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At the behest of @Lord_James, this shall be the thread for general discussion of conventional passive metallic armor.  Whether it's steel, titanium, magnesium, exotic laminates of all three, this is the thread for it.

 

In answer to your earlier question, Lord_James, relatively small amounts of boron, in steels that have the appropriate levels of carbon, form intergranular barriers that dramatically slow the diffusion of carbon out of the austenite crystals during quenching.  Long story short, this means that the depth of material that can be effectively hardened is much greater.

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Between when I asked the question and the creation of the thread, I’ve seen some stuff on boron in steel, though I haven’t explicitly looked for it. I did find that you can only apply a maximum of 0.03% by weight of boron in a steel, past that point it starts to reduce effectiveness. I’d seen boron mentioned in some DTIC articles I found, but I just assumed it was an impurity like sulfur and phosphorus. 

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On 4/14/2021 at 10:17 AM, Collimatrix said:



Rough translation anyone?

This looks like flash processing to me.

 

It uses a mix of medium and high frequency induction heating. With the device they patented, they are able to treat sections of steel up to 3m in length (which is a pretty big deal since it's usually hard to get NC steel plates that large). Talks about the quenching liquid and cooling rate to prevent cracking, but doens't mention which liquid was used, but does mention that Matensite does form in the cooling process. Talks about maintaining the correct space between the induction heater and steel plate and methodology to maintain that distance despite possible warpage from the heating process. Talks about how their methodology allows for the treatment of much thicker plates than what was normally possible before.

 

Basically they use a combination of medium and high frequency induction to allow for better heat treatment deeper into the steel (prior to this methodology single frequency induction was primary a surface treatment and didn't penetrate and heat treat the center of the steel). While the coil is moving around and heating the plate they dump an unknown liquid onto the surface at varying temperatures appropriate to the stage to quench the metal before it is once again passed over by the coil. The rapid heating and quenching at varying temperatures allows for better crystal structure formation as only certain crystal structures form under certain temperatures and conditions so the changing variables allows for more even and diverse crystal formation. Another effect of multiple treatments in rapid succession is that the grains constantly form, break apart then reform smaller each time which allows for the extremely small grain structure required of NC steel.

 

Video showing what induction heat treatment is like

Spoiler

 

 

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Ah, so that does sound like flash processing, or something fairly similar to it.

 

I am... not entirely clear on how flash processing works, chemically speaking.  You can do some interesting things with it that do not make sense to me.  The nanostructured bainite that has been of interest for armor applications lately has an extremely lengthy heat treatment process that requires that the steel be held at temperature for multiple days.  But flash bainite processing can produce a comparably strong microstructure in mere seconds.

 

Given how bainite actually forms, I don't understand how this is not just magic.

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20 minutes ago, Collimatrix said:

Ah, so that does sound like flash processing, or something fairly similar to it.

 

I am... not entirely clear on how flash processing works, chemically speaking.  You can do some interesting things with it that do not make sense to me.  The nanostructured bainite that has been of interest for armor applications lately has an extremely lengthy heat treatment process that requires that the steel be held at temperature for multiple days.  But flash bainite processing can produce a comparably strong microstructure in mere seconds.

 

Given how bainite actually forms, I don't understand how this is not just magic.


Is there any reason why you would use (lower) bainite over martensite? 

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1 hour ago, Lord_James said:


Is there any reason why you would use (lower) bainite over martensite? 


Yes.  Generally speaking, bainitic steels are fairly cheap, at least in terms of the raw materials.  I can't speak for how much the exotic heat-treatment processes needed to create them drive the cost.

 

The martensitic transformation is a diffusionless transformation.  In fact, not only is it diffusionless, but it works substantially better and easier the less diffusion there is.  For this reason, fairly expensive alloying elements like molybdenum and chromium are added to (among other things) prevent the diffusion of carbon out of the austenite crystals during quenching.

The bainitic transformation is the opposite; it's a reconstructive, diffusion transformation.  So, for the most part and with a few exceptions, all the fancy and expensive alloying elements dramatically slow down the bainitic transformation.



Edit:  I suppose there are some other advantages as well.  Quenching and tempering to form a traditional martensitic microstructure can cause significant distortion of the metal.  Not only does plunging hot metal into oil or water potentially cause it to warp, but there's actually a small (but incredibly rapid) volume change associated with the martensitic transformation.  In fact, this is why katanas aren't straight.

 

I don't think that the bainitic transformations are anything like as sensitive to part thickness and geometry as quench and temper heat treatments can be either.

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1 hour ago, Collimatrix said:

For this reason, fairly expensive alloying elements like molybdenum and chromium are added to (among other things) prevent the diffusion of carbon out of the austenite crystals during quenching.


Just to clear this up for me: there are low carbon maraging steels; would these require less alloys? 

