Steel does not combust in the same way organic materials like wood do, which leave behind ash. Instead, steel undergoes significant changes when exposed to high temperatures, affecting its physical properties and leading to chemical reactions. Understanding its melting behavior, loss of strength, and how it can oxidize provides a more accurate picture.
Steel’s Response to High Temperatures
Steel maintains its strength at ambient temperatures, but its properties change as temperatures rise. Steel begins to lose strength around 204°C (400°F), compromising its load-bearing capacity. By approximately 593°C (1100°F), steel can lose about half its original strength, leading to bending or deformation. Above 649°C (1200°F), it can lose up to 90% of its strength, risking structural failure. This softening makes steel pliable for shaping between 815°C and 1200°C (1500°F to 2200°F), depending on the specific alloy.
While steel softens at these temperatures, its melting point is higher. Steel typically melts within a range of 1370°C to 1530°C (2500°F to 2800°F). The exact melting temperature depends on the steel’s specific composition, including its carbon content. For example, carbon steel generally melts between 1425°C and 1540°C (2600°F to 2800°F), while stainless steel melts between 1400°C and 1530°C (2550°F to 2790°F).
When Steel “Burns”: Oxidation and Combustion
When people ask if steel “burns,” they are often referring to a rapid chemical reaction rather than traditional combustion. Unlike wood, which reduces to ash, steel gains mass when it “burns” due to its reaction with oxygen, forming oxides. This process is a form of rapid oxidation, similar to accelerated rusting, occurring at elevated temperatures. For carbon steel, significant oxidation and scaling can begin around 482°C to 510°C (900°F to 950°F), becoming more pronounced above 538°C to 565°C (1000°F to 1050°F).
A common example of steel “burning” in an industrial context is oxy-fuel cutting. In this process, steel is preheated to its ignition temperature, typically between 700°C and 900°C (1292°F and 1652°F) for mild steel. A stream of pure oxygen is then directed at the heated area, initiating an exothermic chemical reaction. This reaction causes the iron in the steel to rapidly oxidize, forming molten iron oxide (slag) and releasing heat, which sustains the cutting process.
Factors Affecting Steel’s Heat Resistance
The heat resistance of steel is not uniform across all types; it is influenced by its specific alloy composition. Alloying elements added to steel alter its thermal properties.
Elements such as chromium enhance steel’s resistance to corrosion and scaling at elevated temperatures, while also increasing its strength and hardness. Nickel improves strength and toughness, especially at high temperatures, and contributes to corrosion resistance. Manganese helps improve mechanical properties and can influence the steel’s response to heat treatment. Other elements like molybdenum and silicon can also boost heat resistance and strength at elevated temperatures. The duration of heat exposure also matters, as prolonged exposure can lead to creep, where steel slowly deforms under constant stress even at temperatures below its softening point.