OFCC University - Metallurgy 101 (Steel)

[willbird]

Remington also hammer forges rifle barrels. Hk was the first mfg. that I ever heard of using the process.

Some of the textbooks written on steel back in the 1930's 40's and 50's are a really good read. They used to take slices off the end of a steel billet to ensure that there were no flaws in it, they acid etched these samples. The billet was poured from one end and it had what was called a "hot top" on one end, a smaller section that was similar to our sprue on a cast bullet, it was supposed to fill shrinkage and any slag or other imperfections would stay in the "hot top". Well they kept slicing the billet until they did not find any more flaws.

When the billet is rolled out to say 1.2" dia. steel for rifle barrels any flaw in the billet will pretty much run the whole length of the resulting steel bars. It was not uncommon in the shop I worked at in Toledo to get 4140 steel that had flaws in it that caused cracks in the finished parts, or nearly finished parts, time to start over . In one case the part was 30" in dia and 18" tall, and had about 40 hours of machining done on it when the crack was discovered.

We also made some parts from inconel 100, aeromet 100 is a close equivalent, In it's annealed state it is roughly as strong as heat treated alloy steel....but it is a lot more difficult to machine, with cnc and carbide tooling it just takes longer . It also retains pretty much these same mechanical properties at 1000 degrees F. The benefit to us from making the clamps from that material was that no heat treating was required.

Bill

[AlanM]

Just a point of, maybe, interest.

The critical temperature, the point where a metal changes from magnetic to non-magnetic or para-magnetic to dia-magnetic or vice versa, is so repeatable and sudden that this property is used to calibrate the temperature reading instrumentation on labratory instuments.

[carmen fovozzo]

.......I LOVE WHEN YOU GUYS TALK DIRTY.

[Morne]

Some of the textbooks written on steel back in the 1930's 40's and 50's are a really good read. They used to take slices off the end of a steel billet to ensure that there were no flaws in it, they acid etched these samples. The billet was poured from one end and it had what was called a "hot top" on one end, a smaller section that was similar to our sprue on a cast bullet, it was supposed to fill shrinkage and any slag or other imperfections would stay in the "hot top". Well they kept slicing the billet until they did not find any more flaws.

When the billet is rolled out to say 1.2" dia. steel for rifle barrels any flaw in the billet will pretty much run the whole length of the resulting steel bars. It was not uncommon in the shop I worked at in Toledo to get 4140 steel that had flaws in it that caused cracks in the finished parts, or nearly finished parts, time to start over . In one case the part was 30" in dia and 18" tall, and had about 40 hours of machining done on it when the crack was discovered.

We also made some parts from inconel 100, aeromet 100 is a close equivalent, In it's annealed state it is roughly as strong as heat treated alloy steel....but it is a lot more difficult to machine, with cnc and carbide tooling it just takes longer . It also retains pretty much these same mechanical properties at 1000 degrees F. The benefit to us from making the clamps from that material was that no heat treating was required.

Bill, you are a treasure trove of knowledge!

You're right - a lot of the textbooks from the WW-II era are excellent. We had many of them in the technical library at my old job.

Yes - hot topping is a way to avoid macro-cracks in further processing. Again, in the casting discussion I'll go into this and more.

The deal with the finding cracks way late is that they are VERY small - often called micro-cracks. You can find these in magnetic steels via Magnetic Particle Inspection (MPI) which is a non-destructive testing (NDT) method for finding flaws. Some folks still use the old "Magnaflux" trade name when talking about it. Basically, you magnetize the steel part and then bathe it in a slurry of flourescent fluid that contains tiny iron filings. Shine a black light on it and any crack (surface or even shallow sub-surface) will show up readily because the iron filings will form a pattern around the mini north-sole magnetic pole created at the crack in the "magnet". Pretty slick stuff.

As to flaws that run the length of a bar - this is most common in the non-remelted (non-ESR/VIM/VAR) and resulphurized steels. Basically you get big sulphur inclusions that are rolled out along with the bar into VERY long strands. I know of a machine shop that cut a whole 12-footer of 3" hex barstock into machined nuts 0.75" in length only to find out at MPI that they were chock full of sulphur inclusions. I understand that some foul language was used when they came back from MPI rejected.

