Heat Treatment Overview

Heat treatment refers to a metal processing technique in which materials undergo heating, holding, and cooling in a solid state to achieve the desired microstructure and properties. Depending on the heating and cooling methods, as well as the characteristics of microstructure and property changes, heat treatment can be categorized into the following types:

• General heat treatment
• Surface heat treatment
• Other heat treatments

Historical Development of Heat Treatment

In the 6th century BC, the use of iron and steel weapons gradually became widespread. To enhance the hardness of steel, quenching techniques rapidly developed. Archaeological finds from Yanshi, Hebei Province, China, include two swords and a halberd, all exhibiting martensite in their microstructures, indicating they were quenched. As quenching technology advanced, it became increasingly apparent that the cooling medium significantly affected the quality of quenching.

During the Three Kingdoms period, a craftsman named Pu Yuan from Shu was said to have forged 3,000 swords for Zhuge Liang, allegedly using water from Chengdu for quenching, demonstrating an early awareness in China of how different water qualities influenced cooling effectiveness. The use of both oil and water for cooling was also noted.

The swords excavated from the tomb of King Jing of Zhongshan (206 BC - 24 AD) during the Western Han Dynasty show a carbon content of 0.15% - 0.4% in the core, while the surface had a carbon content exceeding 0.6%, indicating the application of carburizing techniques. However, this knowledge was considered a personal "craft" secret and was not widely shared, leading to slow development.

In 1863, British metallurgists and geologists demonstrated six different metallurgical structures of steel under a microscope, proving that heating and cooling result in internal structural changes. The high-temperature phases of steel transform into harder phases upon rapid cooling. The isomorphism theory of iron established by the Frenchman Osmond, along with the iron-carbon phase diagram developed by the British scientist Auston, laid a theoretical foundation for modern heat treatment processes.

Heat Treatment Overview

Meanwhile, researchers have explored methods to protect metals during the heating process in metal heat treatment to prevent oxidation and decarburization. Between 1850 and 1880, a series of patents were issued for protective heating using various gases (such as hydrogen, gas, and carbon monoxide). In 1889-1890, an Englishman named Lake obtained patents for bright heat treatment of various metals. Since the 20th century, advancements in metal physics and the application of new technologies have significantly progressed heat treatment processes. A notable advancement occurred between 1901 and 1925, when rotary furnaces were used for gas carburizing in industrial production. In the 1930s, dew point potentiometers emerged, allowing for controllable carbon potential in the furnace atmosphere. Subsequent research introduced methods like carbon potential control using carbon dioxide infrared instruments and oxygen probes. In the 1960s, heat treatment technology incorporated plasma fields, leading to the development of ion nitriding and carburizing processes. The application of laser and electron beam technologies also introduced new methods for surface heat treatment and chemical heat treatment of metals.

The Four Types of Heat Treatment

Tempering

Tempering after quenching results in a microstructure known as tempered sorbite. Tempering is generally not used alone; it is performed after the quenching process on components, primarily to eliminate quenching stresses and achieve the desired microstructure. Depending on the tempering temperature, it can be classified into low, medium, and high-temperature tempering, resulting in tempered martensite, troostite, and sorbite, respectively.

The combination of high-temperature tempering following quenching is known as quenching and tempering, aimed at achieving a balance of strength, hardness, plasticity, and toughness for comprehensive mechanical properties. This process is widely used in important structural components in automobiles, tractors, and machine tools, such as connecting rods, bolts, gears, and shafts. After tempering, the hardness typically ranges from HB200 to HB330.

Annealing

During the annealing process, pearlite transformation occurs. The main purpose of annealing is to bring the internal microstructure of the metal to or near a balanced state, preparing for subsequent processing and final heat treatment. Stress relief annealing is performed to eliminate residual stresses caused by processes such as plastic deformation, welding, and those inherent in castings. Workpieces subjected to forging, casting, welding, and machining contain internal stresses that, if not addressed promptly, can lead to deformation during processing and use, affecting precision.

Using stress relief annealing to eliminate internal stresses generated during processing is crucial. The heating temperature for stress relief annealing is below the phase transformation temperature, so no microstructural changes occur during the entire heat treatment process. Internal stresses are primarily alleviated through natural relaxation during the holding and slow cooling phases.

Quenching

Quenching involves heating the metal workpiece or part above the phase transformation temperature, holding it, and then rapidly cooling it at a rate greater than the critical cooling rate to achieve a martensitic structure. The primary goals of quenching are:

1. Enhancing Mechanical Properties: For example, improving the hardness and wear resistance of tools and bearings, increasing the elastic limit of springs, and enhancing the overall mechanical performance of shaft components.

2. Improving Material Properties: For certain special steels, such as increasing the corrosion resistance of stainless steel or enhancing the permanent magnetism of magnetic steel.

During quenching, it is essential to select the appropriate quenching medium and employ the correct quenching method. Common quenching methods include single-liquid quenching, double-liquid quenching, staged quenching, isothermal quenching, and localized quenching.

Normalizing

Normalizing is characterized by air cooling, meaning that environmental temperature, stacking methods, airflow, and workpiece dimensions all influence the structure and performance post-normalization. The normalized structure can also serve as a classification method for alloy steels. Typically, samples with a diameter of 25 mm are heated to 900°C and air-cooled to achieve structures that categorize alloy steels into pearlitic, bainitic, martensitic, and austenitic steels.

1. For hypoeutectoid steels, normalizing is used to eliminate coarse grain structures and Widmanstätten structures in castings, forgings, and welds; refine grain size; and can serve as pre-heat treatment before quenching.

2. For hypereutectoid steels, normalizing can eliminate networked secondary cementite and refine pearlite, improving mechanical properties and benefiting subsequent spheroidizing annealing.

3. For low-carbon deep-drawing thin steel plates, normalizing can eliminate free cementite at grain boundaries to improve deep-drawing performance.

4. For low-carbon and low-carbon low-alloy steels, normalizing can produce a significant amount of fine lamellar pearlite, increasing hardness to HB140-190, thus avoiding "galling" during cutting and improving machinability. In cases where both normalizing and annealing are applicable for medium-carbon steels, normalizing is more economical and convenient.

5. For ordinary medium-carbon structural steels with less stringent mechanical performance requirements, normalizing can replace quenching followed by high-temperature tempering, offering simplicity in operation while stabilizing the microstructure and dimensions of the steel.

6. High-temperature normalizing (above Ac3, by 150-200°C) can reduce composition segregation in cast and forged parts due to higher diffusion rates at elevated temperatures. Coarse grains from high-temperature normalization can be refined by subsequent lower-temperature normalization.

7. For certain low and medium-carbon alloy steels used in turbines and boilers, normalizing is often employed to achieve a bainitic structure, followed by high-temperature tempering for good creep resistance at 400-550°C.

8. In addition to steel parts and materials, normalizing is also widely used in the heat treatment of ductile iron to achieve a pearlitic matrix, enhancing the strength of ductile iron.