Welding a grade 8 bolt
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  1. #1
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    Oct 2007
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    Welding a grade 8 bolt

    I am making a bracket and wanting to weld a grade 8 bolt to it and was wondering if this is going to weaken it.

    I am making an adapter for a power brake booster that will go from the brake booster to the brake master cylinder. I will be using two 3/8" grade 8 bolts that will pass through a 1/2" plate of C1018. The bolt heads would be welded to keep them from turning.

    Is this okay to do?

  2. #2
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    Re: Welding a grade 8 bolt

    Yes, welding just about any bolt will tend to weaken it. I think if you use very small tacks you'll be ok though. Use the least amount of heat possible. Just enough to keep it from turning with the amount of tourque you're gonna put on it. Better yet, put the bolt in and mark where the head will be. Weld a small tab or two to the plate that will keep the head from turning.
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  3. #3

    Re: Welding a grade 8 bolt

    Grade 8 is way more than you need for that application anyway. It'll be fine.

  4. #4
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    Re: Welding a grade 8 bolt

    Put a wrench on the bolt, then weld the wrench.

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  5. #5
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    Re: Welding a grade 8 bolt

    I would tack it with silicon bronze. Grade 8 bolts don't take kindly to welding in general from my experience.
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  6. #6
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    Re: Welding a grade 8 bolt

    I would tack weld it and not think twice about it. Like Turboblown said, grade 8 is way overkill for that application anyways.
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  7. #7
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    Re: Welding a grade 8 bolt

    Thanks for answering. I was looking at my factory service manual and it shows the torque specs as being 9-15 lb pounds so I guess it is overkill. I just don't want to chance it.

  8. #8
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    Re: Welding a grade 8 bolt

    use grade 5 if thats all the torque grade 5 will stretch before it brakes grade 8 will snap before it stretches

  9. #9
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    Re: Welding a grade 8 bolt

    I've welded grade 8 bolts and nuts, and torqued them enough to stretch the threads ( read ruin them) without failures otherwise. Go for it.

  10. #10
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    Re: Welding a grade 8 bolt

    Quote Originally Posted by prop-doctor View Post
    use grade 5 if thats all the torque grade 5 will stretch before it brakes grade 8 will snap before it stretches
    Correct, higher grade is not always a good thing.

    Other thoughts:
    thread the plate the bolt is going through and use thread lock.
    or
    Use wire locks.
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  11. #11
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    Re: Welding a grade 8 bolt

    I've used literally hundreds of grade 8 bolts for weld in place studs...never heard of one breaking. I like them for studs because the threads will take a lot more abuse than grade 5 and also they weld nicer because they're not plated.

    I'm sure grade 5 is more than sufficient for your application....Whatever you weld in you only need a 1/4 inch of weld to hold it.

  12. #12
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    Re: Welding a grade 8 bolt

    I personally like Scott9116 solution of welding a couple of tabs on the plate next to the bolt.

    After that, welding any bolt becomes a moot point.

  13. #13
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    Re: Welding a grade 8 bolt

    He's only wanting to weld on the head of the bolt, not the bolt shaft itself. If he gets it hot enough to damage it to the point of pulling the head off it'll be a first and should be put on record.

    By the way, what are typical torques for a 3/8ths bolt??

  14. #14
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    Re: Welding a grade 8 bolt

    As far as I know, welding and or tacking a heat threated bolt will weaken it.

    Tacking also melts metal, and when you achieve the melting point of a metal the heat threatment is lost.
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  15. #15
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    Re: Welding a grade 8 bolt

    About, ten years ago I had the bolt that was holding the master cylinder's push rod to the clutch pedal wear through and fail. I ended up with a steel rod in my leg. Granted this was an old truck that required over 90lbs of force on the pedal to engage the clutch and have no idea what grade the bolt was (due to the fact it was welded to the top of the pedal). my thinking is that if it were grade 8 it would have worn through the eye of the rod first and deflected the rod from my calf. My thinking is that welding heat drew out what ever temper the bolt had. Heck, it may have been oxy welded with a coathanger for filller for all I know. But, I do know that I only weld bolts as a last resort now.
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  16. #16
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    Re: Welding a grade 8 bolt

    Thanks for all the replys so far. I can't really weld a tab because the bolt head has to stay flush with the top of the 1018 plate. It will be countersunk. I may just have my machinist mill out the 1018 to the size of the bolt head since he has a CNC machine then there would be no welding. The reason is because the 1018 plate will be sandwiched in between the brake booster and the master cylinder. Once I get this done I will post a pic or two. Will visit him in a couple weeks.

