Exhaustive Review of Materials Science: Engineering Alloys, Iron-Carbon Systems, Heat Treatment, and Corrosion

Introduction to Engineering Alloys and Gear Manufacturing

• Variety of Metal Alloys Used:

  • Plain-carbon steels.

  • Alloy steels.

  • Stainless steels.

  • Cast iron.

  • Copper alloys.

• Specific Industrial Applications for Gears:

  • Chromium Steels: Utilized for automobile transmission gears.

  • Chromium Molybdenum Steels: Utilized for aircraft gas turbine gears.

  • Nickel-Molybdenum Steels: Utilized for earthmoving equipment gears.

  • Copper Alloys: Utilized for manufacturing gears operating at low load levels.

• Selection Criteria for Gear Metals:

  • Size of the component.

  • Stresses involved during operation.

  • Power requirements.

  • Operating environment.

Fundamental Distinctions Between Iron and Steel

• Definition of Iron:

  • The name given to pure ferrite ($Fe$).

  • Also refers to mixtures of ferrite with large amounts of carbon, such as pig iron and cast iron.

• Pig Iron: Produced from iron ore in a blast furnace; serves as the basic raw material for cast iron, wrought iron, and steel.

• Definition of Steel: An alloy of iron and carbon where the carbon content varies from $0\,$ to $2\,$.

• Carbon Content and Material Properties:

  • As carbon percentage increases: Brittleness and hardness increase; ductility and malleability decrease.

  • As carbon percentage decreases: Brittleness and hardness decrease; ductility and malleability increase.

• General Production Statistics: Iron and its alloys (mainly steel) account for approximately $90\,$ of total world metal production due to high strength, toughness, and ductility at a relatively low cost.

Manufacture of Iron and Steel

• Production of Pig Iron (Blast Furnace):

  • Iron is extracted from ores (mainly $Fe_2O_3$) in large blast furnaces.

  • Reducing Agent: Coke (carbon) reduces iron oxides to raw pig iron.

  • Pig Iron Composition: Contains approximately $4\,$ carbon plus impurities.

  • Typical Chemical Reaction: $Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$.

• Steelmaking Process (Basic Oxygen Process):

  • Objective: Oxidize carbon and impurities in pig iron until carbon reaches the required level (usually less than $0.5\,$, maximum $1.2\,$ for plain-carbon steels).

  • Input Material: Pig iron plus up to $30\,$ steel scrap.

  • Converter: Barrel-shaped refractory-lined vessel with an oxygen lance.

  • Reaction: Pure oxygen reacts with the liquid bath to form iron oxide. Carbon then reacts with the iron oxide: $FeO + C \rightarrow Fe + CO$.

  • Refining: Slag-forming fluxes (chiefly lime) are added. Impurities like sulfur and phosphorus are reduced in about $22\,\text{min}$.

• Product Forms and Processing:

  • Continuous Casting: Approximately $96\,$ of steel is cast continuously into long slabs.

  • Ingot Casting: About $4000$ ingots are still cast individually.

  • Recycling: About one-half of raw steel comes from recycled sources (cars, appliances).

  • Processing Steps:

    • Ingots: Heated in a soaking pit and hot-rolled into slabs, billets, or blooms.

    • Slabs: Hot- and cold-rolled into steel sheets and plates.

    • Billets: Hot- and cold-rolled into bars, rods, and wire.

    • Blooms: Hot- and cold-rolled into shapes like I-beams and rails.

The Iron-Iron-Carbide ($Fe-Fe_3C$) Phase Diagram

• Plain-Carbon Steel Definition: Alloys of iron and carbon with $0.03\,$ to $1.2\,$ carbon, $0.25\,$ to $1.00\,$ manganese, and minor amounts of other elements.

• The Phase Diagram Nature: It is not a true equilibrium diagram because iron carbide ($Fe_3C$) is metastable and can decompose into iron and graphite. However, it is treated as an equilibrium phase for practical conditions.

• Solid Phases in the System:

  • $\alpha$ Ferrite: Interstitial solid solution of carbon in BCC iron. Maximum solubility: $0.022\,\%$ at $727\,^{\circ}\text{C}$. Solubility at $0\,^{\circ}\text{C}$ is $0.005\,\%$.

  • Austenite ($\gamma$): Interstitial solid solution of carbon in FCC iron. Maximum solubility: $2.11\,\%$ at $1147\,^{\circ}\text{C}$. Solubility at $727\,^{\circ}\text{C}$ is $0.77\,\%$.

  • Cementite ($Fe_3C$): Intermetallic compound with fixed composition ($6.67\,\%\,\text{C}$, $93.3\,\%\,\text{Fe}$). Extremely hard and brittle.

  • $\delta$ Ferrite: Interstitial solid solution of carbon in BCC iron with a greater lattice constant than $\alpha$. Maximum solubility: $0.09\,\%$ at $1465\,^{\circ}\text{C}$.

