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Thread: Trunnion Survey - Further Analysis

  1. #161
    holescreek's Avatar
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    I'll send you the file when we're closer to done so you can do the housekeeping.

  2. #162
    holescreek's Avatar
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    Just finished sectioning Volk's cracked SW trunnion.

    cuttrunnion002.JPG

    Hard to get a good visual on just how thin the wall is at the relief diameter because the cracked section in front fell out. The section is cut precisely through the middle of the pin.

    cuttrunnion003.JPG

    I also dug out a badly butchered cetme trunnion I received in a kit I bought from a former member here. It arrived with the roller angles ground completely out? Anyway, it measures hard in the back (High 50's/low 60's) and slightly softer in the front (around 43). It also has a large "C" stamped on the bottom in the middle area. Any ideas what the "C" represents?

    Cetmehack004.JPG
    Last edited by holescreek; 09-01-2017 at 10:28 PM.

  3. #163
    Senior Veteran r.erichsen's Avatar
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    Quote Originally Posted by holescreek View Post
    Just finished sectioning Volk's cracked SW trunnion.

    Hard to get a good visual on just how thin the wall is at the relief diameter because the cracked section in front fell out. The section is cut precisely through the middle of the pin.

    The first word in my mind was an expletive, I'll settle for a "golly". That section looks to be no thicker than a business card - a cheap one. The only explanation for that is poor location and little concern as to the implications. Why such a poor spot was chosen and then repeated across all of the SW samples surveyed is disconcerting.

    I'm more convinced now than before that the effort to try to salvage some of these trunnions is going to be a lot of work. We'll have to get a success story on softening the trunnion, drilling a proper flat "spot" and then the cross pin hole. For the 9 mm trunnions, this doesn't make any sense given the reasonably priced options that are out there. For the more specialized/rare 10 mm/.40/.357 SIG, it might be preferable to a get the more costly HK trunnion @ $120ish. The RCM are in the $80-90 range but come with all the problems we've found for the builder to navigate, including excess hardness and an undrilled pin whole.

    R
    Last edited by r.erichsen; 02-05-2012 at 09:40 PM.
    Mangler of loose parts and perfectly good parts kits

  4. #164
    holescreek's Avatar
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    I got the sectioned trunnion in to the microscope. The crack may have originated at the transition between the pin hole and the relief diameter shoulder in the front then moved rearward.

    swsectionedinternal.JPG

    Here's a better pic of the right side (that didn't crack).

    swthinwall.JPG

    OK, a question for the pro's: Since the trunnion counts towards your 922r parts count, why aren't any of them marked with an identification mark to prove origin/maker?
    Last edited by holescreek; 09-01-2017 at 10:30 PM.

  5. #165
    Senior Veteran r.erichsen's Avatar
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    What did you section this thing with anyway? Did you use a bit of belt sanding after the cut to clean up the sections?

    R
    Mangler of loose parts and perfectly good parts kits

  6. #166
    holescreek's Avatar
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    Quote Originally Posted by r.erichsen View Post
    What did you section this thing with anyway? Did you use a bit of belt sanding after the cut to clean up the sections?

    R
    Nope, no post cut grinding. I used a 7" fibre cut-off wheel on my surface grinder. Zip, zip, ting (part falls off). I did deburr the razor sharp edges with a 3M wheel though.

  7. #167
    Senior Veteran r.erichsen's Avatar
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    I think your sectioning leaves little doubt of where the cracks originate and the section thickness makes it clearer than just displaying the section thickness in fractions of an inch. Not all of these trunnions crack, but there is a lot less margin for error and they may or may not last as long as the barrel they are gripping and I get the impression you should be able to go through several barrels before the receiver needs to be scrapped.

    R
    Mangler of loose parts and perfectly good parts kits

  8. #168
    Senior Veteran r.erichsen's Avatar
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    We are nearing the point where Chasgee will have some data to share. Early next week hopefully there will be a very long, detailed post about the trunnions sent for detailed chemical and microhardness anlaysis.

