Basic knowledge of metal heat treatment

Metal heat treatment is a process in which a metal workpiece is heated to an appropriate temperature in a certain medium, and then kept at this temperature for a certain period of time, and then cooled at different speeds.

Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing technologies, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but by changing the microstructure inside the workpiece or changing the chemical composition of the surface of the workpiece , To give or improve the performance of the workpiece. Its characteristic is to improve the internal quality of the workpiece, which is generally not visible to the naked eye.

In order to make the metal workpiece 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 machinery industry. The microstructure of steel is complex and can be controlled by 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 from the Stone Age to the Bronze Age and the Iron Age, the role of heat treatment has gradually become known to people. As early as 770 to 222 BC, the Chinese found in production practice that the properties of copper and iron would change due to temperature and pressure deformation. The softening treatment of white cast iron is an important process for manufacturing farm tools.

In the sixth century BC, steel weapons were gradually adopted. In order to increase the hardness of steel, the quenching process developed rapidly. Two swords and a halberd unearthed in Yanxiadu, Yixian County, Hebei Province, China, have martensite in their microstructures, indicating that they were quenched.

With the development of quenching technology, people gradually discovered the influence of refrigerant on quenching quality. Pu Yuan, a Shu native of the Three Kingdoms, once made 3,000 knives for Zhuge Liang in Xiegu, Shaanxi. According to legend, he sent people to Chengdu to fetch water for quenching. This shows that China has paid attention to the cooling capacity of different water quality in ancient times, as well as the cooling capacity of oil and urine. The sword in the tomb of King Jing of Zhongshan in the Western Han Dynasty (206 BC to 24 AD) unearthed in China 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 “craftsmanship”, he refused to spread it out, so its development was slow.

In 1863, British metallurgists and geologists showed six different metallographic structures of steel under the microscope, which proved that when steel is heated and cooled, the internal structure changes, and the phase of steel at high temperature changes when it is quenched. It is a harder phase. The allotrope theory of iron established by the Frenchman Osmond, and the iron-carbon phase diagram first established by the Englishman Austin, laid a preliminary theoretical foundation for modern heat treatment technology. At the same time, people have also studied the method of protecting the metal during the heating process of the metal heat treatment to avoid the oxidation and decarburization of the metal 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 Laker 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 led to greater development of metal heat treatment processes. A significant progress was the use of rotary drum furnaces for gas carburization in industrial production from 1901 to 1925; a dew point potentiometer appeared in the 1930s to make the carbon potential of the furnace atmosphere controllable. Later, the use of carbon dioxide infrared meters was developed. , Oxygen probe, etc. to further control the carbon potential of the furnace atmosphere; in the 1960s, the heat treatment technology used the role of the plasma field to develop ion nitriding and carburizing processes; the application of laser and electron beam technology also made metal new The surface heat treatment and chemical heat treatment methods.