Heat Treatment of Steel

Heat treatment modifies the microstructure of steel to achieve desired mechanical properties such as hardness, strength, toughness, and ductility. It involves controlled heating and cooling to transform phases like ferrite, pearlite, austenite, bainite, and martensite.

Fundamentals

Critical temperatures: A_c1 (start of austenite on heating) and A_c3 (completion of austenite for hypoeutectoid steels). Eutectoid temperature ~727°C.
Carbon content: Controls hardenability and achievable microstructures. Eutectoid steel ≈0.77 wt% C; hypoeutectoid <0.77%; hypereutectoid >0.77%.
Hardenability vs. hardness: Hardenability is the depth to which martensite forms; hardness is resistance to indentation.
Cooling media: Water, brine, oil, polymer solutions, or air; chosen to control cooling rate and minimize distortion/cracking.
Key transformations: Austenite → martensite (diffusionless), bainite (isothermal), pearlite/ferrite/cementite (diffusional).

Hardening (Quench Hardening)

Hardening aims to form martensite, a supersaturated and hard phase, by austenitizing and rapidly quenching steel.

Purpose

Increase hardness and strength: Wear resistance for tools, dies, gears, and bearing parts.
Create a martensitic matrix: Basis for subsequent tempering to balance toughness.

Typical steps

Austenitize: Heat above A_c3 (hypoeutectoid: ~30–50°C above A_c3) or slightly above A_cm (hypereutectoid: ~30–50°C above A_c1 to avoid grain boundary cementite coarsening).
Soak: Hold for uniform austenite; time depends on section size and alloy.
Quench: Rapid cooling in oil, water, polymer, or air (for air-hardening steels).

Typical temperature ranges

Plain carbon (0.4–0.6%C): 780–840°C.
Alloy steels: 800–900°C (grade dependent).

Microstructure and properties

As-quenched: Martensite (very hard, brittle), retained austenite possible in high-alloy/high-carbon steels.
Properties: Maximum hardness; minimal toughness before tempering.

Common issues

Distortion and cracking: From thermal gradients; mitigated by proper quench media, preheats, and part geometry.
Quench severity: Water/brine more severe than oil; alloys may require controlled quench.

Applications

Cutting tools, springs (followed by tempering), shafts, gears (often combined with case hardening).

Tempering

Tempering follows hardening to reduce brittleness and adjust hardness-toughness balance by controlled heating below A_c1.

Purpose

Relieve internal stresses: Reduce cracking risk.
Improve toughness and ductility: Tailor properties to service requirements.

Typical steps

Heat to 150–650°C: Below A_c1 (commonly 180–220°C for spring temper, 400–600°C for structural parts).
Soak: 30–120 minutes depending on section size.
Cool: Usually still air.

Microstructure and properties

Tempered martensite: Martensite decomposes; carbides precipitate; toughness increases, hardness decreases.

Applications

Almost all quenched steels (tools, dies, machine parts) to achieve usable toughness.

Austempering

Isothermal transformation of austenite to bainite by quenching to a temperature above the martensite start (M_s) and holding.

Purpose

Improve toughness and reduce distortion: Compared to conventional quench-and-temper.
Obtain bainitic microstructure: Good strength-toughness combination.

Typical steps

Austenitize: Similar to hardening.
Quench to 250–400°C salt bath: Above M_s.
Hold isothermally: Until austenite transforms to bainite; then air cool.

Microstructure and properties

Bainite: Upper or lower bainite depending on hold temperature; lower distortion than martensite routes.

Applications

Springs, gears, thin sections requiring dimensional stability and toughness.

Martempering (Marquenching)

Quench to just above M_s, equalize temperature, then cool through martensite range at a controlled rate to minimize thermal gradients.

Purpose

Reduce quench stresses and distortion: Compared to direct quenching.
Enable subsequent tempering: For desired properties.

Typical steps

Austenitize as per steel grade.
Quench to 150–300°C bath: Hold to equalize temperature without transforming significantly.
Air cool through M_s–M_f: Form martensite uniformly; then temper.

Microstructure and properties

As-quenched: Martensite with reduced residual stress; after tempering, tempered martensite.

Applications

Gears, dies, and precision components where distortion control is critical.

Annealing

Annealing softens steel, improves machinability, and enhances ductility by producing a refined, equilibrium microstructure.

Purpose

Reduce hardness and residual stresses: Facilitate forming and machining.
Homogenize microstructure: Refine grains and redistribute carbides.

