Materials Technology (ME-303) - Unit 4 Short Notes

UNIT 4: MATERIALS TECHNOLOGY (Based on Past Exam Analysis)

0.0 Introduction to Engineering Materials

Engineering Materials are substances used in construction or manufacturing to create structures, devices, or products, selected based on required properties and cost.

Classification based on properties & applications:

[!TIP] Exam Focus: Be prepared to classify a given material (e.g., concrete = ceramic composite, polyethylene = polymer) and justify based on bonding and properties.

1.0 Atomic Structure and Bonding

Primary Bond Types & Their Influence:

[!TIP] Common Pitfall: Do not confuse dispersion forces (weak, universal) with dipole-dipole (stronger, requires permanent dipoles). Hydrogen bonding is a special strong dipole-dipole.

2.0 Crystal Structures and Imperfections

Common Crystal Structures & Coordination Number (CN):

• Simple Cubic (SC): CN = 6, APF = 0.52 (Rare in metals)

• Body-Centered Cubic (BCC): CN = 8, APF = 0.68 (e.g., α-Fe, Cr, W)

• Face-Centered Cubic (FCC): CN = 12, APF = 0.74 (e.g., γ-Fe, Al, Cu, Ni, Ag, Au)

• Hexagonal Close-Packed (HCP): CN = 12, APF = 0.74 (e.g., Mg, Zn, Ti, Be)

Atomic Packing Factor (APF) for FCC:

$$APF = \frac{\text{Volume of atoms in unit cell}}{\text{Volume of unit cell}}$$

For FCC: 4 atoms/cell. Edge length \( a = 2\sqrt{2}R \).

$$\boxed{APF_{FCC} = \frac{4 \cdot \frac{4}{3}\pi R^3}{(2\sqrt{2}R)^3} = 0.74}$$

Miller Indices (hkl):

1. Determine intercepts of plane/ direction with axes in terms of lattice parameters \(a, b, c\).

2. Take reciprocals.

3. Clear fractions to smallest integers.

4. Enclose in parentheses ( ) for planes, square brackets [ ] for directions.

Example: Plane intercepts at \(a, \infty, \infty\) → (1 0 0).

Crystal Imperfections:

• Point Defects: Vacancies, interstitials, substitutional/impurity atoms.

• Line Defects (Dislocations): Edge & screw dislocations. Primary carriers of plastic deformation.

• Surface Defects: Grain boundaries, twin boundaries, stacking faults.

• Volume Defects: Pores, cracks, inclusions.

[!TIP] Exam Key: Dislocations are line defects. Vacancies are point defects. Grain boundaries are surface defects. Know how each affects mechanical properties (e.g., grain boundaries strengthen via Hall-Petch).

3.0 Mechanical Properties and Behavior

Engineering vs. True Stress-Strain:

• Engineering: Uses original cross-sectional area \(A_0\) and length \(L_0\).

$$\sigma_{eng} = \frac{F}{A_0}, \quad \epsilon_{eng} = \frac{\Delta L}{L_0}$$

• True: Uses instantaneous area \(A_i\) and length \(L_i\).

$$\sigma_{true} = \frac{F}{A_i}, \quad \epsilon_{true} = \ln\left(\frac{L_i}{L_0}\right)$$

True stress > engineering stress after yielding; true strain > engineering strain.

Stress-Strain Curve Key Points (for mild steel):

DiagramCANVAS: Sketch a typical stress-strain curve for mild steel. Label: Proportional Limit (P), Elastic Limit (E), Yield Point (Upper/Y_u and Lower/Y_l), Ultimate Tensile Strength (UTS), Fracture Point (F). Show linear elastic region, yield plateau, strain hardening, necking region.

• Proportional Limit: Stress ∝ strain (Hooke's Law valid).

• Elastic Limit: Maximum stress with full recovery upon unloading.

• Yield Point: Stress at which plastic deformation begins (distinct upper/lower for low-carbon steel).

• UTS: Maximum engineering stress.

• Fracture Point: Failure.

Ductility & Malleability:

• Ductility: Ability to undergo significant plastic deformation in tension. Measured by % elongation and % reduction in area.

$$\%RA = \frac{A_0 - A_f}{A_0} \times 100$$

• Malleability: Ability to undergo plastic deformation in compression (hammering/rolling into sheets).

Hardness:

• Definition: Resistance to localized plastic deformation (indentation, scratching).

• Measurement:

  • Brinell (HB): Hardened steel/ tungsten carbide ball indenter. Good for coarse structures.

