Steel, a composition of iron and carbon, melts at around 1,370 °C. The torch commonly used by welders is called an oxy-fuel welder or cutter. The fuels used in this type of cutter is Acetylene (most common), Hydrogen, propylene, LPG, natural gases etc..
Acetylene, the most common fuel, can reach a temperatures between 3200 °C to 3500 °C (5800 °F to 6300 °F). However, the major drawback of this fuel is its use underwater, because, if the depth exceeds 33 feet/10 meters underwater, Acetylene becomes unstable, hence Hydrogen becomes its substitute.
Hydrogen on the other hand cannot exceed more than 2,000 °C, but when pre-mixed in a 2:1 ratio with pure oxygen, can reach temperatures up to 2,800 °C.
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Wikipedia under the topic "Oxy-fuel welding and cutting"