Inconel in general is GREAT for high-temps. Technically, it is not a steel, though. It is an iron-nickel-chromium superalloy.

I've also worked with AerMet 100 - tough stuff! Be glad you didn't have to heat treat it. The heat treatment cycle on AerMet 100 is a pain in the keyster (stupid cryogenics) but then again so is trying to break chips on it (the guys on the boring mills used to ask if we were expecting a flock of birds what with all the nests they were making with the stuff). It has both excellent strength AND fracture toughness - which is kind of odd since usually you can get one but not the other. Just goes to show that with enough money you can get anything you want (AerMet costs around $16 a pound - I have eaten REALLY GOOD steak that didn't cost that much).

And, did steel progression over time move up the chart? ie blackpowder was low-carbon, then nickel steel, then x, then y and the top of chart was most recent?

The carbon steels were first, followed shortly by the nickel steels. Manganese additions came next. Between the late 1800's and the 1960's pretty much everything else in that list came about in no particular order (except that in general the second digit indicates sequence, thus 4140 predates 4340 and 8620 predates 8720). Most of the post WW-II improvements haven't been so much in the chemistry as in the method of production. Vacuum remelting started in the 1960's IIRC. Some newer Cobalt containing alloys, like AerMet 100, have only shown up with the introduction of lanthanide oxysulfides to get the disgustingly fine inclusions present in those high-performance alloys. Frankly, metallurgy has sort of left steel behind and moved on to Aluminum and Titanium. Cutting edge work these days are done in things like Metal Matrix Composites.

[willbird]

Back in the day it was a lot more common for a tool and diemaker to actually design, machine, and heat treat the things they made. I recall one book written by a steel maker that had as I recall it 9 steels, they were described as water hard, water tough, oil hard, oil tough, etc.

They had them laid out in a 3x3 matrix and described how moving from the central steel to one of the others changed the properties, and what movement right and left and up and down accomplished. They talked of quenching methods for water hardening tool steels that used a fixture to hold the part, and applied high pressure quench water to the ID of say a bushing so it was glass hard on the surface and progressively softer/tougher going otwards, these bushings would for say drawing wire or round stock to a smaller size.

Machineries handbook is a wonderful resource material, machining inconal or aeromet is as simple as looking up the SFM and doing the math, I did use one 3/4" HSS drill on the aeromet job I mentioned, the textbook SFM as I recall it is between 10 and 20 sfm....that drill is just loafing along at that RPM (53 rpm for 10sfm)...that is about 1/5 the proper rpm for most steels. I used 50 as a general purpose sfm for steels, that allowed me to machine A2, D2, S7, M2, M42, H13, and 4140 pre heat treat (28-32 rockwell) without ever ruining a drill, if I had a job with more than say 5 minutes of cut time I could always tweak the sfm up to a more optimum level, but MOST jobs took longer to program than to run by far. Due to using weenie fadal mills that were not ultra rigid I would use 150sfm for solid carbide tooling.

The superalloys (thats is what the books call Inconel and aeromet) share the trait with D2 that if you get any oil on them from your hands a file just skids and does not cut.

Bill

[Morne]

I intentionally omitted the tool steels (AKA expensive steels) since they don't really pertain much to firearms anyhow.

You could teach a whole college course on tool steels.

[carmen fovozzo]

I intentionally omitted the tool steels (AKA expensive steels) since they don't really pertain much to firearms anyhow.

You could teach a whole college course on tool steels.

...................Yippy, I can hardly wait .

[willbird]

S7 actually is a very good gun steel, I have heard of a guy making 357 magnum 5 shot cylinders out of S7.

The barrel extension and bolt head on a Serbu BFG-50 bolt action 50bmg rifle is also made from S7. S7's high Jominy notch test strength makes it a good steel to use in applications where there are stress risers, and MOST firearms designs are simply LOADED with stress risers. The end result is that a gun part made of S7 can be heat treated 50 rockwell and be just as tough as 4140 heat treated to 44-46 rockwell. S7 is a so called "shock steel", it also has much less distortion in heat treat than traditional alloys, it shares that quality with The A and D series tool steels.

