The choice of the appropriate heat treatment is one of the most critical decisions in the design and manufacture of high-performance metallic components. In sectors such as automotive, aerospace, energy or precision engineering, the optimisation of mechanical and metallurgical properties depends to a large extent on the correct match between the selected alloy, its heat treatment and the end application.

Heat treatment is a process specific to each case: each alloy responds differently to thermal, atmospheric and kinetic parameters. Determining the appropriate cycle requires a deep understanding of phase thermodynamics, diffusion kinetics, chemical composition and the service conditions the part will face during its useful life.

This article examines the fundamental technical criteria for selecting the most appropriate heat treatment according to the type of metallic alloy, with special emphasis on steels, superalloys and advanced alloys, and taking into account the requirements of their final application.

Metallurgical principles of heat treatment

Heat treatment modifies the internal microstructure of the metal in a controlled manner, adjusting the balance between phases, grain size, precipitate distribution and residual stresses. The main objectives are:

• Improve mechanical strength (tension, creep, fatigue).
• Optimise hardness and wear resistance.
• Increase dimensional stability.
• Improve machinability or surface properties.
• Homogenise or relieve internal stresses after forming or machining.

The three metallurgical pillars are:

1. Phase transformations: changes in crystal structure (austenite ↔ martensite ↔ bainite ↔ ferrite ↔ pearlite) induced by temperature and time.

2. Diffusion and segregation: migration of atoms and formation of precipitates (carbides, nitrides, γ’, γ’’ phases).

3. Stress relaxation: elimination of internal stresses by recovery or recrystallisation.

Precise control of thermal and atmospheric parameters enables obtaining microstructures adapted to the function the part must fulfil.

Classification of metallic alloys and their thermal behaviour

Metallic alloys are classified according to their metallic base (Fe, Ni, Ti, Al, Co, Cu) and the metallurgical mechanisms that dominate their behaviour during heating and cooling.

• Carbon and alloy steels: highly hardenable; respond via martensitic or bainitic transformation.
• Stainless steels: balance mechanical properties with corrosion resistance; sensitive to chromium carbide precipitation.
• Nickel or cobalt-based superalloys: precipitation-hardenable; high microstructural stability.
• Lightweight titanium and aluminium alloys: strengthening by precipitation or control of α/β phase proportion.
• Special alloys (copper, magnesium, zirconium): treatments oriented to recrystallisation or solid-solution strengthening.
  Knowledge of equilibrium diagrams (Fe–C, Fe–Cr–Ni, Ni–Al, Ti–Al–V, Al–Mg–Cu–Zn) and TTT/CCT curves (Time-Temperature-Transformation / Continuous Cooling Transformation) is essential to anticipate the metallurgical outcome of the applied heat-treatment cycle.

Selection criteria for treatment according to alloy type
The selection of the heat treatment should balance mechanical performance, microstructural stability, corrosion/oxidation resistance and process cost.

Below, we highlight some technical, microstructural and operational criteria for each alloy family:

Carbon and alloy steels

Steels are the most versatile materials and the most sensitive to heat treatments. Their behaviour depends primarily on carbon content and alloying elements (Cr, Mo, Ni, Mn, V, W).

1. a) Carbon steels (0.1–1.0% C)

• Annealing (650–750 °C): used to soften the material, relieve stresses and refine the grain. Ideal for low-carbon steels (<0.25%C).
• Normalising (850–900 °C): homogenises the pearlitic structure and improves strength without excessive brittleness.
• Quenching + tempering: in medium and high-carbon steels, produces martensite and subsequently adjusts toughness and hardness.

Example: steel C45 (AISI 1045) quenched at 830 °C, oil-cooled and tempered at 550 °C → hardness ~220 HB, good fatigue resistance.

1. b) Alloy steels (Cr, Mo, Ni, V, W)

Alloying elements increase hardenability and temper resistance.

