Heat treatment is a metal thermal processing process that places metal materials in a certain medium for heating, insulation, and cooling, and controls its performance by changing the surface or internal metallographic structure of the material.
Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing processes, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but changes the microstructure inside the workpiece. Or change the chemical composition of the workpiece surface to give or improve the performance of the workpiece. Its characteristic is to improve the intrinsic quality of the workpiece, which is generally not visible to the naked eye. In order to make metal workpieces have the required mechanical properties, physical properties and chemical properties, in addition to the reasonable selection of materials and various forming processes, heat treatment processes are often indispensable. Steel is the most widely used material in the mechanical industry. The microstructure of steel is complex and can be controlled through heat treatment. Therefore, the heat treatment of steel is the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium, etc. and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different performance properties.
In the process of progressing from the Stone Age to the Bronze Age and Iron Age, the role of heat treatment was gradually recognized by people. As early as 770 BC to 222 BC, the Chinese had discovered in production practice that the properties of copper and iron would change due to the influence of temperature and pressure deformation. The softening treatment of white cast iron is an important process for manufacturing agricultural tools. In the sixth century BC, steel weapons were gradually adopted. In order to improve the hardness of steel, the quenching process was rapidly developed. Two swords and a halberd unearthed from Yanxiadu, Yixian County, Hebei Province, China, have martensite in their microstructure, indicating that they have been quenched. With the development of quenching technology, people gradually discovered the impact of quenching agent on quenching quality. Pu Yuan, a native of Shu in the Three Kingdoms, once made 3,000 knives for Zhuge Liang in Xiegu, Shaanxi today. According to legend, he sent people to Chengdu to get water for quenching. This shows that China paid attention to the cooling capabilities of different water qualities in ancient times, and also paid attention to the cooling capabilities of oil and urine. The sword unearthed in China from the tomb of King Jing in Zhongshan during the Western Han Dynasty (206 B.C. to 24 A.D.) has a carbon content of 0.15 to 0.4% in the heart, but a carbon content of more than 0.6% on the surface, indicating that the carburizing process has been applied. But at that time, as a secret of personal "craft", it was not allowed to be disclosed, so its development was very slow. In 1863, British metallographers and geologists demonstrated six different metallographic structures of steel under a microscope, proving that the internal structure of steel will change when it is heated and cooled, and the phase at high temperature in the steel will transform during rapid cooling. It is a harder phase. The allotropy theory of iron established by the Frenchman Osmond and the iron-carbon phase diagram first formulated by the British Austin laid the initial theoretical foundation for modern heat treatment technology. At the same time, people have also studied methods to protect metals during the heating process of metal heat treatment to avoid oxidation and decarburization of metals during the heating process. From 1850 to 1880, there were a series of patents for the application of various gases (such as hydrogen, coal gas, carbon monoxide, etc.) for protective heating. From 1889 to 1890, the British man Lake obtained patents for bright heat treatment of various metals. Since the 20th century, the development of metal physics and the transplantation and application of other new technologies have enabled greater development of metal heat treatment processes. A significant development was the application of rotary drum furnaces for gas carburizing in industrial production from 1901 to 1925. In the 1930s, a dew point potentiometer appeared to control the carbon potential of the atmosphere in the furnace. Later, a carbon dioxide infrared meter was developed. , oxygen probes and other methods to further control the carbon potential of the furnace atmosphere; in the 1960s, heat treatment technology used the role of plasma fields to develop ion nitriding and carburizing processes; the application of laser and electron beam technology gave metals new Surface heat treatment and chemical heat treatment methods.