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Chemical Engineering Basics - Engineering

Q1:

Pick out the wrong statement.

A Both annealing and normalising release the internal stresses of the material besides improving the mechanical properties.

B Low carbon steel does not respond to the heat treatment for hardening of the material, hence it is subjected to case hardening or surface hardening processes like cyniding, carburising, nitriding etc., which produces high carbon outer layers resulting in increase of surface hardness.

C Induction hardening and flame hardening techniques are also used for surface hardening.

D Martempering of a material is a hardening process.

ANS:C - Induction hardening and flame hardening techniques are also used for surface hardening.

Induction hardening and flame hardening are two common techniques used for surface hardening of metals. These techniques are employed to increase the hardness and wear resistance of specific areas of a metal component, typically the surface layer, while maintaining the toughness and ductility of the core material.

  1. Induction Hardening:
    • Induction hardening utilizes electromagnetic induction to heat the surface of a metal component rapidly. An alternating current is passed through a coil, generating a high-frequency magnetic field. When the metal component is placed within this magnetic field, eddy currents are induced within the surface layer, causing rapid heating due to the Joule effect.
    • The heated surface layer reaches a critical temperature, typically above the austenitizing temperature, where the steel transforms into austenite, a phase with higher solubility for carbon and other alloying elements.
    • After heating, the component is quenched rapidly by spraying or immersion in a quenching medium (such as water or oil). The rapid quenching causes the austenitized surface layer to transform into martensite, a hard and brittle phase with high hardness.
    • The hardened surface layer exhibits increased hardness, wear resistance, and improved fatigue strength, while the core retains its original mechanical properties.
  2. Flame Hardening:
    • Flame hardening involves using an oxy-fuel flame to heat specific areas of a metal component to high temperatures. The component is typically rotated or moved relative to the flame to ensure uniform heating of the desired surface.
    • As the surface reaches the austenitizing temperature, the component is quenched rapidly using water or oil to transform the austenitized layer into martensite.
    • Flame hardening is suitable for large or irregularly shaped components and can be easily adapted for selective hardening of specific regions.
    • The hardened surface layer provides increased wear resistance and surface hardness, making it suitable for applications where abrasion and wear are significant concerns.
Both induction hardening and flame hardening are versatile techniques used in various industries such as automotive, aerospace, and manufacturing for enhancing the surface properties of metal components, thereby extending their service life and improving performance in demanding applications.