welding repair of a 4340 crank

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bill jones
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welding repair of a 4340 crank

Post by bill jones »

-I have a 4340 nitrided crank that has a huge wound where two pieces of heavy metal were improperly installed, and the two pieces broke away what little crank metal that was left there to contain the heavy metal after the poor installation job.
-The crank looks ok except that this area needs to be either welded and filled with a lot of filler weld, or a piece of metal will have to be fitted and inserted and welded to repair the counter weight.
-Then there's actually another half a piece of heavy metal involved so this means the crank would then have to be drilled for 2-1/2 pieces of heavy metal right thru the parting lines of the 4340 and the weld repair and thru the very center of the welded repair.
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-So my questions are;
1-to use 100% filler rod and lay a multitude of weld passes to fill the wound?
2-machine the wound of the counterweight to accept a solid block of steel and weld that in the huge gap?
3-what is the proper welding preference assuming either a tig or possibly a mig.
4-what rod would best so that I don't end up with super hard areas where I'm needing to drill?
5-Should there be any special peening of the welds to stress relieve the stresses?
6-any other special considerations because of the 4340 steel and the nitrided surface?
rank
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Post by rank »

To me, it seems you have a couple of options. I would estimate that the problem is not so much with the welding, but rather the tough service environment that the repair will need to endure after welding. I would use GMAW with a 70S wire. Using TIG with these pre-heat temps would be very uncomfortable I imagine.

*Ideally you will want to pre heat the entire crank (in an oven) to 600F and maintain that temperature between passes. After welding, it should not be allowed to fall below 150F before it is stress relieved at 1100F.
*Alternatively, you could locally pre-heat the area to 600F with a torch and plug weld the entire hole in one "pass" if possible to reduce the time spent working with this hot piece of steel. After welding, put the crank in an oven and hold at 750F for an hour.

The above recommendations from Mr. Linnert aside.....if I was feeling lucky......and if I didn't have access to an oven.......and if I could afford to break a crank, I may consider locally pre-heating to 600F, plug weld it in one pass and bury it in anything that will slow the cooling rate way down (fibreglass insulation for example) and pray that the crank won't break the next time you're at redline.

In the end, I think the method you choose kind of comes down to how attached you are to the engine and the rpm involved. Besides, when is the last time you broke a crankshaft?

*source
Welding Metallurgy, Volume 2, George E. Linnert, Chapter 15 "Welding Alloy Steels".

Good luck
jeff
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speedtalk
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Post by speedtalk »

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