Spheroidizing annealing process

Materials suitable for spheroidizing annealing usually refer to eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc., and the general equipment is spheroidizing annealing furnace. Spheroidizing annealing is also called spheroidizing annealing. Incomplete annealing is used to obtain spheroidal pearlite structure of hypereutectoid steel. Spheroidizing annealing is a kind of incomplete annealing with a small temperature range of 20-30 ℃ on the AC1 line. The alloy steel of low and medium carbon steel can have uniform structure, fine grain, low hardness and easy processing after full annealing. High carbon alloy steel contains large amount of carbide, which affects the strength of the matrix. The purpose of spheroidizing annealing is to change the large amount of carbide into fine dispersed carbide and reduce the influence on the strength of the matrix.

The heating temperature of spheroidizing annealing is AC1 + (20-40) ℃ or ACM - (20-30) ℃. After heat preservation, it is cooled isothermal or directly and slowly. During spheroidizing annealing, austenite is incomplete, but the lamellar pearlite transforms into austenite and a small amount of excess carbide dissolves. Therefore, it is impossible to eliminate network carbide. If there is network carbide in hypereutectoid steel, normalizing must be carried out before spheroidizing annealing to eliminate it so as to ensure normal spheroidizing annealing.

There are many spheroidizing annealing methods, the two most commonly used are ordinary spheroidizing annealing and isothermal spheroidizing annealing. The ordinary spheroidizing annealing is to heat the steel to 20-30 ℃ above AC1 for a proper time, then cool it slowly with the furnace, cool it to about 500 ℃ and air cool it out of the furnace. The isothermal spheroidizing annealing process is the same as that of the ordinary spheroidizing annealing process. After the furnace is cooled to a temperature slightly lower than AR1, the isothermal time is 1.5 times of its heating and holding time. After isotherm, it will be cooled to about 500 ℃ with the furnace and air cooled. Compared with ordinary spheroidizing annealing, isothermal spheroidizing annealing can not only shorten the period, but also make the spheroidizing structure uniform, and strictly control the hardness after annealing.

Spheroidizing annealing is mainly used for hypereutectoid carbon steel and alloy tool steel (such as steel grades used for manufacturing cutting tools, measuring tools and dies). Its main purpose is to reduce the hardness, improve the machinability, and prepare for later quenching. This process is good for plastic processing and cutting, and also can improve the mechanical toughness. Especially for bearing steel, tool steel and other steel, if spheroidizing annealing is carried out before quenching, the following effects can be obtained:

Bearing steel: uniform quenching effect; reduction of quenching deformation; improvement of quenching hardness; improvement of workpiece cutting performance; improvement of bearing performance such as wear resistance and pitting resistance.

Tool steel: uniform quenching effect; inhibition of quenching crack and bending; improvement of wear resistance, sharpness of blade and service life.

According to steel grades and annealing purposes, spheroidizing annealing can be divided into the following types:

(1) General spheroidizing annealing, that is, the steel is heated to 730-740 ℃ for enough time, and then slowly cooled to 650 ℃ at a speed of less than 20 ℃ / h. This annealing process is suitable for carbon tool steel near eutectoid composition.

(2) Periodic spheroidizing annealing is also called cyclic annealing. It is heated and cooled repeatedly at the temperature near point a, generally for 3-4 cycles, so that the carbide can be spheroidized in the repeated process of several dissolution and precipitation of lamellar pearlite. The process has a long production cycle, inconvenient operation and is difficult to control. It is suitable for steel with severe pearlite flakes.

(3) Isothermal spheroidizing annealing. Generally, it is heated to 800 ± 10 ℃, cooled to 700 ± 10 ℃ (near A1) after heat preservation for a long time, and then cooled to 600 ℃ at the rate of 30-50 ℃ / h. This process is usually used for bearing steel.

(4) Deformation spheroidizing annealing. By combining plastic deformation with spheroidizing annealing process, the dislocation density and distortion energy in the steel increase due to plastic deformation, which accelerates the dissolution and spheroidization of sheet carbide during annealing, so as to accelerate the spheroidization speed and shorten the spheroidizing annealing time.

According to the different deformation systems, it can be divided into:

(1) The steel is heated to the temperature between ACM and AC1 for plastic deformation, and then cooled to a little lower than AC1 for spheroidizing annealing;

(2) The steel is spheroidized and annealed directly after high temperature finishing and rapid cooling to a certain temperature;

(3) Spheroidizing annealing of steel after cold deformation at a temperature slightly lower than AC1.

The heating temperature of spheroidizing annealing is the key factor to affect the degree of spheroidization. The proper heating temperature can not only ensure the disappearance of lamellar pearlite, but also retain part of the carbide which is not completely dissolved in austenite. As the spheroidizing core, the normal spheroidizing structure of coarser granular carbide is formed. When austenitizing temperature is very high, all carbides are dissolved and homogenized, and flake pearlite is always obtained after cooling. The cooling rate directly affects the size and uniformity of carbide particles. Too fast cooling, too fine carbide particles, and the possibility of forming lamellar carbide, making the hardness higher. When the cooling is too slow, the carbide particles are too coarse.

When the pearlite before spheroidization is thin and the carbide is fine and dispersed, the degenerated pearlite structure obtained by thermomechanical treatment is most likely to be spheroidized, and the spheroidization time can be shortened, the spheroidization quality and the fatigue life of the steel can be improved.