Harvard: Low Hydrogen, (7018 rod), is excellent for repairing everything EXCEPT heavy equipment sometimes.

I have heard other welders talk of equipment welding repairs not succeeding because the rod used was 7018. I'm a big fan of low hydrogen, I've seen it succeed when other rod failed, but I think you're in a whole new ball park.

Research equipment repair a bit and then go with the suggested rod. One thing you have to remember about heavy equipment is that the stress is full contact all the time, not intermediate, much like our bodies. If you weld with 7018 it will last, but not long on something like that. You need something stronger.

Some body that has dealt with heavy equipment, give me hand on this one would you!?

Harvard:
Ususally on any equipment repair I use 8018 but mostly11018. On surface cracks I usually gouge about as deep as I can get, drill the holes on each end and then give a slight preheat before welding. Ive found that if you have access to arc gouging equipment use it! Make sure if you are running multiple passes clean clean clean after each pass. Theres no need to get in a rush. Just keep from getting it too hot or your arc characteritics will get sloppy. Keep checking your post because some of these other guys that post here have some really great methods. I would start out by getting away from that 7018E though.

Please specify what portion of the boom is repeatedly cracking and if possible the orientation of the crack relative to the boom length centerline.  Email me with this information next week and although I will be extremely busy the next ten days, I will provide you with what you need to repair this situation.  Vintage and serial number of the machine along with boom section length would be helpful as well.  You will need some method accurately guaging preheat and interpass tempatures such as an infared pyrometer, direct contact pyrometer or tempstik in preferrably that order of preference.  Additionally you will need some method, as may be availble to you, to slow the cooling rate of completed weld repair.

It's important to know if the continued cracking occurs in the heat affected zone (HAZ) of the base metal next to the weld, or is it actually in the weld metal? 
Was the original crack in the base metal, or in an original weld, or associated with a weld.
Is there a notch of some kind that is acting as the initiation point for the crack?

Its unlikely that the Boom is T1. Most of the time they use A36 depending on what part of the Boom you are working on.

I would venture the fish plate was the correct move to make as long as you do not weld straight across. Use a Diamond shaped fish plate. Tell us more about where the crack is and 7018 is the filler of choice.

Hello
Lots of good advice offered here, now for my 2 cents worth. Cat should provide some degree of support ie weld procedure.
I have worked on alot of heavy equipment and its frustrating when you do your best and a problem still persists. Luckily for you your work enviroment lets you witness the result of your work, and others. So many production shop welders make welds that aren't stressed until thier put into service months later and never experience any direct feedback.
It's a shame your co worker won't offer advise, you on the other hand have taken initiative and are most likely here trying to solve this problem on your own time because you care! See other post (what happened to all the welders).
I remember spending counteless hours scarfing and repairing cracked out welds on draglines, Dozers and loaders, seamed like a never ending job.
On repeating crack locations I like to slow things down a bit and take my time on all aspects of prep and welding.


My procedure
Air Arc out any filler and longitudinally 3x the material thickness, Transition out at 45 degrees +
Grind gouged area and surrounding area at least 3" away from crack provide a 1/4 " root opening
Check prepped area with Dye Pen
With a little finaggling it's possible to slip a 1/4" backup bar into the opening,
I usually tack a welding rod to the bar and slip it in, it has to fit tight against the back of the joint, tack.
I add 2" long start/runoff strips on the excavated ends. 
Use a fresh box of 7018, I hope you haven't been using the open box behind your seat!(hydrogen embritlement)
Preheat the entire area to 200 f and maintain it until complete, stay out of the wind. (500 Max)
I use a back step technique completing each rod into the start of the previous rod, with exception for the cap.
No wide weaves (+2.5 X rod diameter), use stringers and plan every rod placement to aviod a previous start or stop. (soapstone marks)
When placing stringers be sure to allow for the rod diameter on adjacent passes.
Take your time chipping the flux let it cool for a minute. Clean thoroughly after every rod.
I also use a temper bead technique for capping where toeline welds are placed first and work toward the center, the temper bead is the final stringer placed up the middle of the cap.
Cap reinforcement should not exceed 3/32" No undercut/underfill allowed.
Use the runnoff tabs for starts and stops on the ends.
Let cool slowly
Remove runoff tabs and feather grind ends.
This method has worked on alot of previously welded repair locations with much success.
Thats my 2 cents worth, The only time I use a fish plate is when someone else insists. I trust that the engineer did his job.Thanks Randy

My two cents. I worked for Volvo Construction Equipment for 16 year as a weld inspector. You can have all the best procedures in place for the repair but this may not help. The piece of equipment has limits. Is it possible this you are using the machine for something new or different. Maybe a new operator? I do know from past experience if you put that bucket into pushing loads from the side of the bucket you will open up a bad can of worms.