The search button here at M4C doesn't turn up much information on this subject, and the relevant information that can be found is hiding in multiple threads. Since I have spent more time reading about the differences between these three types of stainless steels as they relate to barrel making than I care to admit, I thought I'd take the time to consolidate what I have learned into one thread that others can add to. To be clear, I am not a metallurgist, I have no experience in barrel manufacturing, and the only machining I have ever done as of this point in my life was in my High School metals class... 10 years ago. All of the following information was found online and I am just trying to save others who are researching the topic a little bit of time.
Why stainless steel?
There are two main advantages to a stainless steel barrels. First is a higher corrosion resistance when compared to an unlined, uncoated standard carbon steel barrel. However, don't be fooled into thinking stainless steel barrels are rust proof... They are not. The 400-series stainless steels (also known as martensitic stainless) used for barrel making are more resistant to corrosion than carbon steel, but will still rust if neglected. In fact, a chrome lined carbon steel barrel with a well oiled parkerized finish will usually resist rust better than a bare stainless steel barrel due to the finishes applied to the inside and outside of the barrel. Likewise, melonite treated carbon steel is much more rust resistant than any of the 400-series stainless steels.
The second advantage to stainless steel vs plain carbon steel as it relates to barrels is the "toughness" of the metal. Stainless steel is more resistant to heat and abrasion than plain carbon steel. All else being equal, a stainless steel barrel should theoretically experience less throat erosion than a plain carbon steel barrel, given identical firing schedules. This does not mean that stainless steel will resist throat erosion better than a chrome lined carbon steel barrel - it will not. Chromium is much tougher than 400-series stainless.
At this point many of you are probably thinking, "What about accuracy?" Truth is, as far as I can tell and from what I have gathered from the barrel makers themselves and shooters much more experienced than I, is that accuracy boils down to the barrel making process itself, not the host material. You can make an inaccurate barrel out of stainless steel just as easily as you can make an accurate barrel out of bare carbon steel, however most high end barrels are likely going to made from stainless steel for longer life compared to bare carbon steel. The only notable difference as far as accuracy is concerned is whether the barrel is chrome lined or not. In theory, chrome lining should reduce the accuracy of the host barrel by creating slight variances in bore dimensions. How much depends on how uniformly the chrome lining was applied. That said, FN makes some impressive accuracy claims about their chrome lined, cold hammer forged carbon steel SPR bolt-action rifle barrels...
If you still don't know if you want a stainless steel barrel, you probably don't need one and can stop reading this now. A chrome-lined carbon steel barrel will meet the needs of the majority of shooters, and in many instances may even meet specific needs better than a stainless steel barrel. Unfortunately I do not have the time to go into the differences in carbon steels, so If you decide stainless steel is not for you, you will have to do the rest of the research on your own. This would be a good place to start, specifically post #15, quoted below, where aOcake outlines some questions you should ask yourself when deciding between a stainless steel or chrome lined carbon steel barrel.
Why not stainless steel?
Now that we've established why someone might want to choose a stainless steel barrel, what are some of the reasons they may not? The first and most obvious is because they have determined that a standard carbon steel barrel or a chrome lined carbon steel barrel will meet their needs as well or better than a stainless steel barrel would, and therefore the extra cost of a quality stainless steel barrel is not justified.
Aside from that, there are two main disadvantages to stainless steel barrels. The first is a decreased impact resistance at sub zero temperatures caused by "temper embrittlement" that has the potential to lead to barrel failures. While this is not a common problem, it is still something to consider if you are in a region that frequently sees the mercury drop below 0 degrees Fahrenheit at any time during the year when you may use the rifle. The second disadvantage is created by the addition of sulfur to martensitic stainless steels to make them easier to machine. Higher sulfur content increases the likelihood that the barrel steel could develop "sulphide stringers" which are weak spots in the steel that have the potential to cause barrel failure. I believe this was a contributing factor to the barrel failure problems Tikka was experiencing with their stainless steel barrels a while back.
The following is a quote by Michael Moyer taken from http://www.finishing.com, and is the best summary of the disadvantages to using 400-series stainless steels for barrel making that I have read.
