Welding cast steel to mild steel plate.

There are two main groups of electrodes for welding cast iron; machinable and nonmachinable Machinable type are those whose deposits are soft and ductile enough so they can easily be machined after welding. They are used to repair all kinds of broken castings, correcting for machine errors, filling up defects, or to weld cast iron to steel. There are two common types of machinable electrodes. Characteristics of these AWS classification electrodes are as follows: ( classification based on chemical composition of metal involved. The suffix CI designates an electrode for cast iron).
ENi-CI - Nickel Electrode DCR or AC, general purpose welding especially for thin and medium sections, and casting with low phosphorus content, and where little or no preheat is used.
ENi-FeCI - Nickle Iron Electrode DCR or AC for welding heavy sections, high phosphorus castings, high nickle alloy castings where high-strength welds are required. and welding nodular iron.
This is from THOMAS' Wrought Iron Works in Saskatoon, Sk, Canada or snipped-for-privacy@sasktel.net (306)-653-7796

I think there is a problem of what material we are refering to in the first response.
"cast steel" is a very unfortunate name, as it doesn't mean much in the current context. "cast steel" refers to steel with about 0.8% to 1% Carbon. That is, it is a steel which can be quench-hardened to give very sharp edges like chisels and so on. It would be mass-produced in an electric arc furnace and is no more "cast" then any other steel, be it sheet, plate, billet, rail, etc.
So nothing to do with "cast iron".
Historically, the name "cast steel" came about in the late 1700's and 1800's because the steel was made in crucibles (with charcoal additions) then cast into little ingots which were forged to scythes and the like. The other "steel" of the time was wrought-iron, which was never molten as steel, being reduced to a spongy solid then forged ("wrought") to squeeze out the slag. Wrought iron could be case-carburised giving "blister steel", which was used to make cutting edges, though the homogenous hardenability of crucible steel was advantageous for mass-production.
Look-up the Huntsman crucible process for making tool steel. I come from Sheffield where this stuff started-out.
Welding - your question could be rephrased as "how do I weld a hardenable high-carbon steel to a mild steel?"
I'll duck-out here, as there are others much more expert than me. In general though, you have to avoid high hardnesses and hydrogen cracking. When I have been confronted with this situation, I preheat to 'til there are colour patinas showing on the metal (it must be over 200C (392F)), use a low-hydrogen welding process like (solid-wire, "tradional") MIG then put the weldment in an oven for a few hours to reduce the hydrogen level. Has worked for me in the few instances I have welded high-carbon steel.
Oh yeah -- other trick - never tried it, but a familiar technique - weld using an austenic stainless steel electrode. The weld hydrogen has high solubility and low mobility in the austenitic stainless steel, so what hydrogen does make it into the heat-affected-zone of the parent material is wicked-away relatively extremely fast and the concentrations of hydrogen are very low. All cracking in the weld zone is "hydrogen cracking", it is reckoned. Take away hydrogen and you won't get cracking, however hard the structures you form. This is the trick which caused the Nazi German forces sweeping across Russia to suddenly encounter a huge onslaught of these indigenous Russian tanks better than anything they had - the Russians discovered you could weld armour plate (highly hardenable and cracking-susceptible) with austenitic stainless steel electrodes. OK, the Nazi Germans responded with a better tank (a bit better), but it took something like four times as much effort to build and in the brutality of war they were swamped and everyone there found themselves with a one-way ticket.
Richard Smith

Well, I am confused......... so let me ramble on a bit. I put air suspension (Volvo semi) on an 84 GMC school bus. The ride has been improved vastly, but there is a bit of sway now. To correct the sway I would like to add a .....sway bar..... Got one from the salvage yard. The ends that attach to the axle are cast.....Cast steel maybe? They were cast in the shape of a spring pad, they go between the leaf spring of the original vehicle and the axle housing. What I would like to do is cut these ends a little and weld them to a hunk of steel plate that would be bolted-via the rear suspension u-bolts-to the under side of the axle housing.
So, can these two types of metal be welded and have the necessairy strength to withstand the stress seen by heavy duty suspension components?
I apologize for not being clearer in my original post. But, as a novice, it is hard to know how much information is enough and not too much.
John