Heat Treatment of Steel: Suits and Processes
(a) Steels Best Suited for Heat Treatment:
Heat treatment is particularly beneficial for specific types of steel, primarily those with medium to high carbon content (0.2-1.0%). These steels experience significant changes in their mechanical properties due to the heat-driven transformations in their internal structure. Some examples include:
- Hypoeutectoid steels (carbon content < 0.77%): These steels can achieve significant hardening through quenching, forming martensite with high strength and hardness.
- Eutectoid steel (0.77% carbon): This steel readily transforms into pearlite (a mixture of ferrite and cementite) during slow cooling, achieving a good balance of strength and ductility.
- Hypereutectoid steels (carbon content > 0.77%): These steels can achieve high wear resistance due to the presence of cementite (iron carbide). However, their weldability and ductility are limited.
Steels with very low carbon content (< 0.2%) and high-alloy stainless steels generally benefit less from heat treatment due to their inherent properties or limited transformation potential.
(b) Heat Treatment Processes Explained:
(i) Hardening:
- Process: Steel is heated above its critical temperature (around 723°C), austenitizing its microstructure (transforming it into austenite, a high-carbon phase). This is followed by rapid quenching (usually in water or oil), which traps the high-carbon austenite structure as a metastable phase called martensite.
- Mechanical Properties: Hardening significantly increases the strength and hardness of the steel but at the expense of ductility and toughness. Martensite is very strong and hard but also brittle and prone to cracking.
- Internal Structure: The rapid quenching prevents carbon atoms from diffusing and forming equilibrium phases like pearlite. Instead, austenite transforms into martensite, a distorted tetragonal lattice structure with high carbon content, leading to high strength and hardness.
(ii) Tempering:
- Process: Hardened steel is reheated to a lower temperature (typically between 150°C and 500°C) and then cooled slowly. This allows some of the martensite to transform back into less brittle phases like ferrite and cementite, depending on the tempering temperature.
- Mechanical Properties: Tempering reduces the hardness and strength of the steel compared to the hardened state but significantly improves its ductility and toughness. This makes the steel more resistant to cracking and failure under stress.
- Internal Structure: Tempering allows carbon atoms to diffuse and form more stable phases like ferrite and cementite. The specific phases formed and the degree of transformation depend on the tempering temperature, influencing the final balance of strength, ductility, and toughness.
Therefore, hardening and tempering are complementary processes. Hardening provides high strength and hardness, while tempering improves ductility and toughness, allowing for a tailored balance of properties for specific applications.