Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other machining processes, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but changes the microstructure inside the workpiece or the chemical composition of the surface of the workpiece to give or improve the performance of the workpiece.
The heat treatment process generally includes three processes of heating, heat preservation, and cooling, and sometimes there are only two processes of heating and cooling. These processes are connected and uninterrupted.
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.
Integral heat treatment is a metal heat treatment process that heats the entire workpiece and then cools it at an appropriate speed to obtain the required metallographic structure to change its overall mechanical properties. The overall heat treatment of iron and steel generally has four basic processes: annealing, normalizing, quenching and tempering, that is, the "four fires" of heat treatment.
The quenching of steel is to heat the steel to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel), hold it for a period of time to make it fully or partially austenitized, and Then, the heat treatment process of martensite (or bainite) transformation is performed by cooling faster than the critical cooling rate to below Ms (or isothermal near Ms).
Tempering is a heat treatment process in which the quenched workpiece is reheated to an appropriate temperature lower than the lower critical temperature, and then cooled to room temperature in air, water, oil and other media after holding it for a period of time.
Normalizing is a metal heat treatment process in which the steel is heated to 30-50°C above the critical temperature (the temperature of complete austenitization), and after holding it for an appropriate time, it is removed from the furnace in the air or cooled by water spray or air blowing.
The heat treatment process of heating the metal or alloy to an appropriate temperature, holding it for a certain period of time, and then slowly cooling (usually cooling with the furnace) is called annealing.