Page 8 of 18 FirstFirst ... 678910 ... LastLast
Results 71 to 80 of 174

Thread: Bolt life question.

  1. #71
    Join Date
    Jun 2006
    Posts
    161
    Feedback Score
    0
    One must examine the interaction of specifically the nitrogen with chromium in higher alloys. While it is a generally sound statement that carbo nitriding QPQ will induce a surface compressional stress and as such increase tensile fatigue in many alloys, increased chromium may lead to a step function drop in the impact toughness. This has been seen to the extent that certain military prints will specifically preclude CN hardening on stressed components. Bolt alloys while not typically through hardening grades are very close in chemistry, carbo nitriding QPQ is best applied to case hardening alloys and low alloy steels with substantial bulk of design.

    QPQ is not a new process and has its foundation in cyanide hardening. Melonite became the trade name of the salt bath carbo nitriding which was developed to remove the cyanide base. QPQ extended this process but to be truely effective the polishing phase must refine the surface. Regardless the process remains the diffusion of carbon and nitrogen free radicals into the surface of a steel alloy. The diffusion rate initially creates a sharp boundry between the substate and the underlying material which the quenching then develops by softening the transistion and conversion the alloy phase.

    Bolts exist as high stress, low cycle fatigue components, in that the life is expressed in terms of less than 10^6 cycles under the operating load. What complicates the analysis is that during extraction the lugs are subject to biaxial stress, being both axial shear with tensile moment at the rear but also bending. Typically residual pressures during extraction can be as high as 3000 psi which will both impose load on the lugs but also hold the case to the chamber walls. Where this is leading is that given the load state of a bolt, the life is governed by crack initiation not propagation. Suface treatments that may assist the formation of micro cracks by lowering the impact strength, or alloys with such intergranular inclusions either as the result of treatment or residual tramp elements are best avoided. Hydrogen will dramatically shorten the bolt lfe as will any surface etching effects.

  2. #72
    Join Date
    Jun 2007
    Location
    WA
    Posts
    454
    Feedback Score
    5 (100%)
    Ok, so in English, will QPQ reduce the impact toughness of a bolt and lead to a shorter round count life or not?

    Alloys that create nitrides are typically contain aluminum, chromium, molybdenum and titanium. With this in mind, nitriding works better on metals that contain these metals, but not in excessive amounts as these metals can create very hard and very brittle (especially in the case of aluminum) nitrides that break easily and cause cracks to form. So the real question is at which amounts *will* these nitride creating metals create a situation where the surface loses its impact toughness? There were a lot of "mays" and "mights" in the above post, i'm looking for verifiable data please.

    Same question for barrels and barrel extensions.

    Also, does not the addition of the nitriding process lower the coefficient of friction between the materials, thusly reducing the stresses between parts? And since hydrogen isn't part of the nitriding process, nor is surface etching, how does your final statment factor into the equation?
    Last edited by GrumpyM4; 06-06-12 at 22:59.
    It is missing the point to think that the martial art is solely in cutting a man down; it is in killing evil. It is in the strategem of killing the evil of one man and giving life to ten thousand -Yagyu Munemori

  3. #73
    Join Date
    Jun 2007
    Location
    WA
    Posts
    454
    Feedback Score
    5 (100%)
    doubletap
    Last edited by GrumpyM4; 06-06-12 at 22:58.
    It is missing the point to think that the martial art is solely in cutting a man down; it is in killing evil. It is in the strategem of killing the evil of one man and giving life to ten thousand -Yagyu Munemori

  4. #74
    Join Date
    Apr 2009
    Location
    FL -Where it's summer 10.5 months out of the year
    Posts
    4,114
    Feedback Score
    17 (100%)
    Quote Originally Posted by GrumpyM4 View Post
    Finally fount the micrometer.

    M4 bolt = .512 OD at the cam pin
    and .101 wall thickness at cam pin

    HK bolt = .53 OD at the cam pin
    and 1.11 wall thickness at cam pin.

    M4 bolt = .101 lug thickness

    HK bolt = .101 lug thickness.

    Obviously, my eyes decieved me regarding lug thickness, but I was correct about the bolt diameter difference.

