Tempering of quenched martensitic steel is necessary to improve the ductility of the metal. Tempering is a heat treatment process that involves heating the quenched steel to a specific temperature and then cooling it slowly. This process helps to relieve the internal stresses and reduce the brittleness of the steel, thereby increasing its ductility.

Tempering Process:
During the quenching process, the steel undergoes a rapid cooling, which transforms the austenite phase into the hard and brittle martensite phase. While martensite is known for its high hardness, it is also extremely brittle and prone to cracking under stress. The high hardness of martensite is desirable for certain applications, but its brittleness limits its usefulness in many cases. Therefore, tempering is carried out to balance the hardness and ductility of the steel.

Effect on Ductility:
Tempering involves reheating the steel to a temperature below its critical point, typically between 200°C to 650°C, depending on the desired properties. At this temperature range, the internal stresses in the steel are relieved, and the martensite structure begins to transform. The carbon atoms within the steel diffuse and form small carbide particles, which act as obstacles to dislocation movement. This process, known as tempering, helps to reduce the hardness and increase the ductility of the steel.

The diffusion of carbon during tempering also leads to a decrease in the strength of the steel. The carbide particles act as barriers to dislocation movement, making it easier for the steel to deform plastically. This results in an increased ability of the steel to absorb energy before fracture, thus improving its ductility.

Importance of Ductility:
Ductility is a crucial mechanical property of a material, especially in applications where the material is subjected to dynamic or impact loads. Steel with high ductility can undergo substantial plastic deformation before failure, which makes it less prone to sudden fracture or catastrophic failure. This property is particularly important in structural components or machinery parts that are subjected to varying or unpredictable loads.

In summary, tempering of quenched martensitic steel is necessary to improve its ductility. The tempering process relieves internal stresses, reduces brittleness, and allows the steel to undergo plastic deformation before failure, making it more suitable for various engineering applications.