Welding crack in 304SS wood boiler firebox
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  1. #1
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    Default Welding crack in 304SS wood boiler firebox

    Howdy folks! Long time reading of the wealth of knowledge here, first post though. Normally I'm not the type to ask for help, but I recognize when I need some outside knowledge.

    Long story short, my wood boiler has a 304SS firebox. It's only 2 years old but the compmany is no longer in business so I'm my own warranty at this point sadly. A poorly finished mig weld with a large crater at the end acted as a propagation point for a crack to form early this winter. Before it got silly colt out, I let the boiler burn down and made a repair. Material is 10ga 304SS. Took a 1/16" drill bit and drilled the ends of the crack, used a small burr to v groove out the crack, then GTAW welded it with 308L rod. All was well the rest of the winter. Last week I started noticing moisture in the lower burn chamber again. I let it burn out and cool down for the season over the weekend. Today I went to town with a wire brush and found the my weld cracked down the middle. To my knowledge I did everything correct, but I'm happy to learn I missed a step specific to boiler work. I weld sanitary SS tubing for a living so boiler repair isn't a familiarity.

    Here's the initial crack


    After my first weld. Please don't judge the weld too harshly, it's about as awkward of a welding position as it gets to reach this area waist deep in the loading door of the boiler haha.


    And here's what I found today .


    Did I do something wrong or skip a step? Thank you in advance for any help!
    Last edited by Whitbread; 04-17-2017 at 08:28 PM. Reason: Added pics

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    From what we understand boiler repair is a critical job and cracks may indicate fatal condition of the equupment.

    These things are very dangerous so do not touch it until others who do this for a living chime in.

    Sent from my SAMSUNG-SGH-I337Z using Tapatalk

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    Quote Originally Posted by Tony Quiring View Post
    From what we understand boiler repair is a critical job and cracks may indicate fatal condition of the equupment.

    These things are very dangerous so do not touch it until others who do this for a living chime in.
    This boiler isn't pressurized, so there's essentially zero inherent danger working on it.

    Apparently my first post has to be approved by a moderator? Hopefully it shows up to the party sometime!

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    Since the original posting isn't available yet, all we have to go on is the topic title. So, I'll limit myself to two comments.

    1) If this is really the firebox, and not the boiler, you've got no ASME pressure vessel code issues to worry about. If this is a boiler, better to get someone code qualified to do the work!

    2) 304 is one of the readily weldable alloys of stainless. I've only done it with TIG, which requires exactingly clean surfaces. Prepping a used firebox for TIG welding would require an "intense" cleaning.

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    Quote Originally Posted by Whitbread View Post
    This boiler isn't pressurized, so there's essentially zero inherent danger working on it.

    Apparently my first post has to be approved by a moderator? Hopefully it shows up to the party sometime!
    'weld up the cracks' in heat degraded, oxidized SS is a pipe dream.
    It's no longer SS, and sorta/maybe/kinda weldable, but without any service life.
    On wood stove/ firebox items showing cracking (and there's probably more cracking/distortion than you've noticed or cared to notice), plus heat warpage....is prime indication that service life has come to its end.
    Depending on overall condition, of the no-pressure boiler section and entire unit, it might be possible
    to remove heat degraded parts, fit and weld new in....but that's not a $10 TACK JOB....and still has limited service life.

    [I think this thread is headed in the same direction as the cracked motorcycle wheel thread)

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    Nieghbors weld shop did a couple of these, it was a particular manuf (now out of business
    because of the cracking)

    He found that the configuration of the joint was causing the cracking, and re-welding
    don't really FIX the problem.

    It was a bad design from the get-go, and the joint config placed the weld in a highly restrained area.

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    Stainless loves to crack too, it work hardens easily. because its a boiler shell assuming its not been cooked dry its probably not been hot enough to turn into the unweldable crap, but welding cracks alone is never a long term fix for any repair on anything, if its cracked weld won't ever stop it cracking again, regardless of how good a welder you think you are.

    Only long term fix on stuff like this is to re enginer it so it can move and not suffer the same stresses, some times thats a simple as just cutting off some brackets, other times, its leave a few bolts out so it is no longer restrained in a given direction. Some shapes - things are just fundamentally crap designs and no fixes are possible.

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    'Did I do something wrong or skip a step? Thank you in advance for any help!'

    Mfg. did everything wrong. Poor design, poor execution; as previously discussed in
    above replies. Doubler plate that area and then other areas will crack out.
    Beat the dead horse. Get another boiler with good track record.

    Many have no idea just 'how much' steel plate, let alone SS plate, will warp, grow, distort,
    degrade; when subjected to heat on one side.

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    How do you know its 304? Told by the MFG, Element zappy gun or WAG?

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    Can you take pictures of the assembly around the weld so a better idea of the cause can be determined. These other guys are spot on with their comments, I am just not one to throw something out when a general moment diagram may help solve the bigger problem. There is a restriction of the SS expansion playing into the cause of the failure, but that doesn't look like what caused this second reappearance. The absence of shielding gas on the backside of the weld, plus any contaminants that existed from the dirty backside are what made that weld fail. If it was just an engineering mistake (which it may have been) the repair would have failed in the parent material at the toe of the weld, not in the weld itself. The yield strength of 308l filler when welded properly, is higher than the current state of the parent material. You can overlay the original crack and reappearance crack and see the absolute path, which means the repair was not done properly (no offense of course, just an observation). If you want to save the box, cut a big section out to allow access for cleaning. Clean the parent material really well on both sides of the plate, tack the new plate in, purge the tank with argon then weld real carefully making sure to control heat input as the shrinkage in the weld HAZ will play havoc in the future if not compensated for. Adding some heat into the area after the weld is finished is a good thing to help normalize the weld stress. It sounds odd to add heat to something that will heat on its own, but the grand scale heating will not allow for normalizing. Any stresses that you input with weld heat, will be drastically increased when large scale heat is input. Then again, if you don't want to, of are not comfortable with that scope of repair then toss it and buy a new one. What is your time worth?

    One last thing, there are other filler metal composition better suited for this repair. Call and talk to the guys at Hobart or Lincoln.

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    Try a duplex alloy filler like 2205 or 2209 - duplex alloys are less crack sensitive than full austenitic alloys. The duplex beads solidify as ferrite, which is less crack sensitive, and partially convert to austenite on cooling. The bead will be magnetic. No guarantee the box won't re-crack due to stresses once it put back in use, but your initial welds are less likely to break on cooling.

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