Hardness is a material’s resistance to permanent indentation or scratching. Tempering is a secondary heat treatment process used to refine the properties of hardened steel. Tempering does not increase hardness; it is specifically applied to decrease the hardness achieved in a prior step. This necessary trade-off transforms an extremely hard but fragile material into one that is both strong and reliable for practical applications, dramatically improving its durability and usefulness.
The Necessary Precursor: Creating Maximum Hardness
The process that precedes tempering is quenching, which creates the steel’s maximum possible hardness. This initial step involves heating the steel to a high temperature, typically between 800°C and 1100°C, to transform its internal structure into austenite. The exact temperature depends on the specific carbon content and alloying elements.
Immediately following heating, the material is rapidly cooled, usually in water, oil, or a specialized salt bath. This rapid temperature drop prevents carbon atoms from moving out of the iron crystal lattice to form softer structures like pearlite. The result of this trapped carbon is a distorted, body-centered tetragonal crystal structure known as untempered martensite.
Martensite is the hardest microstructure achievable in steel, but this extreme hardness causes intense brittleness. The internal stresses locked within the lattice structure make the quenched steel highly susceptible to cracking and catastrophic failure under impact or minor stress. This renders the material unsuitable for most real-world uses, making the subsequent tempering process mandatory.
The Direct Effect on Hardness
Tempering involves reheating the quenched steel to a sub-critical temperature, typically ranging from 150°C to 650°C. The primary effect of this reheating is the controlled decomposition of the highly strained martensite microstructure. As the temperature increases, the excess carbon atoms trapped within the iron lattice gain enough energy to begin moving.
This carbon movement allows the internal stresses to relax. During this microstructural rearrangement, extremely fine carbide particles begin to precipitate out of the carbon-supersaturated martensite. These new carbides are far less disruptive to the overall crystal structure than the trapped carbon was, leading to a measurable drop in hardness values, often tested using the Rockwell or Brinell scales.
This change transforms the unstable martensite into a more stable structure known as tempered martensite. The decrease in hardness is a direct consequence of the material releasing the immense internal strain locked in during the rapid quenching process. The higher the tempering temperature, the more microstructural relaxation and carbide formation occurs, leading to a progressively greater reduction in hardness.
The Primary Goal: Increasing Toughness and Reducing Stress
Since tempering intentionally reduces hardness, its primary goal is improving other mechanical properties, specifically toughness and ductility. Toughness describes a material’s ability to absorb energy and plastically deform without fracturing, a property entirely lacking in as-quenched, brittle steel. Tempering achieves this by relieving the massive residual stresses introduced during the rapid cooling phase.
The highly stressed, as-quenched state is prone to micro-cracks and sudden failure. Tempering allows the material to absorb impact and bending forces without shattering, effectively trading a small amount of penetration resistance for a massive gain in reliability. This gain is directly related to the improvement in ductility, which is the material’s ability to undergo significant plastic deformation before failure.
A component that is only hard is fragile, but one that is both hard and tough can withstand repeated and dynamic loading. Engineers seek the final balance between these two properties to create durable tools, springs, and engine components that will not fail unexpectedly in service. The controlled sacrifice of maximum hardness is the means to achieve a durable, functional material.
Controlling the Temper: Temperature and Time
Metallurgists control the final properties of the steel by precisely manipulating the temperature and duration of the tempering process. The relationship between these parameters and the material properties is often visualized on a tempering curve, which plots the resulting hardness against the tempering temperature. Generally, a higher tempering temperature or a longer holding time results in a greater reduction in hardness and a corresponding increase in toughness.
For instance, a low-temperature temper (150°C to 250°C) results in a slight decrease in hardness but a significant improvement in impact resistance, making it suitable for cutting tools. Conversely, a high-temperature temper, sometimes exceeding 600°C, yields a much softer, highly tough material ideal for springs or structural components. In lower-alloy carbon steels, the tempering temperature can be roughly gauged by the thin oxide layer that forms on the surface, which changes color from pale straw to deep blue.