• Cr-Mo steels (42CrMo4, AISI 4140): quench at 850 °C, double temper at 540 °C → hardness 50-55 HRC.
• Nickel-chromium-molybdenum steels: great toughness; used in impact-loaded parts (shafts, crankshafts).
• Vanadium or tungsten steels: designed for tooling; require vacuum quenching and multiple tempering (up to 3 cycles).

Key considerations:

• Control cooling rate to avoid cracks or distortion.
• Prevent surface decarburisation via neutral atmospheres or vacuum.
• Use forced-convection furnaces for thermal uniformity.

Stainless steels

Stainless steels present additional complexity due to their chromium content (>10.5%) and other elements (Ni, Mo, Ti, Nb). Their heat treatment seeks to balance mechanical properties and corrosion resistance without causing sensitisation.

a) Austenitic steels (300-series, 18-8, etc.)

• Solution treatment: 1,050–1,100 °C followed by rapid cooling (water or air). Dissolves carbides and restores ductility.
• Stress-relief: 450–600 °C when dimensional stability is required without losing corrosion resistance.
• Avoid the critical range (600–800 °C): where Cr-carbides precipitate and sensitisation occurs.

Example: AISI 304 annealed at 1,050 °C, rapid cooling → homogeneous austenitic structure and intergranular corrosion resistance..

b) Martensitic stainless steels (AISI 410, 420, 440C)

• Quenching: 950–1,050 °C, oil or air cooled.
• Tempering: 200–350 °C to maximise hardness (up to 60 HRC) or 500–600 °C to improve toughness.
• Applications: valves, blades, plastic moulds.

c) Duplex steels (22Cr–5Ni–3Mo, 25Cr–7Ni–4Mo)

• Solution treat: 1,000–1,100 °C with rapid cooling → balance between ferrite and austenite (50/50).
• Avoid extended ageing over 475 °C (σ phase precipitation and embrittlement).
• Recommended for corrosive environments (seawater, chemical industry).

d) Precipitation-hardening stainless steels (17-4PH, 15-5PH)

• Solution treat: 1,040 °C / rapid cooling.
• Ageing: 480–620 °C (H900, H1025 conditions etc.).
• Precise time control to obtain Cu and Ni₃Al precipitation.

High strength (UTS > 1,200 MPa) and excellent corrosion

Nickel- and cobalt-based superalloys

Superalloys, such as Inconel®, Rene®, Hastelloy® or Haynes®, are designed to operate at temperatures above 70 % of their melting point. They are characterised by creep resistance, thermal fatigue resistance and oxidation resistance.

Their behaviour depends on precipitation hardening by γ’ (Ni₃(Al,Ti)) and γ’’ (Ni₃Nb) phases, or reinforcement by stable carbides (MC, M₂₃C₆).

a) Typical heat-treatment cycles:

1. Solution treat: 1,050–1,200 °C (depending on alloy) → dissolution of secondary phases.
2. Rapid cooling (air or vacuum): retains the supersaturated solid solution.
3. Single or double ageing: 700–800 °C for 4–16 h → controlled precipitation of γ’ and γ’’.

Example: Inconel 718 → 980 °C/1 h + air-cool + 720 °C/8 h + 620 °C/8 h → strength >1,200 MPa at 650 °C.

b) Technical considerations:

• Treatments must be carried out in vacuum or inert atmosphere (N₂, Ar) to avoid oxidation and alloy-element loss.
• Cooling control directly influences precipitate size.
• Overheating may induce grain-growth and loss of creep-resistance.
• Multiple or stepped ageing improves long-term thermal strength

c) Typical applications:

• Turbine blades, compressor discs, combustion chambers, aircraft engine parts.

Lightweight alloys: aluminium and titanium

These alloys combine low density and high specific strength. Their heat-treatments focus on precipitation hardening and control of α/β phases.

a) Aluminium alloys

There are several heat-treatable series (2xxx, 6xxx, 7xxx).

• Solution treat: 450–550 °C (dissolution of intermetallic precipitates).
• Rapid quench (water, polymer): to retain a supersaturated solid solution.
• Natural ageing (T4) or artificial ageing (T6): 100–180 °C, 5–24 h.

