We have Hydro-Vac trucks, in which there is a square water
tank, made of 3/16ths sheet metal, built around a vacuum tank.
We have a fair amount of trouble with cracks in the water tank,
probably due to flexing while being driven, unloaded and the shifting
load, and likely some engineering problem with both the structure
and the welding.
The problem is sometimes on the corners and some times out in
the middle of the sheet (likely against a support welded on the inside,
but I am not sure)
I have done some of the repairs myself and had a few professional
welders weld these cracks. Some hold very well and the crack never
reappears and other re-crack within days.
Basically here is how I fix them. I grind out the crack ( the end of the
crack is sometimes hard to locate) and then run a pass of 6010 or
6011 and then grind out some of that weld and then run a weaving
pass of 7018. These welds are both horizontal and vertical both on
the flat surface and on the outside corners. These havent' been
consistant as of yet, sometimes due to not starting the weld far
enough past the start of the crack and sometimes due to a poor weld.
I am not a trained welder but have a good basic knowledge of it.
I have an Lincoln 250 amp AC/DC welder and Lincoln 255 Mig welder.
I generally have used the stick so far.
I would be interested if these rods are the best ones to use as well
as how I should run these passes up or down. I would guess either
down or up with the 6010 and up with the 7018. Which polarity should
I be using? We had been grinding the top of the weld flat for cosmetics
but quit that.
Any ideas would be helpful and thankyou in advance.
Graham Anderson
Calgary Canada
Hmmm...this is a little different. Im no pro....but a square water tank seems a little odd.....especially if its on the back of a truck..
since you said it was 3/16 thick sheetmetal...im guessing that is mild steel...or perhaps galvinized?
well...if you are having to fix these cracks like every other day..i would suggest investing in some new tanks...like tubular...or oval shaped ones..
But if you have a crack here and there...not very often...then here is how i would fix them.
find the crack and clean the area with a wire brush..
take a drill and drill two very small holes on each end of the crack...drill a little beyond the crack just incase there is an internal crack you dont see.
since the metal is not that thick... i would grind the crack out a little bit...
take some small 6011 rod and weld the crack....make sure you get good penetration on this....go all the way through the metal...a root pass in other words. if its a vertical weld....go upwards...going down may be a chance to trap slag in side the weld.
after that....take a grinder and grind the weld a little bit....leaving clean metal....dont grind it too much..and make sure all the slag is out..
then...make another pass with a small ( 3/32) 7018 ...if its vertical...go upwards again....when finished...the weld should be higher than the surface.....no more than 1/8...
now..you should be good to go...that is the best way i can think of to fix this problem...
By Graham
Date 12-29-2001 16:44
Thanks. The design of the tanks has been changed to plastic tanks
and so far so good. We still have many of the steel tanks and the
trucks with be very serviceable for a few more years.
It isn't as common as every second day although sometimes the repair
will seep that soon afterwards. It leaves just enough water through
to leave an unsightly orange rust trail down the side of the truck.
I had heard of drilling a hole at the end of the crack, but I had thought
that a with a sound weld this may not be needed. I have also used
a torch along the crack to try and find the end. This works somewhat
and I am thinking that I may not be getting a sound bead right at the
start of crack.
This has worked for me-
Burn the crack out with the torch leaving a narrow slot where the crack used to be. Thus you cannot leave any crack under the weld which will always, it seems, grow back up through the weld.
Start welding a little beyond the slot. That way when you drop into the slot everything is nice and hot and good fusion is ensured.
If you have trouble finding the whole crack dye penetrant may help.
Good luck
Bill
By -
Date 01-02-2002 19:47
To give a good answer, we will need why the cracking is occuring. My experience with road tankers would suggest that they are almost certainly due to fatigue.
If this is the case, then you need to do everything to improve the fatigue resistance of the repaired area. Generally the filler itself will not have a great effect. The first and foremost would be to ensure that there are no stress raisers. This means ensuring that there is no excess penetration or re-inforcement, no undercut, lack of fusion, lack of penetration etc. To achieve this you will almost certainly need access to both sides of the weld so that you can dress the inside. Also important is the direction of dressing. Do not grind parallel with the weld. Your grinding marks need to run perpendicular to the weld.
Other issues are:
1) PWHT:- A "stress relieve" treatment tends to prolong fatigue life.
2) Design:- Typically a design that gives more flexibility at the weld allows the stresses to be displaced to an area away from the weld.
A final comment: A weldment in fatigue service will always fail by fatigue sooner or later. (Weldments have no fatigue limmit.) If the vessels are already old, they may be past their fatigue design lives. If this is the case, then you will experience constant fatigue failures.
Hope this helps.
Regards
Niekie Jooste
By -
Date 07-04-2002 19:24
it could be the mounting of the tanks.nomatter what there made of they could still fail, unless proper mounting is used.ie dodge switching to plastic fuel tanks from steel.some still cracked. i had an 84 .stress from the straps cracked the plastic tank.never mount a tank directly.
