The most commonly used gas in welding is acetylene. Acetylene, when combined with oxygen, produces a flame with a high temperature, making it ideal for various welding applications, particularly in oxy-acetylene welding. This gas can melt metals quickly, facilitating the joining process. Acetylene is favored because it burns hotter than many other gases, allowing for efficient welding of different materials, including steel and aluminum. Its versatility and effectiveness have made it a standard choice in industries involving metal fabrication, automotive repair, and construction, where precise and strong welds are crucial.