between repair/replacement but everything wears out eventually. Usually you pick the cheapest part but sometimes you pick the easiest part to replace. I always chuckle when I read of someone getting a wear resistant coating on an AR-15 bolt carrier. Tell me, rifleman, which is easier to replace - the bolt carrier or the UPPER RECEIVER?!?!?! 8)

TiN - AKA IonBond
Popular on Deagles, drill bits and general issue pimp gear this is NOT the tin coatings your grandpa was used to seeing on steel. This is TiN (titanium nitride) not tin (latin = stannum) - VERY different animals. TiN is deposited via a vapor deposition method (either physical or chemical) in a heated vacuum chamber. The result is VERY, VERY THIN usually gold-colored ceramic coating of exceptional hardness. Given its rigidity (like most ceramics) it is vulnerable to chipping. Also, as the PVD/CVD process isn't the greatest for adhesion if pretreat cleanliness isn't maintained poor QC will lead to flaking problems right quick.

Chrome
Chrome plating is a time-tested wear coating. It is deposited from a chromic acid bath, catalyzed with sulfuric acid, onto steel parts. You need to have special tooling, called anodes, made from lead that roughly conform to the shape of the surface to be coated. The part is made cathodic using a DC power supply and the current causes the chrome in solution to "plate-out" on the steel part. While surface prep is always important the inherent rigoroursness of forward-current electroplating makes for good ahesion even on very smooth (non-grit blasted) surfaces. Thus, you can get the surface nice and smooth first in the steel and then plate chrome over it (and polish the chrome, too). After plating most heat treated low-alloy steels will need to be "baked" to remove the hydrogen they picked up during the processing. If not, the hydrogen will "embrittle" the steel which is bad news. Interestingly, chrome is deposited as one crystal structure but changes (usually during the "bake") to another shortly after deposition. The resulting specific volume change and almost total lack of ductility cracks the chrome. While EXCELLENT process control (even some aerospace-grade platers don't know all the tricks to it) can limit the severity of the cracking and prevent them from going all the way through the chrome plating you must figure that some through cracks exist. Thus, chrome by itself is not considered a barrier to corrosion.

Electroless Nickel
There are three types of electroless nickel - hi-phos, mid-phos and lo-phos. The "phos" indicates the "phosphorus" content of the plating - the higher the "phos" the harder the plating. Electroless nickel, unlike chrome, doesn't require any electrical current to "throw" the plating onto the part. Instead, the nickel hypophosphite plating solution autocatalytically decomposes and "drops" nickel out of solution onto the steel part. Since this is not the greatest method for good adhesion the pre-treat cycle is critical. Done right you can take a ballpien hammer to the nickel on a corner and it won't chip - do it wrong and it'll come off during normal assembly. Sometimes the "right" surface prep includes such nasty chemicals as hydrofluoric acid (wanna give your EH&S guy a heart attack just ask him about THAT chemical). Just as with chrome you have to bake heat treated steels afterwards but this process also hardens, and cracks, the nickel plating. Since it is inherently cracked (the harder the more cracks) it is not considered to be an effective barrier against corrosion. Still, with the right "phos" level it can be about as hard as chrome plating.

Carburizing - AKA Case Hardening
This is not so much a finish as a heat treatment process. Back in the old days (think mid to late 1800's) the carbon steel frames on guns could rust pretty easily. The best "bluing" process they had back then was a sort of "plum" bluing that was essentially controlled red rust - really not that great of a process. As such, they were looking for a way to prevent red rust from "spalling" off of their guns and so borrowed a trick from the old sword makers - carburizing (a form of case hardening). Basically, they would take the part and pack it with ground up charcoal or (hopefully animal) bones and then put it into a furnace. The high temps would make the carbon in the charcoal/bone mobile and allow it to diffuse into the surface of the steel. When it cooled the surface was now much harder (since it had more carbon) while the core of the part was still resilient (having the original lower carbon content). The hard surface could still rust, but because the steel matrix was so bloody hard it resisted "spalling" or "flaking off". Thus, you could take some steel wool to the rust and then oil it and be fine - unlike the car bodies we've all watched bubble up with rust over the years. Obviously being a harder surface gave it wear resistance, something useful for all them holster-draws the cowboys were doing. In modern times we don't do the "packing" method anymore but instead use a carbon-rich heat treating atmosphere (some mixture of cracked natural gas) to accomplish the same thing.

