A Comprehensive Overview of Heat Treatment: Key Knowledge and Applications

Heat treatment is a fundamental manufacturing process in the metalworking industry, which optimizes material performance to meet diverse engineering requirements. This article summarizes core knowledge of heat treatment, covering basic theories, process parameters, microstructure-performance relationships, typical applications, defect control, advanced technologies, and safety & environmental protection, based on industry-specific expertise.

1. Fundamental Theories: Core Concepts & Classification

At its core, heat treatment alters the internal microstructure of metallic materials through heating, holding, and cooling cycles, thereby tailoring properties like hardness, strength, and toughness.

Steel heat treatment is primarily categorized into three types:

Overall Heat Treatment: Includes annealing, normalizing, quenching, and tempering—four basic processes that modify the entire workpiece’s microstructure.

Surface Heat Treatment: Focuses on surface properties without changing the bulk composition (e.g., surface quenching) or alters surface chemistry (e.g., chemical heat treatment like carburizing, nitriding, and carbonitriding).

Special Processes: Such as thermomechanical treatment and vacuum heat treatment, designed for specific performance needs.

A key distinction lies between annealing and normalizing: annealing uses slow cooling (furnace or ash cooling) to reduce hardness and relieve internal stress, while normalizing employs air cooling for finer, more uniform microstructures and slightly higher strength. Critically, quenching—used to achieve hard martensitic structures—must be followed by tempering to mitigate brittleness and balance hardness-toughness by relieving residual stress (150–650°C).

2. Process Parameters: Critical Factors for Quality

Successful heat treatment depends on precise control of three core parameters:

2.1 Critical Temperatures (Ac₁, Ac₃, Acm)

These temperatures guide heating cycles:

Ac₁: Start temperature of pearlite-to-austenite transformation.

Ac₃: Temperature at which ferrite fully transforms to austenite in hypoeutectoid steel.

Acm: Temperature at which secondary cementite completely dissolves in hypereutectoid steel.

2.2 Heating Temperature & Holding Time

Heating Temperature: Hypoeutectoid steel is heated to 30–50°C above Ac₃ (full austenitization), while hypereutectoid steel is heated to 30–50°C above Ac₁ (retaining some carbides for wear resistance). Alloys require higher temperatures or longer holding times due to slower alloy element diffusion.

Holding Time: Calculated as workpiece effective thickness (mm) × heating coefficient (K)—K=1–1.5 for carbon steel and 1.5–2.5 for alloy steel.

2.3 Cooling Rate & Quenching Media

Cooling rate dictates microstructure:

Fast cooling (>critical rate): Forms martensite.

Medium cooling: Produces bainite.

Slow cooling: Results in pearlite or ferrite-cementite mixtures.

Ideal quenching media balance "fast cooling to avoid softening" and "slow cooling to prevent cracking." Water/saltwater suits high-hardness needs (but risks cracking), while oil/polymer solutions are preferred for complex-shaped parts (reducing deformation).

3. Microstructure vs. Performance: The Core Relationship

Material properties are directly determined by microstructure, with key relationships including:

3.1 Martensite

Hard but brittle, with a needle-like or lath-like structure. Higher carbon content increases brittleness, while retained austenite reduces hardness but improves toughness.

3.2 Tempered Microstructures

Tempering temperature defines performance:

Low-temperature (150–250°C): Tempered martensite (58–62 HRC) for tools/dies.

Medium-temperature (350–500°C): Tempered troostite (high elastic limit) for springs.

High-temperature (500–650°C): Tempered sorbite (excellent comprehensive mechanical properties) for shafts/gears.

3.3 Special Phenomena

Secondary Hardening: Alloys (e.g., high-speed steel) regain hardness during 500–600°C tempering due to fine carbide precipitation (VC, Mo₂C).

Temper Brittleness: Type I (250–400°C, irreversible) is avoided by fast cooling; Type II (450–650°C, reversible) is suppressed by adding W/Mo.

