Structural steel can withstand approximately 425°C before it begins to soften. Between 600°C and 650°C, the steel will lose half of its strength and will pose a risk of failing (depending on the load it bears). Unsurprisingly, even a house fire will reach very high temperatures of around 600°C or just under. Of course, it depends on what the material being combusted is. A simple candle will often burn at 600°C or more, whilst propane can cause a fire to reach almost 2000°C.