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43 minutes ago, Lord_James said:


Just to clear this up for me: there are low carbon maraging steels; would these require less alloys? 

 

No, maraging steels require a pretty good amount of alloying elements, usually (a whole metric buttload of) nickel, cobalt and molybdenum, and then a dash of something strange like titanium or aluminum which forms the small, dispersed intermetallic inclusions that form during the final heat treatment.

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Maraging steels in general are low carbon, as they do not get their strength from carbon distorting the lattice but rather from metallic precipitates like Ni3Mo. Carbon is in fact not desired in Maraging steels.

For the Maraging steel to work properly you need a minimum of 19% Ni, which is very expensive, but you get what you pay for.

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Indeed.  One of the problems that occurs in steels with a lot of carbon and a lot of alloying elements is that instead of doing their respective jobs, the alloying elements and the carbon want to go off and play together and form carbides.  This is a particular problem in high-carbon stainless steels, as the chromium and carbon very much want to form chromium carbide instead of staying in solution and providing corrosion resistance and interstitial hardening, respectively.  There are ways to mitigate, but not entirely solve this problem.

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Is friction stir welding a (technologically or economically, take your pick) viable way to mate maraging steel plates (let's say something like 18Ni2400) or does it require extensive side treatment to make sure the crystalline structure is not weakened in and around (HAZ) the weld due to the heat? I also seem to recall reading some time ago that there were concerns about the durability of FSW equipment when working with MS, but I don't know if they're still relevant nowadays.

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1 hour ago, Renegade334 said:

Is friction stir welding a (technologically or economically, take your pick) viable way to mate maraging steel plates (let's say something like 18Ni2400) or does it require extensive side treatment to make sure the crystalline structure is not weakened in and around (HAZ) the weld due to the heat? I also seem to recall reading some time ago that there were concerns about the durability of FSW equipment when working with MS, but I don't know if they're still relevant nowadays.

 

I've seen some papers demonstrating that it can be done at a small scale, but I haven't seen anything saying yea or nay about large-scale, long-term durability.  So I don't know for sure.

 

Assuming that the weld itself is sound, one of the interesting properties about maraging steels is that the heat treatment can be "reset."  The strengthening mechanism in maraging steels is the precipitation of tiny inclusions of intermetallic compounds (which is why maraging steels usually have weird shit in them like titanium; that's what helps form the intermetallic).  By heating up the steel, these intermetallics can be re-dissolved, the steel cooled back down, and then heated up again to a somewhat lower temperature to re-precipitate out the intermetallics and re-harden the metal.

It is my understanding that doing this will basically erase any HAZ from a weld, although it doesn't get rid of any mechanical defects of the weld if it has incomplete penetration.

 

This also assumes that you have a big enough oven to fit the entire object that you stir-friction welded together out of maraging steel.

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4 hours ago, Collimatrix said:

Assuming that the weld itself is sound, one of the interesting properties about maraging steels is that the heat treatment can be "reset."  The strengthening mechanism in maraging steels is the precipitation of tiny inclusions of intermetallic compounds (which is why maraging steels usually have weird shit in them like titanium; that's what helps form the intermetallic).  By heating up the steel, these intermetallics can be re-dissolved, the steel cooled back down, and then heated up again to a somewhat lower temperature to re-precipitate out the intermetallics and re-harden the metal.

It is my understanding that doing this will basically erase any HAZ from a weld, although it doesn't get rid of any mechanical defects of the weld if it has incomplete penetration.

 

Is Aermet also capable of this "heat reset"? IIRC it does not use intermetallic precipitation to attain high hardness, but carbide [(Mo,Cr)2C?] precipitation. 900°F-aged Aermet 100, in particular, has fracture and notch toughness exceeding that of maraging steel (I believe A100 was selected for the landing gear of the F/A-18E/F because the Navy was not satisfied with maraging steel), which makes me wonder if it was ever considered for inclusion into armor packages (even in small amounts) despite its very high price.

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6 minutes ago, Renegade334 said:

Is Aermet also capable of this "heat reset"?

Per The spec sheet, Aermet is a brand name for a Maraging steel. So like all* solution heat treat and age alloys, it should be capable of said reset.

 

*In certain alloys and geometries the reset is not possible as the solution heat treat and quench cause cracking.

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43 minutes ago, Renegade334 said:

 

Is Aermet also capable of this "heat reset"? IIRC it does not use intermetallic precipitation to attain high hardness, but carbide [(Mo,Cr)2C?] precipitation. 900°F-aged Aermet 100, in particular, has fracture and notch toughness exceeding that of maraging steel (I believe A100 was selected for the landing gear of the F/A-18E/F because the Navy was not satisfied with maraging steel), which makes me wonder if it was ever considered for inclusion into armor packages (even in small amounts) despite its very high price.