4. Typical Applications: Tailored Processes for Key Components

Heat treatment processes are customized to match the performance requirements of specific components and materials:

For automotive gears made of alloys like 20CrMnTi, the standard process is carburizing (920–950°C) followed by oil quenching and low-temperature tempering (180°C), which achieves a surface hardness of 58–62 HRC while maintaining a tough core.

For die steel such as H13, the workflow includes annealing, quenching (1020–1050°C, oil-cooled), and double tempering (560–680°C). This sequence relieves internal stress and adjusts hardness to around 54–56 HRC.

High-speed steel like W18Cr4V requires high-temperature quenching (1270–1280°C) to form martensite and carbides, followed by triple tempering at 560°C to convert retained austenite to martensite, resulting in a hardness of 63–66 HRC and excellent wear resistance.

Ductile iron can be treated via austempering at 300–400°C to obtain a microstructure of bainite and retained austenite, balancing strength and toughness.

For 18-8 type austenitic stainless steel, solution treatment (1050–1100°C, water-cooled) is critical to prevent intergranular corrosion. Additionally, stabilization treatment (adding Ti or Nb) helps avoid carbide precipitation when the material is exposed to temperatures between 450–850°C.

5. Defect Control: Prevention & Mitigation

Common heat treatment defects and their countermeasures are as follows:

Quenching Cracks: Caused by thermal/organizational stress or improper processes (e.g., rapid heating, excessive cooling). Prevention measures include preheating, adopting graded or isothermal quenching, and tempering immediately after quenching.

Distortion: Can be corrected via cold pressing, hot straightening (local heating above tempering temperature), or vibratory stress relief. Pre-treatments like normalizing or annealing to eliminate forging stress also minimize distortion.

Burning: Occurs when heating temperature exceeds the solidus line, leading to grain boundary melting and brittleness. Strict temperature monitoring (especially for alloy steels) with thermometers is the key prevention method.

Decarburization: Results from reactions between the workpiece surface and oxygen/CO₂ during heating, reducing surface hardness and fatigue life. It can be controlled by using protective atmospheres (e.g., nitrogen, argon) or salt bath furnaces.

6. Advanced Technologies: Innovation Drivers

Emerging heat treatment technologies are reshaping the industry by enhancing performance and efficiency:

TMCP (Thermomechanical Control Process): Combines controlled rolling and controlled cooling to replace traditional heat treatment, refining grain structures and forming bainite—widely used in shipbuilding steel production.

Laser Quenching: Enables localized hardening with precision up to 0.1mm (ideal for gear tooth surfaces). It uses self-cooling for quenching (no need for media), reducing deformation and increasing hardness by 10–15%.

QP (Quenching-Partitioning): Involves holding below the Ms temperature to allow carbon diffusion from martensite to retained austenite, stabilizing the latter and improving toughness. This process is key for manufacturing third-generation automotive TRIP steel.

Nanobainitic Steel Heat Treatment: Austempering at 200–300°C produces nanoscale bainite and retained austenite, achieving a strength of 2000MPa with better toughness than traditional martensitic steel.

7. Safety & Environmental Protection

Heat treatment accounts for approximately 30% of total energy consumption in mechanical manufacturing, making safety and sustainability critical priorities:

Safety Risk Mitigation: Strict operational protocols are implemented to prevent high-temperature scalds (from heating equipment or workpieces), exposure to toxic gases (e.g., CN⁻, CO from salt bath furnaces), fires (from quenching oil leaks), and mechanical injuries (during hoisting or clamping).

Emission Reduction: Measures include using vacuum furnaces (to avoid oxidative burning), sealing quenching tanks (reducing oil mist volatilization), and installing exhaust gas purification devices (for adsorption or catalytic decomposition of harmful substances).

Wastewater Treatment: Chromium-containing wastewater requires reduction and precipitation treatment, while cyanide-containing wastewater needs detoxification. Comprehensive wastewater undergoes biochemical treatment to meet discharge standards before release.

Conclusion

Heat treatment is a cornerstone of material engineering, bridging raw materials and high-performance components. Mastering its principles, parameters, and innovations is critical for improving product reliability, reducing costs, and advancing sustainable manufacturing in industries like automotive, aerospace, and machinery.