There is no practical way that I know of to weld steel with a typical air-propane torch. Even though the flame temperature is a bit over 3000 degrees F, and should be hot enough, the heat transfer is not enough to bring the steel up to welding temperatures. Too much heat is lost through conduction to surrounding material (heat sink effect) or dissapation into the surrounding air. Plus, I believe only the small inner flame cone gets that hot. The secondary cone is quite a bit cooler.

Oxygen-propane flames can be used to weld steel but even that is trickly because the temperatures are marginal. Oxygen-acetylene does a better job because the flame is hotter, but, again there are limitations. It is usually not used on thick materials that would need multiple passes to weld. Electric arc processes do a much better job.

A propane flame would not normally contaminate steel since the main byproducts are water vapor and carbon dioxide. These would not appreciably combine with steel unless it was molten and the water vapor was dissociated into hydrogen and oxygen. It won't be hot enough to do that. You may see some surface rust if there is enough water available after the steel cools below the boiling point.

Same thing with brazing, air-propane torches typically are not hot enough. It might work on thin material but oxy-propane will be better. Contamination won't be a problem.

I cannot answer the aluminum question because I have no info on that. If no one else answers, I think there must be something written about the subject somewhere. Start by talking to your local welding supply shop.

Hope this helps,
CHGuilford