The process of heating metal or alloy to a suitable temperature, holding a certain period of time and then slowly cooling is called annealing.
The essence of annealing is to turn the steel into pearlite after austenitizing, and the microstructure after annealing is close to the equilibrium microstructure.
The purpose of annealing:
(1) reduce the hardness of the steel, improve the plasticity, easy to machine and cold deformation processing;
(2) the chemical composition and microstructure of the homogeneous steel, refining the grain, improving the properties of the steel or making the microstructure for quenching
(3) eliminate internal stress and work hardening so as to prevent deformation and cracking.
Annealing and normalizing are mainly used for preparatory heat treatment, and annealing and normalizing can also be used as the final heat treatment for parts with little stress and low performance requirements.
Classification of annealing methods
The commonly used annealing method is divided into two parts according to the heating temperature:
Recrystallization annealing at critical temperature (Ac1 or Ac3) above: full annealing, diffusion annealing, incomplete annealing, spheroidizing annealing.
Annealing under critical temperature (Ac1 or Ac3): recrystallization annealing and stress relieving annealing.
1, full annealing
Process: the steel is heated to Ac3 or above 20~30 C, after a period of time and then cooled slowly (along with the furnace) to obtain the heat treatment process close to the equilibrium structure (complete austenitizing).
Full annealing is mainly used for sub eutectoid steels (wc=0.3~0.6%), generally medium carbon steels and low, medium carbon alloy steel castings, forgings and hot rolled profiles, and sometimes for their weldments. The lower hardness of low carbon steel after full annealing, is not conducive to the cutting process; hypereutectoid steel is heated to above Accm austenitic state slow cooling annealing, Fe3C II will mesh along the grain boundaries, the strength, hardness, plasticity and toughness of the steel is significantly reduced, to the final heat treatment to stay hidden.
Objective: to refine grain size, uniform microstructure, eliminate internal stress, reduce hardness and improve machinability of steel. The microstructure of the eutectoid steel after annealing is F+P.
In actual production, in order to improve the productivity, the annealing cooling is about 500 degrees Celsius, then the air cooling is released.
2. Isothermal annealing
Full annealing takes a long time, especially for undercooled and relatively stable alloy steels. If the austenite steel is cooled rapidly to a little lower than the Ar1 temperature isothermal, A is converted to P, and then air cooled to room temperature, which can greatly shorten the annealing time. This annealing method is called isothermal annealing.
Process: heating the steel above the Ac3 (or Ac1) temperature, heat preservation time after appropriate, rapid cooling to a temperature of pearlite zone, and isothermal transformation of austenite to pearlite, the heat treatment process, and then cooled to room temperature.
Applicable to A stable steel: high carbon steel (wc>0.6%), alloy tool steel, high alloy steel (alloy >10% total). Isothermal annealing is also beneficial to obtain uniform microstructure and properties. But it is not suitable for large section steel parts and large batch of burden, because isothermal annealing is not easy to make the workpiece internal or batch work to achieve isothermal temperature.
3, incomplete annealing
Process: the steel is heated to Ac1~Ac3 (hypo eutectoid steel) or Ac1~Accm (super eutectoid steel) and cooled slowly after heat preservation to obtain the heat treatment process close to the equilibrium structure.
It is mainly used in hypereutectic steel to obtain spherical pearlite structure, so as to eliminate internal stress, reduce hardness and improve machinability. Spheroidizing annealing is a kind of incomplete annealing.
4. Spheroidizing annealing
A heat treatment process for spheroidization of carbides in steel and obtaining granular pearlite.
When the temperature is above 20~30, the holding time should not be too long, and 2~4h is suitable. The cooling method usually adopts the furnace cooling, or the isothermal time is longer than Ac1 or less than 20 Ar1.
It is mainly used in eutectoid steel and super eutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc.. The microstructure of eutectoid steel after cold rolling and forging is lamellar pearlite and reticulated cementite, which is hard and brittle, not only difficult to be machined, but also easy to deform and crack in the process of quenching. Spherical pearlite is formed by spheroidizing annealing, and cementite particles are spherical and dispersed on the ferrite matrix in spherical pearlite. Compared with flake pearlite, spheroidal pearlite has low hardness, easy cutting, and the austenite grain is not easy to be coarse when quenching heating, and the deformation and cracking tendency is small when cooling. If the eutectoid steel has network cementite, it must be eliminated by normalizing process before spheroidizing annealing, so as to ensure the normal annealing of spheroidizing.
5, diffusion annealing
Heat treatment process of ingot, casting or forging billet heated to a little lower than solidus temperature for long time insulation, and then slowly cooling to eliminate the uneven chemical composition phenomenon. It is applied to some high quality alloy steels and seriously segregated alloy steel castings and ingots.
6. Stress relieving annealing
Heat the steel to a temperature below Ac1 (usually 500~650 C), heat it up, and then cool down with the furnace. The stress relieving annealing temperature is less than A1, so the stress relieving annealing does not cause the change of microstructure.
7. Recrystallization annealing
Recrystallization annealing and annealing, the metal is heated after cold deformation to the recrystallization temperature above the proper time, the deformed grain to uniform equiaxed grain and heat treatment technology for eliminating residual stress and work hardening.
Selection of annealing method
(1) the annealing is usually used for all kinds of steels with sub eutectoid structure. In order to shorten the annealing time, isothermal annealing can be used;
(2) spheroidizing annealing is usually used in hypereutectic steel, and incomplete annealing can be used when the requirement is not high. Spheroidizing annealing is often used in tool steel and bearing steel. The cold extrusion parts and cold heading parts of low carbon steel or medium carbon steel are sometimes annealed by spheroidizing;
(3) recrystallization annealing can be used to eliminate the work hardening;
(4) in order to eliminate the internal stress caused by various kinds of processing, can be used to stress annealing; large steel castings Z some high-grade quality alloy steel, in order to improve the uniformity of microstructure and chemical composition, often used in diffusion annealing.