Metallurgy Study Guide

📖 Core Concepts Metallurgy – the science + technology of metals: how metallic elements, intermetallics, and alloys behave chemically & physically, and how we shape them into useful parts. Metallurgist – a specialist who applies physics, chemistry, and engineering to extract, process, and design metal components. Subdisciplines Chemical metallurgy: reduction/oxidation reactions, extraction, thermodynamics, electrochemistry, corrosion. Physical metallurgy: crystal structures, phase transformations, mechanical behavior, failure mechanisms. Ferrous vs. Non‑Ferrous – iron‑based (steel, cast iron) vs. all other metals (Al, Cu, Ti, Ni alloys). Ferrous metals ≈ 95 % of world production. Key Alloy Families Iron‑Carbon: plain carbon steel, cast iron, Hadfield (Mn‑Cr) steel. Stainless: austenitic grades → high corrosion resistance. Lightweight: Al‑Cu, Mg alloys – automotive/aerospace. Superalloys: Ni‑based (Inconel) – retain strength at > 800 °C. Metalworking Categories – casting, forging, rolling, extrusion, machining, sintering, additive manufacturing, cold‑working. Heat‑Treatment Basics – anneal (soften), quench (hard martensite), temper (reduce brittleness). Surface Modification – electroplating, electroless deposition, shot peening, thermal spraying. Characterization – metallography, X‑ray/electron diffraction, SEM, TEM, EBSD, atom‑probe tomography. --- 📌 Must Remember Ferrous dominance: 95 % of global metal output is iron‑based. Iron‑Carbon system: carbon is the only intentional alloying element in plain carbon steels; > 2 % C → cast iron. Quench‑tempering cycle: rapid cooling → hard martensite → reheating (tempering) → tougher, slightly softer metal. Shot peening effect: introduces compressive surface stresses → improves fatigue & stress‑corrosion resistance. Electroplating vs. Electroless – electroplating needs an external current; electroless works on non‑conductors via chemical reduction. Casting variants: sand (low cost, rough), investment (high accuracy, complex), die (high volume, thin walls), centrifugal (high‑density parts). Superalloys: Ni‑based alloys maintain strength at > 800 °C; used in turbines & pressure vessels. Surface‑engineered coatings: thermal spray > electroplating for high‑temperature service. Characterization hierarchy: metallography (macro/microstructure) → diffraction (phase ID) → SEM/TEM (morphology & composition) → EBSD/APT (crystallography & atomic composition). --- 🔄 Key Processes Extractive Metallurgy (Metal from Ore) Physical reduction: crush → concentrate → melt. Chemical reduction: add carbon or CO to metal oxides (e.g., Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂). Electrolytic reduction: pass current through molten/aqueous salts (e.g., Al³⁺ → Al). Leaching: dissolve ore in acid/base → filter → recover metal from solution. Casting Workflow Melt metal → pour into mold → solidify → remove casting → clean/finish. Forging Sequence Heat billet → place in die → hammer/press → cool → (optional) reheating for multiple passes. Heat‑Treatment Cycle Anneal: heat → hold above transformation temp → slow cool. Quench: heat → rapid water/oil/oil‑air quench → martensite. Temper: reheat to 150‑650 °C → hold → cool → reduced brittleness. Shot Peening Accelerate steel shot → impact surface → create overlapping dimples → compressive residual stress field. Electroless Deposition Immerse substrate in metal‑salt bath + reducing agent → autocatalytic metal layer grows uniformly. --- 🔍 Key Comparisons Chemical vs. Physical Metallurgy – Chemical: reactions (reduction, corrosion). Physical: structure & mechanical properties. Ferrous vs. Non‑Ferrous Alloys – Ferrous: iron base, magnetic (except austenitic stainless), high strength. Non‑Ferrous: lighter, non‑magnetic, often superior corrosion resistance. Annealing vs. Quenching – Anneal: slow cooling → soft, ductile. Quench: rapid cooling → hard, brittle martensite. Electroplating vs. Electroless Deposition – Electroplating: requires current, works best on conductive surfaces. Electroless: chemical reduction, coats non‑conductors. Casting vs. Additive Manufacturing – Casting: molten metal fills mold; limited geometry complexity. 3‑D printing: layer‑by‑layer powder sintering/melting; high design freedom, slower for large parts. --- ⚠️ Common Misunderstandings “All steel is magnetic.” → Austenitic stainless steels are austenite (γ‑Fe) and are non‑magnetic. “Quenching always makes metal stronger.” – It increases hardness but also brittleness; tempering is needed for toughness. “Higher carbon = better steel.” – Excess carbon (> 2 %) makes cast iron, not ductile steel. “Shot peening weakens the surface.” – It actually introduces compressive stresses that improve fatigue life. “Electroplating works on plastics.” – Plastics must first be made conductive (e.g., via a primer) or use electroless deposition. --- 🧠 Mental Models / Intuition “Heat‑treat as a traffic light.” – Red (heat) → Yellow (hold) → Green (cool). The speed of cooling decides the “stop” (martensite) or “go” (ferrite/pearlite) microstructures. “Extraction is a three‑step ladder.” – Lift (crush/grade) → Separate (leach/reduce) → Collect (refine). “Surface coatings are a shield hierarchy.” – Thin electroless → medium electroplated → thick thermal spray = increasing protection against temperature & wear. --- 🚩 Exceptions & Edge Cases Hadfield steel – high Mn & Cr; remains non‑magnetic and work‑hardens dramatically under impact. Titanium alloys – cannot be water‑quenched; they oxidize quickly, so inert gas or oil quenching is used. Aluminum casting – prone to hot‑tearing; requires rapid solidification (die casting) or controlled cooling (investment casting). Shot peening depth – limited to 0.5 mm; deeper defects need other methods (e.g., deep rolling). --- 📍 When to Use Which Choose alloy → High strength, low cost: plain carbon steel. Corrosion‑critical: austenitic stainless. Lightweight, high‑strength: Al‑Cu or Mg alloy. > 800 °C service: Ni‑based superalloy. Select forming process → Complex internal cavities: investment casting. High‑volume thin walls: die casting. Large structural parts: forging or rolling. Custom geometry, low volume: additive manufacturing. Pick surface treatment → Mild corrosion, decorative: electroplating. Non‑conductive substrate or uniform coating: electroless deposition. High‑temperature wear: thermal spray. Fatigue‑critical component: shot peening. --- 👀 Patterns to Recognize “High carbon → cast iron” – any Fe‑C composition > 2 % C signals cast iron, not steel. “Rapid cooling + high‑hardness = martensite” – look for quench steps followed by high hardness values. “Compressively stressed surface = shot peening” – dimples in micro‑fracture surface microscopy indicate shot peening. “Layered coating = additive process” – alternating metal/ceramic layers in microscopy suggest thermal spray or 3‑D printing. “Oxidation‑resistant alloy = Ni‑based or Cr‑rich” – presence of Ni/Cr in composition points to superalloy or stainless. --- 🗂️ Exam Traps Distractor: “All stainless steels are magnetic.” – Only ferritic and martensitic grades are; austenitic are not. Distractor: “Quenching always improves ductility.” – It actually reduces ductility; only tempering restores it. Distractor: “Electroplating can be applied to ceramics directly.” – Requires a conductive seed layer or electroless pre‑coat. Distractor: “Higher alloying element = higher strength for any alloy.” – Excessive alloying can cause brittleness or processing difficulties (e.g., too much carbon → cast iron). Distractor: “All casting defects are due to mold design.” – Many arise from improper pouring temperature, turbulence, or solidification shrinkage, not just mold shape. ---