Metal Heat Treatment Operation Process Steps

Heat treatment process generally includes heating, insulation, cooling three processes, sometimes only heating and cooling two processes. These processes are linked to each other and cannot be interrupted.
Heating is one of the important steps in heat treatment. There are many heating methods for metal heat treatment, the earliest is the use of charcoal and coal as a heat source, and then the application of liquid and gas fuels. The application of electricity enables easy control of heating and no environmental pollution. These heat sources can be used for direct heating, or indirect heating through 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 impact on the surface properties of the parts after heat treatment. Therefore, the metal should usually be heated in a controlled or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging methods.
Heating temperature is one of the important process parameters of heat treatment process, and selecting and controlling heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the metal material being treated and the purpose of the heat treatment, but it is generally heated above the phase transition temperature to obtain the desired tissue. In addition, the transformation takes a certain time, so when the surface of the metal workpiece reaches the required heating temperature, it is necessary to maintain a certain time at this temperature, so that the internal and external temperature is consistent, so that the microstructure transformation is complete, this time is called insulation time. When high energy density heating and surface heat treatment are used, the heating speed is very fast, and generally there is no holding time or holding time is very short, while the holding time of chemical heat treatment is often longer.
Cooling is also an indispensable step in the process of heat treatment. The cooling method varies from process to process, mainly to control the cooling rate. General annealing cooling speed is the slowest, normalizing cooling speed is faster, quenching cooling speed is faster. However, there are different requirements for different types of steel, such as air-hard steel can be quenched with the same cooling rate as normal.
Metal heat treatment process can be divided into overall heat treatment, surface heat treatment, local heat treatment and chemical heat treatment. According to the different heating medium, heating temperature and cooling method, each category can be distinguished into a number of different heat treatment processes. The same metal adopts different heat treatment process, can obtain different structure, thus has different properties. Iron and steel is the most widely used metal in industry, and the microstructure of iron and steel is the most complex, so there are many kinds of iron and steel heat treatment processes.
Integral heat treatment is a metal heat treatment process that heats the workpiece as a whole and then coals it at an appropriate rate to change its overall mechanical properties. The overall heat treatment of iron and steel roughly has annealing, normalizing, quenching and tempering four basic processes.

Normalizing is to heat the workpiece to the appropriate temperature and then cool it in the air. The effect of normalizing is similar to annealing, but the obtained structure is finer. It is often used to improve the cutting performance of materials, and sometimes used for some parts with low requirements as final heat treatment.
Quenching is the workpiece heating and insulation, in water, oil or other inorganic salts, organic aqueous solution and other quenching medium rapid cooling. After quenching, the steel becomes hard, but at the same time becomes brittle.
In order to reduce the brittleness of steel parts, the quenched steel parts are kept warm for a long time at an appropriate temperature higher than room temperature but lower than 710℃, and then cooled. This process is called tempering.
Annealing, normalizing, quenching and tempering are the “four fires” in the overall heat treatment, among which quenching and tempering are closely related, often used together, none is dispensable.
“Four fires” with different heating temperatures and cooling methods, and evolved into different heat treatment processes. In order to obtain a certain strength and toughness, the process of quenching and high temperature tempering is called quenching and tempering. After some alloys are quenched to form supersaturated solid solutions, they are kept at room temperature or slightly higher appropriate temperature for a long time to improve the hardness, strength or electrical magnetism of the alloy. Such a heat treatment process is called aging treatment. The pressure machining deformation and heat treatment effectively and closely combined, so that the workpiece to obtain a good strength, toughness with the method called deformation heat treatment; Heat treatment in negative pressure atmosphere or vacuum is called vacuum heat treatment. It can not only make the workpiece not oxidized, not decarburized, keep the surface of the workpiece clean after treatment, improve the performance of the workpiece, but also carry out chemical heat treatment through the infiltration agent.
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 only heat the surface of the workpiece without making too much heat into the workpiece, the heat source used must have a high energy density, that is, to give greater heat energy on the workpiece per unit area, so that the workpiece surface or local can reach high temperature for a short time or instantaneous. The main methods of surface heat treatment are laser heat treatment, flame quenching and induction heating heat treatment, commonly used heat sources such as aerobic acetylene or propylene oxide flame, induction current, laser and electron beam.
Chemical heat treatment is a metal heat treatment process by changing the chemical composition, structure and performance of the workpiece surface. 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 put the workpiece in the medium containing carbon, nitrogen or other alloying elements (gas, liquid, solid) heating, insulation for a long time, so that the workpiece surface layer into carbon, nitrogen, boron and chromium and other elements. After infiltration of elements, other heat treatment processes such as quenching and tempering are sometimes carried out. The main methods of chemical heat treatment are carburizing, nitriding, metal burburizing, compound burburizing and so on.
Heat treatment is one of the important processes in the manufacturing of mechanical parts and tooling. In general, it can ensure and improve the performance of the workpiece, such as wear resistance, corrosion resistance and so on. It can also improve the structure and stress state of the blank, so as to facilitate all kinds of cold and hot processing. For example, white cast iron can be annealed for a long time to obtain malleable cast iron and improve plasticity. The gear adopts the correct heat treatment process, the service life can be multiplied or dozens of times higher than the gear without heat treatment; In addition, the cheap carbon steel through the infiltration of some alloying elements has some price of alloy steel properties, can replace some heat-resistant steel, stainless steel; Almost all tools and moulds need to be heat treated before they can be used.