Salt Bath NitroCarburizing - AKA Tennifer & Melonite
This is a subset of case-hardening. Regardless of the fancy-pants marketing name slapped on it the process is the same. You take a steel part and immerse it in a molten salt bath that contains cyanide (2 more things your EH&S guy will just love - molten salts and cyanide). The high temperature from the molten salt (around 1,000 F IIRC) makes the nitrogen and carbon mobile and they diffuse into the surface of the steel. Both nitrogen and carbon almost equally strengthen steel so having them both do it gets the steel harder faster (and time at temp is money to a heat treater). Just like carburizing this does NOT prevent corrosion, it just keeps down on the ugly red rust flaking off. Its advantages over "carburizing" include a faster heat-up time, quicker achieving of hardness and greater case-depth uniformity.

NOTE - Stainless steel parts that get EITHER carburized OR nitrocarburized are, metallurgically speaking, no longer stainless. The reason is because the carbon combines with the chromium to form chromium carbides. This decreases from the available "free" chromium content needed to form the tightly adherent chromium oxide film. Lacking that, the steel becomes just a very expensive low-alloy steel. Why firearms makers bother to make parts from stainless steel and then nitrocarburize them is completely beyond me. They'd get virtually identical performance from a low-alloy steel like 4140 that got nitrocarburized and it would cost a LOT less. For you welders out there - this same phenomenon can happen when welding certain grades of stainless steel. That's why there are "L" grades of the 300-series stainless steels, the "L" indicates low carbon content so as to avoid making the weld-zone non-stainless.
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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby THUNDERKISS » Mon Dec 22, 2008 11:50 pm

Wow. Excellent information there, Morne. Thanks.

Quick question: How does cobalt plating fare against chromium and electroless nickel plating in terms of hardness and durability? Is H2 embrittlement still an issue or is it more a function of the lower alloy being plated?

Thanks.

:)
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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby Morne » Tue Dec 23, 2008 9:02 am

I didn't know anyone was using cobalt plating on firearms. Is this OEM or aftermarket?

Elemental cobalt and elemental chromium have almost identical hardness values (electroless nickel is hard because of precipitated nickel phosphides). That said, the plating process does affect the hardness of the coating. What little I could find on cobalt indicates that it is also an electroless process and as such it should be fairly dense. Unlike electroless nickel it doesn't need any impurities to give it hardness so it might not even crack right away like electroless nickel and chrome do.

And yes - I'm sure that an embrittlement relief bake would be required. If nothing else the pre-treat steps are likely embrittling to the steel.

Cobalt gets used as an additive in several other coatings (like tungsten carbide) that are common outside of the firearms world. If you want to go grinding on a cobalt-containing coating you need special grinding coolant - otherwise regular coolant will leach the cobalt right out of the coating (bad for both the coating and the poor guy running the machine).

As an aside - cobalt is very expensive. The biggest deposits of it are found in places where the politics are even less stable than Jell-O.
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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby Mr. Glock » Tue Dec 23, 2008 10:11 am

Another good chapter, Morne.

So, basically, the corrosion-resistant coatings prevent oxygen from reaching the steel, while wear-resistant coatings create a harder coating (which may be perimiable due to cracking)?

I understand that salt, water and steel creates a low-powered "battery" effect, which is why saltwater or winter-time road salt increases steel corrosion....how does this happen to accelerate the O2 penetration?

Kimber used thier "KimPro" finish on the recent SIS line, over stainless steel. It was very slippery, but said to resist human sweat very well...does this fall into your last category (Salt-Bath Nitrocarburizing)?

On the Glock Tennifer: Early Glocks had a "rough" flat finish on the slide, while today's are "smooth" shiney surface...any difference?

Early XD slides used a process that did not resist wear/corrsion as well as a current day production...was this the same process, just not done as well or correctly?

Let's say you were in a sailboat, and carrying a Glock....since Tennifer is not corrosion resisitant, but obviously more flaky corrosion resistant that plain steel, what form does the corrosion take (ie how do I see it visually)?
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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby Morne » Tue Dec 23, 2008 12:05 pm

Mr. Glock wrote:So, basically, the corrosion-resistant coatings prevent oxygen from reaching the steel, while wear-resistant coatings create a harder coating (which may be perimiable due to cracking)?