Types and temperatures

Full annealing: Heat above A_c3 (hypoeutectoid) or above A_c1 (hypereutectoid), slow furnace cool to ~550°C, then air cool.
Process annealing: Subcritical (~550–700°C) to restore ductility after cold work.
Isothermal annealing: Austenitize, then hold just below A_r1 for uniform pearlite.

Microstructure and properties

Pearlite + ferrite (hypoeutectoid) or pearlite + carbides (hypereutectoid): Soft, ductile, machinable.

Applications

Pre-machining condition for forgings, castings, and welded structures.

Stress Relieving

Low-temperature heat treatment to reduce residual stresses without significant microstructural change.

Purpose

Minimize distortion and cracking in service: Especially after welding, machining, or cold working.

Typical steps

Heat to 500–650°C: Below A_c1.
Hold: Commonly 1–2 hours depending on section size.
Cool: Typically still air or controlled furnace cool to reduce gradients.

Microstructure and properties

Minimal phase change: Residual stress reduction with negligible hardness change.

Applications

Weldments, large machined parts, fixtures, and dies.

Spheroidizing

Transforms lamellar cementite into spheroidal carbides in a ferritic matrix to maximize softness and machinability, especially in high-carbon steels.

Purpose

Improve machinability and cold formability: Reduce cutting forces and tool wear.

Common methods

Prolonged subcritical hold: ~650–700°C for several hours.
Cyclic heating: Alternate just above and below A_c1.
From quenched state: Tempering near A_c1 to coarsen carbides.

Microstructure and properties

Spheroidized carbides in ferrite: Lowest hardness, excellent machinability.

Applications

High-carbon tool steels before final hardening; bearing steels prior to machining.

Normalizing

Air cooling from above the critical range to refine grain size and homogenize microstructure.

Purpose

Improve mechanical properties uniformly: Strength and toughness balanced with fine pearlite.
Refine grains and dissolve banding/segregation: Especially in rolled or forged products.

Typical steps

Heat: ~30–60°C above A_c3 (hypoeutectoid) or above A_c1 (hypereutectoid).
Soak: Uniform austenite formation.
Air cool: Still air to room temperature.

Microstructure and properties

Fine pearlite + ferrite: Stronger than full annealed; good machinability.

Applications

Pre-heat-treatment conditioning, castings, forgings, and weldments for property uniformity.

Case Hardening

Creates a hard, wear-resistant surface (case) over a tough core by increasing surface carbon or nitrogen and then hardening.

Purpose

High surface hardness + tough core: Resist wear and contact fatigue while maintaining impact resistance.

Methods

Carburizing: Diffuse carbon at 880–950°C in solid, liquid, or gas mediums; quench to form martensitic case.
Carbonitriding: Carbon + nitrogen at ~800–900°C; good for low-carbon steels, thin cases.
Nitriding: Nitrogen diffusion at 500–580°C without quenching; forms hard nitrides with minimal distortion.
Ferritic nitrocarburizing: ~560–580°C; thin compound layer for scuff resistance.

Microstructure and properties

Carburized parts: High-carbon martensitic case; low-carbon tempered core.
Nitrided parts: Compound (white) layer + diffusion zone; excellent wear and fatigue resistance.

Applications

Gears, camshafts, crankshafts, pins, rollers, and high-wear components.

Process Selection Guide

Goal	Recommended process	Notes
Maximum surface wear resistance	Case hardening (carburizing/carbonitriding/nitriding)	Choose based on distortion tolerance and case depth.
High hardness with reasonable toughness	Hardening + tempering	Temper temperature sets toughness-hardness balance.
Low distortion, good toughness	Austempering	Bainite; ideal for thin sections.
Distortion control during hardening	Martempering	Equalize just above M_s before martensite formation.
Softening and machinability	Annealing or spheroidizing	Spheroidizing best for high-carbon steels.
Uniform properties and grain refinement	Normalizing	Air cool; finer pearlite than annealed condition.
Reduce residual stresses	Stress relieving	Below A_c1; minimal property change.

Glossary of Microstructures

Ferrite (α-Fe): Soft, ductile iron phase with low carbon solubility.
Pearlite: Lamellar ferrite-cementite mixture; balance of strength and ductility.
Bainite: Non-lamellar ferrite-carbide aggregate formed isothermally; strong and tough.
Martensite: Supersaturated body-centered tetragonal phase; very hard, brittle in untempered state.
Carbides (Fe₃C and alloy carbides): Hard particles providing wear resistance.