Escavator boom

Date 07-11-2008 00:56

Where is the crack located? If its possible depending on location Ive drilled a hole on each end of the crack, gouge the crack about 1/4 of the base metal thickness depending once again on the crack location. Then give it a little pre heat about 250 300 and run 11018 all the way back out. This will all depend on where its located and the size of the crack.

By gawelder (**)

Date 07-11-2008 01:02

The crack is about 12" back from where the second section of boom attaches. crack is on the right side of boom and it is about 6"to8" long. The escavator is a 320B Cat.

By rig welder6 (**)

Date 07-11-2008 01:34

the drilled hole thing has worked for me in the past, and try hooking your bead at the end and start of pass

By 803056

Date 07-11-2008 03:49

You may have failed to excavate the crack completely when you prepared the fracture for repair.

If you failed to excavate in the through thickness direction, any remnant of the crack or incomplete joint penetration left in the root would act as a stress riser from which the crack could propagate.

The crack tip can extend a considerable distance into the base metal, but still be too small and too tight to see with the naked eye. The ends of the crack must be located, preferably with the aid of dye penetrant or acid etching and magnification. Still the actual crack tip may extend beyond what can be seen even with the dye penetrant. It is for that reason AWS D1.1 requires the area excavated be extended two inches beyond the crack tip (ends).

It is interesting to look at a cracked sample that has been polished and etched with the aid of high magnification. Even though the crack tip looks to be intact when viewed with the unaided eye, the crack extends a substantial distance into the base metal. Under high magnification the crack is easily seen and individual grains appear to be separated from the surrounding base metal. The metal is already cracked, but you can't see it!

Failure to completely remove the entire crack before making the welded repair is an invitation for the repair to fail again in short order. Attempting to weld a part that is contaminated with soil, grease, oils, etc. are likely to affect the weld's integrity as well.

As for your choice to use E7018 electrode, I would expect it to hold up pretty well even if the excavator boom was constructed of stronger material. If the E7018 low hydrogen electrode was properly stored and the weld properly made, there should have been some localized yielding before failing. However, if the weld contained inclusions, incomplete fusion, undercut, if the weld face was not ground smooth, etc. the notch effect could cause the repair weld to fail without substantial yielding. Close examination of the failed weld can provide useful clues as to the reason it failed after such short service.

Equipment repair is always a challenge considering the working environment, materials, and ever present dirt and grime. Nothing beats a thorough steam cleaning before attempting a repair.

Best regards - Al

By Shane Feder (****)

Date 07-11-2008 04:13

gawelder,
You didn't mention whether you applied preheat or not ?
Regards,
Shane

By gawelder (**)

Date 07-11-2008 11:10

No I didn't pre-heat the area. I just ground the crack out about 3/8" deep,then I used the 7018's to weld it back out. I'm going back out to the site this morning to check and see what happened. I think I will gouge the crack out about 2" past the visible crack,that will give it some pre-heat, then I will weld it back out with some 11018's.
Thanks alot to all for the information !
Gawelder

