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  • #16
    Lou,What is the general PWHT for the 4130,

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    • #17
      Hi Mark, Welding process such as oxy/fuel (high heat input type) and multi run welding would cause weldment made and its wider heat afftected zone to have coarser grain structure (technically, it involves microstructure changes inside the steel). Mechanical properties-wise, it will be hard and brittle and less tough. Therefore, preferably, to refine its structure and stress relieving it and getting back to normalise and temper state similar to its parent metal. This is done performing PWHT by heating up to amber again, and within controlled environment (e.g no force air cooling fan or windy surrounding directly or indirectly subject weldment to rapid cooling) and lastly to let the weldment slow cooled to ambient temperature. In short, if OF (oxy/fuel welding) employed to weld 4130 steel, I"ll perform PWHT.Alternatively, TIG or MIG welding processes (low heat input type) would cause weldment made lesser of coarse grain structure and narrower heat affected zone as compared to above. The disadvantage employing these will be moisture contamination from surrounding atmosphere to cause hydrogen embrittlement (also known as hydrogen induced cracking) inside the weld if the shielding gas is "flawed" (ie, improper flow rate, wrong selection of shielding gas and contaminated shielding gas at source etc etc). This microfissure crack will eventually propagate outward and fail the joint completely. With the modern welding equipment and user"s proper management of the welding parameters (ie, voltage, current, wire feed speed(MIG only), proper shielding gas type (ie, argon or mixed CO280%/argon20% - for steel welding; helium or argon - afor luminium welding) and proper joint configuration design (ie, full penetration or partial penetration

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      • #18
        Lou, be aware also that the MIG process always begins cold, that first burst is flawed where as the TIG process uses the "molten pool" method giving a more uniform weld depending on the skill of the operator.

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        • #19
          Hi Chooky, it was nice to have met you and Ian in Bindoon, long weekend. I hope the wet weather did not dampen your flying with Ian on that day? Yeah, you are right to take note the starting portion of MIG welding. I remembered my "shfu" taught me during apprenticeship to start an arc outside intended start run and dragged back to create molten pool. Similarly, when ending weld run, also, to puddle back the molten pool momentarily and extinguish arc without pulling out shielding nozzle away immediately. The exiting shielding gas (after the arc stopped)shielded the solidifying molten puddle from external atmospheric contamination and slow cool the weld. This reduced or eliminated start/stop shrinkage crack and porosities.Nowadays, with the introduction of "pulsed spray transfer mode" and "modified short circuiting transfer mode" in the modern day MIG welding machine to weld thin section, these have greatly improved the quality of weld and, also, integrity (soundness) of weld.

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          • #20
            Lou,Thanks for the info on the pwht, it is how i thought it would be, how ever(nearly put BUT) what is the difference between the pwht and just allowing the actual weld to cool the same way as the pwht. I realise there will be a reason just wanting to know thats all. ???

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            • #21
              Hi Mark, it"s interesting you have asked. I asked the similar question during my apprenticeship. My supervisor replied with an analogy story.

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              • #22
                Hi Guys,Sorry I haven"t replied sooner but I have been off flying across Australia and enjoying myself in WA generally. (actually I"m not sorry at all! come to think about it)On the subject of "mild steel" for the mast I am not quite sure exactly what grade steel it is but it is not 4130 chrome moly.

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