Ferrous Alloys and Their Production

Ferrous Alloys

• Ferrous alloys are a major component in metal production, primarily due to a combination of good strength, toughness, and ductility at a relatively low cost.
• Iron is the main constituent in ferrous alloys.
• They are produced in larger quantities than other metals.
• Their widespread use is attributed to abundant iron-containing compounds in the Earth's crust.
• Ferrous alloys are often produced using economically efficient extraction, refining, alloying, and fabrication techniques.
• Ferrous alloys exhibit a diverse range of mechanical and physical properties.
• Ferrous materials are categorized into two groups depending on carbon content: cast irons (carbon content > 2%) and steels (carbon content < 2%).

Types of Metal Alloys

• Metal alloys incorporate metals and have various useful engineering properties applicable in design.
• Iron and their alloys, predominantly steel, account for around 90% of global metal production.

Production of Ferrous Alloys

• The process begins with a blast furnace.
• Iron ore, limestone, and coke (carbon) are combined in the furnace.
• These reactants result in pig iron.

Reactions in the Blast Furnace

• Heat generation: Carbon reacts with oxygen to produce carbon dioxide.
• Reduction of iron ore to pig iron: Carbon monoxide reduces iron oxide to molten iron.
• Purification: Limestone decomposes into calcium oxide, which helps remove impurities from the molten iron.
• Forming of slag: Calcium oxide combines with impurities to produce slag.

Slab, Bloom, Billet

• Cast steel ingots are rolled into different shapes; then rolled, extruded, or drawn to produce differing shapes.

Steels

• Steels are versatile materials encompassing various properties like strength, temperature resistance, corrosion resistance, impact resistance, and abrasion resistance.
• Steels are categorized based on their composition.

Plain Carbon Steels

• These steels have relatively low strength but superior ductility and toughness.
• Applications include structural shapes (I-beams), sheets used in pipelines, buildings, bridges, and tin cans.
• These steels are inexpensive to produce.
• These steels are frequently employed in sheet material and forming applications.

High Strength, Low Alloy (HSLA) Steels

• Developed as replacements for conventional low carbon steels.
• Characterized by high strength-to-weight ratios, good ductility, formability, and machinability.
• Superior resistance to corrosion compared to conventional low carbon steels.

Medium Carbon Steels

• Contain up to 0.60% carbon
• Mechanical properties can be improved by heat treatments.
• Applications include various machinery, tractors, mining equipment, railway wheels, tracks, gears, crankshafts, and machine parts.

High Carbon Steels

• Possessing carbon contents between 0.60% and 1.4%
• These steels are the hardest, strongest, and least ductile among carbon steels.
• Frequently used in hardened and tempered states.
• Applications include tools, carbon die steels, cutters, springs, and high-alloy components.
• Chromium, vanadium, tungsten, and molybdenum are commonly used as alloying elements.

Stainless Steels

• Exhibit excellent corrosion resistance in various environments, particularly atmospheric environments.
• Corrosion resistance is attributed to high chromium content, which forms a protective chromium oxide layer that protects the underlying alloy from corrosion.
• Nickel and molybdenum additions further enhance corrosion resistance.
• Generally, stainless steels contain very low carbon content (≤0.15%).
• Increasing carbon content decreases corrosion resistance due to the formation of chromium carbide.

Specialty Steels

• Maraging steel: Has low carbon and high nickel content, thus exhibits high strength and toughness, and is formable, weldable, machinable
• Free-machining steels: Include lead, sulfur, and other elements to enhance machinability, lubricate cutting operations, and reduce chip formation.
• Interstitial-free (IF) steel: Low carbon levels, resulting from alloying elements that combine with carbon and leave the steel virtually free of interstitial atoms; thus, exhibit excellent ductility.

Designation of Steels

• AISI and SAE specifications utilize numerical designations that categorize steels based on their chemical compositions and alloy content.
• The initial digits typically indicate alloying elements, and the final digits refer to carbon content multiplied by 100.

Cast Irons

• Cast irons are a class of ferrous alloys with carbon content greater than 2.14%.
• Various types such as gray iron, ductile (nodular) iron, white iron, malleable iron, and compacted graphite iron exist; each with unique microstructures and properties.
• Gray iron exhibits a gray appearance due to graphite flakes; it has relatively low impact resistance and ductility but still has high compressive strength, good machinability, and high resistance to wear and thermal fatigue.
• Ductile iron—produced by adding magnesium—has spheroidal graphite particles and excels in castability, machinability, and wear resistance; is also comparable to steel in strength, toughness, and ductility.
• White iron—formed by rapid cooling—contains cementite instead of graphite. It is extremely hard and brittle, but exhibits excellent wear resistance.
• Malleable iron arises from the heat treatment of white iron, which produces rounded graphite clumps and makes it readily machinable with superior ductility compared to gray and white iron.
• Compacted graphite iron is relatively recently added in the cast iron family, which has worm-like graphite and higher thermal conductivity compared to others.