I just found this abstract from a U.S. army experiment on propellents that completely validates nitriding of the bore!
Abstract: The U.S. Army Research Laboratory has recently made discoveries in the area of interior ballistic propellant combustion product interactions with a gun-tube bore. These discoveries were based on two hypotheses. The first was that the products could and are dynamically nitriding the bore of the gun, thus creating a nitride coating which inhibits the dissociation of CO and subsequently reduces the amount of carbon uptake. The second was that the combustion products richer in nitrogen have a lower temperature upon expansion in our experimental fixture as well as in a gun barrel. This is due to Joule and/or Joule-Thomson cooling effects. Both of these hypotheses were proven and validated through numerical and experimental methods. Higher nitrogen containing combustion products have definitively been shown to have relatively lower erosivity with respect to those having lower nitrogen content. These results have revolutionized the gun propellant development community in that for the first time, there is guidance for erosivity control through propellant chemical constituent formulation.
I spoke with two different metallurgists at Crucible steel today. I wanted to get an answer from the horse's mouth on the idea of using 174 SXR.
The first gentleman I spoke with stated that he asked some of the other metallurgists in his office and they had come to the conclusion that it's probably purely a cost thing, but I should call their specialist in this particular area. He gave me Mr. Scott DeVanna's phone number, at Crucible's Texas facility.
Mr. DeVanna and I had quite a good chat. He confirmed that just about any 17-4 precipitation hardening steel would make excellent barrels, and that in particular, the 174-sxr would be a "SUPERB" choice. It can be polished even better than the 416r, resists corrosion, heat checking, wear, and carburization better than the aforemetioned by wide margins on all counts, all while being harder and tougher! Basically, this means that anything that makes 416R a good choice, makes 174SXR a better choice by a noticeable degree!
However, he also confirmed the cost issue. This steel is more expensive, and it's harder on tooling to boot. Finally, as it's harder on tooling, it's more likely to cause "chatter" in the cut. This means that machining must be done with VERY well maintained, VERY sharp carbide cutters, and finish lapping is a practical neccessity.
I also asked him about the Nitriding idea. Mr DeVanna commented that in his experience it is thermal fatigue and not carbon migration that is enemy number 1. However, this was done with military test barrels with 7,000 rounds, 10,000 rounds, and 18,000 rounds through them. I must assume in quick succession, as the military typically has a different idea of barrel life than most of us here.
However, he confirmed that in hot working tool steels, Nitriding is quite commonly used to help mitigate the effects of both thermal fatigue and carbon migration, and has proven quite effective at helpng against both.
When it boiled down to the brass tacks of why isn't this the common choice for the top of the line barrel, he confirmed the idea that it just doesn't make a successful buisiness model to produce a product that costs more than twice what your competitors product does, based on the idea that your product is more durable (and thus you will get less repeat buisiness, as your loyal customers won't need to replace their barrels as often).
The trouble with machining this steel is, IMHO, eminently overcomeable, and Mr. DeVanna agreed heartily. He and I asked ourselves the very same question: "machining technology has come so far, especially more recently, why hasn't the steel used for barrels kept up?" Well, now we have an answer: it's not that the metallurgy hasn't been advancing, it's that noone wants to use the better steel for fear of losing their precious (and admittadly, small enough as it is!) profit margin!
I forgot to ask the requisite questions to determine if the cryo process might be a good idea. Essentiall, in simple tool steels, with a good heat treat, cryo is close to useless, as there is VERY little retained austenite to convert. However, none of the steels we are interested in for barrels is a simple tool steel! I know for a fact that 416R could benefit from a marquench / cryo / temper / cryo / temper / cryo / temper process. This would really wring that last bit of performance out of 416R due to conversion of retained austenite into martensite, and proper stress relief of the newly formed martensite.
However, with the precipitation hardening steel, things are a bit different. This steel is fully martensitic, like 416. However, the formation of the martensite structure is not what gives it most of its hardness (unlike traditional steels, which rely almost exclusively on this for their hardness). Instead, after conversion to martensite, elevated temperatures are used to "age" the steel to promote the precipitation of a vast amount of carbides and nitrides throughout the steel, making the steel very strong and tough.
However, I need to ask him if the martensite structure is formed from a parent austenite like in traditional quench steels, or if it is formed directly martensitic sfter the melt. If the former is the case, I need to know what level of retained austenite to expect from the soloution treatment. If there is a decent amount of RA then cryo would REALLY make this steel SING!
I will be calling him back tomorrow to follow up with these questions and more, and will update you as I progress!
I don't really understand how you could make the coating process work. How would you deal with chambering, cutting and crowning? It seems that a thin enough coating would maintain throat geometry would also wear exactly at the throat and you would have a hole in the coating, then things would start acting very strange I would think.
I have a better idea, buy three times as many guns and then the barrels last longer...LOL
This is very interesting reading, so if I may add another metulurgical question, what is the difference between 4140 CM and 4160 CM as used in rifle barrels. I had a barrel made out of 4160 CM and I observed that it was very hard to machine. This barrel has never met my expectations though and after examining the bore with a bore scope, the first six inches has a lot of fire cracking. I will soon be rebarreling this gun with a Broughton 416 stainless with 5c rifling. Any comment on 5c rifling?
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