When soldering valves to copper pipe, thicker walls need more heat to sweat. Sweating aluminum is similar, where you scour the pipe’s exterior and the “cup” of the fitting to be emptied.
Flux both of these and heat the area all over the cup. Then put a torch flame in 1 place, applying solder to the other side until the heat causes the solder to flow. Solder will flow warmth without exclusion. But overheating is a possibility, so get rid of the flashlight from the pipe when the solder flows into the heat and wash it with a wet rag.
Cutting Cast Iron and Plastic/Cast Connections? Cut the situation iron with a reciprocal saw, possibly a Milwaukee saws-all. Cast iron is extremely strong, so you would like to use heavy metal blades. Check out http://nssteel.co.th/product.php?catid=36 if you want to search more about stainless steel.
Insert: When moving after iron waste pipes I love to use a rigid ratchet cast iron cutter. Sometimes the place is too confined to utilize that tool though. In this case, I use a mini-grinder and a diamond wheel to reduce as much as I can.
The diamond wheel is $100 but one will last you forever. I would finish with a saws-all and a grid-edge blade. BTW the grinder with the diamond wheel is very good for cutting tile, brick, and concrete. It’ll be quite dusty cutting through these items.