Gunsmithing Metallurgy - recoilless rifle

[Evolution 9]

I’m looking for steel options for a recoilless rifle. I’m fairly competent in the design theory of RRs but materials isn’t my area of expertise and I’m hoping it is for some of you.

Questions:

1. Is there any reason not to simply pick the highest tensile psi rated steel that’s machinable? In other words, 17-7 has twice the tensile and yield strength of 4140, yet 4140 is chosen for gun barrels and I don’t know why. School me.

2. Are there specific steels I should be considering?

3. Titanium is used in the current Carl Gustaf even though there are steels with a much higher psi to weight ratio... thoughts? And if titanium, which titanium?

4. I’ll eventually need to ask similar questions of really knowledgeable experts. Any steel manufacturer can tell me about the properties of their products, but they likely don’t also happen to be experts on the forces involved inside a gun’s chamber. Any suggestions for who to talk to?


Disclaimers:

1. I’m in the very early stages here, and no, I won’t be risking anyone’s life based off of advice from the internet. I’ll get confirmation from professionals and lots of proof testing before actual use.

2. There are at least two legal ways to go about this and I’ll definitely be following all applicable laws.

3. Yes, I know that recoilless rifles operate at very low chamber pressure such that any pot metal will work, but I’m wanting a safety margin of several hundred percent since a failure will most definitely kill the user.

 

[chevy_man]

Being machinable and being able to easily machine are two different things.


Generally you'll see the lowest grade steel used that's safe to save on tooling costs. There are also some surprisingly strong steels in one category, that will completely fail when exposed to the heat of combustion under high pressure. Take into account you may damage any heat treating the mill has done for that particular product to get those specs.


4150 is a good place to look. As something that's industry proven to work it would at least make a good starting point. Wall thickness is where you can get your strength.

Titanium is expensive. Don't use it for prototyping. Wait till you've got the design figured out before you invest in the tools to machine it.


Forces inside the chamber are going to be heat and pressure. Should be able to calculate off the cartridge used.

 

[Ledzep]

Strength isn't the only criteria.

Ductility, toughness, resilience, ductile to brittle transition temperature, corrosion/erosion resistance....

All trade-offs, and EVERY steel can vary wildly in all of the above depending on heat treat and temper.

Check the specs on 4140 quenched and tempered at 400 for an hour. Likely as strong or stronger than 17-7 (I'm not familiar with that one off the top of my head).

 

[LRI]

You need toughness and hardness as these parts literally beat the piss out of each other. The tensile strength is great, but if the stuff is bubble gum soft then it'll deform, gall, and make a mess of things.

That's why chromoly is used so heavily. It's a simple chemistry material that is very predictable to machine and heat treat. Heat treatable stainless is a whole lot more complicated. (416R, 15-5, 17-4ph, etc) Think of it as Gramma's bread. Its always good, but some loafs are better than others. Unless your going to cowgirl up and buy the really good stuff (listed materials with certs) then you just never really know. On its best day you'll be looking at 42-44 rockwell and that's really not hard enough for a semi auto. Gas, recoil, or blow back. Doesn't really matter. The stuff will want to booger to itself and its stupid expensive.

Titanium is about the worst stuff to use. Like SS, it's like rubbing hot two boogers together and expecting them not to stick to one another. -It hates impact as well.

8620 is another nice choice because it machines nice and takes nitriding very well. It does move around a bit so finish grinding is often the go to choice for post heat finishing. Just be aware that you'll need to comp for that when you spec out the depth of the case you want.

Good luck. Sounds fun.

 

[Ledzep]

Stainless branches into several categories. The two we see in guns most often are martensitic and precipitate hardening. Mostly what makes stainless steels "stainless" is a higher chromium content that forms oxides on the surface of the part that block destructive oxidation with the iron.

Martensitic stainless steels, like 416 typically follow a pretty traditional steel heat treating regiment. Heat to above the normalizing temperature, then quench. Quench cooling rate is also a balancing act. Too fast and you can cause cracks, too slow and the center of the part cools slow enough to not form martensite (the hard phase of steel). Martensite is UBER strong in a tensile test machine with controlled strain rates, but uselessly brittle in the real world. So you have to temper the steel. Usually 400-800 degrees for a relatively short time soak. This allows some (most) of the steel to form into pearlite (softer, normalized phase). How hot and how long you temper controls the balancing beam between hard & strong vs. soft & ductile. Toughness rides somewhere between the two.

Precipitate hardened steels harden the same way most 6061 and 7075 aluminum is hardened ("T6" in aluminum refers to the state of hardness). With precipitate hardened materials you take advantage of supersaturation at higher temps to allow for the formation of impurities in the material. The impurities create barriers to slip planes and prevent deformation. Diffusuion of those impurities into the host material is increased with temperature so the material starts very hard, and the more temperature its exposed to, and the longer its held at temp, the softer it gets as the impurities dissolve and disperse throughout the material. It's a 2 ways street. ALL precipitation hardened materials are constantly getting weaker/softer over time, albeit at a very very slow rate at room temp, but they often lack the negative characteristics with cold temperatures that martensitic steels have (brittleness with cold temp). Probably the poorest material option for follow-on nitride treatment.

Chrome-moly steels have the Chromium, Molybdenum, vanadium, etc... added to help with heat treatability. The alloying elements allow slower quenching (even air quenching in some circumstances) to produce martensite. It's especially helpful the thicker the part is. In sheet-metal parts, plain carbon steel, 1040, for example, can be made pretty much as strong/hard as 4140. However, as the thickness increases, the plain carbon steels are not able to get cool enough fast enough in the center to form martensite, and the core remains soft, where the 4140 in thicker parts will thru-harden.

I guess I should've said this at the beginning... Iron and carbon are the main driver of hardness and pure tensile strength in steels. Mostly carbon content (general statement, I wouldn't be surprised if there were exceptions). Hence the similar characteristics between 4140 and 1040 steels (both have 0.40% carbon content nominally) in thin parts. The higher chromium content in stainless steels limits the amount of carbon and iron, so in pretty much every case that I'm aware of, stainless steels in their hardest, strongest state are softer and weaker than alloy and plain carbon steels (depending on thickness and heat treat).


And then there's work hardening and grain structure..... Why cold forging is stronger than hot forging is stronger than casting (mostly).

 

[Ledzep]

As far as Titanium goes... It has its place, but it's not the uber material a lot of people think it is. Hard to machine, as Chad said, easy to gall, and it leaves a LOT to be desired in the ductility department. Strength-to-weight is kind of the biggest thing it has going for it. It has a different crystalline structure than most all steels and aluminum and behaves differently. I'm sure someone else here knows more; I'm not very well versed in Ti and its alloys/treatments.