I wouldn't weld the brackets to the ductile iron pipe. What I would do is to install two clamps around the pipe on each side of the joint. The clamps should be made of flat bars, which should be split into two equal parts. Bend both ends of each part at 90° and make a hole on each end. Use bolts and nuts to keep both halves firmly in place, bolted to each other. Now that you have the two steel clamps firmly in place, weld the brackets to them.
In this manner, you wouldn't have to worry about feasibility, cautions, reducing the pressure rate etc, for the weld wouldn't even touch the pipe wall. The size and spacement of the flat bars will be suggested by your good engineering judgement.
As a matter of fact, I've never liked to weld anything on cast iron, whether gray or ductile.
By the way, your e-address suggests that you work for Jacobs Engineering, is that right?
Giovanni S. Crisi
Sao Paulo - Brazil

As Giovanni Crisi said, if you can do what you want mechanically, that is without welding, you might be better off. If you must weld A36 to ductile iron pipe, you might want to contact the makers of the pipe for their recommendations.
We recently had a similar job (A36 rings to 12" ductile iron, cement lined pipe) and the manufacturer was very helpful. They had a Technical Data Sheet to follow that specified Ni-Rod 44. Also very slow cooling was required. We eventually completed the job but had a lot of cracking to deal with before we were through.

CHGuilford

I would have to say I agree with the other post. You may want to check on the dimensions of some riser clamps. These may work for you right off the shelf.

Of the different cast irons, ductile iron is one of the easiest to weld because the base metal is less susceptible to cracking in the parent metal. Having said this, welding to cast iron is always a risk, because of purity problems of the base metal.

If you have a rather pure base metal, with very little S, P or other low melting point contaminats then, if you take the proper care, you can get a reasonable weld. On the other hand, some cast irons are rendered unweldable due to the contaminats in the base metal, irrespective of the steps taken to mitigate cracking during welding. It is difficult to know this before you start welding. This means that you should always see welds to cast irons of "commercial origin" as being a high risk weld. You may have to scrap your pipe / job entirely.

Hope this helps

Regards
Niekie Jooste