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  1. #1
    Join Date
    Sep 2013
    Posts
    11

    Default Trouble with 304 exhaust

    I have been making myself exhaust systems for a few years now. but on my last system I have noticed what looks like rust. Its only happening in the heat effected areas next to the welds. Is there something I am missing? I try to use as little heat as I can and back purge with argon.

    Here is a picture, its looks better in person but this gets the problem across. The parts looked great for the first few months.

    photo_zpsc8611589.jpg

  2. #2
    Join Date
    Oct 2012
    Location
    yuba city, CA
    Posts
    49

    Default rust bloom ss welds

    If a steel-not SS brush is used before/after welding, this can occur.
    Any brush or abrasive used on SS, must not have been used on
    steel-prior, to avoid embedding particles of steel/iron into the SS.
    Do not allow SS to come in contact with steel dust and ideally-not
    contact steel during work. Multiple wet wiping, before/during/after
    processing helps a lot. Gloves just for SS. Clean cardboard, etc. over
    work area.

    Passivation after welding can remove the iron particles, but not
    allowing contamination in the first place is easier.

    The rust seen is reacting with the iron particles in the SS matrix.
    Trying to buff, sand, polish off the embedded iron particles removes
    some of them and pushes the rest of them deeper into the SS matrix.
    At the worst--this rust blooming will accelerate corrosion of the SS.

  3. #3
    Join Date
    Sep 2013
    Posts
    11

    Default

    Ive been using a chop saw with a metal blade to try and reduce contamination. What tools are best to clean up the cuts afterwards? Something hard like a grinding wheel or soft like a flap wheel?

  4. #4
    Join Date
    Oct 2012
    Location
    yuba city, CA
    Posts
    49

    Default abrasives to use with SS?

    Quote Originally Posted by mrk1 View Post
    Ive been using a chop saw with a metal blade to try and reduce contamination. What tools are best to clean up the cuts afterwards? Something hard like a grinding wheel or soft like a flap wheel?
    Huh??? Did my previous answer confuse you?

    Whatever you use, it cannot have touched steel. If in doubt-use new. Segregate the finishing materials, ID them, keep them covered. Wrap SS with plastic wrap or wax paper, before holding in saw vise, etc. Clean grinder guards and grinders of any steel swarf.

    That pic you posted looks like the joints were brushed with a steel brush; or steel laden sanding disc--but you're not revealing what you used. For the education of everybody--not just yourself, what did you use on those joints?

    FWIW....I'm pretty religious about handling SS--as I briefly
    described.....and I don't get rust effects after the fact. On really fussy, primo-stuff,
    I may clean and passivate the new SS--before fabbing, then passivate-again; after fabbing.
    The extra care, cleaning, handling and attention SS needs is another reason 'why?' SS fab can cost 3-5X the same in steel fab.

  5. #5
    Join Date
    Sep 2013
    Posts
    11

    Default

    My process has been,

    -cut with SS metal bladed chop saw
    -grind off burs with bench grinding wheel (unsure of type, just what sears had)
    -brush with designated SS brush
    -wipe off with acetone
    -purge with argon, using purge caps when I can and blue tape otherwise
    -tig weld with 40-45 amps no pulse, #8 cup and gas lens, little to no filler of either 308L or 347 hard to say it was a long time ago. A 3/32nd tungsten was used.

    Even if I have a get a separate grinding wheel for SS can the wheel itself be a source of contamination? The chop saw can leave some decent sized burs especially if the blade is getting older.

    I have two of my exhausts in the shop both made by me and one looks like crap and the other looks great, I definitely need to track this contamination down. Also a side note, at the time I made these I was sharing my shop space so I cant say 100% that say the saw wasn't used for something else.

    I really only work in SS and Aluminum so my tools don't see much mild steel unless I build a jig or something. The chop saw has lead to a lot of issues for me and Im thinking of switching to a horizontal band saw but don't know how straight of cuts they can make.
    Last edited by mrk1; 09-11-2013 at 06:16 AM.

  6. #6
    Join Date
    Nov 2003
    Posts
    3,101

    Default

    Quote Originally Posted by mrk1 View Post
    I have two of my exhausts in the shop both made by me and one looks like crap and the other looks great, I definitely need to track this contamination down. Also a side note, at the time I made these I was sharing my shop space so I cant say 100% that say the saw wasn't used for something else.

    I really only work in SS and Aluminum so my tools don't see much mild steel unless I build a jig or something. The chop saw has lead to a lot of issues for me and Im thinking of switching to a horizontal band saw but don't know how straight of cuts they can make.
    It is quite likely that some of the contamination occurs during manufacture shippment and handling long before you even see the tube...some may be due to poor QC in manufacture... do you know where the tube is made??

    as said before... Nitric acid passivation or even Phosphoric acid etch after the weld would help to reduce it...

    In my experience Electropolish would afford the best protection with the bonus of a bright shiny finish...
    .

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