Sources and References

Primary texts: Physical Metallurgy Principles by Abbaschian, Abbaschian, and Reed-Hill (Cengage); Steels: Microstructure and Properties by Bhadeshia and Honeycombe; Physical Metallurgy edited by Cahn and Haasen.

Supplementary texts: Metallurgy for the Non-Metallurgist by Krauss (ASM); Corrosion Engineering by Fontana; Principles of Heat Treatment by Higgins; Light Alloys: Metallurgy of the Light Metals by Polmear.

Online resources: ASM Handbook Volumes 1, 2, 4, 9, 11, 13; NACE corrosion resources; Granta Design EduPack; MIT OpenCourseWare 3.40J Physical Metallurgy.

Chapter 1: Metals and Alloy Systems

1.1 Metallic Bonding and Crystal Structure

Metals owe their ductility, electrical conductivity, and thermal conductivity to delocalised valence electrons in a periodic lattice. The dominant crystal structures in engineering metals are FCC (aluminium, copper, nickel, austenitic steel), BCC (ferritic steel, chromium, molybdenum), and HCP (magnesium, titanium, zinc).

1.2 Solid Solutions

Alloying modifies properties through substitutional or interstitial solid solutions. Hume-Rothery rules set the conditions for extensive substitutional solubility: similar atomic size, electronegativity, valence, and crystal structure. Interstitial atoms such as carbon and nitrogen occupy gaps between host atoms and strengthen iron at low concentrations.

1.3 Phase Diagrams

Binary and multicomponent phase diagrams map equilibrium phases as a function of composition and temperature. Critical features include solubility limits, eutectic and peritectic reactions, intermetallic compounds, and miscibility gaps. Thermodynamic software (Thermo-Calc, JMatPro) extends diagram interpretation to multicomponent commercial alloys.

Chapter 2: Iron-Carbon Alloys

2.1 The Fe-Fe\(_3\)C Diagram

Iron exhibits polymorphism: BCC ferrite, FCC austenite, and high-temperature delta ferrite. Carbon dissolves interstitially up to 0.022% in ferrite and up to 2.14% in austenite. The eutectoid reaction at 727°C transforms austenite to ferrite plus cementite, producing pearlite.

2.2 Steels

Plain carbon steels are classified by carbon content as low (< 0.25%), medium (0.25–0.6%), and high (> 0.6%). Alloy steels add Mn, Si, Cr, Ni, Mo, V, and others to improve hardenability, toughness, and special properties. Standard naming systems (SAE/AISI) encode composition; for example, SAE 4140 denotes a Cr-Mo steel with 0.4% C.

2.3 Cast Irons

Cast irons contain 2–4% C and typically 1–3% Si. Graphite morphology—flake (grey iron), nodular (ductile iron), temper (malleable iron), or compacted (CGI)—controls mechanical behaviour. White cast iron retains all carbon as cementite, yielding high hardness with low toughness.

Chapter 3: Heat Treatment

3.1 Annealing Processes

Full annealing, normalising, stress-relief annealing, and spheroidising modify microstructure for specific purposes: softening, grain refinement, stress reduction, or improved machinability. Each process is defined by heating above or below transformation temperatures followed by controlled cooling.

3.2 Quenching and Tempering

Quenching austenite to room temperature produces martensite, a supersaturated BCT solid solution of carbon. Hardness rises with carbon content. Tempering at 150–650°C precipitates carbides, trading hardness for toughness. The Hollomon–Jaffe parameter

correlates tempered hardness across temperatures and times.

3.3 Surface Hardening

Carburising, nitriding, carbonitriding, and induction hardening produce hard cases over tough cores. Case depth depends on process temperature, time, and diffusivity. Shot-peening augments these with compressive residual stresses beneficial for fatigue.

Chapter 4: Function of Alloying Elements

4.1 Effects on Transformation

Alloying shifts TTT and CCT curves, modifying hardenability. Mn, Cr, Ni, and Mo raise hardenability; V and Ti pin austenite grains and refine microstructure. Boron at 5–30 ppm dramatically increases hardenability at minimal cost.

4.2 Effects on Properties

Alloying tunes strength, toughness, hardness, weldability, and corrosion resistance. Ni promotes toughness at low temperature; Cr provides oxidation and corrosion resistance; Si improves strength; Al and Ti form age-hardening precipitates; S and Pb enhance machinability.

4.3 Microalloyed Steels

Microalloyed (HSLA) steels add small amounts (< 0.1%) of Nb, V, Ti, or Al to produce fine-grained ferritic-pearlitic or bainitic structures with high strength at moderate cost. Controlled rolling synergises chemistry and thermomechanical processing.

