JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV ADVANCED MATERIALS TECHNOLOGY Previous Question Paper (July - 2021 - R16)

Plain carbon steels are classified based on their carbon content, which significantly influences their mechanical properties and heat treatment response:

• Low Carbon Steel (Mild Steel): Contains 0.05% to 0.30% carbon. These are ductile, malleable, and possess excellent weldability and formability. They are widely used in structural steel shapes, automobile body panels, pipes, and wire products.
• Medium Carbon Steel: Contains 0.30% to 0.60% carbon. These offer a balance between strength and ductility and can be heat-treated to improve hardness. Applications include gears, axles, shafts, railway wheels, and structural components requiring higher strength.
• High Carbon Steel: Contains 0.60% to 1.50% carbon. These are very hard and possess high wear resistance but have low ductility. They are used for cutting tools, knives, springs, high-strength wires, and dies.

The general properties of plain carbon steels include good tensile strength, reasonable ductility, and low cost compared to alloy steels. They are easily machined and fabricated using standard industrial processes.

The primary limitations of plain carbon steels are:

• Poor Hardenability: They require rapid quenching (often in water) to achieve high hardness, which increases the risk of distortion or cracking.
• Low Corrosion Resistance: They are highly susceptible to oxidation and rusting in atmospheric conditions, requiring protective coatings.
• Strength-Weight Ratio: They have lower strength-to-weight ratios compared to high-strength low-alloy (HSLA) steels.
• Temperature Sensitivity: They lose strength rapidly at elevated temperatures and exhibit brittle behavior at cryogenic temperatures.

Stainless steels are a group of iron-based alloys containing at least 10.5% chromium by weight, which provides excellent corrosion resistance by forming a thin, self-healing, passive chromium oxide layer on the surface. They are primarily engineered for environments where resistance to oxidation and corrosion is critical.

Stainless steels are classified into five main categories based on their microstructural characteristics:

1. Austenitic: Face-centered cubic (FCC) structure, achieved by adding nickel. Non-magnetic, excellent ductility, and superior corrosion resistance.
2. Ferritic: Body-centered cubic (BCC) structure, primarily chromium-based. Magnetic, good ductility, and moderate corrosion resistance.
3. Martensitic: Body-centered tetragonal (BCT) structure. High carbon content allows for hardening via heat treatment, resulting in high strength and hardness.
4. Duplex: A balanced mixture of austenite and ferrite phases, providing high strength and excellent resistance to stress corrosion cracking.
5. Precipitation Hardening (PH): Alloys that can be strengthened through heat treatment to form fine precipitates, offering high strength combined with good corrosion resistance.

The chemical composition varies by grade; for example, the common 304 grade contains approximately 18% Cr and 8% Ni, while 410 grade contains 12% Cr and low nickel. Key properties include high tensile strength, excellent toughness, high-temperature oxidation resistance, and ease of fabrication. Applications span across various industries: Austenitic steels are widely used in food processing and chemical equipment; Ferritic steels are used in automotive trim and heat exchangers; Martensitic steels are utilized for cutlery, surgical instruments, and turbine blades; Duplex steels are essential for offshore oil and gas infrastructure; and PH steels are employed in aerospace components.