ME3010 FINAL

Quenching

Continuous, rapid cooling of austenitized steel in a medium (water, oil, air) to form martensite.

Martensite Formation

Occurs by quenching: heating, holding at elevated temperature, and rapid cooling transforms austenite into hard martensite.

Hardenability

The ability of an alloy to form martensite upon quenching; higher hardenability means martensite forms at greater depths.

Jominy End‑Quench Test

A test where hardness is measured along a quenched bar; greater distance of high hardness indicates higher hardenability.

Alloying Elements Effect on Hardenability

Elements like Cr, Mo, Ni are added primarily to increase hardenability, allowing martensite formation with slower cooling.

Carbon Content Effect on Hardenability

Higher carbon content increases both hardness and hardenability of steel.

Austenite Grain Size & Hardenability

Larger grains reduce grain‑boundary nucleation sites for pearlite, delaying transformation and increasing hardenability, but may produce coarse martensite with lower toughness.

Quenching Media: Air

Low quench severity, minimal stresses; used for high‑alloy steels that harden easily.

Quenching Media: Oil

Moderate severity, moderate internal stress and less cracking; typical for thin sections and knives.

Quenching Media: Water

High severity, high hardenability, but high internal stress, distortion and cracking risk; used for low‑carbon steels.

Quenched vs. Annealed Parts

Quenched parts are harder, stronger, more brittle, and contain more internal stress than annealed parts.

Part Size & Hardenability

Smaller parts and higher surface‑to‑volume ratio (e.g., holes) cool faster, increasing hardenability and hardening effect.

Case Hardening

Produces a hard, wear‑resistant surface with a tough core by diffusing carbon or nitrogen into the steel surface at high temperature.

Diffusion Factors

Small atoms diffuse faster; BCC structure allows faster diffusion than FCC; diffusivity increases exponentially with temperature.

Gas Carburizing

Carbon supplied by a carbon‑rich atmosphere; case depth increases non‑linearly with time.

Precipitation Hardening

Strengthening by extremely fine, dispersed second‑phase particles that block dislocation motion; achieved by solution treatment and aging.

Strengthening Mechanisms (Other)

Grain size reduction adds more grain boundaries; solid‑solution strengthening adds local strain fields; strain (work) hardening increases dislocation density.

Precipitation Hardening Heat Treatment Steps

Solution treatment: heat to dissolve all solute, quench to supersaturated solid solution. Aging: reheat to form fine precipitates.

Overaging

Excessive aging time causes precipitates to coarsen, resulting in a loss of strength.

Artificial vs. Natural Aging

Artificial aging uses elevated temperature; natural aging occurs at room temperature (slower, no overaging).

Superalloys

Ni‑base or Co‑base alloys with exceptional creep and oxidation resistance, precipitation hardened for high‑temperature service (e.g., turbines).

Annealing

Heating, holding, then slow cooling (furnace) to relieve stresses, increase ductility, and refine grain structure.

Process Annealing (Intermediate Annealing)

Used between cold‑working steps; involves recovery and recrystallization to restore ductility, terminated before grain growth.

Full Annealing of Steels

Austenitize, then furnace cool → coarse pearlite; yields a soft, ductile microstructure for machining or forming.

Normalizing

Austenitize, then air cool → fine pearlite; improves machinability, refines grains, and modifies residual stresses.

Normalized vs. Annealed

Normalized: stronger, harder, slightly more brittle. Annealed: softer, more ductile, greater stress relief.

Steels (Ferrous Alloys)

Fe–C alloys with 0.008–2.14 wt% C. Low‑carbon (<0.25%): ferrite+pearlite, cold worked. Medium‑carbon (0.25–0.60%): quenchable. High‑carbon (0.60–1.4%): hard, wear resistant.

Stainless Steels

Alloy steels containing at least ~11% Cr for corrosion resistance.

AISI/SAE Steel Designation

First two digits indicate alloy type (10 = plain carbon); last two digits give carbon content in hundredths of a percent (e.g., 1020 = 0.20% C).

Cast Irons

Ferrous alloys with >2.14% C; types include gray, ductile, white, and malleable.

Nonferrous Alloys: Aluminum

Lightweight, good crashworthiness; strengthened by cold work and alloying.

Nonferrous Alloys: Copper

Pure Cu is soft, ductile, highly conductive; brass is Cu–Zn, bronze is Cu–Sn.

Nonferrous Alloys: Titanium

High strength‑to‑weight ratio, good ductility.

Refractory Metals

Metals like W, Mo with high strength and hardness at all temperatures.

Noble Metals

Au, Ag, Pt; soft, ductile, expensive, resist corrosion; used in jewellery and electrical contacts.