A. To understand this subject, one must understand high-strength steel. For the record, I am a metallurgist by trade, and heat treat Quality Manager so I know enough about these steels. I do not like AISI 416 SS for gun barrels and I will explain why:
To achieve the high strength the steel must possess to withstand the forces produced during firing, AISI 416 SS and/or AISI 4140/4150/4340 must be austenitized, quenched and tempered. After quenching, the average 416 SS will be about 40 Hardness Rockwell C (HRC) and 4000 grades about 50 HRC (To benefit those who do not know this scale, a file will be about 60 HRC, and a hammer will be about 30 HRC). In the "as quenched" state, the material is brittle and unstable. Tempering is employed to reduce the hardness to a "tough" state and stabilize the newly formed martensitic structure. In the case of 416 SS, and to get the hardness to about HRC 30 so it is able to be machined, one must temper at about 1075 °F. This is not desirable as 416 SS shows a marked reduction in impact resistance when tempered between 700 °F and 1100 °F (temper embrittlement). It will also show a marked decrease in corrosion resistance. 416 SS does still, however, exhibit better wear characteristics and corrosion resistance than the 4000 series high-strength grades mostly due to the higher chromium content. It is also readily available, inexpensive, and it looks good so manufacturers use it. The big problem though is that it is not as free-machining as the 4000 series grades so generally sulphur is added to alleviate that problem. What you then have is a microstructure with "sulphide stringers" in it that has been tempered in a bad tempering range so the impact resistance of the steel is very poor. Failures happen, and are not really wide-spread, but I will not buy a 416 SS barrel for that reason. The AISI 4140/4150/4340 grades do not have this temper embrittlement problem, and show superior impact resistance when tempered to about 30 HRC. They are cheaper to buy in a production rifle. One who takes good care of a firearm will never have any major corrosion and wear issues with the 4000 grade steel barrels anyway. And if you do use it an awful lot and it begins to wear out, well then you got your money's worth from the product, just buy a new barrel. Nothing lasts forever anyway. As a note, AISI 410 SS is a better alternative to 416 SS as is does not generally have the sulphur issue, however the temper embrittlement issue is still a concern.
Here is my opinion: Unless you are competition shooter, buy the non-stainless grade barrels. If you are a professional match shooter find a good AISI 17-4PH barrel as it is a much better choice if one wants corrosion resistance, wear resistance, and impact resistance.
For my money and safety, it is a 4000-series material.
Respectfully submitted, Mike M.
- Green Lane, Pennsylvania
So what are the differences?
Now that we have talked about the main advantages and disadvantages to stainless steel barrels in general, what are the differences between 410, 416, and 416R stainless steels?
410 Stainless Steel:
410SS.jpgGrade 410 is the basic martensitic stainless steel; like most non-stainless steels it can be hardened by a "quench-and-temper" heat treatment. It contains a minimum of 11.5 per cent chromium, just sufficient to give corrosion resistance properties. It achieves maximum corrosion resistance when it has been hardened and tempered and then polished. Grade 410 is a general purpose grade often supplied in the hardened, but still machinable condition, for applications where high strength and moderate heat and corrosion resistance are required.
Martensitic stainless steels are optimised for high hardness, and other properties are to some degree compromised. Fabrication must be by methods that allow for poor weldability and usually the need for a final heat treatment. Corrosion resistance of the martensitic grades is lower than that of the common austenitic grades, and their useful operating temperature range is limited by their loss of ductility at sub-zero temperatures and loss of strength by over-tempering at elevated temperatures.
Main Source: http://www.azom.comCorrosion Resistance
410 resists dry atmosphere, fresh water, mild alkalies and acids, food, steam and hot gases. Must be hardened for maximum heat and corrosion resistance. Performance is best with a smooth surface finish. Less corrosion resistant than the austenitic grades and also less than 17% chromium ferritic alloys such as Grade 430.
Good resistance to scaling up to approximately 650°C, but generally not recommended for use in temperatures between 400 and 580°C, because of the reduction in mechanical properties.