Pretty much. Zinc Phosphating actually chemically resists corrosion even though it is porous by letting the Zinc get attacked before the steel. The really, really good corrosion resistant coatings used in other industries don't get used on firearms (like Cadmium Plating with a chromate conversion coating). Frankly, the environment for a handgun just isn't that aggressive compared to other industries (nuclear power generation, chemical processing, effluent scrubbing, desalination, etc...). Thus, we can get by with non-optimum solutions most of the time. Also, just using a stainless steel like 17-4 PH and then passivating it will solve darn near any corrosion problem a gun design will ever see. Simple solutions are often the best.

I understand that salt, water and steel creates a low-powered "battery" effect, which is why saltwater or winter-time road salt increases steel corrosion....how does this happen to accelerate the O2 penetration?

Water that has impurities dissolved in it (pure water is actually not conductive) acts as a conductor. All corrosion mechanisms are just electro-chemical reactions - providing a liquid circuit board for them just speeds them up. Saltwater conducts even better and thus permits the electro-chemical process to progress even faster. The oxygen diffusion isn't necessarily any faster, but the rate of oxide formation sure speeds up.

Kimber used thier "KimPro" finish on the recent SIS line, over stainless steel. It was very slippery, but said to resist human sweat very well...does this fall into your last category (Salt-Bath Nitrocarburizing)?

I believe the KimPro finish is just a phenolic resin coating - nothing but a fancy paint-like layer. Marketing hype strikes again.

On the Glock Tennifer: Early Glocks had a "rough" flat finish on the slide, while today's are "smooth" shiney surface...any difference?

The amount of polishing before Tennifer determines the degree of glossiness.

Early XD slides used a process that did not resist wear/corrsion as well as a current day production...was this the same process, just not done as well or correctly?

Not sure what happened with the early XD slides.

Let's say you were in a sailboat, and carrying a Glock....since Tennifer is not corrosion resisitant, but obviously more flaky corrosion resistant that plain steel, what form does the corrosion take (ie how do I see it visually)?

You'd start seeing dark pinpoints that would slowly spread until the whole surface was a smooth red/plum color. At some point, the pitting corrosion mechanism would override the hardness in some spot and blistering would begin. After that, it would be just like a car body. The difference is that it would take longer for the blistering to start than for an untreated carbon steel.
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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby THUNDERKISS » Tue Dec 23, 2008 2:25 pm

Morne wrote:I didn't know anyone was using cobalt plating on firearms. Is this OEM or aftermarket?

Elemental cobalt and elemental chromium have almost identical hardness values (electroless nickel is hard because of precipitated nickel phosphides). That said, the plating process does affect the hardness of the coating. What little I could find on cobalt indicates that it is also an electroless process and as such it should be fairly dense. Unlike electroless nickel it doesn't need any impurities to give it hardness so it might not even crack right away like electroless nickel and chrome do.

And yes - I'm sure that an embrittlement relief bake would be required. If nothing else the pre-treat steps are likely embrittling to the steel.

Cobalt gets used as an additive in several other coatings (like tungsten carbide) that are common outside of the firearms world. If you want to go grinding on a cobalt-containing coating you need special grinding coolant - otherwise regular coolant will leach the cobalt right out of the coating (bad for both the coating and the poor guy running the machine).

As an aside - cobalt is very expensive. The biggest deposits of it are found in places where the politics are even less stable than Jell-O.



Morne-

It is being applied "aftermarket" by:

http://www.overlandplating.com/services.php

Their prices don't seem to be too "over the top" given the additional expense of using Co over Cr and I assume (rightly or wrongly) that their use of "triple hard" Cr implies the presence of an increased quantity of the apropriate hardening impurities. (Phosphides, too?) In the past, I had toyed with the idea of getting one of my Arsenal milled underfolder AKS-47s plated, but eventually decided against the measure due to the (small) risk of damage that H2 embrittlement posed. Thanks for the answers, too, they confirmed my concerns and I am now convinced that I made a good decision. :D

While we are "talking shop" would you mind explaining the difference in relative hardness between cold-rolled 70/30 cartridge brass and low carbon steel alloy? Specifically what is the differential hardness between the two on a similar scale ike Rockwell C or B ?

Thanks again!