• Invariant Reactions:

  • Peritectic Reaction ($1495\,^{\circ}\text{C}$): $\text{Liquid} (0.53\,\%\,\text{C}) + \delta (0.09\,\%\,\text{C}) \rightarrow \gamma (0.17\,\%\,\text{C})$.

  • Eutectic Reaction ($1147\,^{\circ}\text{C}$): $\text{Liquid} (4.3\,\%\,\text{C}) \rightarrow \gamma \text{ austenite} (2.11\,\%\,\text{C}) + Fe_3C (6.67\,\%\,\text{C})$.

  • Eutectoid Reaction ($727\,^{\circ}\text{C}$): $\gamma \text{ austenite} (0.77\,\%\,\text{C}) \rightarrow \alpha \text{ ferrite} (0.022\,\%\,\text{C}) + Fe_3C (6.67\,\%\,\text{C})$.

Slow Cooling of Plain-Carbon Steels

• Eutectoid Steels (0.77% C):

  • Austenitizing: Heating to approx. $750\,^{\circ}\text{C}$ to form homogeneous austenite.

  • Transformation: Cooling below $727\,^{\circ}\text{C}$ transforms austenite into Pearlite, a lamellar structure of alternate plates of $\alpha$ ferrite and cementite.

• Hypoeutectoid Steels (< 0.77% C):

  • Example ($0.4\,\%\,\text{C}$): Between high temperature and $727\,^{\circ}\text{C}$, proeutectoid ferrite nucleates at austenite grain boundaries.

  • At $727\,^{\circ}\text{C}$, the remaining austenite (now at $0.77\,\%\,\text{C}$) transforms into pearlite.

  • Final structure: Proeutectoid ferrite (white) + pearlite (dark).

• Hypereutectoid Steels (> 0.77% C):

  • Example ($1.2\,\%\,\text{C}$): Below the $\text{A}_{cm}$ line, proeutectoid cementite forms at austenite grain boundaries.

  • At $727\,^{\circ}\text{C}$, remaining austenite transforms to pearlite.

  • Final structure: Proeutectoid cementite (white network) + pearlite.

Other Forms of Iron and Steel

• Wrought Iron:

  • Purest form of iron (up to $99.9\,\%$).

  • Production: Melting white cast iron and passing an oxidizing flame over it.

  • Properties: Melting point $1485\,^{\circ}\text{C}$, excellent corrosion resistance, high ductility/malleability, cannot be cast (only forged).

• Cast Iron ($2\,$ to $4.5\,$):

  • Contains Silicon ($Si$), Sulfur ($S$), Phosphorus ($P$), and Manganese ($Mn$).

  • Grey Cast Iron: Carbon as graphite flakes; excellent damping capacity and machinability.

  • White Cast Iron (Chilled): No graphite; hard, brittle, wear-resistant; used for railway brake blocks.

  • Mottled Cast Iron: Mixture of grey and white cast iron.

  • Malleable Cast Iron: Produced by annealing white cast iron; cementite decomposes to ferrite.

  • Nodular (Ductile) Cast Iron: Addition of magnesium makes graphite form nodules instead of flakes; high ductility.

• Steel Classification by Carbon Content:

  • Dead Mild Steel: up to $0.10\,$.

  • Low Carbon Steel (Mild Steel): $0.10\,$ to $0.30\,$.

  • Medium Carbon Steel: $0.30\,$ to $0.70\,$.

  • High Carbon Steel: $0.70\,$ to $1.5\,$.

Alloy and Stainless Steels

• Common Alloying Elements and Effects:

  • Nickel ($Ni$): Increases toughness, strength, and corrosion resistance.

  • Chromium ($Cr$): Improves corrosion resistance (stainless property), toughness, and hardenability.

  • Manganese ($Mn$): Improves strength/hardness but decreases weldability.

  • Molybdenum ($Mo$): Increases high-temperature strength.

  • Tungsten ($W$): Increases hardness and wear resistance.

• Stainless Steel Types:

  • Ferritic: $18\,$ to $30\,$; high corrosion resistance, used in food plants.

  • Martensitic: $12\,$ to $17\,$; can be heat-treated for high strength (surgical knives).

  • Austenitic: $16\,$ to $26\,$, $6\,$ to $22\,$; non-magnetic, excellent corrosion resistance (aircraft industry).

  • Precipitation Hardening (PH): High strength at maximum temperatures.

  • Duplex: Mixture of austenite and ferrite; used in heat exchangers.

Non-ferrous Metals and Alloys

• Copper ($Cu$):

  • Extraction: Heated to remove $H_2O$, $CO_2$, and $S$, then melted and refined by electrolysis.

  • Brass: Alloy of $Cu$ and $Zn$. Alpha brass (<36\,\%\,\text{Zn}) for cold working; Alpha-beta brass (>36\,\%\,\text{Zn}) for hot working.

