The type of gas used in welding is acetylene. Acetylene is a colorless, flammable gas that is commonly used in oxy-acetylene welding, which combines oxygen and acetylene to create a high-temperature flame capable of melting metals. This process is particularly effective for welding steel and other alloys. The flame can be adjusted to produce different temperatures based on the welding needs. Acetylene is produced industrially by the reaction of calcium carbide with water and is widely used in various welding techniques and for cutting metals.