9310 Bolt Data

[lysander]

The biggest issues I’ve seen with 9310 is the result of improper heat treat, and 9310 is much less forgiving with the treatment temperature range. 9310 is harder than C158 but it’s also more brittle, it’s fatigue life is also notably shorter than C158.

I have a number of reports that stated that 9310 is actually a very forgiving steel to heat treat compared to say 8620 which no one has every accused of being a difficult steel to treat. If fact, 9310 response to thin section quenching is better that Carpenter 158.

And, taking the leap between this statement:

". . . by comparison to Carpenter 158, AISI 9310 has several elements present in its composition that are detrimental to fatigue while not being evident in the physical properties . . ."

and this statement:

". . . 9310 is harder than C158 but it’s also more brittle, it’s fatigue life is also notably shorter than C158 . . . "

Is an unsupportable leap.

First, I am going to take issue with "several elements present in its composition that are detrimental to fatigue", the only element in 9310 that is not in 158 is molybdenum which is not "detrimental to fatigue", the other two elements , as stated earlier are impurities that will be present to some degree in all alloys.

Second, 9310 is not "harder". It is hardened to achieve the proper strength, and that happens to be the same hardness as Carpenter 158, around 36-40 HRc.

And last, I have not seen any study of 9310 that states it has a short fatigue life. In fact one of the reasons it is popular as a gear material is because it can withstand high cyclic loading over a long period of time.

So, stop requoting the myth that 9310 is "harder to heat treat", "is harder and more brittle", and unless you can cite a study that shows 9310 has a poor fatigue behavior, stop that too.

[lysander]

"Low Cycle Bending Fatigue of AISI 9310 Steel Spur Gears"

In a fatigue test of AISI 9310 steel gears the information shown below on cycles to crack initiation was reported. The steel in question was carburized at 1650° F for 8 hours, austentized at 1550° F for 2.5 hours, quenched in oil, frozen at -120° F for 3.5 hours and tempered at 350° F. Since 9310 AR bolts are proprietary, I can't say what process they used to heat treat their bolts, but the above is almost exactly the heat treatment used for the M60 bolts.



In "Failure Analysis of an M16 Rifle Bolt", the calculated maximum stress in the most heavily loaded locking lugs is shown below:



The red area in the model is 1070 MPa, or around 156,000 psi.

Between these two bits of information, I would say that expecting a 9310 bolt to last at least 5,000 cycles (which is about the earliest point where Carpenter 158 bolts first start to show cracks), is not unbelievable.

[sinister]

Lysander, thank you for all these great posts!

I would add the government doesn't like adding cost to a per-unit item price, whether that is the price of an individual replacement bolt or a complete rifle -- unless they are looking for a specific improvement. I have no idea if there is a price delta between Carpenter-158 and 9310 bolts.

Something else (and I don't know whether or not it matters) to consider -- isn't Crucible the only foundry that batch-makes Carpenter 158?

[SeriousStudent]

I have often wondered the exact same thing. Great question, I'm going to move this thread the AR Technical Discussion, and make it a sticky.

Thanks very much for the data here, it's much appreciated.

[SeriousStudent]

Lysander, thank you for all these great posts!

I would add the government doesn't like adding cost to a per-unit item price, whether that is the price of an individual replacement bolt or a complete rifle -- unless they are looking for a specific improvement. I have no idea if there is a price delta between Carpenter-158 and 9310 bolts.

Something else (and I don't know whether or not it matters) to consider -- isn't Crucible the only forge that batch-makes Carpenter 158?

I remember a conversation with Will Larsen years ago about minimum order size regarding Carpenter 158 steel. It was one of those late night talks as Will was driving across country to teach.

I think you are right, but it was really late at night so I'm not positive. I'll see what I can find out. I recall him saying availability has sometimes been a challenge, due to minimum order quantities.

I also deleted a couple of dupe posts - sorry for the website lagging at times.

I love this thread. Many thanks to the participants.

[prepare]

Didn't the M240 replace the M60?

Any idea if the M240 bolts are still 9310?

[lysander]

Didn't the M240 replace the M60?

Any idea if the M240 bolts are still 9310?

The M249 bolt is AISI 9310.

M240 bolts were never 9310. The M240 bolt is actually two parts, the breech block itself, and the locking flap. Both are specified in a Belgian steel specification, but these steels are similar to AISI 4130 for the breech block and AISI 8620 for the lock.

[lysander]

Lysander, thank you for all these great posts!

I would add the government doesn't like adding cost to a per-unit item price, whether that is the price of an individual replacement bolt or a complete rifle -- unless they are looking for a specific improvement. I have no idea if there is a price delta between Carpenter-158 and 9310 bolts.

Something else (and I don't know whether or not it matters) to consider -- isn't Crucible the only foundry that batch-makes Carpenter 158?

It cost about half a million dollars for the Army to change a drawing. The return on investment needs to positive in three or five years, let's say five as it makes the math easier.

A bolt cost the Govt about $40, and let's say they buy 100,000 a year, and let's assume that 10% of that is the raw material. So, in order for this to meet the return on investment requirement, the raw material would have to be 25% cheaper.

Carpenter 158 is not that much more expensive than 9310 . . .

Carpenter Technologies of Wyomissing, PA.

[Spooky1]

I'm gonna sit right here, be quiet, and learn something new from yall. Thank you all for sharing hour knowledge! This is why I joined.

🙂

[Todd.K]

I would have to say that 9310 is a perfectly adequate substitute material.

Yet many are sold as some new wonder improvement vs C158. That doesn’t make me trust the company with the other critical processes.

To the OP on Nitride. I’m very much against nitride on any critically heat treated part. The nitride is hot enough to alter the hardness.