Heat Treatment | Engineering Materials - Mechanical Engineering PDF Download

Introduction

Heat treatment is a group of controlled heating and cooling operations applied to metals and alloys in the solid state to obtain desired mechanical and physical properties. For steels, most common heat-treating processes involve the transformation or decomposition of austenite. The nature, morphology and distribution of the transformation products (for example pearlite, bainite, martensite, carbides) determine the final hardness, strength, ductility and toughness of the steel.

• Heat treatment is used to relieve internal stresses, change hardness, alter ductility and toughness, and produce specific microstructures.
• When heating parts of varying cross-section, differences in temperature rise between thin and thick sections must be considered to minimise thermal stress and distortion; where possible, heating rates may be reduced for thinner sections.
• Successful heat treatment requires knowledge of equilibrium transformation temperatures (critical temperatures), diffusion processes and cooling rates for the alloy being treated.

Annealing

Annealing is a heat-treatment family whose principal aims are to relieve internal stresses, soften the material, increase ductility and toughness, and produce a desired, often more workable, microstructure.

• Typical uses: preparing steels for forming or machining, restoring ductility after cold work, and producing particular microstructures for further processing.
• General sequence: heating to a specified temperature, holding (soaking) at that temperature to homogenise and complete transformations, then controlled cooling (often slow cooling in the furnace) to achieve the required microstructure.

1. Heating to a desired temperature appropriate for the steel grade and intended transformation.
2. Soaking for sufficient time to complete transformation and homogenise temperature and composition.
3. Cooling, commonly very slowly (for example in a furnace) to avoid hard phases and produce coarse, soft microstructures.

Types of annealing

• Full annealing: Steel is heated to a temperature above the upper critical temperature (typically tens of degrees above the austenitising temperature) to form homogeneous austenite, held to complete transformation, then cooled slowly in the furnace. The result is coarse pearlitic structure with low hardness and improved ductility.
• Process (or recovery) annealing: Low-temperature anneal performed below the lower critical temperature to relieve residual stresses developed during cold working without changing the bulk microstructure. Common for low-carbon steels prior to further cold working.
• Spheroidising annealing: Performed for medium and high carbon steels to convert hard lamellar carbides into a globular (spheroidal) form to improve machinability and cold formability. Typically involves long holds just below the lower critical temperature.
• Diffusion annealing: Used to remove chemical inhomogeneity (segregation) in cast or heavily worked steels. The material is heated to high temperature (for example around 1150 °C for some steels), held for several hours (commonly many hours, e.g., 6-8 hours as a guide), then cooled slowly to promote long-range diffusion and composition uniformity.

Normalizing

Normalizing is a heat treatment used to refine grain size and produce a uniform, fine pearlitic microstructure after deformation or non-uniform processing. It is applied when the as-worked microstructure contains coarse, irregular grains or pearlite.

• The workpiece is heated to a temperature above the upper critical temperature (for many steels this is typically tens of degrees above the critical line; practical guidelines list ranges such as 55 °C to 85 °C above the upper critical temperature for some operations) to form austenite throughout the section.
• After sufficient soak to transform the structure to austenite, the part is removed from the furnace and cooled in still air (air cooling). Cooling in air produces faster cooling than furnace cooling and results in finer grains and higher strength and hardness than full annealing, but with somewhat lower ductility.
• The resulting microstructure is generally fine pearlite with the appropriate proeutectoid phase depending on composition. Normalizing is commonly used after forging, hot rolling or other operations that produce a non-uniform or coarse microstructure.

Heat treatment

Tempering

Tempering is a heat treatment used on quenched steels to reduce brittleness and relieve residual stresses while retaining a controlled level of hardness and strength. Tempering is performed by reheating martensitic steel to a temperature below the eutectoid (austenitising) temperature, holding for a specified time, and then cooling-commonly in air.

• Primary objectives: relieve residual stresses produced by quenching, increase ductility and toughness, and modify hardness to the desired value.

Classification by tempering temperature and effects

• High-temperature tempering (≈ 500-700 °C): Produces a structure known as sorbite (tempered bainite/tempered martensite with fine carbide dispersion). Residual stresses from quenching are greatly reduced or removed and toughness is significantly improved. Typical applications include components requiring high toughness with moderate hardness.

Tempering of martensite
• Medium-temperature tempering (≈ 300-500 °C): Results in a tempered martensite often described as troostite or tempered martensite with fine carbide precipitates. Endurance (fatigue) strength may increase after appropriate tempering and subsequent cooling. Used for spring steels and some die steels where a balance of strength and toughness is required.
• Low-temperature tempering (≈ 150-300 °C): Typical hold times are 1-3 hours depending on section size. This tempering reduces internal stresses, slightly increases toughness and gives improved wear resistance while keeping relatively high hardness. It is used for cutting and measuring tools and where dimensional stability and hardness must be maintained with some resistance to brittleness.

Case hardening (Surface hardening) methods

The aim of case hardening (or surface hardening) is to produce a hard, wear-resistant surface (the case) while retaining a relatively soft, tough core. This combination gives good wear resistance, fatigue life and impact resilience for components such as gears, shafts and bearings. Common methods are described below.

Carburizing (case carburizing)

Carburizing is used to increase the carbon content of the surface layer of low-carbon steels (typically steels with up to about 0.18% C) by diffusion of carbon into the surface at a temperature where austenite is stable. The part is then quenched to obtain a hard, high-carbon martensitic case while the core remains lower in carbon and relatively tough.