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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby Morne » Tue Dec 23, 2008 4:06 pm

THUNDERKISS wrote:Their prices don't seem to be too "over the top" given the additional expense of using Co over Cr and I assume (rightly or wrongly) that their use of "triple hard" Cr implies the presence of an increased quantity of the apropriate hardening impurities.

Their "triple chrome" is decorative, not hard. They call it "triple" because it has 3 layers - copper, then nickel, then (very thin) chrome. That is normal for decorative/automotive chrome. Oddly, they seem to use the same scheme for hard chrome and just make that top layer thicker. That's just plain weird. Most hard chrome platers use a reverse current etch in the chromic acid plating solution to activate the bare steel and then just plate immediately. That way it is chrome on steel ONLY. The correct chrome to put on a gun is HARD CHROME.

In the past, I had toyed with the idea of getting one of my Arsenal milled underfolder AKS-47s plated, but eventually decided against the measure due to the (small) risk of damage that H2 embrittlement posed. Thanks for the answers, too, they confirmed my concerns and I am now convinced that I made a good decision. :D

Don't let me run you off of getting your AK chromed. Hydrogen embrittlement, while a very real phenomenon, can be handled properly with a bake. Chrome is permeable to hydrogen and will bake right out through it. Now, copper and nickel are NOT permeable to hydrogen and as such "triple chrome" will trap hydrogen like a rising credit card interest rate. That is why hard chrome is usually put on bare steel - it gives the hydrogen an escape path to relieve the hardened steel underneath.

While we are "talking shop" would you mind explaining the difference in relative hardness between cold-rolled 70/30 cartridge brass and low carbon steel alloy? Specifically what is the differential hardness between the two on a similar scale ike Rockwell C or B ?

Definition = HR_ = Rockwell Hardness Scale (underscore has the scale indicator)

Here are a couple of resources:
http://www.ejbmetals.com/pdf/datasheets-metal_strip/Ca260.pdf
This shows that fully hardened brass is about 70-73 on the HR30T scale.

http://www.hardnesstesters.biz/article/008.html
Here we can convert that range to approximately 81.5-86.0 HRB. Please note that such a conversion is not technically kosher and as such should be taken with a grain of salt.

Low carbon steel, say something with around 0.20% carbon and heat treated to about 75,000 psi ultimate tensile strength, would be roughly HRB 81.5. This is too low to even register properly on the HRC scale. The very top 3 HRB numbers (98-100) correspond roughly to the bottom 3 HRC numbers (20-22). Thus, virtually anything on the Rockwell "C" scale is going to be harder than virtually anything on the Rockwell "B" scale.

Does that help?
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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby willbird » Tue Dec 23, 2008 4:55 pm

Then there is ion nitride. Not sure if it is used much in firearms, but it can achieve much the same effects as melonite does.

It is used on Engine crankshafts and on Cat 40 milling cutters for cnc mills.

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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby THUNDERKISS » Tue Dec 23, 2008 7:04 pm

Morne wrote:
THUNDERKISS wrote:Their prices don't seem to be too "over the top" given the additional expense of using Co over Cr and I assume (rightly or wrongly) that their use of "triple hard" Cr implies the presence of an increased quantity of the apropriate hardening impurities.

Their "triple chrome" is decorative, not hard. They call it "triple" because it has 3 layers - copper, then nickel, then (very thin) chrome. That is normal for decorative/automotive chrome. Oddly, they seem to use the same scheme for hard chrome and just make that top layer thicker. That's just plain weird. Most hard chrome platers use a reverse current etch in the chromic acid plating solution to activate the bare steel and then just plate immediately. That way it is chrome on steel ONLY. The correct chrome to put on a gun is HARD CHROME.

In the past, I had toyed with the idea of getting one of my Arsenal milled underfolder AKS-47s plated, but eventually decided against the measure due to the (small) risk of damage that H2 embrittlement posed. Thanks for the answers, too, they confirmed my concerns and I am now convinced that I made a good decision. :D

Don't let me run you off of getting your AK chromed. Hydrogen embrittlement, while a very real phenomenon, can be handled properly with a bake. Chrome is permeable to hydrogen and will bake right out through it. Now, copper and nickel are NOT permeable to hydrogen and as such "triple chrome" will trap hydrogen like a rising credit card interest rate. That is why hard chrome is usually put on bare steel - it gives the hydrogen an escape path to relieve the hardened steel underneath.