By aevald

Date 07-11-2008 14:43

Hello gawelder, if you will refer back to Al's post you will notice a statement that he made concerning "completely" excavating the crack as opposed to simply grinding or gouging partially before welding it back in. A couple of points for you to consider, using 11018 will not necessarily solve the cracking issue, it may make it worse, the material that boom is made of is very likely A-36, you would just end up over-welding the repair and causing the stress to be shifted and crack again outside of the weld area, likely right along side of it. Secondly, a more important item and the one that Al was more specifically referring to, you need to completely take the cracked area out all the way through the thickness of the material. A carbon arc gouging unit is best suited for this. Definitely gouge the entire crack the length of it and slightly past both ends and completely through the thickness of the material. When you are done you will have the edges slightly beveled and you will be able to "see" into the boom, be sure to grind and clean this excavation well before proceeding. Next, cut a piece of backing material of 1/4" to 3/8" or so in thickness that could be turned sideways and fished into the crack and then pulled back up tight against the inside of the boom. This can be accomplished by stubbing a rod out against the backing piece and will allow you to hold onto and position it from the outside while you are putting it into place. Once you have gotten this inside the boom and pulled back up against it, tack it into position as required and then remove the rod portion and properly clean the backer and any tacks before proceeding with the weld-out of the excavated area. Preheat would be a good idea, 200F to 250F should be sufficient. To further understand why Al made his statement about partial penetration versus complete penetration you could do a simple test for yourself on some scrap material. take a couple pieces of flat bar, bevel their edges about halfway through, butt them up tight and weld them together. Then place this piece into a vice and clamp it slightly below the weld, using a hammer apply blows to the side of the plate that doesn't have the weld on in and watch how easily it will break. The un-welded portion of the joint will provide the notch that will allow the weld to break rather readily. Do this same test again, yet in this one make the weld on the side with the bevel and then use a grinder to grind the un-welded portion of this same joint out and then weld that in. Now put this part in the vice the same way and try to break it. It will likely bend but not break, this is how the repair to the boom can react when it has only a partial penetration repair as opposed to a full penetration repair. Hope this post can be of some use to you. Best regards, aevald

By Metarinka (****)

Date 07-11-2008 15:15

+1 to everything said, also sometimes it is advantageous to repair the metal with a filler material that has equal or lower tensile strength than the base material. As long as the modulus of elasticity is greater. If the unit was cracked there was either a stress concentration or too large of a force in that specific area. Using a more elastic material will allow the material to yield without breaking. WE used this concept several times for repair especially if we found brittle fracture, a lot of welds where failing due to expansion and contraction of the base material, simply using more elastic filler allowed the base material to elongate it's .080 without cracking

By aevald

Date 07-11-2008 18:09

Hello Metarinka, if you would like gawelder to receive a notification post so that he will be aware of the additional information you have included, make your post as a direct reply to his. I received a notification of your post and appreciate the additional information yet, he may not see it unless it comes up as a notification to him. Best regards, aevald

By darren

Date 07-12-2008 06:39 Edited 07-12-2008 20:23

all very good responses from some very seasoned and expert tradesmen.
as we all do things our own way and having repaired some booms (some more than once in the same spot) define the cracks limits as Al suggests it is out side where it seems to terminate mag particle and die penatrant if you can, (you can wrap your lead around the boom a half dozen times around the cracked area and use grinding dust for you indicating powder and put your leads on a dead short on a fence or some other big piece of metal with some resistance and sprinkle the grinding dust on the cracked area after you have thoroughly sanded the surface of paint ) drill your holes an inch past where the crack indication stops. preheat to take any paint off the area and to help eliminate growing the crack, gouge from the holes into the centre of the crack, depending on your confidence with a gouging rod you can gouge a bevel and a root opening or grind one in. with a 6010 rod put in a root make sure its a good one or grind it out and put in another the root is critical on these cracks. then weld it out with a good 7018 type rod (in my opinion there is no better rod than arctec 223 for this job with a small nickel component to give it some "boing factor" or some resistance to repeated tension and compression loads) as was stated start your weld off to the side and curve back into the start of the crack/hole making a 2 inch at least recurve before you are traveling in the direction of the crack. put in as small of stringer welds as you can while still being effective, don't weld it to fast as you don't want heat to build up. this next part is controversial, some guys say sand it down flush with sharp sanding discs as to generate no heat and some say to leave the weld on, i cant speak to either one being better i have had success with both and failures with both.
and if you never want to see the job again fish plate the crack ( a whole other story),
darren
p.s. what is the application of the hoe and what are the operators style because unless in severe service or being abused or factory defect cats "should" hold up. and if there is one crack. there are more you cannot see yet that are already started

By Sberry

Date 07-12-2008 15:18

Good stuff in this thread and for the most part I would like to see it ground smooth, I also go to the 7018 with this type of repair in the field.