  • Rockwell (HR): Depth of penetration. Different scales (C for steels, B for softer metals). Quick, no prep.

  • Vickers (HV): Square-based diamond pyramid. Micro to macro range, single scale.

Creep:

Time-dependent deformation under constant load at elevated temperature.

DiagramCANVAS: Sketch a typical creep curve (strain vs. time). Label three stages: 1) Primary (decelerating), 2) Secondary (steady-state, minimum creep rate), 3) Tertiary (accelerating to rupture).

• Stage 1 (Primary): Strain rate decreases due to work hardening.

• Stage 2 (Secondary): Equilibrium between work hardening & recovery. Minimum creep rate is key design parameter.

• Stage 3 (Tertiary): Strain rate accelerates due to void formation, necking, leading to rupture.

• Effect of Temperature: Increases creep rate exponentially (Arrhenius relationship). Above ~0.3-0.4 \(T_m\) (absolute melting temp), creep becomes significant.

Fatigue:

• Definition: Progressive, localized structural damage under cyclic loading, leading to failure at stresses < UTS.

• Fatigue Limit (Endurance Limit): Maximum stress amplitude for infinite life (e.g., \(10^6\) or \(10^7\) cycles). Not all materials have one (e.g., Al).

• S-N Diagram: Plot of stress amplitude (S) vs. number of cycles to failure (N) on log scale.

  DiagramCANVAS: Sketch an S-N curve for a ferrous alloy showing a fatigue limit (horizontal asymptote). For a non-ferrous alloy, show curve continuing downwards.

• Factors Affecting Fatigue Strength: Surface finish, size, stress concentration, material properties, environment, mean stress.

Impact Toughness (Charpy/Izod):

• Test: Specimen with a V-notch struck by pendulum. Energy absorbed (J) to break specimen = impact toughness.

• Specimen Logic: Notch creates a stress concentration, simulating a flaw. Test measures material's resistance to rapid crack propagation.

• Notch Sensitivity: Tendency for a material's fracture strength to be reduced by a notch. Brittle materials show high notch sensitivity.

• Charpy vs. Izod: Charpy: specimen supported as a beam, struck behind notch. Izod: specimen clamped vertically, struck at notch.

Fracture:

• Ductile: Significant plastic deformation, "cup-and-cone" morphology, dimples on fracture surface (microvoid coalescence).

• Brittle: Little/no plastic deformation, cleavage facets, often transgranular.

• Griffith's Theory (Brittle Fracture): Crack propagates when reduction in elastic strain energy (release) ≥ energy required to create new crack surfaces (surface energy). For a crack of length \(2a\):

$$\sigma_c = \sqrt{\frac{2E\gamma_s}{\pi a}}$$

Where \(\sigma_c\) = critical stress, \(E\) = Young's modulus, \(\gamma_s\) = specific surface energy, \(a\) = half-crack length.

[!TIP] High-Yield: Be able to sketch & label the stress-strain curve, creep curve, and S-N diagram. Know the difference between Charpy and Izod specimen orientation.

4.0 Phase Diagrams and Solidification

Binary Phase Diagram Interpretation:

• Phases: Single, physically & chemically homogeneous regions (e.g., α, L, α+β).

• Phase Regions (Fields): Areas where specific phase(s) exist (liquid, solid solution, two-phase).

• Lever Rule: Calculates relative amounts of phases in a two-phase region at a given T/composition.

$$\text{Weight fraction of phase } \alpha = \frac{C_0 - C_\beta}{C_\alpha - C_\beta}$$

Where \(C_0\) = overall composition, \(C_\alpha, C_\beta\) = compositions of phases at the tie-line ends.

Solid Solutions:

• Substitutional: Solute atoms replace solvent atoms. Requires: Hume-Rothery Rules:

  1. Atomic size difference < ±15%.

  2. Same crystal structure.

  3. Similar electronegativity.

  4. Same valence.

• Interstitial: Solute atoms occupy interstices (e.g., C in γ-Fe (austenite)). Size factor critical (solute < 1.0 Å larger than interstitial site).

Intermediate Phases vs. Solid Solutions:

• Solid Solution: Complete/partial solid solubility, single phase, variable composition within limits.

• Intermediate Phase: Fixed or narrow composition range, distinct crystal structure, often ordered (e.g., σ phase, intermetallic compounds like Ni₃Al).