While we are "talking shop" would you mind explaining the difference in relative hardness between cold-rolled 70/30 cartridge brass and low carbon steel alloy? Specifically what is the differential hardness between the two on a similar scale ike Rockwell C or B ?

Definition = HR_ = Rockwell Hardness Scale (underscore has the scale indicator)

Here are a couple of resources:
http://www.ejbmetals.com/pdf/datasheets-metal_strip/Ca260.pdf
This shows that fully hardened brass is about 70-73 on the HR30T scale.

http://www.hardnesstesters.biz/article/008.html
Here we can convert that range to approximately 81.5-86.0 HRB. Please note that such a conversion is not technically kosher and as such should be taken with a grain of salt.

Low carbon steel, say something with around 0.20% carbon and heat treated to about 75,000 psi ultimate tensile strength, would be roughly HRB 81.5. This is too low to even register properly on the HRC scale. The very top 3 HRB numbers (98-100) correspond roughly to the bottom 3 HRC numbers (20-22). Thus, virtually anything on the Rockwell "C" scale is going to be harder than virtually anything on the Rockwell "B" scale.

Does that help?



Morne-

As usual I never cease to be amazed by the thoroughness with which you answer these types of questions.

Yeah, it helps. I ask for a "post it note" size answer and receive an encyclopeadic response. Wow, dude. Simply...Wow. And thanks. Can't forget that.

After reviewing the material that you posted and perusing the links that you posted I think that I have a pretty decent "handle" on what you are trying to convey. If I've absorbed correctly that which you've explained above my understanding is that while not technically "appropriate" cross scale conversions between the HRB and HRC measurements are almost mutually exclusive of each other in that they cover disparate ranges of hardness and that any "overlap" between them appears to be largely an artifact of the conversion process from one scale to another.

The resulting conversion of the cold rolled fully hardened "70/30" cartridge brass which lays within the 70-73 range upon the HR30T scale and "converts" :wink: to an "approximate range" of 81.5-86.0 HRB which in turn falls into the "ballpark" of the hardness of low carbon steel (0.20% carbon, heat treated to about 75,000 psi UTS), at roughly HRB 81.5. I then take it then that both could be then be assumed to possess the same (relatively) hardness for all practical purposes, the issue of "conversion" notwithstanding.

This addresses an issue that I have long suspected to be the case, in that cartidge brass jacketed bullets can be just as hard on barrels as those fabricated from "mild" steel which I believe to be the same animal as low carbon steel. Perhaps I've made a faulty assumption here and if so, I expect to be corrected on it.

As for the chrome plating of my AKS-47, you didn't "run me off" from anything since the decision was made well in advance of this discussion. I had contacted a few aftermarket sources for the service and after running into two that just didn't instill any confidence in me by way of their expressed, "purported" expertise over the phone, I began educating myself about the process and found them (the suppliers) to be kind of "flimsy" and I backed out. My "Chem-Fu" and "Metallurgy-Fu" is not nearly as refined as yours, but I am no fool and when I start hearing "double talk" my "B.S. meter" "pegs" and I leave. Just another benefit of being an overly cynical, suspicious-for-no-good-reason, retired cop. :mrgreen: Now that I've heard confirmation that their "triple hard" chrome is most likely anything but, in addition to the "weird" label bein' trotted out, I am less impressed now than I was before with it especially since there is copper and nickel underneath and I am glad that I didn't do it. Besides, I am an AK "purist" and it was a "psychically" painful thought to begin with. :wink:



You stated earlier that: "Stainless steel parts that get EITHER carburized OR nitrocarburized are, metallurgically speaking, no longer stainless. The reason is because the carbon combines with the chromium to form chromium carbides.

I hope that I am not over-simplifiying (though I fear that I am), but is this effectively a "form" of passivation of the chromium content of the stainless steel that doesn't require as high a chromium content as actual passivation since it utilizes a slightly different, yet nitrogenous, reaction?

Again thanks for the information, Morne. I'll be up until the "wee" hours poring over this stuff thanks to you. :roll: It is almost as if you've conspired to make me lose sleep by inundating me with fascinating material. I'll get you for this, I promise... :wink:

TK
:)

TK

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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby jgatsios » Tue Dec 23, 2008 7:11 pm

Awesome, Morne.

Question: I am considering the purchase of a DSA FAL. They offer the DuraCoat as a $300 upgrade over the standard Parkerized finish. Is it worth it?

