Metallurgy: Cast Iron and Steel Types

Grey Cast Iron

• Grey cast iron is produced when liquid cast iron cools slowly
• It contains about 3.5% carbon, which makes it brittle
• Grey cast iron fractures easily from sharp blows
• It has a grey crystalline colour when fractured
• Grey cast iron is used for large pipes, steam radiators, water hydrants, and machine frames

White Cast Iron

• White cast iron is produced by rapidly cooling or chilling liquid iron
• It has a carbon content of 2% to 3.5%
• White cast iron is extremely hard and cannot be machined except by grinding
• It is used for castings needing abrasion and wear resistance

Pig Iron

• Pig iron is formed by pouring melted iron from the blast furnace into sand molds
• The molds are parallel trenches with a connecting channel, forming "pigs" connected to a long bar called the "Sow"
• Pig iron is about 93% pure iron, 3-5% carbon, and smaller amounts of silicon, phosphorus, sulfur, and manganese

Carbon Steel

• Adding 1% or less of carbon to iron produces carbon steel, which is vastly superior to plain iron
• An alloy is a base metal (like iron) with small amounts of other metals added to improve its properties
• Carbon steel is made by heating pig iron, wrought iron, and scrap metal in an open-hearth furnace
• The furnace uses hot air and gas to keep the iron liquid and carbon levels are adjusted before pouring the metal into ingots

Crucible Steel

• Crucible steel is high-carbon steel or alloy steel made by melting wrought iron and scrap steel in a crucible
• Carbon or other alloying elements are added to the crucible and melted with the iron
• The steel is then poured into ingot molds
• The electric arc furnace has largely replaced the crucible furnace for making high-quality steel

Electric Furnace Steel

• The electric furnace is used when precise temperature control and alloying element addition is crucial
• It achieves higher temperatures than other steel-making furnaces
• High-carbon steel, special alloy steel, and high-speed steel are produced in electric furnaces
• Electric furnaces allow for control of the steel's grain structure

Basic Oxygen Process

• The Basic Oxygen Process uses oxygen to quickly refine iron ore, steel scrap, and molten iron
• Oxygen is blown down from the top through a lance into the molten metal bath
• Burnt lime and other fluxing agents are added during the process to refine the steel

Annealing

• Annealing is a softening process that relieves internal stress in steel
• It brings the steel into its softest possible condition when cold
• Annealing refines the steel's grain structure
• Full annealing involves heating the steel above its upper critical temperature, soaking it, and cooling it slowly
• Process annealing is similar but involves lower temperatures

Normalizing

• Normalizing refines the steel's grain structure and relieves stress
• It is used before other heat treatments
• Normalizing involves heating the steel above its upper critical temperature, soaking it, and cooling it in still air

Hardening

• Hardening produces a fine grain structure and high hardness
• It involves heating the steel above its critical temperature, soaking it, and rapidly quenching it in a medium like brine, water, oil, or air

Case Hardening

• Case hardening involves producing a hard, wear-resistant surface while keeping the core tough
• Common methods of Case Hardening include carburizing, cyaniding and nitriding
• Carburizing involves soaking the metal in a carbonaceous material
• Cyaniding involves immersing the steel in a molten bath of cyanide salts
• Nitriding involves heating the steel in anhydrous ammonia to produce a hard surface

Tempering

• Tempering, also called drawing, relieves internal strain in hardened steel, increasing its toughness
• It involves heat treating hardened steel below its lower critical temperature, soaking it, and cooling it in air or a quenching medium
• Tempering prevents cracking or fracturing under stress

Uses of Different Heat Treated Materials

• Ferrous metals (iron and steel) are commonly heat treated
• Some aluminum alloys can be heat treated while others require cold working for hardening
• Heat treating high-temperature nickel-based alloys is done in some cases depending on composition
• The temperatures and process of heat treatment vary among different metals

Heat Treatment

• Heat treatment hardens or strengthens metals
• It also makes metals softer and more ductile
• Heat treatment types include hardening, tempering, annealing, normalizing, and case hardening

Anodic Treatment

• Anodic treatment is used to provide corrosion resistance and other properties to aluminum parts
• The three types of anodic treatment specified by MIL-A-8625R are Chromic Anodise Coating, Sulphuric Anodise Coating, and Hard Anodising Coating
• Chromic Anodise Coating is a dark-grey coating used for sealing and corrosion resistance
• Sulphuric Anodise Coating is best for dyeing, has a dull yellow-green appearance, and is used for sealing
• Hard Anodising Coating provides electrical insulation and abrasion resistance to parts like hydraulic cylinders and actuator cams

Cadmium Plating

• Cadmium plating is a non-porous, electrolytically deposited layer that resists corrosion on steel
• Type 1 is pure, silver-coloured cadmium plating
• Type 2 is cadmium plating followed by a chrome treatment for improved corrosion resistance
• Type 3 is cadmium plating followed by a phosphate treatment, primarily for paint base

This quiz explores the characteristics and applications of various types of cast iron, including grey, white, and pig iron, as well as carbon steel. You'll learn about their chemical compositions, production methods, and uses in industries. Test your knowledge on these essential materials in metallurgy.