The purpose and process description of recrystallization annealing process

The recrystallization annealing process of steel is: slowly heating to Ac3 (subeutectoid steel) or Ac1 (eutectoid steel or hypereutectoid steel) above 30 ~ 50℃, keeping appropriate time, and then slowly cooling down. By heating during pearlite (or there are proeutectoid ferrite and cementite) into austenitic phase transformation recrystallization (back) and cooling occurred in the process of phase transition back to the contrary, recrystallization, form the crystal grain more fine, slice layer thicker, organize uniform pearlite (or there are proeutectoid ferrite and cementite). Where the annealing temperature is above Ac3 (subeutectoid steel), the recrystallization of steel is called annealing, and the annealing temperature is between Ac1 and Ac3 (subeutectoid steel) or between Ac1 and Acm (supereutectoid steel), and the partial recrystallization of steel is called non-annealing. The former is mainly used for subeutectoid steel castings, forging and rolling parts, welding parts, in order to eliminate the microstructure defects (such as weytenstein structure, banded structure, etc.), to make the microstructure finer and more uniform, so as to improve the plasticity and toughness of steel parts. The latter is mainly used for forging and rolling of medium carbon and high carbon steel and low alloy structural steel. If the cooling speed of the forging and rolling parts is larger, the pearlite formed is smaller and the hardness is higher. If stop forging, stop rolling temperature is too low, there is a large internal stress in the steel. At this time can be used to replace annealing annealing, pearlite recrystallization, grain thinning, but also reduce the hardness, eliminate the internal stress, improve the machinability. In addition, the spheroidization annealing of supereutectoid steel at the annealing temperature between Ac1 and Acm is also annealing.

One of the main process parameters of annealing is the high heating temperature (annealing temperature). The selection of annealing heating temperature of most alloys is based on the phase diagram of the alloy system, such as carbon steel based on the iron-carbon balance diagram. The annealing temperature of all kinds of steel (including carbon steel and alloy steel) is at a certain temperature of Ac3 above, Ac1 above or below, depending on the specific purpose of annealing. The annealing temperature of various non-ferrous alloys is below the solid phase line temperature, above the solid solubility line temperature or below a certain temperature.

Recrystallization annealing is applied to alloys in which solid phase transition (recrystallization) occurs during equilibrium heating and cooling. The annealing temperature is a temperature above or below the phase transition temperature range of the alloy. Heating and cooling are slow. Recrystallization occurs in the process of heating and cooling, so it is called recrystallization annealing, which is often referred to as annealing.