Cooling Curves:

Plot of temperature vs. time during controlled cooling. Thermal arrests (plateaus) indicate phase transformations (e.g., liquid→solid, eutectoid). Used to construct phase diagrams.

Fe-Fe₃C (Iron-Cementite) Diagram - Key Features:

DiagramSEARCH: "iron iron carbide phase diagram labeled"

• Phases: δ-ferrite (BCC), γ-austenite (FCC), α-ferrite (BCC), Fe₃C (cementite, orthorhombic).

• Key Reactions:

  • Eutectoid (727°C, 0.76% C): γ (0.76% C) → α (0.022% C) + Fe₃C (6.67% C) = Pearlite (lamellar).

  • Eutectic (1147°C, 4.3% C): L (4.3% C) → γ (2.11% C) + Fe₃C (6.67% C) = Ledeburite.

  • Peritectic (1495°C, 0.16% C): δ (0.09% C) + L (0.53% C) → γ (0.16% C).

• Important Microstructures:

  • Pearlite: Lamellar mixture of α + Fe₃C. Strength/hardness increases with fineness.

  • Martensite: Supersaturated solid solution of C in BCT (distorted BCC) from rapid quench of austenite. Very hard & brittle.

  • Bainite: Forms at intermediate temps. Upper (feathery, α+Fe₃C) & Lower (plate-like, α+Fe₃C).

  • Spheroidite: Cementite spheres in α matrix (from prolonged annealing). Very soft/machinable.

Time-Temperature-Transformation (T-T-T) Diagram for Steel:

DiagramSEARCH: "TTT diagram for eutectoid steel"

• Construction: Isothermal transformation of austenite. Plot start/finish of transformation vs. time (log scale) at constant T.

• "Nose": Shortest incubation time. Cooling faster than nose → martensite (no diffusion).

• Relevance vs. Equilibrium Diagram: TTT shows kinetics (how fast, what forms at constant T). Equilibrium diagram shows final phases at slow cooling. TTT guides heat treatment (e.g., quench to avoid nose for martensite, hold in bainite region for ausforming).

Partial Al-Cu Diagram (up to 10% Cu) & Age-Hardening:

DiagramSEARCH: "aluminum copper phase diagram up to 10 percent"

• Key Feature: Maximum solid solubility of Cu in Al is ~5.7% at eutectic temp (548°C). Solvus line marks boundary of α (Al-rich solid solution).

• Age-Hardening Alloys (4-5% Cu): Quench from single α region → supersaturated solid solution (metastable). At room temp (natural aging) or elevated temp (artificial aging), fine precipitates (θ'' → θ' → θ (Al₂Cu)) form, hindering dislocation motion → increase in hardness/strength.

Eutectoid Solid Solution:

• Definition: A solid solution that undergoes a eutectoid reaction upon cooling (γ → α + β).

• Significance in Steel: Austenite (γ) is the eutectoid solid solution. Its decomposition into ferrite (α) and cementite (Fe₃C) forms pearlite, the fundamental microstructure of hypo/hyper-eutectoid steels. Control of pearlite spacing is key to property tuning.

[!TIP] Critical: Practice lever rule calculations on Fe-Fe₃C diagram. Know the eutectoid composition (0.76% C) and temperature (727°C). Understand the difference between equilibrium cooling (phases from diagram) vs. continuous cooling (microstructures from TTT/CCT).

5.0 Solid-State Transformations

Types:

1. Diffusional (Nucleation & Growth): Requires atomic diffusion. Examples: Austenite→Pearlite, Austenite→Bainite. Controlled by T & time (TTT).

2. Diffusionless (Martensitic): Shear transformation, no long-range diffusion. Atoms shift cooperatively. Example: Austenite→Martensite. Controlled by cooling rate (must be fast enough to avoid diffusional reactions). Results in distorted lattice (BCT in steel).

Martensite Transformation in Steel:

• Formation: Rapid quench of austenite (γ, FCC) below \(M_s\) (martensite start temp). Diffusionless shear.

• Characteristics: Supersaturated C in BCT structure. Hardness increases with C content (up to ~1%). Very high dislocation density. Untempered martensite is extremely brittle.

• Effect on Properties: High hardness & strength, low ductility/toughness. Must be tempered for most applications.

[!TIP] Exam Link: Martensite formation is directly linked to hardening (quenching) in Unit 6. Its brittleness necessitates tempering.

6.0 Heat Treatment Processes

Objectives: Relieve stresses, soften for machining, harden, toughen, refine grain, improve machinability, enhance wear/corrosion resistance.

