Introduction to Metal Heat Treatment Processes

Introduction to Metal Heat Treatment Processes

1. Annealing

Objective: 1. Reduce hardness, increase ductility, and improve machinability and formability; 2. Refine the grain structure, enhance mechanical properties, and prepare for subsequent processing steps; 3. Eliminate residual stresses induced by cold and hot working.

Key points of application: 1. Applicable to forgings, weldments, and raw materials in non‑conforming condition for alloy structural steels, carbon tool steels, alloy tool steels, and high-speed steels; 2. Annealing is generally performed in the as‑received (blank) condition.

2. Normalizing

Objective: 1. Reduce hardness, increase ductility, and improve machinability and formability; 2. Refine the grain structure, enhance mechanical properties, and prepare for subsequent processing steps; 3. Eliminate residual stresses induced by cold and hot working.

Key points of application: Normalizing is typically used as a preliminary heat‑treatment process for forgings, welded components, and carburized parts. For low‑carbon and medium‑carbon carbon structural steels and low‑alloy steel parts with modest performance requirements, it may also serve as the final heat treatment. However, for most medium‑ and high‑alloy steels, air cooling can result in full or local quenching; therefore, normalizing should not be employed as the final heat‑treatment step.

3. Quenching

Objective: Quenching is generally performed to obtain a high‑hardness martensitic microstructure; in certain cases, for specific high‑alloy steels, quenching is carried out to produce a single, uniform austenitic microstructure, thereby enhancing wear resistance and corrosion resistance.

Key points of application: 1 It is generally used for carbon steels and alloy steels with a carbon content exceeding 0.3%; 2 Quenching can fully exploit the strength and wear resistance of steel, but it also generates significant internal stresses and reduces the material’s ductility and impact toughness; therefore, tempering is necessary to achieve improved overall mechanical properties.

4. Tempering

Objective: 1. Reduce or eliminate residual stresses after quenching, thereby minimizing workpiece distortion and cracking; 2. Adjust hardness, enhance ductility and toughness, and achieve the mechanical properties required for service; 3. Stabilize workpiece dimensions.

Key points of application: 1. Low‑temperature tempering is used to maintain the high hardness and wear resistance of steel after quenching; medium‑temperature tempering is employed to enhance the steel’s elasticity and yield strength while preserving a certain level of toughness; and high‑temperature tempering is applied when the primary objective is to retain high impact toughness and ductility, along with adequate strength. 2. For ordinary steels, tempering in the range of 230–280°C should be avoided as much as possible, and for stainless steels, tempering between 400–450°C should be avoided, since these temperature ranges can give rise to first‑kind temper embrittlement.

5. Quenching and tempering

Objective: 1. Improve machinability and enhance surface finish; 2. Reduce distortion and cracking during quenching; 3. Achieve favorable overall mechanical properties.

Key points of application: 1. It is suitable for alloy structural steels, alloy tool steels, and high-speed steels with high hardenability; 2. It can serve not only as the final heat treatment for various critical components but also as a preliminary heat treatment for certain precision parts, such as lead screws, to minimize distortion.

6. Statute of limitations

Objective: 1. Stabilize the microstructure of steel parts after quenching, thereby reducing deformation during storage or service; 2. Relieve residual stresses induced by quenching and grinding, ensuring dimensional and shape stability.

Key points of application: 1. Suitable for all steel grades after quenching; 2. Commonly used for precision components that must maintain their shape, such as precision lead screws, measuring tools, machine bed housings, and the like.

7. Cold treatment

Objective: 1. Convert all or most of the residual austenite in quenched steel parts into martensite, thereby enhancing the hardness, strength, wear resistance, and fatigue limit of the steel; 2. Stabilize the microstructure of the steel to ensure dimensional and shape stability of the component.

Key points of application: 1. After quenching, steel parts should be immediately subjected to cryogenic treatment, followed by low-temperature tempering, to eliminate internal stresses induced during low-temperature cooling. 2. Cryogenic treatment is primarily suitable for precision cutting tools, measuring instruments, and precision components made of alloy steels.

8. Flame surface hardening

Objective: To enhance the surface hardness, wear resistance, and fatigue strength of steel components while maintaining toughness in the core.

Key points of application: 1. Primarily used for workpieces made of medium-carbon steel, with a typical hardening depth of 2–6 mm; 2. Suitable for large workpieces produced in single pieces or small batches, as well as for workpieces requiring localized quenching.

9. Induction Surface Hardening

Objective: To enhance the surface hardness, wear resistance, and fatigue strength of steel components while maintaining toughness in the core.

Key points of application: 1. It is primarily used for parts made of medium-carbon steel and medium-carbon alloy structural steel. 2. Due to the skin effect, the hardened layer depth in high-frequency induction hardening is typically 1–2 mm, in medium-frequency hardening 3–5 mm, and in high-frequency hardening generally exceeds 10 mm.

10. Carburizing

Objective: To enhance the surface hardness, wear resistance, and fatigue strength of steel components while maintaining toughness in the core.

Key points of application: 1. This process is used for parts made of low-carbon steel and low-alloy steel with a carbon content of 0.15% to 0.25%; the typical case depth after carburizing is 0.5 to 2.5 mm. 2. Following carburization, quenching must be performed to transform the surface into martensite, thereby achieving the intended purpose of carburization.

11. Nitridation

Objective: To enhance the surface hardness, wear resistance, fatigue strength, and corrosion resistance of steel components.

Key application points: This process is commonly used for medium-carbon alloy structural steels containing alloying elements such as aluminum, chromium, and molybdenum, as well as for carbon steels and cast irons. Typically, the nitrided layer depth is… 0.025~0.8mm.

12. Nitrocarburizing

Objective: To enhance the surface hardness, wear resistance, fatigue strength, and corrosion resistance of steel components.

Key points of application: 1. It is mainly used for parts made of low-carbon steel, low-alloy structural steel, and tool steel; the typical nitrided layer depth ranges from 0.02 to 3 mm. 2. After nitriding, quenching and low-temperature tempering are also required.