Example: Al 7075-T6 → solution at 475 °C, water quench + artificial ageing at 120 °C/24h → tensile strength 540 MPa.

Key points:

• Avoid over-ageing (coarse precipitates and strength loss).
• Control distortion during quench to minimise warping.
• In aerospace components, convection furnaces and exact ±3 °C temperature-control are preferred.

b) Titanium alloys (Ti-6Al-4V and similar)

Titanium combines α (hexagonal close-packed) and β (body-centred cubic) phases. Heat-treatments allow modification of its proportion and morphology.

• Annealing: 700–800 °C → homogenisation and stress relief.
• α+β treatment: 900–950 °C → improved strength and ductility.
• Ageing: 480–600 °C following cooling → precipitation of secondary α, increased strength.

Technical considerations:

• Treatments should be done in vacuum or inert atmospheres to avoid contamination by O, N, H.
• Time at temperature should be minimal to prevent grain-growth.
• Cooling control determines α morphology: acicular (martensitic) or globular (ductile).

Applications: aerospace, biomedical components, high-reliability transmission elements.

Special alloys

Other high-technology alloys (Cu-Be, Zr, Mg, Co-Cr) also require specific treatments.

• Copper–beryllium: solution treat at 760–800 °C + ageing at 300–350 °C → precipitation hardening (BeCu₂).
• Magnesium: stress-relief anneal (250–350 °C) to avoid residual deformation.
• Co–Cr–Mo alloys: quench and age for carbide control, high-wear resistance.

Factors linked to the end application

Practical selection examples

Example 1: Gear made of steel 42CrMo4 (AISI 4140)

• Requirement: high wear resistance and toughness.
• Treatment: oil quench from 850 °C + double temper at 550 °C.
• Result: tempered martensite with hardness 50-55 HRC and good fatigue resistance.

Example 2: Turbine of superalloy Inconel 718

• Requirement: creep and oxidation resistance at 700 °C.
• Treatment: solution at 980 °C, air‐quench + double ageing (720 °C/8 h + 620 °C/8 h).
• Result: controlled precipitation of γ’ and γ’’ with excellent thermal stability.

Example 3: Titanium Ti-6Al-4V for biomedical implant

• Requirement: high biocompatibility and fatigue resistance.
• Treatment: vacuum anneal at 700 °C to homogenise α+β phases.
• Result: excellent ductility and behaviour under cyclic stresses.

Process control and quality assurance

The effectiveness of a heat treatment depends as much on the design of the cycle as on process control. Critical variables include:

• Actual temperature and thermal uniformity: control via calibrated thermocouples and PID systems.
• Atmosphere and pressure: in vacuum or controlled‐atmosphere treatments, leaks and gas‐purity are monitored.
• Cooling rate: directly affects martensite formation or precipitate size.
• Metallographic and hardness tests: verification of microstructures, hardenability curves and hardness profiles.
• Traceability and repeatability: essential in certified environments (ISO 9001, NADCAP).

The use of state-of-the-art furnaces with digital control, pressure cooling and recording systems ensures reproducibility and compliance with specifications.

Conclusions

The choice of the appropriate heat treatment for a given metallic alloy is a strategic decision that combines metallurgy, process engineering and knowledge of the application. There are no universal treatments: each combination of alloy and end-function requires a tailor-made thermal design.
Steels, superalloys and advanced alloys offer enormous versatility, but also demand precision in thermal and atmospheric control. Factors such as chemical composition, part geometry, service loads and compatibility with coatings or subsequent machining determine the success of the process.

In industrial practice, vacuum heat treatments and controlled-atmosphere technologies have enabled quality and repeatability levels unimaginable a few decades ago. Integrated into digital control and full traceability systems, they form the basis of modern precision metallurgy, where every degree, every minute and each cooling parameter directly translate into performance and reliability.

As a services company, at TTT Group we offer our clients a full management of their needs in part‐treatment, providing the optimal solution in the selection and combination of technical solutions for their products.