Core Processes:

Tempering Theory & Stages:

1. Stage 1 (150-250°C): Relief of high internal stresses, slight decrease in hardness.

2. Stage 2 (250-350°C): Decomposition of martensite into tempered martensite (ferrite + ε-carbide). Drop in hardness.

3. Stage 3 (400-650°C): Cementite (Fe₃C) precipitates. Toughness increases, hardness decreases further.

Types: Low-temp (150-300°C, tools), Medium (300-450°C), High (450-650°C, structural parts).

Specialized Processes:

• Austempering: Austenitize → quench to bainite region (just above \(M_s\)) → hold until transformation to ausferrite (lower bainite + retained austenite) → cool in air. Result: Good toughness, minimal distortion. Used for springs, gears.

• Martempering (Marquenching): Austenitize → quench to just above \(M_s\) (in "nose" of TTT) → hold for temperature uniformity → cool in air through \(M_s\). Result: Minimizes thermal stresses/cracking, transforms to martensite upon final cooling. Used for complex parts.

Hardenability:

• Definition: Ability of a steel to be hardened (form martensite) to a given depth under a specified set of conditions. NOT the same as hardness.

• Factors: Carbon content (↑ increases), Alloying elements (Cr, Mo, Ni, Mn, Si, V - ↑ by slowing diffusion), Austenite grain size (↑ ↑), Quench severity (medium/aggressiveness of quench medium).

• Jominy End-Quench Test: Standardized test. Specimen austenitized, one end quenched with water. Hardness measured along length → hardenability curve. Depth to a given hardness indicates hardenability.

Quench Severity: Ability of a quench medium to extract heat. H value (Grossmann's). Water (H=1.0) > Oil (H=0.35-0.5) > Air (H=0.15-0.25). Affects depth of hardening.

Common Heat Treatment Defects & Precautions:

• Distortion/Cracking: Due to thermal stresses. Precautions: Use martempering/austempering, proper quench media, uniform heating/cooling.

• Decarburization: Loss of surface C in air. Precautions: Use protective atmosphere, salt bath, or pack in carbonaceous material.

• Overheating/Oversizing: Coarse grain. Precautions: Control temperature/time.

• Insufficient Austenitizing: Incomplete transformation. Precautions: Hold at correct temp/time.

[!TIP] Key Distinction: Annealing (soft, slow cool) vs. Normalizing (stronger, air cool). Hardening (martensite, quench) vs. Tempering (toughen, after quench). Austempering (bainite) vs. Martempering (martensite with less stress).

7.0 Surface Hardening and Treatment Techniques

Fundamental Difference:

• Case Hardening (Diffusion-based): Adds new layer (case) by diffusing interstitial atoms (C, N) into surface. Case & core remain separate phases. Case depth controlled by time/temp.

• Surface Hardening (No Diffusion): Modifies existing surface layer by rapid heating & quenching. No composition change. Depth shallow (~1mm).

Case Hardening Processes (Diffusion):

1. Carburizing (Case Carburizing): Add C to low-carbon steel surface.

   • Process: Pack (solid), liquid (cyanide bath), or gas (endothermic atmosphere). T ≈ 850-950°C.

   • Variables: Time (controls depth), Temperature, Carbon Potential (controls surface C%).

   • Result: High C surface → martensite after quench → hard, wear-resistant case. Tough, ductile core.

2. Nitriding: Add N to surface (alloy steels like 4140, 4340, tool steels).

   • Process: Gas (NH₃), plasma (ion), or salt bath. T ≈ 500-570°C (lower than carburizing).

   • Advantages: No quench needed (forms hard nitrides ε, γ'), minimal distortion, excellent wear/corrosion resistance.

3. Cyaniding: Short-duration (15-90 min) carburizing + nitriding using molten cyanide salts.

   • Application: Thin case (0.025-0.75 mm) on low-carbon steels. Fast, but toxic salts.

4. Carbo-nitriding: Simultaneous diffusion of C & N (often gas process). Combines benefits. Used for gears, cams.

Surface Hardening Processes (No Diffusion):

1. Flame Hardening: Direct oxy-acetylene flame heats surface rapidly → quench (water spray). Shallow case, localized, economical for large parts (gears, rails).

2. Ion Implantation: Bombard surface with high-energy ions (N, B, C) in vacuum. No melting, modifies top 0.1-0.5 µm. Improves wear, fatigue, corrosion. Used for tools, biomedical implants.

