A nickel or stainless rod will act as a sort of a bonding agent between the two different metals, because the structures of the cast iron molecules don't combine well with those of the mild steel. The reason for pre-heating is so that you get a nice uniform grain structure along the mating surfaces of your weld - increasing strenght and reducing welding induced stress.
So anytime I weld dissimilar metals I should use a nickel rod?
I have seen my buddy weld his diff in my garage with just the lincoln rods I have laying around, I guess nickel rods would have been a good idea!
It all depends on how dissimilar the two metals are. If you are welding carbon steels like any 10xx,11xx,12xx or 15xx series then you can just stick them together but once you start wanting to weld different alloys together then you have to watch. In most cases you can just stick things together; but if your life depends on it the right rod can make all the differance.:welder
as far as what kind of rod to use you can easily get away with a low hydrogen rod like the common 7018, the secret... as has already been hinted at is to use post heat and control the cooling . i think the best way for you to control the cooling on a piece this big is to cover it completly with sand after the welding is done.you will need to have a good supply of sand on hand and i would recommend a pit with sand in it 3-4 inches and them place your diff. in the hole and cover it . this may sound like a lot of work but this is the best way to control the cooling, don't be in a rush to uncover it either!!! most large castings are placed in an oven to control the cooling but i doubt you have access to an oven that big! go down to your local welding shop and get a temp. crayon that will melt at the temp. you need and make sure you heat the whole piece to this temp or the weld will cool too fast and cause under bead cracking and it won't be long till the welds fail. the other way... weld no more than 3/4 of an inch and then peen the sh*t out of the weld with a dull chipping hammer. this controls the shrink rate in the weld zone and avoids the need to post heat. don't try and go past one inch of weld before peening!!! best of luck hope this helps! cam:welder
Just to begin I am a welder in the:canadian Forces. Generally speaking we do not use alot of nickle or stainless rods except for certain jobs on certain vehicles. Just because metals are dissimilar does not mean that you would use either of those welds. Take note if you need to grind out your weld for whatever reason good luck:violin Yes preheating, concurrent heating and post heating are very important, you can pick up temperature crayons or sticks for relatively cheap at your local welding suppy store, they will allow you to reach the appropriate temperatures required. Peening is also very important. This is basically stress relieveing of the welded partent material the filler metal and the heat affected zone. If you plan on doing alot of welding pick up a text book or even some literture from one of the welding machine manufactures Esab, Lincoln, Miller etc you can alwys use the knowledge of the rod makers also BOC,Certanium Acklands etc. You should have no problems welding the cast together. Just my 2 cents:canadian
I was going to ask if this needed to be done on the axle tubes, like if you where going to put on new perches, but answered my own question the tubes aren't cast dummy. For a laymen welder like myself would this be a good idea for anything that is cast(barring aluminuim or other extremely unsimilair metals)?