JNTU B.Tech III HEAT TREATMENT Previous Question Paper (October - 2020 - R16)

For hypereutectoid steels, which contain more than 0.8% carbon, the heat treatment temperature ranges are defined relative to the A₁ (lower critical) and A_cm (upper critical) lines on the iron-iron carbide phase diagram.

Normalizing: The temperature range for normalizing hypereutectoid steel is typically 30°C to 50°C above the A_cm line. Heating into this single-phase austenite region ensures the complete dissolution of proeutectoid cementite into the austenite matrix, which then transforms into a uniform, fine pearlitic structure upon air cooling.

Hardening: The hardening temperature range is generally 30°C to 50°C above the A₁ line. Unlike normalizing, hardening is performed within the austenite plus cementite (α + Fe₃C) region. This range is selected to transform the pearlite into austenite while leaving the excess proeutectoid cementite undissolved, which increases the wear resistance and hardness of the final martensitic structure.

Criteria for Selection: The selection of these ranges is based on the following factors:

• Microstructural Control: Normalizing aims for a homogeneous grain structure, requiring full austenitization above A_cm. Hardening aims for maximum hardness, where undissolved cementite particles act as hard phases to improve abrasion resistance.
• Grain Growth Prevention: Temperatures are kept strictly near the critical lines (30-50°C above) to prevent excessive austenite grain growth, which would otherwise result in a brittle, coarse martensitic structure upon quenching.
• Energy Efficiency: Operating as close to the critical lines as possible minimizes energy consumption and thermal stress during the heating and cooling cycles.

Hardening by quenching involves rapid cooling of steel from the austenitizing temperature to form martensite, a body-centered tetragonal structure. While martensite provides high hardness and strength, it is inherently brittle, possesses high internal stresses due to the volume expansion during transformation, and lacks sufficient ductility and toughness for structural applications. Tempering is essential to transform this metastable martensite into a more stable tempered martensite, which relieves internal stresses and restores ductility.

The tempering process involves reheating the quenched steel to a temperature below the lower critical temperature (A1). The variations in mechanical properties with increasing tempering temperature are as follows:

• Hardness and Strength: These properties decrease progressively as the tempering temperature increases. The decomposition of martensite into ferrite and fine cementite particles reduces the lattice strain and dislocation density, leading to a loss in hardness.
• Ductility and Toughness: These properties increase significantly with higher tempering temperatures. The relief of internal quenching stresses and the growth of carbide particles allow for greater plastic deformation, thereby improving the material's impact resistance and elongation.
• Internal Stresses: Residual stresses induced during quenching are progressively relieved as the tempering temperature rises, which significantly improves the dimensional stability of the component.

In summary, tempering allows for a controlled trade-off between hardness and toughness, enabling the material to meet specific engineering requirements by selecting an appropriate tempering temperature.