Bill

[Morne]

Since we're gonna discuss them, here's a quick thumbnail on tool steels:

Family-------------------Letter Designators-----------Uses
"High-Speed Steels"----M & T--------------------------High-speed machining tools
"Hot-Work Steels"------H-------------------------------High-temp forming applications (think forging dies)
"Air-Hardening"---------A-------------------------------Shear knives, punches, blanking dies
"Deep-Hardening"-------D-------------------------------Drawing dies, brick molds, gages, rolls (GREAT wear resistance & resist hi-temp softening)
"Oil-Hardening"---------O-------------------------------Trimming, blanking, drawing, flanging tools (Good room temp wear resistance - often weld repairable)
"Shock-Resisting"-------S--------------------------------Chisels, rivet sets, punches, driver bits (great strength & toughness)
"Mold-Steels"------------P--------------------------------Die casting dies, plastic injection molds (very high polishability)
"Water-Hardening"-----W-------------------------------Wood-working tools, cutlery, cold heading (shallow, hard case with a soft, tough core)

Anytime you see a "X-hardening" that indicates what you should be using for cooling the tool steel from the austenitizing temperature (orange-red hot) during heat treatment.

[willbird]

S-7 is a general purpose, air hardening, tool steel with a high resistance to impact and shock. It is moderately "red-hard" which makes it one of those rare steels that does very well as either a hot work or a cold work steel. The breakdown is as follows:

TYPE ANALYSIS Element Percentage
Carbon 0.50%
Manganese 0.70%
Silicon 0.30%
Chromium 3.25%
Molybdenum 1.40%

The critical temperature for S-7 is 1700 degrees to 1750 degrees F. A heat soak of 20 minutes plus 5 minutes per additional inch of thickness is recommended for a complete hardening (shorter times may be used for smaller pieces, for example chisels Ed.). Pieces up to 2" thick can be air cooled but at thicknesses greater than that an oil quench to 150 F is recommended. Tempering should be done as quickly as possible following the hardening process. The factory specification for cold work is an Rc hardness of 55-57, for hot work a Rc of 50-53 is suggested.

TEMPERING TABLE Temperatures
in Degrees
Fahrenheit Rockwell
Hardness
(C scale)
As Hardened 59-61
300 57-59
400 55-57
500 53-55
600 52-54
700 51-53
800 51-53
900 51-53
1000 50-52
1100 43-48
1200 37-40

S-7 is one of those hot work/cold work steels that many smiths "forge and forget" It can be difficult to find scrap sources of S-7, but it is readily available from steel suppliers and well worth the cost.

Author's Notice:
References for this article are from, but not limited to, the material specification sheets from Carpenter Technology Corporation, Steel Division and lecture material from Robb Gunter's 1994 ABANA conference demonstration on scrap tool steels.

MY memory bank was telling me heat treat for S7 and A2 were very similar, there is an article at page 14 in this newsletter at the link below that tells how to heat treat common tool steels, and it lumps S7 and A2 together. I have heard of a guy named Rick Rowlands (do a google search for Tod Engine) casting whole anvils from S7. More or less S and D steels are just as "air hardening" as A2 is. The tempering temperatures for S7 and A2 are slightly different. But S7 heat treated to typical gun receiver hardness could withstand at least a 1000 F temperature without suffering any loss in hardness.

http://www.bamsite.org/Index/BAM%20NovDec08.pdf" onclick="window.open(this.href);return false;

[haspelbein]

Wow, this is an interesting thread, and maybe I can get an answer to some of the questions that I've recently seen pop-up in European forums. There have been some discussions regarding the quality of the steel or the treatment of US-made barrels in popular rifles in Europa, such as the Remington 700.

Many claim that the lifetime of the US-made barrels is lower, and that their effective use for long-range shooting is therefore lower and limited to 5000 rounds, and this would not be the case with some higher-priced European manufacturers.

My understanding has always been that there are very limited options regarding the barrel steel, and that the main factors were mainly the form of rifling as well as surface treatments, such as lining the barrel.

So, any input regarding the type of steel used vs. the longevity of the barrel?

Thanks a lot in advance!

[Morne]

There have been some discussions regarding the quality of the steel or the treatment of US-made barrels in popular rifles in Europa, such as the Remington 700.

