Post
by maplebones » Wed Dec 05, 2007 11:38 am
I have a large stainless weld on the final drive housing on my 420c, done by an unknown previous owner, and I've often wondered if they just got lucky, or if the composition of the housing allowed itself to be welded. There are different types of cast iron, gray cast used in engine blocks and low stress housings, ductile cast used in C clamps and pipe wrenches, and what's called semi-steel cast iron used in machinery and a lot of agricultural applications. They each require different approaches and so I'm wondering if anyone knows for sure what Deere used for in their final drive housings. There are ways to tell, a spark test will usually identify gray, or more definitively, a look at an etched sample under a microscope.
The idea of using stainless steel welding rod on gray cast iron is a back-woods approach that may or may not work. Nickle is the element that is responsible for the succesful welding of gray cast iron, and depending on the type of stainless alloy used [ there are over 30 types out there] there may or may not be enough nickle to insure success.
If you're going to take the trouble to clean, prepare and weld gray cast iron do yourself a favor and buy pure nickle rod. E-Ni CI [ coated rod] or ER-Ni CI [bare rod for tig]. This is the industry standard for repair of gray cast. They're not cheap, probably over $5.00 a rod, but if you consider your time, they're well worth the money.
The procedure is to vee out the edges, ideally with a sharp chisel. A grinder will work, but gray cast contains a lot of graphite and this will tend to get smeared on the surface. If you find the filler metal rolls off the base metal, it's because of the graphite. I always sear the joint with an oxidizing flame before welding. Let it cool.
Do Not Pre-heat. Pre-heat has it's place in other welding procedures, but not when electric welding cast with nickle. Weld no more than 1/2 inch long beads. Let it cool so you can touch it with your hand. Peening the bead with light blows from a small ball peen hammer as it's cooling will expand the bead and reduce the shrinking stess on the cast iron. Move over and deposit another half inch bead, and so on until it's done.
If this sounds slow, it is, and many welders won't have the patience for this, but this is the established procedure worked out by metallugists after extensive testing, and it works.
You may find that the nickle will not wet the base metal if using the above procedure on exhaust manifolds, especially diesels. This is because after long usage the sulfer in the fuels has migrated into the cast iron and this starts to come out preventing fusion. You can smell it. I have had success in this situation using oxy-acetylene and cast iron rods. These rods are actually cast iron, they are square with a rough cast surface, and brittle.
They were used a lot at one time, but after the evolution of nickle rods they more or less dissapeared and you may have trouble finding them, but this is one application where they are definitely superior. I have repaired at least 50 manifolds without failure.
If the part is operating at under 500 degrees, the geometry allows it, and color match is not a factor, then oxy-acetylene brazing with brass should always be your first choise. It causes no metallurgical changes or hardening of the cast and is always stronger than the original, and in the case of ears or protrusions it's a piece of cake. Vee out from both sides, sear off the graphite, clamp it down, tin it and weld it. The brass stays ductile and stretches until a low temperature, taking the strain off the cast iron.
There are other factors to consider, welding always causes distortion, and the geometry of the part may force enough of the shrinkage stresses onto the base metal to make welding impractical, but a lot of parts are needlessly replaced for lack of the proper technique
420c