  • Bronze: Alloy of $Cu$ and $Sn$ ($5\,$ to $25\,$). Types include Beryllium, Phosphor, Aluminium, and Manganese bronze.

• Aluminium ($Al$):

  • Ore: Bauxite ($Al_2O_3 \cdot nH_2O$).

  • Properties: Melting point $660\,^{\circ}\text{C}$, non-magnetic, high conductivity, corrosion-resistant.

  • Alloys:

    • Duralumin: $3.5\,$ to $4.5\,\%\,\text{Cu}$, $0.4\,\%$ to $0.8\,\%\,\text{Mg}$; used for aircraft bodies.

    • Y-alloy: $4\,\%\,\text{Cu}$, $2\,\%\,\text{Ni}$, $1\,\%\,\text{Mg}$; used for engine pistons.

    • Magnalium: $1\,\%$ to $6\,\%\,\text{Mg}$; very light weight.

• Zinc ($Zn$): Melting point approx. $480\,^{\circ}\text{C}$; used for die casting and galvanizing.

• Tin ($Sn$): Melting point $230\,^{\circ}\text{C}$; non-toxic, used for food packaging foils.

• Nickel ($Ni$): Melting point $1455\,^{\circ}\text{C}$; extracted from sulfide ores.

• Lead ($Pb$): Melting point $326\,^{\circ}\text{C}$; high density, used in batteries and bearings.

Heat Treatment of Materials

• Definition: Controlled heating and cooling in the solid state to change physical/mechanical properties.

• Stages: 1. Heating to temperature, 2. Soaking (holding), 3. Cooling at a specific rate.

• Primary Processes:

  • Annealing: Done to soften metal. Includes Full annealing (above critical temp), Process annealing (below lower critical), and Spherodise annealing (converting cementite to spheres for high-carbon steel).

  • Normalizing: Heating to $40\,^{\circ}\text{C}$-$50\,^{\circ}\text{C}$ above upper critical temp followed by air cooling to refine grain size.

  • Hardening: Quenching from above critical temp into water/oil to form martensite.

  • Tempering: Reheating hardened steel below lower critical temp to improve toughness and reduce brittleness.

  • Isothermal Treatments: Austempering (forming tough structure from molten salt quench) and Martempering (stepped quenching to reduce distortion).

• Surface and Case Hardening:

  • Induction Hardening: High-frequency current heats the surface, followed by water spray.

  • Flame Hardening: Oxyacetylene flame used for localized heating.

  • Carburizing: Pack (solid), Liquid, or Gas methods for increasing surface carbon.

  • Nitriding: Heating in ammonia ($NH_3$) at $650\,^{\circ}\text{C}$ to form hard nitrides.

  • Cyaniding: Simultaneous absorption of carbon and nitrogen in a sodium cyanide bath.

Corrosion of Metals

• General Definition: Destructive attack of a metal by chemical or electrochemical reaction with its environment.

• Factors Governing Corrosion: Metal type, surface treatment, and environment.

• Types of Corrosion Damage:

  • General/Uniform Attack: Reported in $mm/y$ or $g/m^2/day$.

  • Pitting: Localized attack; the pitting factor is the ratio of deepest penetration to average penetration.

  • Dealloying (Dezincification): Selective removal of one element (e.g., zinc from brass).

  • Intergranular Corrosion: Rapid attack at grain boundaries; "weld decay" in stainless steels.

  • Cracking: Stress-corrosion cracking (static stress) and corrosion fatigue (cyclic stress).

• Cavitation-Erosion: Loss of material due to collapse of vapor bubbles at a metal-liquid interface.

• Prevention Methods:

  • Inherent Resistance: Using alloys that form solid solutions.

  • Metallic Coatings: "Direct" protection (e.g., tin on steel) or "Sacrificial" protection (e.g., zinc on iron).

  • Cathodic Protection: Making the structure a cathode by using sacrificial anodes or impressed D.C. current.

  • Surface Treatments: Phosphating, chromating, or blueing.

Numerical Problems and Formulae

• Lever Rule for Phase Weight Percentages:

  • $W_{\alpha} = \frac{C_{Fe_3C} - C_{0}}{C_{Fe_3C} - C_{\alpha}} \times 100$

  • $W_{Fe_3C} = \frac{C_{0} - C_{\alpha}}{C_{Fe_3C} - C_{\alpha}} \times 100$

• Corrosion Rate Conversions: Knowledge of metal density is required to convert from weight loss ($gmd$) to penetration ($mm/y$).

  • Excellent resistance classification: < 0.15\,\text{mm/y}.

  • Satisfactory classification: $0.15\,\text{to}\,1.5\,\text{mm/y}$.

  • Unsatisfactory classification: > 1.5\,\text{mm/y}.