    R
    Mangler of loose parts and perfectly good parts kits

  9. #169
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    The lab results are in! The results are summarized in the table below, but here are the highlights:

    1. The CETME, HK, and PTR trunnions are all made from the same material. There isn't a US grade equivalent, but it is like 9310 steel but with 20 carbon and lower molybdenum. I believe that the CETME trunnion was later determined to be an HK, but Holes will have to clarify. The RCM and SW trunnions are made from 8620. All of them were manufactured from bar stock (no evidence of casting for the SW trunnion).

    2. All parts were carburized. Case depth varied with the manufacturer but the factory parts had case depths of around 0.035 inch, PTR at around 0.030, and the other two around 0.020 inch. This variation wouldn't really affect the propensity for cracking, but it might affect the long term wear properties of the ramp area. The deeper case depths would hold up longer.

    3. The factory parts were masked on the OD at the end with the barrel pin hole. This is why Holes found lower hardness readings when he measured the surface. The rest of the parts were carburized on all surfaces. This high surface hardness would make it harder to drill the barrel pin hole, and could cause other dimensional problems because of drill wander, etc. The factory obviously changed this when switching from CETME to HK because of this problem.

    4. If the US manufacturers would heat treat their parts correctly, they could mimic the core and case hardness found on the factory parts. It appears that the factory was aiming for a case hardness of around HRC 58-60, with a resultant core hardness of HRC 37-39. Even the factory had some QA issues as evidenced by the HK sample with slightly lower hardness. The RCM trunnion missed the boat here with hardness values significantly lower. There is no reason why you would carburize a part and harden it to a lower hardness range. You want to maximize case hardness to ensure good wear/fatigue resistance. I believe the RCM trunnion was a QA problem that just slipped through.



    In summary:

    - The PTR trunnions come closest to factory spec because they are machined from the same material and heat treated to the same parameters except case depth.

    - The factory masked the OD at the pin end to aid in drilling the barrel pin hole. It doesn't seem that this is really necessary since the CETME trunnions don't have this feature, but I'm sure that it helped with their manufacturing process.

    - Even though the other US manufacturers use a different type of steel, their parts should perform as well as the factory parts except with the possibility of less life in the ramp areas due to a shallower case depth.

    The big issue here is, of course, why do the US trunnions crack after assembly? It appears that it is indeed related to the problems associated with having to drill the barrel pin hole in the trunnion and the dimensional issues that Holes uncovered with respect to section thickness. If you re-use the factory trunnion, you eliminate the need to deal with drilling it, and eliminate the other problems associated with proper alignment. If the parts are properly heat treated (and it seems that the RCM trunnion I analyzed was not) and you can drill a properly aligned and located hole through the carburized surface, the US parts should not crack in this area.

    As a side note, localized heat treatment of the pin end, as Holes attempted earlier, would not be that effective because the carburized surface hardness will not be reduced significantly. The core hardness would also drop drastically, as illustrated in the results found on the RCM trunnion. You can try and shorten the tempering time to only affect the carburized areas, but it may not be worth the risk of creating a weak core without sufficient strength to keep everything in place.

    I hope this helps everyone better understand what is going on with the trunnions we can use for our builds. I'll attach a few pictures to help illustrate the findings.

    Thanks,

    Chuck
    Last edited by chasgee; 02-19-2012 at 11:01 PM.

  10. #170
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    Here's the cross section of the HK trunnion:



    You can see the carburized layer (grey band) at all surfaces of the part except the OD at the pin hole side and at the welds at the locking piece side (arrows). The welds diffuse the carburized case away. Sorry for the lousy picture quality, but the grey band goes all the way around.


    Here's a photo of the microstructure, typical for all of the parts. Tempered martensite, as expected. The arrows indicate linear inclusions, typical for bar stock. All parts had these.


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