Many claim that the lifetime of the US-made barrels is lower, and that their effective use for long-range shooting is therefore lower and limited to 5000 rounds, and this would not be the case with some higher-priced European manufacturers.

My understanding has always been that there are very limited options regarding the barrel steel, and that the main factors were mainly the form of rifling as well as surface treatments, such as lining the barrel.

So, any input regarding the type of steel used vs. the longevity of the barrel?

Haspy - good to hear from you again! Seems like it has been forever. Been spending time on the Jovian Moons again?

First off, comparing a Remington 700 to an expensive custom made rifle from anywhere is grossly unfair. A better comparison would be against the Tikka T3 family.

Obviously, barrel linings like chrome will have a HUGE impact on useful life. That said, most people never shoot a true "long-range rifle" enough to ever worry about wearing out the first barrel regardless of its shortcomings.

As to the rifling form, I'm certain that has an impact on barrel life as well. Hammer forged rifling, which gives you polygonal rifling, is probably the best in my eyes. In general, a little compressive mechanical stress yields benefits when working high-strength low alloy steels. Think about it in terms of thread root radius rolling and shot peening.

As to the steels - I think you've encountered a "My daddy can beat up your daddy" attitude. Barring something tangible to discuss it is just an unsubstantiated claim. The only negative experience I've had with European steel makers was an issue with the old British Engineered Steels (now part of Corus). They got conned into adding titanium as an alloying element into a 43XX series steel, which caused a HORRIBLE ductility loss for a client of mine. Sure, the Ti formed Ti-carbonitrides that upped the macrohardness (though without any benefit to UTS) but it also caused a stress riser that was disastrous when testing for % elongation and reduction of area.

I've also had problems with domestic steel makers. I'm in an argument right now with Republic. Although PART of that problem is that my client didn't specify a very good grade of steel in the first place. Order a chevy and get a chevy - don't comlain that it ain't a cadillac.

Now there ARE bad nations for steel production. China, for one. Part of it is that their different standards are not easily converted to what we westerners are used to (once had a client ask me if I could "whip up" a translation spreadsheet in an hour - that is a project that would require a year+ of work). But part of it is that their internal customers do not demand high quality (partially) due to a lack of litigiousness. Without a fear of lawsuits, you can cut ALL KINDS of corners. Inspect a heat of chinese steel to western standards and you'd have one VERY LONG rejection report to read. Might still be useful, depending upon the application, but you have to walk into that situation "eyes open" to the pitfalls.

[haspelbein]

No, I have not been to the "Jovian" Moons, but spent a lot of time working in downtown Chicago, which feels about as alien in many regard. I'm also in the process of putting an AR lower together, but get interrupted every time before I start in earnest. But I'm still trying to decide on which stock and upper to use. But back to the thread topic:

Agreed, comparing a Remington 700 to a custom rifle is not fair comparison, but the claim was that even a Steyr or Merkel in the $1300 to $2000 price range would have a much better barrel than lasts much longer. I've never owned a Steyr or Merkel to verify such a claim, and I know that may Steyr and Merkel rifles are beautifully machined, but that argument about barrel longevity never made much sense to me.

I have a suspicion that the rumor started because benchrest shooters in the US consider a barrel lifetime to last about 5000 to 10000 rounds. This number seems to appear low to European competitive shooters. Yet I don't see any "secret sauce" either in the selection of steel or the rifling that would substantiate claims that barrels from higher-priced European manufacturers last longer.

I should approach the 5000 round limit with my Remington sooner than later and will see if I can notice a change. So far it has been very steady in it's accuracy. (a tad over 1 MOA)

[Pessimist]

Good explanation!

I've been away from mechanical engineering for a while, and never very good at metalurgy anyway, so this was a good refresher.

One question: "HRC"? Is that the same as Rockwell C? If so, what's the "H" stand for?

I always wondered how hard barrels were. I guess 33-36 RC leaves them reasonablly machinable.

I would love to see how they actually gun drill a long gun barrel in practice. That, and what their tolerances are.

Thanks again!

[curmudgeon3]

One question: "HRC"? Is that the same as Rockwell C? If so, what's the "H" stand for?

Hardness.