  17. #17
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    Re: Welding a grade 8 bolt

    You are only welding the head of the bolt, I would just make 2 passes on one side of the hex. The second pass will add more heat to the first one and should keep it from cracking.

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  18. #18
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    Re: Welding a grade 8 bolt

    The grade 8 bolt may be slightly weaker after you weld it than it was new, but it will still be stronger than a grade 5 bolt is when new. Grade 8 bolts get their strength from the steel alloy, not from heat treatment.

    If they were heat treated, they would have ground threads- rolled threads wouldn't work due to warpage from the heat treatment process.
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  19. #19
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    Re: Welding a grade 8 bolt

    This is an improvised clamp that I made in the field. One inch grade 8 nuts welded to scrap channel with 7018. I used a 3' bar to spin it with about all the strength I could muster without a trip to the hospital. It took longer to find the parts than it did to build it and it worked great. Ugly, but got the job done.Unless you will have stresses near the design limit of the fasteners I doubt that any weakening is all that significant.
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  20. #20
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    Re: Welding a grade 8 bolt

    If grade 8 bolts don't break after being welded on these days it is probably because these Chinese made bolts are not true grade 8. Back in the days when bolts were made in America I worked at a shop that was rigging up a truck with a side boom winch. The guys working on it mounted the winch late one afternoon and welded the bolt heads so you wouldn't need a backup if you had to pull the winch to work on it. Imagine everyone's suprise the next morning when we came to work and the winch was laying on the ground. All the bolts had snapped off at the heads where they were welded.

  21. #21
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    Re: Welding a grade 8 bolt

    Interesting...

  22. #22
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    Re: Welding a grade 8 bolt

    This question got me thinking, so I did a bit of looking and found this:http://www.portlandbolt.com/faqs/wel...h-anchor-bolts
    In another topic they say that ASE grade 8 is equivalent to ASTM A490.
    I'll probably still weld to them until it bites me in the a$$, but figured I'd pass this along.

  23. #23
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    Re: Welding a grade 8 bolt

    Quote Originally Posted by Oldtimer View Post
    If grade 8 bolts don't break after being welded on these days it is probably because these Chinese made bolts are not true grade 8. Back in the days when bolts were made in America I worked at a shop that was rigging up a truck with a side boom winch. The guys working on it mounted the winch late one afternoon and welded the bolt heads so you wouldn't need a backup if you had to pull the winch to work on it. Imagine everyone's suprise the next morning when we came to work and the winch was laying on the ground. All the bolts had snapped off at the heads where they were welded.
    I had a very similar experience last week. Tig welded 3 1/2" Grade 8 bolts to a 1/4" plate and mounted a vice on it. Tightened them with an impact, one broke immediately, the bolt not the weld! It looked crystalized, the other two bolts were holding so I didn't worry about it right away just did what I had to do. The next day all that was holding the vice on the plate was gravity!
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  24. #24
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    Re: Welding a grade 8 bolt

    Quote Originally Posted by prop-doctor View Post
    use grade 5 if thats all the torque grade 5 will stretch before it brakes grade 8 will snap before it stretches
    A short dissertation as to why that statement is not true:


    http://www.pirate4x4.com/tech/billav...lts/index.html

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  25. #25
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    Re: Welding a grade 8 bolt

    Quote Originally Posted by riley mcmillan View Post
    I would tack it with silicon bronze. Grade 8 bolts don't take kindly to welding in general from my experience.
    You are right.

    Almost any material that is to be machined into a bolt. Will tend to weld poorly. The harder the bolt the worse it will weld.

    Just like most of us know not to weld leaf springs, drill bits, or screw drivers. We should know not to weld hardened bolts.

    The steel I believe is a medium carbon steel made to harden. You should avoid welding such a material, hardened or not.

    It can literally fall off when done welding it well.



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