Steel is an alloy of iron and carbon, with carbon content typically ranging up to 2.14% by weight. When exposed to heat, steel initiates a complex sequence of physical and internal changes that fundamentally alter its structure and performance. The severity of these alterations depends entirely on the maximum temperature reached and the duration of the exposure. Effects range from visible color changes and physical expansion to atomic rearrangement and the eventual degradation of mechanical integrity.
Observable Changes During Heating: Color and Expansion
One of the first and most noticeable effects of heating steel is physical expansion, driven by increased atomic vibration. As the temperature rises, atoms within the steel’s crystal structure vibrate with greater amplitude, pushing the material’s boundaries outward and causing it to increase in volume. This thermal expansion must be accounted for in any design where steel components are subjected to large temperature fluctuations.
The most intuitive indicator of steel’s temperature is the color it emits, a direct result of incandescence. Steel begins to emit a visible dull red glow at around 900°F (480°C). As the temperature climbs, the color shifts predictably, becoming cherry red around 1,300°F (700°C), and progressing through orange and yellow.
At very high temperatures, such as above 2,000°F (1,100°C), the steel appears white hot. This visible color scale has historically been used by blacksmiths and metallurgists to estimate temperature. At lower temperatures, a phenomenon called “heat tint” or “temper color” occurs, where a thin oxide layer forms. This produces a rainbow of colors (straw, blue, purple) that correspond to specific temperatures between approximately 400°F and 600°F (200°C and 315°C).
Microstructural Reorganization: Phase Transformations
Beyond the visible changes, heating steel triggers a profound internal reorganization, known as a phase transformation. At room temperature, the iron atoms in steel typically form a body-centered cubic (BCC) structure called ferrite, which is relatively soft and has a limited ability to dissolve carbon. This structure is stable until a certain critical temperature is reached.
When steel is heated above its lower critical temperature (A1), approximately 1,333°F (723°C) for carbon steel, the crystalline structure begins to change. The BCC ferrite structure transforms into a face-centered cubic (FCC) structure called austenite. This transformation is a rearrangement of the iron atoms into a more compact arrangement.
The austenite phase has a significantly greater capacity to dissolve carbon atoms within its lattice structure. As the steel is heated toward its upper critical temperature (A3), this process continues until the entire volume of the steel has transformed into a uniform phase of austenite. This internal rearrangement is a diffusion-controlled process, preparing the steel for subsequent thermal processing.
This transformation is the basis for most heat treatment processes because the carbon atoms are fully dissolved and ready to be locked into a new, harder structure if the steel is cooled rapidly. Without achieving the austenitic state, it is impossible to fundamentally alter the steel’s mechanical properties through heat treatment. The final properties are determined by how this high-temperature austenite is subsequently cooled.
Consequences of Heat: Strength Loss and Oxidation
The intentional heating of steel for processing is distinct from the negative consequences of prolonged exposure to high temperatures, which include degradation of strength. As steel is heated, especially when it transforms into the high-temperature austenite phase, its mechanical strength, including yield strength and hardness, decreases substantially. This loss of strength is a major concern in structural applications, as prolonged exposure to fire can cause steel support members to soften and fail under load.
Another consequence of high-temperature exposure is creep, the permanent, time-dependent deformation of the material under constant mechanical stress. Unlike a sudden failure, creep is a gradual process where the material slowly deforms, even when the applied stress is well below the normal yield strength. The rate of creep accelerates significantly as temperatures increase, often becoming a serious design consideration above 0.4 times the steel’s absolute melting temperature.
Simultaneously, steel reacts vigorously with oxygen at elevated temperatures in a process known as high-temperature oxidation. This reaction forms a layer of iron oxide, commonly referred to as “scale,” on the steel’s surface. For carbon steel, noticeable oxidation begins around 900°F (482°C), and the rate of scaling becomes excessive above 1,000°F (538°C). This scaling reduces the effective thickness of the component and compromises the integrity of the surface, leading to material loss and eventual failure.