Steel Study Guide

📖 Core Concepts Steel – an iron‑carbon alloy (0.02 %–2.14 % C) whose mechanical properties exceed those of pure iron. Ferrite (α‑Fe) – body‑centred cubic (BCC) iron stable at ≤ 910 °C; dissolves very little C (≤ 0.021 % at 723 °C). Austenite (γ‑Fe) – face‑centred cubic (FCC) iron stable > 910 °C; can hold up to 2.1 % C at 1,148 °C. Cementite (Fe₃C) – hard, brittle iron‑carbide that forms when carbon exceeds solubility in ferrite/austenite. Pearlite – lamellar mixture of ferrite + cementite that forms at the eutectoid composition (≈ 0.8 % C). Martensite – supersaturated, highly strained BCC (≤ 0.2 % C) or BCT (higher C) phase created by rapid quenching of austenite. Alloying Elements – Mn, Ni, Cr, Mo, etc., added to tailor hardenability, corrosion resistance, high‑temp strength. Heat‑Treatment Steps – annealing (stress relief), quenching (martensite formation), tempering (reduce brittleness). 📌 Must Remember Carbon range for steel: 0.02 %–2.14 % (plain carbon). > 2.1 % → cast iron. Eutectoid point: 0.8 % C → ferrite + cementite → pearlite. Martensite crystal: BCC if C ≤ 0.2 %; BCT if C > 0.2 %. Ferrite carbon solubility: 0.005 % at 0 °C, 0.021 % at 723 °C. Austenite carbon solubility: up to 2.1 % at 1,148 °C. Trade‑off: More C or alloying → higher strength, lower ductility. Key production methods: Basic Oxygen Steelmaking (BOS) and Electric Arc Furnace (EAF) recycling. 🔄 Key Processes Smelting & Refinement Reduce iron ore with CO → pig iron (high C). Blow O₂ through melt → oxidize excess C & impurities → desired steel composition. Basic Oxygen Steelmaking (BOS) Inject pure O₂ into molten iron → rapid oxidation, impurity removal, shorter cycle than open‑hearth. Electric Arc Furnace (EAF) Melt scrap steel with electric arcs → efficient recycling, low CO₂ per tonne. Heat Treatment (Anneal → Quench → Temper) Anneal: Heat → hold above transformation temp → slow cool → relieve stresses. Quench: Heat austenite → rapid water/oil quench → lock C → martensite. Temper: Re‑heat martensite to sub‑critical temperature → precipitate fine carbides → reduce brittleness. 🔍 Key Comparisons Ferrite vs. Austenite BCC vs. FCC crystal lattice. Low vs. high carbon solubility. Stable at low vs. high temperature. Martensite (≤0.2 % C) vs. Martensite (>0.2 % C) BCC lattice vs. body‑centred tetragonal (BCT). Lower vs. higher tetragonal distortion → greater hardness but more residual stress. BOS vs. EAF Primary iron feedstock vs. scrap. Oxygen blowing vs. electric arcs. Faster production vs. higher recycling rate. ⚠️ Common Misunderstandings “All steel is stainless.” – Only steels with ≥ 11 % Cr are stainless; most structural steels lack sufficient Cr. “Higher carbon always means stronger steel.” – Beyond 0.8 % C, strength rises but ductility falls dramatically; heat‑treatment is needed to exploit hardness. “Quenching always yields the hardest steel.” – Without subsequent tempering, martensite is excessively brittle and may crack under service loads. 🧠 Mental Models / Intuition “Carbon as a traffic jam”: Ferrite is a wide‑lane road (low C capacity). Austenite is a multi‑lane highway (high C capacity). When you freeze traffic (quench), cars (C atoms) get stuck in place → martensite lattice distortion. “Phase diagram as a temperature‑C map”: Move horizontally (change C) → shift eutectoid point; move vertically (temp) → cross ferrite ↔ austenite boundaries. 🚩 Exceptions & Edge Cases Very low‑C steels (<0.02 %): Behave more like pure iron; limited hardenability. High‑alloy tool steels: Even with low C, added W, Mo, V produce carbides that give extreme hardness despite low overall carbon. Maraging steels: Strength derives from precipitation of intermetallics, not carbon‑based martensite; they contain 0.01 % C. 📍 When to Use Which Select a production route: BOS → when you have abundant molten iron and need large, uniform billets. EAF → when scrap availability is high and you aim for lower CO₂ footprint. Choose alloy class: Carbon steel → cost‑sensitive, general construction. Low‑alloy / HSLA → need higher strength without large weight increase. Dual‑phase, TRIP, TWIP → automotive panels demanding high energy absorption. Stainless → corrosion‑critical environments (≥ 11 % Cr). Tool / Maraging → cutting tools, aerospace parts needing extreme hardness at elevated temperatures. 👀 Patterns to Recognize Cooling rate ↔ microstructure: Slow → coarse pearlite; moderate → fine pearlite/bainite; fast → martensite. Carbon content ↔ phase presence: ≤ 0.02 % → mostly ferrite; ≈ 0.8 % → eutectoid pearlite; > 0.8 % → excess cementite + pearlite. Alloying element effect: Cr ↑ → corrosion resistance; Mo ↑ → hardenability; Ni ↑ → toughness at low temps. 🗂️ Exam Traps “> 2.1 % C = steel” – Actually > 2.1 % defines cast iron, not steel. “All martensite is BCC” – Only low‑C martensite is BCC; higher C gives BCT. “Quenching always increases hardness without side effects” – Ignores tensile stresses in surrounding ferrite and the need for tempering to avoid brittleness. “Stainless steel must contain nickel” – Minimum 11 % Cr suffices; Ni is common but not mandatory. “Higher cooling rate always yields finer pearlite” – At very high rates you skip pearlite formation altogether and get martensite.