Question

Railway wagon axles are made by heating iron rods embedded in charcoal powder. This process is known as:
A. Tempering
B. Case hardening
C. Sherardising
D. Annealing

Answer 1

Hint: Iron rods are heated by embedding in charcoal powder to make the railway wagon axles. This is a heat treatment method used to improve the durability and strength of the metal. There are various heat treatment processes. Each process has a different method of heating. Using its heating method we can identify the heating process used to make railway wagon axles.

Complete answer:
Tempering: Tempering is a heat treatment process. This process increases the toughness of hard, brittle steel. In this process metal is heated gradually and then at a temperature below the critical temperature metal is held at a fixed period. After heating for a fixed period of time metal is cooled. In this process, metal is heated in air, vacuum or inert atmosphere.
In the tempering process as metal rods are not heated by embedding in charcoal powder so option (A) is an incorrect answer.

Case hardening: The case hardening process is also known as the surface hardening process. This process increases the durability of metal by adding a thin layer of metal alloy on the metal surface. It resists the wear and tear of the steel and increases its strength. Out of various heat treatment processes, case hardening is a very efficient heat treatment process.
Using this process the surface of wrought iron can be hardened by depositing a layer of steel on its surface. In this process, iron rods are heated by embedding them in charcoal powder.
Thus, option (B) case hardening is correct.

Sherardising: Sherardising is also a surface coating process like case hardening. However, in this process iron surface is coated with zinc-iron alloy. This coating of alloy improves resistance to corrosion. In this heating, process metal is heated in zinc powder that causes diffusion of zinc in the metal surface.
In sherardising process, as metal rods are not heated by embedding in charcoal powder so option (C) is an incorrect answer.

Annealing: Annealing is also a heating process. In this process, metals are heated at a specific temperature and then cool it slowly at a controlled rate. The annealing process softens the metal. So metal can be used for cold working.
In the annealing process, as metal rods are not heated by embedding in charcoal powder so option (D) is an incorrect answer.

Note:
There are various heat treatment processes used to alter the properties of the metal. Heat treatment process improves strength, hardness, toughness, ductility, elasticity, formability etc. By varying heating and cooling rate we can alter the desired property of the metal. Based on heating techniques, the heat treatment processes are classified as tempering, case hardening, sherardising and annealing.