[!TIP] Contrast: Nitriding (low T, no quench) vs. Carburizing (high T, requires quench). Flame hardening is surface-only, no diffusion.

8.0 Engineering Alloys and Their Applications

Ferrous Alloys:

Steels Classification & Applications:

Influence of Specific Alloying Elements in Steel:

• Cr: Increases hardenability, wear/corrosion resistance (stainless). Forms carbides.

• Ni: Increases toughness, strength, corrosion resistance (austenitic stainless). Stabilizes austenite.

• Mo: Increases hardenability, high-temperature strength, reduces temper brittleness.

• Mn: Deoxidizes, combines with S (MnS), increases hardenability & strength.

• Si: Deoxidizes, increases strength/elastic limit (springs), reduces magnetism.

• V: Forms stable VC, refines grain, increases hardenability & strength.

• Cu: Increases corrosion resistance (weathering steels), precipitation hardening.

• Al: Deoxidizer, grain refiner (Al-killed steel), nitriding steels.

Hadfield Manganese Steel (Mangalloy):

• Composition: ~12-14% Mn, 1% C.

• Properties: High impact strength & ductility + work hardening (surface hardens under impact/abrasion).

• Applications: Railway switches, crusher jaws, wear-resistant liners.

Cast Iron Types:

Non-Ferrous Alloys:

Aluminum Alloys (1xxx-7xxx Series):

• 1xxx: ~99% Al. High conductivity, corrosion resist. Electrical wires.

• 2xxx (Al-Cu): 2000 series. Heat-treatable (age-hardenable). Moderate strength, poor corrosion. Aircraft (2024).

• 3xxx (Al-Mn): 3000 series. Moderate strength, good workability. General sheet, beverage cans (3003).

• 4xxx (Al-Si): 4000 series. Low melting, good fluidity. Welding rods, brazing.

• 5xxx (Al-Mg): 5000 series. Non-heat-treatable, excellent corrosion (marine), weldable. Shipbuilding, tanks (5083, 5086).

• 6xxx (Al-Mg-Si): 6000 series. Heat-treatable, good strength/corrosion, weldable. Extrusions, automotive (6061, 6082).

• 7xxx (Al-Zn-Mg-Cu): 7000 series. High strength, heat-treatable. Aerospace (7075, 7050).

Copper Alloys:

• Copper-Tin Bronze (Phosphor Bronze): 80-95% Cu, 5-20% Sn + P. High strength, corrosion, fatigue resist. Bearings, springs, gears.

• Babbitt Metals (White Metals):

  • Tin-base: Sn + Sb + Cu. Excellent embeddability, conformability. High-speed bearings.

  • Lead-base: Pb + Sb + Sn. Cheaper, lower load capacity. General bearings.

  • Copper-base (Lead Bronze): Cu + Pb + Sn. High load, low speed. Heavy-duty bearings.

Nickel Alloys:

• Monel (Ni-Cu): ~67% Ni, 30% Cu. Excellent corrosion (sea water, acids), high strength. Marine hardware, chemical equipment.

• Inconel (Ni-Cr-Fe): High Cr. Excellent oxidation/corrosion at high T. Jet engines, nuclear reactors.

• Hastelloy (Ni-Mo-Cr): Superior corrosion in reducing/oxidizing acids. Chemical processing.

Titanium Alloys:

• Alpha Alloys: Al (α-stabilizer) + Sn, Zr. Properties: Low density (~4.5 g/cm³), excellent corrosion, good creep up to ~400°C. Applications: Aerospace (airframe), biomedical implants (Ti-6Al-4V most common).

• Alpha-Beta & Beta Alloys: Higher strength, heat-treatable.

[!TIP] Alloy Series: Remember Al 2xxx (Cu), 6xxx (Mg-Si), 7xxx (Zn) are heat-treatable. Cu-Sn is bronze. Ni-Cu is Monel. Ti-6Al-4V is the workhorse alpha-beta alloy.

9.0 Materials Testing and Characterization

Destructive Testing:

• Tensile: Stress-strain curve → UTS, yield strength, %EL, %RA, \(E\).

• Hardness: Brinell (HB), Rockwell (HR), Vickers (HV). Non-destructive in most cases (small indent).

• Impact: Charpy/Izod → energy absorbed (J). Temperature-dependent (DBTT - Ductile-to-Brittle Transition Temperature).

• Fatigue: Rotating beam (R.R. Moore), axial load. S-N curve → fatigue limit/strength.