    Like I said earlier, an M4 bolt fits freely into the MR556 carrier even with gas rings, but the HK bolt will only fit part way into an M4 carrier. It stops just before the cam pin hole is fully inside the carrier. This is caused by the piston ring bore on the inside of the carrier itself.

    An examination of the HK bolt and the M4 bolt (noveske) show that the bolt lug joints (angles at the rear of the lugs where they transition from the rear of the lug into the rest of the bolt, a 90 degree transition)) are more radiused on the HK bolt, allowing for better fatigue resistance. Matter of fact, every angle and joint on the HK bolt is radiused, and to a better extent then anything on the DI bolt. Any metal worker will tell you that sharp angles create weak points in your material, and HK seems to have made every attempt to minimise this issue.


    Not to beat the nitriding drum, but here are some thoughts regarding a real easy way to PiP the M4 system:

    At this time, I have a single HK bolt that has been QPQ nitrided. I had my MR fully QPQ nitrided when I understood that this process imparted better corrosion resistance the chrome, had a lower coefficient of friction then chrome, and penetrated twice as deep as chrome plate adds to the steel. All great things.

    What I learned AFTERWARDS, is that nitriding also increases the fatigue strength of the base material by nearly three times as much. Around 270%. Meaning that with the lower coeficient of friction (especially with lube added) between both a QPQ nitrided barrel/barrel extension and bolt, coupled with the increased fatigue strength of the nitriding process, my nitrided bolt should last an easy 20k rounds or more.

    Looks like I need to send my spare off for nitriding as well, and I think that I'm going to start having all of my DI parts nitrided too.

    At this point, I can't find a single negative aspect of nitriding gun parts.

    Best part is that even a single dip nitride will enhance the base material greatly and it's a cheap process and when done in bulk, doesn't add a whole hell of a lot to the overall cost of a rifle.

    If this became part of the TDP for Mil guns, I think they would find the overall performance of the M4/M16 platform increase greatly, with very little cost increase.
    Great post and thanks for the measurements.

    However, I should note that not all DI bolts are the same. My LMT bolt has undercut lugs (a round radius at the rear of the bolt lugs) and very slightly 'softer' radii than a BCM bolt on a few other angles (lug roots, etc).

  5. #75
    Join Date
    Jun 2006
    Posts
    161
    Feedback Score
    0
    Quote Originally Posted by GrumpyM4 View Post
    Ok, so in English, will QPQ reduce the impact toughness of a bolt and lead to a shorter round count life or not?

    Alloys that create nitrides are typically contain aluminum, chromium, molybdenum and titanium. With this in mind, nitriding works better on metals that contain these metals, but not in excessive amounts as these metals can create very hard and very brittle (especially in the case of aluminum) nitrides that break easily and cause cracks to form. So the real question is at which amounts *will* these nitride creating metals create a situation where the surface loses its impact toughness? There were a lot of "mays" and "mights" in the above post, i'm looking for verifiable data please.

    Same question for barrels and barrel extensions.

    Also, does not the addition of the nitriding process lower the coefficient of friction between the materials, thusly reducing the stresses between parts? And since hydrogen isn't part of the nitriding process, nor is surface etching, how does your final statment factor into the equation?
    In English, you are looking at a very complex system and then asking questions about metallurgy and fatigue analysis. The answer is statistical and dependent upon the bolt alloy, geometry, surface treatment conditions and loading cycles amongst many others. If you want a brief answer I would not use this type of treatment on an AR bolt and you should note that none of the competitors in the recent competitions for the military submitted this. The current TDP precludes the use of CN for bolts specifically for a reason.

    I will answer this out of order. You are looking for verifiable data, for a very case specific application. This can only be achieved by an extended series of testing and given the fact that the life is governed by first stage fatigue in a low cycle environment it is not a viable proposition to accelerate the tests. The only recourse is to real time testing to collect the data. In English you need to shoot a minimum of 32 rifles to failure using a certified batch of ammunition. Ideally many more. Your verifiable data costs considerable money. Unless the party concerned is conducting an academic exercise or writing a research paper the existence of the data is of no help as you will have no ability to either access it or duplicate the result.