By makeithot (***)

Date 07-13-2008 00:32

You have pretty much gotten alot of information to do this repair but the method that I use is as follows. 1- locate the ends of the cracks and drill stop.(drill a hole at the end of the crack) 2 - Once you have drilled the holes use a flash light to see if you have gotten the hole at the end of the crack, if you have not you will need to drill onother hole to get to the end of the crack. (If you don't it will crack again). 3- gauge out or grind out the crack to the depth of your prefferd root land. 4 - use a zip disc to open your root land. 5 - preheat the area to be welded I normally go to 350. 6 - root pass with 5P+ OR 6P+ (these are my favorites) 7 - weld out with E7018. 8 - then grind flat and install a dupler plate at least half the thickness of the parent material. 9 - radius the ends of the dupler before it is installed. It is important to take your time on each step to insure your have followed your welding procedure to the letter or you will have to re-do it. It would be best if you have done some open root welding before you attempt this. At the risk of bragging , if I've done this once I've don it a thousand times and never had to re-weld one (knock on wood)

By gawelder (**)

Date 07-13-2008 22:26

I went back out to the site and it was cracked at the same site as before. This time I drilled a hole about 1 1/2" past the crack at both ends. I then took my torch and cut through the crack, I opened it up about 3/8". inside of the boom there was a solid triangular piece in the end of the boom, the crack went around the end of the solid piece, so I did not have room to fish a backing place inside. I ground a V- groove in the crack area that I cut through,then I welded the groove back out with some 7018's.I did not grind the weld back down smooth. The area where the crack was about 12" back from the end of the main boom, there was no room to put a fish plate over it as the second section of boom comes back over it when the boom comes back in. The machine is being abused by the operator. They are using it clearing trees. I think the crack is being caused by the operator swinging the bucket into the trees from the side causing extensive stress on the main boom. Thanks to everyone for the information!
Gawelder

By webbcity (***)

Date 07-14-2008 18:23

gawelder , i just scanned this thread so may have missed some . there are some no no's . when welders work on equipment # 1 is don't use welding machine to jump start ( espically with computers now days ) #2 protect hydraulic cylinder rods from spatter , sparks and grinding . i will try to read completely when i get back this week . good luck . willie

By Shane Feder (****)

Date 07-14-2008 20:02

gawelder,
I spent 12 months in the coalfields in Australia working for an engineering firm that did nothing but repair cracks on massive earthmoving machinery (some up to 7000 tons of mobile machinery).
This work varied from 1/2" up to 8" thick butt welds.
First thing, it is imperative you preheat the steel, AWS D1.1 should give you the required preheat based on material type, material thickness and type of consumable to be used. (If you don't have it one of the gentlemen on the forum may help if you list the info)
Secondly, do not put a doubler plate on for at least 48 hours due to the possibility of delayed cracking. If a crack is repaired properly the repair should be as strong as the parent metal and there is no need to put a plate on. A doubler over a repair is like a band aid, it may hide it but if it recracks you will not know until it propogates out from under the plate (possibly in all directions)
Generally if a weld cracks through the weld metal it is something the welder has done wrong (no preheat, incorrect consumable,insufficient removal of crack etc,etc) but if it cracks beside the weld in the parent metal it is possible that the material is that fatigued that it will keep cracking regardless of repair methods. That is generally where an engineer gets involved and decides to cut a section of defective plate out and replace.
Hope that helps,
Regards,
Shane

By Wrench Tech (**)

Date 07-15-2008 03:27

gawelder. At one time we had a lot of trouble with cracks in our Hitachi excavator booms. Two things that I remember were major factors. One thing is that you can`t golf rocks, logs or anything wilth an excavator - cause if you do, the boom will crack. Another thing was that when we cut into a boom to have a look there were baffle plates welded across the inside of the boom and where they attached to the sides is where the boom seemed to crack. The inside welds were really terrible - badly undercut - so between that and the golfing it was no wonder we had our share of problems. Eventually, the operators learned to quit with the golf - or at least golf less - and we arced the cracks right through and past the ends with the air arc before welding it up. And if possible weld a nice rounded fish plate over the top. I still weld a boom crack once in a while but we used to have to do it way more often.