Final answer:
The flame that should be used is the oxyacetylene flame, which is produced by mixing oxygen and acetylene gases. This flame provides a concentrated, high-energy heat source suitable for applying hard-surfacing materials to a metal surface.
Step-by-step explanation:
The flame that should be used to apply hard-surfacing materials to a metal surface is the oxyacetylene flame. An oxyacetylene flame is produced by mixing oxygen and acetylene gases in the correct proportions and igniting them.
The flame produced by this mixture is very hot and can reach temperatures of up to 6,000 degrees Fahrenheit, making it suitable for high-temperature applications such as applying hard-surfacing materials. This type of flame is commonly used in welding and cutting processes, as it provides a concentrated, high-energy heat source that can effectively melt and fuse metals.