Thanks.
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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby Morne » Tue Dec 23, 2008 9:06 pm

willbird wrote:Then there is ion nitride. Not sure if it is used much in firearms, but it can achieve much the same effects as melonite does.

Billmeister,

As usual you have hit a nail on the head that others might miss.

Nitriding, either via gas from cracked ammonia or the ion process you mention, is an EXCELLENT case-hardening method for stainless steels. Unlike carburizing or nitrocarburizing this is a carbon free process; thus allowing the "stainless" to retain its "stainless" character. I've only ever personally spoken to the chief metallurgist of one firearm manufacturer (S&W) but if I ever get Glock on the line they'll get an earful about needing to switch to gas or ion nitriding of those stainless slides. They can call it "Glockifer" or "Glockite" for all I care.

The big reason people like nitrocarburizing over plain-jane nitriding is that it is FASTER which equals CHEAPER. The technology is decades old, far from being cutting edge, but it gets you high hardness so quickly that you can shorten the time at temperature. Remember, to a heat treater time at temp is money.

THUNDERKISS wrote:This addresses an issue that I have long suspected to be the case, in that cartidge brass jacketed bullets can be just as hard on barrels as those fabricated from "mild" steel which I believe to be the same animal as low carbon steel. Perhaps I've made a faulty assumption here and if so, I expect to be corrected on it.

Mild steel is a low carbon steel. It is cheap (its most attractive quality) and not terribly strong. In fact, there are certain aluminum alloys that have a greater UTS than mild steel. Again, hardness isn't everything but it certainly is a part of the equation.

You stated earlier that: "Stainless steel parts that get EITHER carburized OR nitrocarburized are, metallurgically speaking, no longer stainless. The reason is because the carbon combines with the chromium to form chromium carbides.

I hope that I am not over-simplifiying (though I fear that I am), but is this effectively a "form" of passivation of the chromium content of the stainless steel that doesn't require as high a chromium content as actual passivation since it utilizes a slightly different, yet nitrogenous, reaction?

This is actually anti-passivation. Since the chromium gets tied up into carbides it is incapable of forming the oxide film that stops oxygen from diffusing inward. Thus, the (expensive) chromium content is virtually wasted. After this happens the "stainless" now behaves much more like a nitrocarburized low-alloy steel than it does a nitrided stainless steel. If it were my firearm company I would either use a low-alloy steel in the first place (then nitrocarburize it) or I'd use a stainless and nitride it ONLY. Mixing soup with "fertilizer" is not very appetizing, though both have their roles in the food industry.

jgatsios wrote:Question: I am considering the purchase of a DSA FAL. They offer the DuraCoat as a $300 upgrade over the standard Parkerized finish. Is it worth it?

IMHO it is not worth it. If you really want a DuraCoat finish then do it yourself (assuming you are comfy with detail stripping the gun). Phosphating (parkerizing) is an excellent base for DuraCoat and the application is cake. If you have an air compressor and an airbrush you should be able to do it. I have applied DuraCoat several times, including quite recently, and wouldn't dream of charging someone $300 for one rifle. Then again, if painting with an airbrush isn't your bag maybe paying someone else is a good idea. Personally, I grew up painting model cars, model planes and miniature figurines so it was no great leap.

I really like DuraCoat. I am a fan. I also like working on my own guns and magazines.

ETA - I just went to the DSA page and saw their camo DuraCoat patterns - NICE AS ICE!!! :D :D :D
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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby THUNDERKISS » Tue Dec 23, 2008 10:41 pm

Morne,

As always, thank you for taking the time to answer my questions. Have a Merry Christmas!
TK

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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby cromanos » Tue Dec 23, 2008 11:49 pm

I vote we make he Metallurgy series a sticky - there's some great info here!
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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby Greg Focker » Wed Dec 24, 2008 2:56 am

Where would the boron carbide coating that I have on a couple of my pocket knives fit into this?
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Re: OFCC University - Metallurgy 102 (External Coatings)

Postby Morne » Wed Dec 24, 2008 5:33 am

Greg Focker wrote:Where would the boron carbide coating that I have on a couple of my pocket knives fit into this?

Boron Carbide is a vapor-deposited coating in the same fashion as TiN. It has similar drawbacks. In the toolmaking industry these are common coatings, not so much in firearms.
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