Popular science 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 after it is kept at this temperature for a certain period of time, it is 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 been recognized. As early as 770 to 222 BC, the Chinese found 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 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 quenching agent 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 over 0.6% on the surface, indicating that the carburizing process has been applied. But at that time, as a secret of personal “handsmanship”, 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 brought new metal Surface heat treatment and chemical heat treatment methods.

Metal heat treatment process

The heat treatment process generally includes three processes of heating, heat preservation and cooling, sometimes only two processes of heating and cooling. These processes are connected and uninterrupted.

Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment. The earliest was to use charcoal and coal as heat sources, and then to use liquid and gas fuels. The application of electricity makes heating easy to control without environmental pollution. These heat sources can be used for direct heating, or indirect heating by molten salt or metal, or even floating particles.

When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization often occur (that is, the carbon content on the surface of the steel parts is reduced), which has a very negative effect on the surface properties of the parts after heat treatment. Therefore, metals should usually be heated in a controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be heated by coating or packaging methods.

The heating temperature is one of the important process parameters of the heat treatment process. The selection and control of the heating temperature is the main issue to ensure the quality of the heat treatment. The heating temperature varies with the metal material being processed and the purpose of the heat treatment, but it is generally heated above the phase transition temperature to obtain a high-temperature structure. In addition, the transformation takes a certain time. Therefore, when the surface of the metal workpiece reaches the required heating temperature, it must be maintained at this temperature for a certain period of time to make the internal and external temperatures consistent and complete the microstructure transformation. This time is called the holding time. When high-energy density heating and surface heat treatment are used, the heating speed is extremely fast and there is generally no holding time, while the holding time of chemical heat treatment is often longer.

Cooling is also an indispensable step in the heat treatment process. The cooling method varies from process to process, and the main thing is to control the cooling rate. Generally, annealing has the slowest cooling rate, normalizing cooling rate is faster, and quenching cooling rate is faster. However, there are different requirements due to different steel grades. For example, hollow hard steel can be quenched at the same cooling rate as normalizing.

Metal heat treatment processes can be roughly divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. According to the different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. The same metal uses different heat treatment processes to obtain different structures and thus have different properties. Steel is the most widely used metal in the industry, and the microstructure of steel is also the most complex, so there are many types of steel heat treatment processes.

Overall heat treatment is a metal heat treatment process that heats the entire workpiece and then cools it at an appropriate speed to change its overall mechanical properties. The overall heat treatment of steel has four basic processes: annealing, normalizing, quenching and tempering.

Annealing is to heat the workpiece to an appropriate temperature, use different holding times according to the material and workpiece size, and then perform slow cooling. The purpose is to make the internal structure of the metal reach or close to the equilibrium state, obtain good process performance and use performance, or for further quenching Prepare for organization. Normalizing is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to annealing, except that the resulting structure is finer. It is often used to improve the cutting performance of materials, and sometimes used for parts that are not demanding. As the final heat treatment.

Quenching is to quickly cool the workpiece in a quenching medium such as water, oil, other inorganic salts, and organic aqueous solutions after heating and holding the workpiece. After quenching, the steel becomes hard, but at the same time it becomes brittle. In order to reduce the brittleness of steel parts, the quenched steel parts are kept at an appropriate temperature higher than room temperature and lower than 650°C for a long time, and then cooled. This process is called tempering. Annealing, normalizing, quenching, and tempering are the “four fires” in the overall heat treatment. Quenching and tempering are closely related and are often used in conjunction with each other.

With the difference in heating temperature and cooling method, the “four fires” have evolved different heat treatment processes. In order to obtain a certain strength and toughness, the process of combining quenching and high temperature tempering is called quenching and tempering. Some alloys are quenched After the supersaturated solid solution is formed, it is placed at room temperature or a slightly higher appropriate temperature for a longer period of time to improve the hardness, strength, or electrical magnetic properties of the alloy. This heat treatment process is called aging treatment.

The method of effectively and tightly combining pressure processing deformation and heat treatment to obtain good strength and toughness of the workpiece is called thermomechanical heat treatment; heat treatment in a negative pressure atmosphere or vacuum is called vacuum heat treatment, which can not only make The workpiece is not oxidized or decarburized. It keeps the surface of the workpiece smooth and clean after treatment and improves the performance of the workpiece. It can also be injected with a penetrant for chemical heat treatment. Surface heat treatment is a metal heat treatment process that only heats the surface of the workpiece to change the mechanical properties of the surface. In order to heat only the surface of the workpiece without causing too much heat to be transferred into the workpiece, the heat source used must have a high energy density, that is, to give a larger heat energy to the workpiece per unit area, so that the surface or part of the workpiece can be short-term or instantaneous Reach high temperatures. The main methods of surface heat treatment include flame quenching and induction heating heat treatment. Commonly used heat sources include flames such as oxygen acetylene or oxypropane, induction current, laser, and electron beam.

Chemical heat treatment is a metal heat treatment process that changes the chemical composition, organization and performance of the surface of the workpiece. The difference between chemical heat treatment and surface heat treatment is that the latter changes the chemical composition of the surface of the workpiece. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, nitrogen or other alloying elements, and keep it for a long time, so that the surface of the workpiece is infiltrated with elements such as carbon, nitrogen, boron and chromium. After infiltration of elements, other heat treatment processes such as quenching and tempering are sometimes required. The main methods of chemical heat treatment are carburizing, nitriding and metalizing.

Heat treatment is one of the important processes in the manufacturing process of mechanical parts and molds. Generally speaking, it can guarantee and improve various properties of the workpiece, such as wear resistance and corrosion resistance. It can also improve the structure and stress state of the blank to facilitate various cold and hot processing.

For example, white cast iron can obtain malleable cast iron after long-term annealing treatment, which can improve plasticity; gears adopt the correct heat treatment process, and the service life can be doubled or tens of times longer than gears without heat treatment; in addition, cheap carbon steel can be penetrated Certain alloying elements have the properties of some expensive alloy steels, which can replace certain heat-resistant steels and stainless steels; almost all tools and molds require heat treatment before they can be used.