    You have half answered you own question in that you have researched and found that the nitrogen free radicals can work adversely with certain elements. As to the percentages of elements present that will be detrimental to a metal this will depend upon the loading and the geometry as well as the chemistry. Potential micro surface cracks in a 25lb retaining piece that is 8" thick are of little consequence, as an illustration, but such initiation points in a bolt would be detrimental. Bolts are typically, what is generally referred to as surface hardening alloys, in that the carbon content is lower and they are usually used in an application where the surface will be hardened. But they are also higher alloys so in this instance they are not clearly classified.

    Barrels, good ones, are CMV through hardening alloys, draw your own conclusions! Dropping the alloy specification can allow a barrel to be effectively CN treated. The manufacturer must determine this. Stainless steel is out and the ability of the nitrogen to combine with the chromium will frequently leave the resulting part more susceptible to corrosion that the virgin part.

    Barrel extensions are good if run in isolation. They are a very suitable alloy but must be taken in the as machined form. CN over the existing case hardening is a variable proposition.

    To address friction between components, again the information that you are using is case specific example. In this case the tribological results from a controlled standard test. The information should be regarded as indicative of properties that can be achieved. If you can definitively quantify that a CN treated bolt/barrel extension will impose less bending stress on the lugs than a classical case hardened combination, when there is substantial contamination from combustion gasses and also lubricant, either as applied or burnt, then you have the perfect answer.

    The comments wrt hydrogen and surface etching were general information and correctly are not pertinent to the direct discussion of CN on alloys. They were related to the consideration of surface treatments by other methods and how problems may occur.

    I hope that you realize that I am not being evasive in trying to provide answers it is just that a simple yes or no does not exist. I also hope that you do not think that I am being detrimental to the use of CN. Many parts benefit from this finish but like NiB, Chrome, Nickel and Ionbond it is not a universal fix all. The gun industry has the tendency to "find" a process/lube/finish and then finding a single good application, pounce upon it without any further examination and use it for everything they can throw into the vat. This said I am examining CN QPQ for barrels in one instance and I think that bolt carriers may be another great application. I like NiB for through hardened parts and use both chrome and Ionbond in my products.

  6. #76
    Join Date
    Nov 2010
    Location
    MI
    Posts
    3,055
    Feedback Score
    21 (100%)
    Quote Originally Posted by Bill Alexander View Post
    Barrels, good ones, are CMV through hardening alloys, draw your own conclusions! Dropping the alloy specification can allow a barrel to be effectively CN treated.

    Barrel extensions are good if run in isolation. They are a very suitable alloy but must be taken in the as machined form. CN over the existing case hardening is a variable proposition.

    This said I am examining CN QPQ for barrels in one instance and I think that bolt carriers may be another great application. I like NiB for through hardened parts and use both chrome and Ionbond in my products.
    Bill,

    with 8620 barrel extensions, the QPQ temperature seems to be at odds with the carburize/harden/temper that they normally get.

    With QPQ alone, it appears 8620 does not develop sufficient core hardness for strength.

    A material like 4140 seems more suitable for through harden + QPQ treatment.

    What's your take?
    Black River Tactical
    BRT OPTIMUM HFCL Barrels - Hammer Forged Chrome Lined 11.5", 12.5", 14.5"
    BRT OPTIMUM Barrels - 16" MPR, 14.5" MPC, 12.5" MRC, 11.5" CQB, 9" PDW
    BRT EZTUNE Preset Gas Tubes - CAR and MID
    BRT Covert Comps 7.62, 5.56, 6X, 9mm
    BRT MarkBlue Gas Tubes - BRT EXT, EXC and PDW Lengths
    BRT MicroPin Gas Blocks - .750" & .625"
    BRT MicroTUNE Adjustable Gas Blocks
    BRT CustomTUNE Gas Ports

  7. #77
    Join Date
    Jan 2008
    Location
    Roaming
    Posts
    683
    Feedback Score
    0
    Quote Originally Posted by Clint View Post
    Bill,

    with 8620 barrel extensions, the QPQ temperature seems to be at odds with the carburize/harden/temper that they normally get.

    With QPQ alone, it appears 8620 does not develop sufficient core hardness for strength.

    A material like 4140 seems more suitable for through harden + QPQ treatment.

