CAT 330D boom repair

[Mccahilldozing]

Does anyone have the repair instructions from CAT on how to repair a boom crack on a 330d excavator? Any help is appreciated

 

[Welder Dave]

Looks like that boom has had some extensive repairs in the past. The lengthwise weld looks really ropey. I would suspect it needs to be gouged and/or ground out for 100% penetration and a 2-300 deg's. preheat. 7018 would be the rod of choice or maybe 8018C-3 which has 1% nickel for extra toughness. Peening each pass while still hot would also be beneficial.

 

[Nige]

A machine S/N would be a good start. There are a few procedures kicking around but knowing the boom type (mass, reach, etc) would help to narrow down the field a bit..
Is the crack located as shown by the area marked "1" on the drawing below..?



Without looking too deeply into it my suspicion is that in order to do a decent repair job: -
a) The boom will have to come off so it can be rotated - no vertical or overhead welding.
b) The boom box structure will need opening up so that the weld roots can be back-gouged and welded up with no possibility of leaving residual root cracks. Spendy but Eutectic Xuper Nucleotec 2222 electrodes are the dog's bollox for repairs like this. The weld has an eleongation spec of something like 50%.

 

[Mccahilldozing]

Serial number is MWP02904. We section repaired it before and replaced it with A514 steel. This is the third failure in the same spot. I’ve not heard of Euctitic rods but damn I’m going to try some. Because I cant be down I found a heavy duty boom that I’m going to replace. I’m going to try to completely repair the old boom though to have as a spare. CAT is being secretive about the repair procedures because they want the work

 

[Bls repair]

Do you use this just for digging or are you using implements on it?

 

[Mccahilldozing]

I use it for orchard removal.

 

[Bls repair]

It looks to me like you are side loading the boom causing the boom to twist and crack

 

[Welder Dave]

A514 is T-1 steel. 11018 is the recommended electrode. The existing welds on the boom aren't the greatest. Preheat and interpass temperatures are very important. Peening each pass and wrapping with a blanket to slow cool is also recommended. A competent welder may be able to do some of the welds out of position but 100% penetration is imperative. Here's a good article on welding T-1 and recommendations from Lincoln. Not following these recommendations may have contributed to the boom cracking.

https://www.thefabricator.com/thefabricator/article/arcwelding/welding-astm-a514-or-a514m-05-steelr#:~:text=ASTM A514 is a specification,P, Q, and S.

https://www.lincolnelectric.com/en-us/support/welding-solutions/pages/t1-steels-detail.aspx

 

[Mccahilldozing]

Yes there is some side loading involved but it’s the nature of what we do

 

[Mccahilldozing]

Thank you! Great article! The welds aren’t failing. It’s cracking in the top plate and into the side plate. The base metal was gr50 and we used Hobart Fabco excel arc 71 on the section repair.

https://www.thefabricator.com/thefabricator/article/arcwelding/welding-astm-a514-or-a514m-05-steelr#:~:text=ASTM A514 is a specification,P, Q, and S.[/QUOTE]

 

[Welder Dave]

How far across the top does the crack go? It looks like the thick top plate struck something pretty hard as there is a dent at the crack. Hard to say where the crack started but it could have started in the heat affected zone of the vertical weld.

 

[Nige]

Here's the deal. Cat booms, as well as most of their other structures, don't use anything in the way of special steels. The general usage plate for a fabricated box section like this one is a 28mPa yield structural steel plate. So don't for a moment go thinking you need something like AS14/T-1, all it will cause you is headaches.
I'll send you a welding procedure for the boom repair as well as something I have from when we welded a load of box structures (boom & sticks IIRC) on 336D machines. It might give you some ideas.

 

[Mccahilldozing]

I don’t think anything hit it. I think it’s twisting and loaded up. When we sectioned it it moved about 1 inch as we were cutting out the bad section. That and the center pin was bound up and had to be pressed out

 

[Bls repair]

Only suggestion I can make is the reinforce the area so it doesn’t twist as much( heavier plate or interior cross bracing of the boom) . Doing this may just move the problem to another spot . An engineer may be useful for this.

 

[Welder Dave]

Was it preheated before welding and were the welds peened? There could have been built up stress and pulling the steel together to weld (restraining it) added even more stress.

 

[John C.]

You boom problems are part of a large update package that was deemed fix as fail. Caterpillar had a design and manufacturing problem with nearly all excavator booms and provided factory procedure and in many cases monetary help of resolving the problems. Many of those boom were fixed as machine warranty or policy review. The problem with your boom is structural and cannot be repaired simply by arcing out the cracks and re-welding or adding plates to the outside of the boom. In essence, if you contact your local dealer, they should provide you with the repair procedure at the least. If not, you should go over their heads to the regional factory representative for resolution.

 

[Nige]

Wot he said............

 

[heymccall]

REBE6770 ?

Cat isn't gonna pay or subsidize now, but, when did the original failure occur?

 

[Welder Dave]

Curious if Cat ever specified what the problem was, bad steel, not heavy enough for the application, poor welding technique, design flaw, etc.

 

[Nige]

Curious if Cat ever specified what the problem was, bad steel, not heavy enough for the application, poor welding technique, design flaw, etc.

If what I saw on various machines is any guide it was poor welding. We opened up loads of cracks where root penetration was little or even non-existent. The problem with trying to weld only from the outside is that the structure contains baffles inside it and in many instances the cracks had progressed into the welding between the baffle plates and the outer skin of the boom. In the worst cases the cracks were in the baffle plates themselves. In that case there was no option but to cut a hole in the structure and get inside it. Typical of this sort of structures it's what you can't see that will come back and bite you in the a$$ if it's not fixed correctly. A plasma torch is your friend in cases like this.

Here's an example. Root pentration was nil. The chalk marks in the 2nd photo are cracks in the welds between the baffles and the outer skin. The curved strip in that same photo is the backup strip for the external weld that had no root pentration.