On 2018-08-08 14:48:54
Key points of integral heat treatment process
Metal heat treatment is a kind of technology which can heat, heat and cool the metal workpiece in a certain medium, and control its performance by changing the surface or internal structure of the metal material. Heat treatment process generally includes three processes: heating, heat preservation and cooling, sometimes only heating and cooling. These processes are interlinked and uninterrupted. Heating is one of the most important processes in heat treatment.
When the metal is heated, the workpiece is exposed to the air, which often occurs oxidation and decarburization (that is, the carbon content on the surface of steel parts is reduced), which has a very adverse effect on the surface performance of parts after heat treatment. Therefore, the metal should be heated in controllable or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging.
Heating temperature is one of the important process parameters of heat treatment process. The selection and control of 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 heat treatment, but it is generally heated above the phase change temperature to obtain high temperature structure. In addition, the transformation needs a certain time, so when the surface of the metal workpiece reaches the required heating temperature, it must be kept at this temperature for a certain time, so that the internal and external temperatures are the same, and the microstructure is completely transformed, which 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 heat preservation time, while the heat preservation time of chemical heat treatment is often longer.
Cooling is also an indispensable step in the process of heat treatment. The cooling method varies with the process, mainly controlling the cooling speed. Generally, the cooling speed of annealing is slower, that of normalizing is faster, and that of quenching is faster. But there are different requirements due to different kinds of steel. For example, air hardening steel can be quenched at the same cooling rate as normalizing.
Metal heat treatment process can be generally divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. Next, we will focus on the overall heat treatment of the relevant process points!
The whole heat treatment is a metal heat treatment process that heats the whole workpiece and cools it at a proper speed to change its whole mechanical properties. There are four basic heat treatment processes of iron and steel: annealing, normalizing, quenching and tempering.
Annealing is a metal heat treatment process that heats the workpiece to a proper temperature, holds it for a certain period of time, and then cools it slowly. The purpose is to make the internal structure of the metal reach or close to the equilibrium state, and obtain good process performance and service performance. In addition, annealing is often used as the pretreatment of quenching.
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, but the microstructure obtained is finer, which is often used to improve the cutting performance of materials.
Quenching is the rapid cooling of workpiece in water, oil or other inorganic salt, organic water solution and other quenching medium after heating and heat preservation. After quenching, the steel becomes hard but brittle at the same time. The purpose of quenching is to improve the hardness of steel.
In order to reduce the brittleness of quenched steel, the quenched steel is kept warm for a long time at a proper temperature higher than room temperature but lower than 710 ℃, and then cooled. This process is called tempering.