    What's your take?
    I'll agree with you. QPQ does not bring the core hardness up high enough to be a stand alone heat treat. IMO the core should be near 50 for toughness without being brittle.
    QPQ for barrels is perfect since they are 32-40 as made and then QPQ makes the surface apx 60 for wear resistance.
    The 8620 carriers we had were heat treated as normal then went out for melonite, the melonite treatment dropped the surface hardness apx 1 point but gave them a better corrosion resistant finish.
    AR15performance
    TRUMP 2020
    The 6.8 is the best choice for hunting deer and hogs with an AR15.

  8. #78
    Join Date
    Jun 2006
    Posts
    161
    Feedback Score
    0
    8620 alloy is not a through hardening grade, rather it is specifically a surface hardening alloy and regardless of wether it is a gas carburize, solution carburize or CN QPQ it is very difficult to develop the core beyond its natural state. The alloy is selected specifically for this reason and the designer must account for this when using such alloys. The ability for the core not to become excessively hard is key but grain growth and inter-granular precipitation of carbides, nitrides must be considered when looking at the heat treatment. Phase diagrams become very important. Remember when considering gun parts fatigue is the dominant failure mode so a hard core may be detrimental

    4140 carries more carbon and will through harden. CN-QPQ should be approached with caution or the resulting part may simply become brittle. This is not to preclude such an alloy. Slight alloy alterations may help. For a barrel one must restrain grain growth to prevent loss of ductility over that of the heat treatment. A good hard surface and nice tempered core is of little help if the elongation to break is reduced disproportionately.

    It is difficult to examine any material simply in terms of hard or soft, ductile or brittle etc. Fracture toughness is a related but independent property. S-N diagrams are considered for the fatigue, and a material with extremely high tensile yield point may fail rapidly in fracture. In mechanisms there are considerations for strain rate dependancy. The barrel extension is subject to compressional loading under the lugs as well as shear and a bending moment. so compressional strength and galling must be examined.

    It is way to easy to let the complexity of material examination become an impenetrable cloud. Once the basics are known, testing becomes a good ally.

  9. #79
    Join Date
    Jun 2007
    Location
    WA
    Posts
    454
    Feedback Score
    5 (100%)
    Quote Originally Posted by Bill Alexander View Post
    In English, you are looking at a very complex system and then asking questions about metallurgy and fatigue analysis. The answer is statistical and dependent upon the bolt alloy, geometry, surface treatment conditions and loading cycles amongst many others. If you want a brief answer I would not use this type of treatment on an AR bolt and you should note that none of the competitors in the recent competitions for the military submitted this. The current TDP precludes the use of CN for bolts specifically for a reason.
    I find the last part interesting. When was the specific requirement for no CN added to the TDP? Is it a requirement that's been there since the beginning and simply been a carry-over or was it added later?

    I will answer this out of order. You are looking for verifiable data, for a very case specific application. This can only be achieved by an extended series of testing and given the fact that the life is governed by first stage fatigue in a low cycle environment it is not a viable proposition to accelerate the tests. The only recourse is to real time testing to collect the data. In English you need to shoot a minimum of 32 rifles to failure using a certified batch of ammunition. Ideally many more. Your verifiable data costs considerable money. Unless the party concerned is conducting an academic exercise or writing a research paper the existence of the data is of no help as you will have no ability to either access it or duplicate the result.
    Exactly. I was hoping that somebody had actually done a test for this data and I didn't know if you were explaining the theory or the data, which is why I asked.

    So far from what I'm reading, I should simply shoot the shit out of my nitrided bolt, but keep my non-nitrided bolt handy should the CN bolt fail.

    You have half answered you own question in that you have researched and found that the nitrogen free radicals can work adversely with certain elements. As to the percentages of elements present that will be detrimental to a metal this will depend upon the loading and the geometry as well as the chemistry. Potential micro surface cracks in a 25lb retaining piece that is 8" thick are of little consequence, as an illustration, but such initiation points in a bolt would be detrimental. Bolts are typically, what is generally referred to as surface hardening alloys, in that the carbon content is lower and they are usually used in an application where the surface will be hardened. But they are also higher alloys so in this instance they are not clearly classified.
    So much of what has been discussed is still clearly in the realm of theory as there have been no actual tests. While the known properties of various materials and processes under other conditions clearly point to *possible* issues in this situation, the fact is that nobody knows for sure and nobody wants to put in the time, effort, and finances to find out.

