Metallic Alloys Lecture Notes

Metallic Alloys: Composed mainly of metallic elements, metallic alloys exhibit enhanced mechanical properties compared to pure metals, making them essential in various industries. These alloys typically provide increased strength, hardness, corrosion resistance, and improved ductility, which are crucial for structural and load-bearing applications across sectors such as construction, manufacturing, and automotive.

Classification of Steels
Low Carbon Steels (C < 0.25%):

• Produced in large quantities and are relatively inexpensive to manufacture, making them widely available for various applications.

• Unresponsive to heat treatments due to their low carbon content, which retains their soft and ductile nature.

• The microstructure predominantly consists of ferrite and pearlite, which contributes to properties like formability and weldability but reduces tensile strength.

• Common examples, including AISI 1010, 1020, and A36, are utilized in applications such as automobile panels, nails, and structural steel for buildings and bridges.

Medium Carbon Steels (C = 0.25% - 0.60%):

• These steels can be heat treated to significantly enhance their mechanical properties, allowing for greater versatility in applications requiring moderate strength.

• The microstructure generally includes a tempered martensite structure, which offers a balance between hardness and ductility.

• Examples such as 1040 and 1080 are commonly used in critical applications like crankshafts, connecting rods, and chisels due to their improved strength characteristics.

• Generally exhibit lower ductility compared to low carbon steels, requiring careful handling during fabrication.

High Carbon Steels (C = 0.60% - 1.40%):

• These are the strongest and hardest among carbon steels, typically used in hardened and tempered states to achieve maximum toughness and wear resistance.

• Alloyed frequently with elements like chromium and vanadium to enhance hardness and resistance to deformation.

• Examples include tool steels like A2 and D2, which are essential for manufacturing cutting tools, dies, and molds that require high durability.

• Due to their brittleness, they require meticulous heat treatment processes to preserve mechanical integrity in finished products.

Stainless Steels:

• Characterized by the inclusion of 11% or more chromium (Cr), stainless steels offer remarkable corrosion resistance, which makes them ideal for harsh environments where moisture and corrosive agents are present.

• They are classified into major types: ferritic (magnetic, good weldability), austenitic (non-magnetic, superior formability and corrosion resistance), and martensitic (high strength and hardness but lower corrosion resistance). Each type exhibits unique properties tailored to specific applications.

• Common examples include AISI 304 and 316, widely used in kitchenware, food processing plants, medical instruments, and construction materials due to their durability and aesthetic appeal.

Cast Irons:

• Defined by their high carbon content (C > 2.14%), cast irons differ from steels mainly in their microstructure and mechanical properties, providing excellent fluidity and complex shape capabilities during casting.

• Variants include gray, ductile, and malleable cast iron, with properties regarding tensile strength and brittleness adapting to different industrial needs.

• Optimal for applications requiring good fluidity during pouring and low shrinkage during solidification, making them commonly used in engine blocks, pipes, and heavy machinery components.

Non-Ferrous Alloys:

• Aluminum Alloys:

  • Characterized by low density (approximately 2.7 g/cm³) and high electrical and thermal conductivity, these alloys are corrosion-resistant, although they may be less strong than steel.

  • Commonly utilized in aerospace and automotive manufacturing, examples include Alloy 2024 (high strength) and Alloy 6061 (versatile and weldable).

• Copper Alloys:

  • Renowned for their superior electrical and thermal conductivity, copper alloys are essential in electrical applications.

  • They include brass (Copper-Zinc) and bronze (Copper-Tin), frequently used in electrical fittings, plumbing fixtures, and artistic sculptures due to their aesthetic surface finishes.

• Titanium Alloys:

  • Notable for their low density and high strength compared to steel, titanium alloys exhibit excellent corrosion resistance, making them ideal for aerospace and biomedical applications.

  • An example is Ti-6Al-4V, utilized in jet engines, medical implants, and high-performance sporting goods.

• Magnesium Alloys:

  • Lightweight and often utilized in applications where reducing weight is crucial; nevertheless, their susceptibility to corrosion limits usage in marine environments.

  • Enhanced properties can be achieved when alloyed with Aluminum and Zinc, making them suitable for specific automotive parts and electronic housings.

Production and Processing:

• Forming Operations:

  • Includes methods like hot working, cold working, forging, rolling, and extrusion that influence the final mechanical properties of the alloys, with each method tailored for specific requirements like shape and strength.

• Thermal Processing:

  • Encompasses varied heat treatments aimed at relieving residual stresses, enhancing ductility, refining the microstructure, and altering mechanical properties to optimize performance in intended applications.

  • Annealing is one crucial form of thermal processing which involves heating the metal alloy and then slowly cooling it to relieve internal stresses and improve ductility, vital for further fabrication steps.

• Welding Techniques:

  • Metallurgical bonding processes are essential for joining different alloy types, crucial in large-scale manufacturing and construction projects.

  • However, welding may induce residual stresses, requiring post-weld heat treatment to restore ductility and prevent issues like cracking and brittleness in the welded joint.

Hardening Techniques:

• Hardenability:

  • Refers to the ability of steel to harden in depth through quenching, profoundly influenced by alloy composition and the specific heat treatment processes applied.

• Jominy End-Quench Test:

  • A standardized method used to measure hardenability by evaluating how hardness varies with distance from the quenched end of a steel sample, revealing critical data on its suitability for different applications based on required hardness characteristics.