Sand Casting

Casting using a sand mold, good for complex shapes; mold broken after solidification.

Die Casting

Metal mold, high‑pressure injection; high productivity for large quantities.

Investment Casting (Lost‑Wax)

Wax pattern coated with ceramic shell; exceptional detail and accuracy.

Continuous Casting

Directly casts long strands in an automated process; better quality than ingot casting.

Casting Defects

Projections (fins, swells) and cavities (blowholes, shrinkage); sometimes repairable.

Powder Metallurgy

Metal powders compacted and sintered below melting point; porous, ideal for high‑melting, low‑ductility metals.

Powder Production Methods

Atomisation (gas, water, centrifugal) or mechanical milling.

Sintering Mechanisms

Solid‑state diffusion and vapour‑phase transport bond powder particles.

Hot Working

Deformation above recrystallisation temperature; no strain hardening, poor surface finish, lower energy required.

Cold Working

Deformation below recrystallisation temperature; increases strength, reduces ductility, high quality finish, higher energy required.

Cold‑Work–Anneal Cycle

Cold‑worked parts are intermediate annealed to restore ductility before further deformation.

Forging

Shaping by compressive forces (hammering, pressing); excellent grain flow. Open‑die: unrestricted flow; closed‑die (drop forging): dies enclose workpiece.

Rolling

Metal passes between rotating rolls to reduce thickness. Hot rolling breaks down cast structures; cold rolling produces sheet, strip, foil with superior finish.

Extrusion

Metal forced through a die by a ram to create long products with uniform cross‑section; tubes use a mandrel.

Drawing

Metal pulled through a die in tension; used for wire, rod, tubing; wire drawing starts from hot‑rolled rod.

Cast vs. Wrought Alloys

Cast: solidification microstructure, may be inhomogeneous. Wrought: mechanically worked → finer, uniform grains, better properties.

Welding

Fuses two parts with or without filler to create a metallurgical bond; Heat Affected Zone (HAZ) adjacent to weld may soften or form brittle martensite.

Welding Cracking Factors

Restrained contraction, low ductility filler, hydrogen, high thermal expansion promote cracking.

Brazing & Soldering

Join without melting base metal; filler wets and diffuses. Soldering uses fillers melting below 450°C.

Additive Manufacturing Advantages

Enables complex geometries, rapid production, customisation, tool‑less builds, lightweight designs.

Vat Photopolymerisation (SLA)

UV‑cured resin process, high resolution.

Material Jetting

Resin droplets selectively deposited and UV cured; needs post‑processing.

Material Extrusion (FDM)

Filament melted and deposited layer by layer.

Powder Bed Fusion (SLS/SLM)

Laser or electron beam selectively melts powder in a bed.

Directed Energy Deposition (DED)

Focused thermal energy melts material as it is continuously fed; used for repair and building.

Binder Jetting

Liquid binder printed onto powder bed, then part is sintered for strength.

3D‑Printed Fuel Nozzle Example

Consolidated 18 parts into 1, added intricate cooling channels, achieved 5× durability over conventional manufacturing.

Heat Treatment Stages

Heating to an elevated temperature followed by holding and then cooling to room temperature.

Melting

Heating charge material (raw material or scrap) in a prepared atmosphere (air, inert gas, or vacuum) to achieve a homogeneous molten liquid.

Recovery

The stage of process annealing where internal strain energy is relieved without forming new crystals.

Recrystallization

The stage of process annealing where new strain-free equiaxed grains form and consume cold-worked crystals.

Bronze

A copper alloy containing tin.

Brass

A copper alloy containing zinc.

Mechanical Comminution

A method to obtain fine metal particles by reducing solid material from larger to smaller average particle size using milling or crushing.

HAZ Microstructural Changes

The heat-affected zone may experience recrystallization and grain growth that reduces strength in cold-worked parts or may produce brittle martensite depending on the alloy composition.

Primary vs. Secondary Processing

Primary processing involves creating the initial shape via ingot or powder metallurgy; secondary processing includes forming operations (forging, rolling) and joining.

Gas Carburizing Carrier Gas

Carbon is delivered to the steel surface by a carrier gas, typically a mixture of argon and methane.

Material Jetting Green State

Parts created via material jetting require post-processing because the newly printed object is initially in a fragile green state.

Cast Alloy Homogenization

Cast alloys require significant soaking or sintering time to achieve homogeneity because there is no hot work to accelerate diffusion.

Melting Fuel (Coke)

Internal heating for melting can involve mixing fuel directly with the charge, such as mixing coke (a refined coal) with iron.

Extrusion Mandrel

To produce hollow tubing during the extrusion process, a mandrel is required on the extrusion ram to define the inner wall cleara