• Creep: Constant load & T (≥0.3 \(T_m\)). Creep curve → steady-state creep rate \(\dot{\epsilon}_s\). Rupture life.

Non-Destructive Testing (NDT) for Cracks:

[!TIP] NDT Selection: UT for internal flaws in thick sections. PT/MT for surface cracks (ferrous vs. non-ferrous). RT for weld inspection (2D view). ET for surface cracks in tubes/wires.

10.0 Chemical Analysis in Metallurgy

Qualitative Analysis:

• Spot Test Method: Drop reagent on metal surface. Observation: Color change, precipitate, effervescence.

  • Principle: Specific chemical reaction between reagent and element/ion.

  • Procedure: Clean surface, apply reagent, observe immediate/after heat.

  • Application: Rapid identification of metals/alloys in scrap, shop floor. (e.g., HNO₃ for Cu/Cu alloys → blue-green; KCN for Ag/Au).

Quantitative Analysis:

• Gravimetric Analysis:

  • Principle: Convert analyte to insoluble compound, filter, dry, weigh. Mass → amount.

  • Procedure: Dissolve sample, precipitate, filter, wash, dry/ignite, weigh.

  • Applications: Determining %C (as CO₂), %S (as BaSO₄), %Ni (as NiDMG).

• Volumetric Analysis (Titrimetry):

  • Principle: React analyte with standard solution of known concentration. Volume at endpoint → amount.

  • Procedure: Titration with indicator/pH meter. Common: Acid-base, redox, complexometric (EDTA).

  • Applications: %C (combustion, then titrate CO₂ with Ba(OH)₂), %Mn (redox), %Cu (iodometric or EDTA).

Instrumental Methods:

• Optical Emission Spectroscopy (OES) / Spark Analysis:

  • Principle: Spark excites atoms in sample → emit characteristic wavelengths → spectrometer measures intensity → composition.

  • Use: Rapid, accurate multi-element analysis of solids (metals). Standard for foundries, quality control.

• Spectrophotometric Analysis:

  • Principle: Measure absorption of specific wavelength by colored solution. Beer-Lambert law: \(A = \epsilon c l\).

  • Use: Trace elements after dissolution & complexation (e.g., Cu with BCA, Ni with dimethylglyoxime).

• Colorimetric Methods:

  • Principle: Form colored compound, compare color intensity with standards (visual or photometer).

  • Applications: Quick field tests, e.g., for Cr⁶⁺ (yellow diphenylcarbazide), Cu (ammonia complex deep blue).

[!TIP] Distinguish: Gravimetric = weigh precipitate. Volumetric = measure volume of titrant. OES = direct solid analysis, multi-element. Spectrophotometry = solution, single element, sensitive.

11.0 Industrial Metallurgy: Ores and Reagents

Metallic Ores:

Manganese:

• Main Ores:

  • Pyrolusite: MnO₂ (most important, 63% Mn).

  • Rhodochrosite: MnCO₃ (48% Mn).

  • Hausmannite: Mn₃O₄.

  • Braunite: Mn₂O₃·MnSiO₃.

• Properties: Hard, brittle, silvery-gray. Oxidizes easily.

• Industrial Applications:

  1. Steelmaking: Deoxidizer, sulfur fixer, alloying (Hadfield steel). ~90% of Mn used.

  2. Batteries: Dry cell (MnO₂ cathode).

  3. Chemicals: MnSO₄ (fertilizer, animal feed), KMnO₄ (oxidizing agent).

Chromium:

• Main Ore: Chromite (FeCr₂O₄ or (Fe,Mg)Cr₂O₄). Often as solid solution with magnesite.

• Properties: Hard, brittle, high melting point (2030°C), corrosion-resistant.

• Significance in Metallurgical Industry:

  1. Stainless & Alloy Steels: Major alloying element (≥10.5% for stainless). Forms hard carbides, increases hardenability, corrosion/oxidation resistance.

  2. Refractories: Chromite-magnesite bricks for steelmaking furnaces (high T, slag resistance).

  3. Plating: Chrome plating (decorative, hard) from CrO₃ solutions.

  4. Pigments: Chrome yellow (PbCrO₄), chrome green.

Chemical Reagents in Metallurgy:

[!TIP] Ore Focus: Chromite is the only commercial Cr ore. Pyrolusite is the main Mn ore. Know their chemical formulas and primary metallurgical uses (Cr for stainless steel, Mn for steel deoxidation/alloying).