I can't talk to the metallurgy beyond an amateur level, but my impression is that the juice just isn't worth the squeeze when considering these ultra-expensive, ultra-high-strength steels. Even a TE of 2 versus RHA doesn't get you all that much when a) it's going to have to go over an RHA backing and lose some efficiency in the process, b) any repair work becomes a chore due to it not being easily cut, welded or otherwise worked without ruining the secret sauce and c) the TE of reactive elements is higher still.

 

The result is, I think, that super-steels remain a niche item on armored vehicles - reserved for those few places where you absolutely cannot do it any other way.

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6 hours ago, Renegade334 said:

 

Is Aermet also capable of this "heat reset"? IIRC it does not use intermetallic precipitation to attain high hardness, but carbide [(Mo,Cr)2C?] precipitation. 900°F-aged Aermet 100, in particular, has fracture and notch toughness exceeding that of maraging steel (I believe A100 was selected for the landing gear of the F/A-18E/F because the Navy was not satisfied with maraging steel), which makes me wonder if it was ever considered for inclusion into armor packages (even in small amounts) despite its very high price.

 

 

That's a good question, and I'm not sure.  Per the spec sheet N-L-M posted, it is a solution hardening (which is the same thing as "age hardening;" metallurgical terminology is nonsense sometimes) alloy.  I bet that small titanium addition is what's doing the trick.  So the precipitation hardened part could be reset.  However, it also has a bit of carbon in it, unlike a lot of other maraging steels.

 

If you tried to "reset" the heat treatment, that carbon could cause some problems.  Some carbides form at higher temperatures than the intermetallic precipitates, and if the metal is hot enough that the carbon is mobile and can diffuse (basically, the hotter the alloy gets, the more random kinetic energy the carbon atoms have, and the more they can drift around), then the carbon may start to form larger and larger carbide inclusions through a process called Ostwald Ripening.

 

There's an ideal size of carbide or intermetallic inclusion particle size.  If they're too small, they don't do much of anything.  If they're too big, they tend to embrittle the steel because the carbides themselves are very hard but brittle.  If they're just right they tend to pin dislocations and prevent plastic deformation thereby.

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2 hours ago, Collimatrix said:

If you tried to "reset" the heat treatment, that carbon could cause some problems.  Some carbides form at higher temperatures than the intermetallic precipitates

While this is true, the solution heat treat is above that temperature, and everything should dissolve.

Forming the wrong precipitates is an issue if you age wrong or if you accidentally age via heat input such as welding later in the process.

 

welding itself is also an issue for precipitation hardened metals if done post-age as it remelts the HAZ had effectively solutionizes it, of course. This is what causes 6061, for example, to lose so much of its strength in the HAZ.

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12 hours ago, N-L-M said:

welding itself is also an issue for precipitation hardened metals if done post-age as it remelts the HAZ had effectively solutionizes it, of course. This is what causes 6061, for example, to lose so much of its strength in the HAZ.

 

I havent been able to find anything, but can you age only select areas (for instance, the weld zone) of precipitation alloys? from what I know, you would have to send the whole damn thing through the oven again; but if you have an oven big enough for the whole piece, why would you weld it after you aged it? 

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23 minutes ago, Lord_James said:

but can you age only select areas (for instance, the weld zone) of precipitation alloys?

Not a practical proposition, really. You'll get a naturally vs artificially aged condition if you let it sit around for a while without messing with it any more, which for some precipitation hardened metals can be around halfway to the full artificial age.

 

25 minutes ago, Lord_James said:

but if you have an oven big enough for the whole piece, why would you weld it after you aged it?

That's quite the "if". And even if you *do*, there are a few reasons. Fabrication efficiency, cost, repairs, and the like all come to mind.

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  • 3 months later...

Hello (long time lurker), I was wondering how one would calculate the protection provided against long rods by a highly sloped piece of RHA like that found on most modern MBT hull and turret roofs. To this end I have two basic questions.

 

Is it simply the LOS thickness with no shell normalization?  

https://www.researchgate.net/publication/242632628_THE_PENETRATION_PROCESS_OF_LONG_RODS_INTO_THIN_METALLIC_TARGETS_AT_HIGH_OBLIQUITY

This paper makes the following claim about HHS steel angled at 73-77 degrees:

 

"The Mass efficiency (Ef) of the target plates was calculated from the DOP results related to the baseline performance of the rod at the relevant impact velocity. The reference penetration is 95 mm in an RHA block at normal incidence. Based on that, the DOP data indicate Ef in the range of 1.07 to 1.09. That efficiency is mainly related to the obliquity advantage, described above."

 

Would it be reasonable to estimate that non-HH RHA with a greater angle (80-83 degrees) could replicate this mass efficiency?

 

What angle does RHA need to be to deflect/ricochet/shatter/etc. long rods?

I've seen papers that seem to have different definitions of "ricochet" and different testing setups (which makes interpretation difficult). I was wondering if this forum has reached any conclusion in this area.

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