Chapter 5: Stainless, Tool, and High-Temperature Steels

5.1 Stainless Steels

Minimum 10.5% Cr forms a passive Cr\(_2\)O\(_3\) film that resists corrosion. Austenitic (304, 316), ferritic (430), martensitic (410), duplex, and precipitation-hardening grades offer distinct combinations of strength, toughness, and corrosion resistance. Sensitisation during welding can cause intergranular corrosion; stabilised (Ti, Nb) or low-carbon grades mitigate this.

5.2 Tool Steels

Tool steels combine hardness, wear resistance, and hot hardness for cutting, forming, and die applications. Classes include water-hardening, oil-hardening, air-hardening, high-speed, and hot-work. Carbides of W, Mo, V, and Cr provide wear resistance; Co improves hot hardness.

5.3 Creep-Resistant Alloys

At elevated temperatures, creep dominates life. Ferritic-martensitic steels (9Cr-1Mo) serve to about 600°C. Austenitic stainless and nickel-based superalloys (Inconel, Waspaloy, single-crystal CMSX) extend service to 700–1100°C, with gamma-prime \( \gamma' \) and gamma-double-prime \( \gamma'' \) precipitates sustaining strength.

Chapter 6: Non-Ferrous Alloys

6.1 Copper and Nickel Base

Copper alloys include brasses (Cu-Zn), bronzes (Cu-Sn, Cu-Al, Cu-Be), and cupronickels (Cu-Ni). Applications span electrical, marine, architectural, and springs. Nickel-base alloys (Monel, Inconel, Hastelloy) provide exceptional corrosion resistance in severe chemical environments and high-temperature strength in gas turbines.

6.2 Light Metals

Aluminium alloys are classified by principal alloying addition: 1xxx (pure Al), 2xxx (Cu), 3xxx (Mn), 5xxx (Mg), 6xxx (Mg-Si), 7xxx (Zn-Mg-Cu). Age-hardening in 2xxx, 6xxx, and 7xxx series produces high strength-to-weight ratios. Magnesium alloys offer lower density at the cost of flammability during machining and reduced strength. Titanium alloys (Ti-6Al-4V) combine strength, corrosion resistance, and biocompatibility.

6.3 Casting and Wrought Forms

Casting alloys contain silicon or other elements to improve fluidity; wrought alloys balance workability with strength. Designations encode composition and temper; for aluminium, T6 indicates solution-treated and artificially aged.

Chapter 7: Metal Processing

7.1 Casting

Sand, investment, die, and continuous casting each shape metal from the liquid state. Solidification structure—columnar, equiaxed, or refined by inoculation—influences properties. Porosity, shrinkage, and segregation require careful riser and gating design.

7.2 Hot and Cold Working

Hot working above recrystallisation temperature enables large strains and produces wrought structures with beneficial grain flow. Cold working increases strength and improves surface finish at the cost of ductility. Process selection considers geometry, quantity, and target properties.

7.3 Joining

Soldering uses filler metals melting below 450°C; brazing, above. Welding processes (SMAW, GMAW, GTAW, SAW, resistance, electron beam, laser) produce fusion or solid-state bonds. Heat-affected zones may be weaker, harder, or more brittle than the base metal; weld procedure specifications define parameters ensuring code-compliant joints.

Chapter 8: Corrosion and Failure Analysis

8.1 Electrochemical Corrosion

Corrosion proceeds by coupled anodic (metal oxidation) and cathodic (typically oxygen reduction or hydrogen evolution) reactions. The rate is set by the polarisation of both half-reactions; Butler–Volmer and Tafel relations describe the kinetics. Galvanic, pitting, crevice, intergranular, selective leaching, erosion, and stress-corrosion cracking are common morphologies.

8.2 Oxidation and High-Temperature Attack

Dry oxidation follows logarithmic, parabolic, or linear rate laws. Parabolic kinetics \( x^2 = k_p t \) indicate a protective, diffusion-controlled scale. Alloying with Cr, Al, or Si forms protective oxides that resist high-temperature attack, as used in turbine components.

8.3 Failure Analysis

A failure investigation proceeds from visual examination to fractography, metallography, chemical analysis, and mechanical testing. Fatigue, brittle fracture, ductile overload, creep, corrosion, and wear produce characteristic surface features. The outcome informs redesign, material selection, operational changes, or inspection strategy, closing the loop from service to metallurgy to improved practice.