    Barrels, good ones, are CMV through hardening alloys, draw your own conclusions! Dropping the alloy specification can allow a barrel to be effectively CN treated. The manufacturer must determine this. Stainless steel is out and the ability of the nitrogen to combine with the chromium will frequently leave the resulting part more susceptible to corrosion that the virgin part.
    The above I knew. So far the only reason I could see for someone to CN stainless is for a color change, but then again, black oxide treatment can do that without compromising the base material.

    Barrel extensions are good if run in isolation. They are a very suitable alloy but must be taken in the as machined form. CN over the existing case hardening is a variable proposition.
    Interesting. I was under the impression that the temperatures involved in CN-QPQ not only acted as a stress relief for the base metal, but also had no effect on pre-existing case hardening.

    I know that case hardening goes deeper then CN, though, so might it be a case of CH preventing CN from penetrating as deep and not getting the full effect from the CN due to the CH?

    One would think that in this case, "more is better". Taking information from the bolt discussion above, if a part is case hardened (deeper penetration the CN), and then CN treated, even if the CN were to develop fatigue cracking, wouldn't the CH prevent the cracks in the CN to develop further into the base material? Or does CN remove the CH or destroy its properties?

    That being said, wouldn't this also apply to bolts as well?

    To address friction between components, again the information that you are using is case specific example. In this case the tribological results from a controlled standard test. The information should be regarded as indicative of properties that can be achieved. If you can definitively quantify that a CN treated bolt/barrel extension will impose less bending stress on the lugs than a classical case hardened combination, when there is substantial contamination from combustion gasses and also lubricant, either as applied or burnt, then you have the perfect answer.
    Sorry, that made me laugh. In other words, there is no perfect answer. For all the number crunching and understanding of metallurgy and processes, what happens with the end user simply cannot be accounted for sometimes.

    The comments wrt hydrogen and surface etching were general information and correctly are not pertinent to the direct discussion of CN on alloys. They were related to the consideration of surface treatments by other methods and how problems may occur.
    Roger that.

    I hope that you realize that I am not being evasive in trying to provide answers it is just that a simple yes or no does not exist. I also hope that you do not think that I am being detrimental to the use of CN.
    Absolutly not! I just like to understand, and while I have a base knowledge of metallurgy (former DoD shipfitter), the higher end stuff like what we are currently discussing is, other then the homework i've done myself, not fully within my realm of understanding. This is why I ask questions.

    I hope my questions didn't come across as "challenging". I just tend to be pretty direct about things when I want to learn.


    Many parts benefit from this finish but like NiB, Chrome, Nickel and Ionbond it is not a universal fix all. The gun industry has the tendency to "find" a process/lube/finish and then finding a single good application, pounce upon it without any further examination and use it for everything they can throw into the vat. This said I am examining CN QPQ for barrels in one instance and I think that bolt carriers may be another great application. I like NiB for through hardened parts and use both chrome and Ionbond in my products.
    Outstanding information.

    Thank you for taking the time to explain things a little further in depth, I appreciate it. I hope you don't mind me asking questions.
    It is missing the point to think that the martial art is solely in cutting a man down; it is in killing evil. It is in the strategem of killing the evil of one man and giving life to ten thousand -Yagyu Munemori

  10. #80
    Join Date
    Jan 2008
    Location
    Roaming
    Posts
    683
    Feedback Score
    0
    The company that carburizies the bolts and carriers we machine brings the core of both up to apx 50 for maximium toughness of the parts. I'm not saying what our exterior is but those 2 parts can be either to hard and brittle or not hard enough.
    I've seen some bolts on the market that are so hard they chip at the edges around the cam pin hole and I've seen another brand that (I believe are 8620) are so soft that the lugs compressed. I have cut a carrier from that same company in half with a $9 hacksaw, I'm not sure it was tempered at all.
    I would like to see some company Melonite a bolt to see how it holds up ...in their own rifle.
    AR15performance
    TRUMP 2020
    The 6.8 is the best choice for hunting deer and hogs with an AR15.

Page 8 of 18 FirstFirst ... 678910 ... LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •