Jawaharlal Nehru Technological University Hyderabad B.Tech III METALLURGY FOR NON METALLURGISTS Previous Question Paper (November/December - 2020 - R16)

Dislocations are line defects or irregularities within a crystal lattice that significantly influence the mechanical properties of materials, particularly their ability to undergo plastic deformation. These defects represent the boundary between slipped and unslipped regions of the crystal.

1. Edge Dislocation

An edge dislocation can be visualized as the insertion of an extra half-plane of atoms into an otherwise perfect crystal lattice. The dislocation line is defined as the edge of this extra half-plane.

• The lattice above the slip plane is in compression, while the lattice below is in tension.
• The Burgers vector (b) is perpendicular to the dislocation line.
• When a shear stress is applied, the dislocation moves through the crystal by breaking and reforming atomic bonds, allowing the crystal to deform plastically.

2. Screw Dislocation

A screw dislocation is formed by shearing the crystal lattice such that the atomic planes trace a spiral or helical path around the dislocation line. It resembles the threads of a screw.

• The dislocation line is parallel to the direction of the applied shear stress.
• The Burgers vector (b) is parallel to the dislocation line.
• Deformation occurs as the screw dislocation moves across the slip plane, effectively shifting one part of the crystal relative to the other.

Note: Most real crystals contain mixed dislocations, which possess characteristics of both edge and screw types, where the Burgers vector is neither parallel nor perpendicular to the dislocation line.

Plain carbon steels are classified primarily based on their carbon content, which dictates their mechanical properties, heat treatment response, and industrial utility. They are categorized into three main groups:

• Low Carbon Steels (Mild Steels): Containing up to 0.30% carbon. These are the most widely used steels due to their excellent ductility, toughness, and weldability. They are relatively soft and possess low tensile strength. Applications include structural steel shapes (I-beams, channels), automobile body panels, pipes, and wire products.
• Medium Carbon Steels: Containing between 0.30% and 0.60% carbon. These steels offer a balance between ductility and strength. They can be heat-treated (quenched and tempered) to improve their mechanical properties. They are commonly used for machine parts such as gears, axles, connecting rods, crankshafts, and railway wheels.
• High Carbon Steels: Containing between 0.60% and 1.50% carbon. These steels are very hard and have high wear resistance but low ductility. They are difficult to weld and are typically used in the hardened and tempered condition. Applications include cutting tools, springs, high-strength wires, knives, and dies.

The relationship between carbon content and mechanical properties is summarized as follows:

In addition to carbon, these steels contain small amounts of manganese (0.25% to 1.65%), silicon (up to 0.60%), and trace amounts of sulfur and phosphorus, which are kept low to prevent brittleness. The selection of a specific grade depends on the required balance of strength, formability, and cost-effectiveness for the intended engineering application.