4140?
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Thread: 4140?

  1. #1
    Join Date
    Feb 2010
    Location
    ontario
    Posts
    8

    4140?

    Hi guys,

    I built an excavator bucket this winter. Haven't used it yet but i did notice a crack in a joint.

    Here is the deal. There is a round pin, diam. about 3", welded into a hole in 1" mild steel. The pin is 4140 tool steel.

    Anyway, the machinist who made the pin for me told me to preheat the pin before welding. He was concerned with cracking.

    I used .045 solid migwire, preheated the pin and mild steel. The weld went well.

    After i was done, i post-heated the weld area to red hot and let it cool slowly in air.

    During paint prep some time later, i noticed a crack in the weld area.

    What can i do? How do i prevent it from cracking?

  2. #2
    Join Date
    Mar 2010
    Location
    Monroe, Washington
    Posts
    284

    Re: 4140?

    Hey, I'm no expert but I remember somewhere about having to post heat above ~500degF but you have to hold it there to prevent hydrogen embrittlement - it takes a while. I'm sure there are tables out there somewhere in some metallurgical books that'd tell you. I believe it's introduced by either using the wrong rod or wet/contaminated rod.

    I'm sure somebody else with more experience will chime in but those are a couple things you might research a little.
    Millermatic 135
    Syncrowave 250

  3. #3
    Join Date
    Oct 2007
    Location
    Vandalia, Ohio near Dayton
    Posts
    1,861

    Re: 4140?

    Excuse me for nit-picking, but 4140 isn't a tool steel in the technical sense of the term. It's a chrom-moly alloy.

    But that's a side matter, what most likely happened is that your weld cooled too rapidly in the air. Thermal contraction stresses and/or formation of a hard phase called martensite caused your weld to crack.

    Your post says your machinist warned you to pre-heat the weld. Did you? What temperature did you pre-heat it to? Post heating the weld to red hot and air cooling probably didn't help, and likely made the problem worse, not better.

    I recommend that you grind out the weld, pre-heat the joint to 350°F. Check the temp of the surrounding metal with an optical pyrometer, thermocouple gage, or tempil stick. Make your weld quickly, using your MIG welder again. Assuming this is a single pass weld, no need to worry about inter-pass temps. Given the size of the components, I hope you're using a 220V MIG welder rated for 250-300 amps of welding current. Anything smaller is probably not up to the job.

    You need to immediately insulate the finished weld and slow the cooling. You could apply low heat with a torch to delay cooling. Don't heat the weld up more, just warm it to retard the cooling. Or pack the joint in insulating glass wool. If possible, bury the part in clean, dry sand or vermiculite. Wait several hours or overnight, then unpack the weld joint and inspect. You should be good to go, assuming your weld parameters(voltage, wire feed speed/amperage) and welding techique were good.

    Hope this helps...
    Mobile Welding at your worksite or place of business. Serving Dayton, Ohio and the surrounding metro areas.
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  4. #4
    Join Date
    May 2009
    Posts
    214

    Re: 4140?

    4140 of that thickness should be post weld stress relieved at 50 - 100°F below the temper temperature (should be about 1200°F) for at least 2 hours and allowed to cool in still air or cover with a weld blanketor several inches of DRY sand.

  5. #5
    Join Date
    Feb 2010
    Location
    ontario
    Posts
    8

    Re: 4140?

    ok, thanks a lot guys.

    I did pre heat to 'hot to the touch', not 350 for sure.

    I post heated, but not evenly. I did it with a torch. Then i air cooled it.

    I guess what i need to do it weld it, then keep it hot hot hot until i can bury it in the sand. Gotta think about how to do that.

    My welder is 250 amp.

    Thanks a ton for explaining this to me.

    Ken

  6. #6
    Join Date
    Apr 2010
    Location
    Galesburg, il USA
    Posts
    1,649

    Re: 4140?

    Hi watglen,

    mot krig has it right on about the hydrogen embrittlement, and A_DAB_will_do has it right about fixing this. I'll add a little...

    If you asked an RPE, he would probably advise that you "fasten" an end plate (if this is a keeper for the pin). If you insist it must be welded, and you're not in a production setting the only procedures he will sign and stamp would be SMAW with 8018-B2(or better) filler, TIG or MIG "spray" (the last two would need to be observed). The RPE would never approve any weld that is not "Low Hydrogen".

    Welding direct hardening materials with traditional short circuit mig and hard wire is not a low hydrogen process. It can be done at 600-900F to slow the "weld quench" with a controlled cool down to prevent "secondary hardening", but if it fails in service all you need to find out if it was a "quick and dirty MIG" weld is a brush and some water.

    Matt

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