Pure oxygen, instead of air, is used to increase the flame temperature to allow localized melting of the workpiece material (e.g. steel) in a room environment. A common propane/air flame burns at about 2,250 K (1,980 °C; 3,590 °F),[2] a propane/oxygen flame burns at about 2,526 K (2,253 °C; 4,087 °F),[3] and an acetylene/oxygen flame burns at about 3,773 K (3,500 °C; 6,332 °F)

Torches that do not mix fuel with oxygen (combining, instead, atmospheric air) are not considered oxy-fuel torches and can typically be identified by a single tank (oxy-fuel cutting requires two isolated supplies, fuel and oxygen). Most metals cannot be melted with a single-tank torch. As such, single-tank torches are typically used only for soldering and brazing, rather than welding.

https://en.wikipedia.org/wiki/Oxy-fuel_welding_and_cutting[1]

Do you see that bold sentence? Without force feeding pure oxygen, not even a blowtorch can melt steel, and the fuel used in blow torches burns a lot hotter in air than even jet fuel.

The simple fact is, the colour of the flames coming out of the WTC shows they were burning at around 600o C. Firefighters in the middle of the WTC fires reported small fires that could easily be handled with a few hoses.

If the fires in the core of the WTC were hot enough to soften steel, no firefighter would have been able to get anywhere near those floors, and there certainly wouldn't have been identifiable bodies lying in the middle of it, as the firefighters reported.