Final answer:
No, an oxidizing flame is not always used to weld plain carbon steel. In fact, a reducing flame is typically used to prevent excessive oxidation and the formation of brittle oxides on the steel surface.
Step-by-step explanation:
An oxidizing flame is not always used to weld plain carbon steel. In fact, an oxidizing flame is often avoided because it can lead to excessive oxidation and the formation of brittle oxides on the surface of the steel. Instead, a reducing flame is typically used, which contains a higher amount of fuel gas and less oxygen.
An acetylene torch, for example, produces a reducing flame by combining acetylene gas with oxygen. The high heat of the combustion reaction in the torch melts the metal being welded or cut.
So, in conclusion, the statement that an oxidizing flame is always used to weld plain carbon steel is false.