• Typical carburizing temperature range: about 870-950 °C (an austenitising range for many steels), where austenite can dissolve more carbon.
• After carburizing, the depth of carburized case depends on time, temperature and carbon potential of the environment; practical case depths are often up to a few tenths of a millimetre (for example up to ≈ 0.3 mm) depending on process parameters, with surface hardnesses commonly in the range 55-65 HRC after quenching and tempering.

Pack carburizing

• Pieces to be carburized are surrounded by a solid carburizing mixture (carbon-bearing compound) and packed in a sealed steel box.
• The box is heated to the carburizing temperature and held for the required time to produce the desired case depth.
• A typical pack carburizing mixture (by weight) can include charcoal as the carbon source and additives to promote gas generation: for example 50% charcoal + 20% BaCO₃ + 5% CaCO₃ + 5-12% Na₂CO₃.

Pack carburizing

Gas carburizing

• The workpiece is treated in a controlled atmosphere furnace containing carbonaceous gases or hydrocarbons such as methane (CH₄), propane or butane. The gas composition and furnace conditions control the carbon potential at the surface.
• The part is heated to the carburizing temperature (commonly ≈ 900-950 °C) and held for several hours (commonly 3-12 hours depending on case depth required).
• A simplified representation of methane decomposition at high temperature is: CH₄ → C (atomic) + 2 H₂, indicating the availability of carbon for surface diffusion.

Liquid carburizing

• The part is immersed in a molten salt bath containing cyanide or other carbon-bearing salts at carburizing temperature (≈ 900-950 °C).
• The salt bath supplies carbon and sometimes nitrogen to the surface; for example a common bath for liquid carburizing/cyaniding contains NaCN together with other salts.
• Carbon (and some nitrogen) diffuses into the surface to build up the case; the part is then quenched and tempered as required for final properties.

Nitriding

• Nitriding is a low-temperature surface hardening process in which nitrogen is introduced into the steel surface to form hard nitrides; it is most effective on alloy steels that form stable nitrides (for example steels containing aluminium, chromium, molybdenum, or vanadium).
• Typical nitriding temperatures are in the range 500-600 °C, much lower than carburizing temperatures; processes may use gaseous ammonia (NH₃), plasma (ion) nitriding, or salt-bath nitriding.
• In gaseous nitriding, ammonia dissociates at the heated surface to produce reactive nitrogen species that diffuse into the steel surface. A simplified global reaction is: 2 NH₃ → N₂ + 3 H₂, while reactive (atomic or nascent) nitrogen species are responsible for nitride formation and diffusion.
• Nitriding produces a hard, wear-resistant case and, because it is done at relatively low temperature, minimal distortion occurs and no quenching is required.

Cyaniding

• Cyaniding (also called liquid carburizing or cyanide hardening when used at relatively low temperature) is a rapid case hardening process in which the component is immersed in a molten cyanide salt bath (sodium cyanide containing salts) at temperatures around 820-860 °C.
• The bath provides both carbon and nitrogen to the surface; the process is typically followed by rapid quenching to obtain a hard case.
• A representative composition (by weight) of a cyanide salt bath can be: 20-30% NaCN + 25-50% NaCl + 25-50% Na₂CO₃.

Induction hardening

Induction hardening is a rapid surface hardening method using electromagnetic induction to heat only the surface layer of a component, followed by immediate quenching to form a hard martensitic case.

• The workpiece is placed within or near a copper coil and a high-frequency alternating current is passed through the coil. The changing magnetic field induces eddy currents in the surface layer of the conductive workpiece and heat is generated by resistive losses in the surface.
• Because heating is concentrated at the surface, the core remains relatively cool and retains its original microstructure and toughness.
• After induction heating to the austenitising range at the surface, the surface is rapidly quenched (often by water or polymer quenchant) to form martensite in the case while the core remains tough.
• Induction hardening uses very high heating rates and short times; typical heat times are on the order of a few seconds to a few tens of seconds (for example 2-50 s depending on size and frequency). Heating rates and required quench temperatures depend on steel grade and mass; higher heating rates reduce time to transform the surface and affect required austenitising temperatures.

Flame hardening

Flame hardening is a localized surface hardening technique in which a high-temperature gas flame (commonly oxy-acetylene or oxy-fuel) is used to rapidly heat the surface layer to the austenitising temperature, followed by immediate quenching to produce a martensitic case.

• The flame heats the surface rapidly so that only the required surface depth is brought to austenite; the core remains relatively cool and tough.
• After rapid heating, the surface is quenched-often by water spray or a water jet-to produce a martensitic layer typically 2-4 mm thick, depending on flame parameters, heating time and part geometry.
• Flame hardening is suitable for large components or those requiring selective hardening of particular areas such as gear teeth, shafts and cams.

Flame hardening

Notes on transformations and selection of heat-treatments

• Understanding the equilibrium and non-equilibrium transformations of austenite (to pearlite, bainite, martensite, etc.) and the effect of cooling rate is central to selecting the correct heat treatment for property targets.
• Quenching media (water, oil, polymer, air) and quench severity control the cooling rate and therefore the final structure; they must be chosen to balance hardness with distortion and cracking risk.
• Post-treatment tempering is commonly required after quenching to adjust toughness and relieve residual stresses while retaining adequate hardness.
• Surface hardening methods (carburizing, nitriding, cyaniding, induction and flame hardening) allow a hard, wear-resistant outer layer with a ductile core, improving component life for sliding and rolling contacts.

For practical work, always consult alloy-specific phase diagrams, continuous cooling transformation (CCT) or time-temperature-transformation (TTT) diagrams, and the steel manufacturer's recommendations for temperatures, hold times and cooling media to obtain the intended microstructure and properties with minimal distortion.