Full anneal - 815-900°C, slow furnace cool to 600°C and then air cool.
Process Anneal - 650-760°C and air cool.
Heat to 925-1010°C, followed by quenching in oil or air. Oil quenching is necessary for heavy sections. Temper, generally within the range 200-400°C, to obtain a wide variety of hardness values and mechanical properties as indicated in the accompanying table.
The tempering range 400-580°C should generally be avoided.
416 Stainless Steel:
416SS.jpgGrade 416 has the highest machinability of any stainless steel, at about 85% of that of a free-machining carbon steel. As for most other free-machining stainless steels the improvement in machinability is achieved by addition of sulphur which forms manganese sulphide inclusions; this sulphur addition also lowers the corrosion resistance, weldability and formability to below that of its non-free machining equivalent Grade 410.
Grade 416 is sometimes used in the unhardened or hardened and highly tempered condition because of its low cost and ready machinability.
Martensitic stainless steels are optimised for high hardness, and other properties are to some degree compromised. Fabrication must be by methods that allow for poor weldability and usually also allow for a final harden and temper heat treatment. Corrosion resistance is lower than the common austenitic grades, and their useful operating temperature range is limited by their loss of ductility at sub-zero temperatures and loss of strength by over-tempering at elevated temperatures.
Main Source: http://www.azom.comCorrosion Resistance
Useful resistance to dry atmospheres, fresh water and mild alkalies and acids, but less resistant than the equivalent non-free-machining grades. Less corrosion resistant than the austenitic grades and also less than 17% chromium ferritic alloys such as Grade 430. High sulphur content free machining grades such as 416 are totally unsuitable for marine or other chloride exposure.
Maximum corrosion resistance is achieved in the hardened condition, with a smooth surface finish.
Fair resistance to scaling in intermittent service up to 760°C and up to 675°C in continuous service. Not recommended for use in temperatures above the relevant tempering temperature, if maintenance of mechanical properties is important.
Full Annealing - Heat to 815-900°C for ½ hour per 25mm of thickness. Cool at 30°C per hour maximum to 600°C and air cool.
Sub-Critical Annealing - Heat to 650-760°C and air cool.
Hardened by heating to 925-1010°C, quenching in oil, and tempering to suit the mechanical requirements. See accompanying table.
Note: The tempering range 400-580°C should be avoided, due to poor ductility.
416R Stainless Steel:
416R is a proprietary steel produced by Crucible specifically designed for rifle barrels. The main difference between 416R and standard 416 stainless steels is the addition of Molybdenum (which is present in 410 but not 416 stainless steels) and a lower sulfur content than 416 stainless steel (but still higher than 410 stainless steel). Crucible also states that barrels made from 416R may be safely used at temperatures as low as -40 degrees Fahrenheit. For more information on 416R stainless steel, please see Crucible's 416R Datasheet.
In conclusion, if you are in the market for a stainless steel rifle barrel I would offer the following recommendations:
- Avoid lightweight or thin profile stainless steel barrels. This recommendation is echoed from multiple barrel manufactures due to the potential issues with temper embrittlement in martensitic stainless steels. The thicker walls of a medium, heavy, or bull profile barrel will strengthen the barrel and make up for the shortcomings of 400-series stainless steels as a barrel steel.
- Avoid standard 416 stainless steel barrels. 410 and 416R stainless steels both have a lower sulfur content, making them less prone to developing sulphide stringers which may result in catastrophic barrel failure.
- If the rifle will never see freezing temperatures, 410 stainless steel will likely be your best option. 410 stainless steel has the lowest sulfur content of the three grades we discussed, and will be the least likely to develop sulphide stringers. Avoiding sub-zero temperatures and using a barrel of adequate thickness should also minimize the temper embrittlement issue.
- If the rifle may see freezing temperatures, 416R stainless steel will likely be your best option. 416R stainless steel is rated for use at temperatures as low as -40 degrees Fahrenheit and has a lower sulfur content than standard 416 stainless steel. However, you should still ensure the barrel is of adequate thickness.