What is ANNEALING heat treament?

Anneal Process Definition:

Annealing mainly refers to the heat treatment process in which the material is exposed to high temperature for a long time and then cooled slowly. The main purpose is to release stress, increase material ductility and toughness, produce special microstructure, etc.

The annealing process includes complete annealing, spheroidizing annealing, isothermal annealing, graphite annealing, diffusion annealing, stress relief annealing, incomplete annealing, post-weld annealing, etc.

What is annealing?

Annealing heat treatment is divided into complete annealing, incomplete annealing and stress relief annealing, diffusion annealing, spheroidizing annealing, recrystallization annealing. The mechanical properties of annealed materials can be detected by tensile test or hardness test. Many steel products are supplied in the state of annealing and heat treatment. Rockwell hardness tester can be used to test the hardness of steel. For thinner steel plates, steel strips and thin-walled steel pipes, surface Rockwell hardness testers can be used to test HRT hardness. Heating the steel to a certain temperature above or below the critical point Ac1, holding it for a period of time, and then cooling it slowly in a furnace or buried in a furnace or a medium with poor thermal conductivity to obtain a stable structure close to an equilibrium state.

Annealing Purpose:

① Improve or eliminate various structural defects and residual stresses caused by steel casting, forging, rolling and welding, and prevent deformation and cracking of workpieces;

② soften the workpiece for cutting;

③ refine the grain and improve the structure to improve the mechanical properties of the workpiece.

④ Make organizational preparations for final heat treatment (quenching, tempering).

Anneal Classification:

●Fully Annealing:

Objective To refine grains, uniform structure, eliminate internal stress and processing defects, reduce hardness, improve cutting performance and cold plastic deformation ability.

It is used to refine the coarse superheated structure with poor mechanical properties after casting, forging and welding of medium and low carbon steel. Heat the workpiece to 30-50°C above the temperature at which ferrite is completely transformed into austenite, keep it warm for a period of time, and then cool slowly with the furnace. During the cooling process, the austenite will transform again to make the steel structure thinner .

●Spheroidizing annealing:

It is used to reduce the high hardness of tool steel and bearing steel after forging. The workpiece is heated to 20-40°C above the temperature at which the steel begins to form austenite, and then slowly cooled after heat preservation. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thereby reducing the hardness.

●Isothermal annealing:

It is used to reduce the high hardness of some alloy structural steels with high nickel and chromium content for cutting. Generally, it is first cooled to the most unstable temperature of austenite at a faster rate, and the austenite is transformed into troostite or sorbite for an appropriate time, and the hardness can be reduced.

●Recrystallization annealing is used to eliminate the hardening phenomenon (increase in hardness and decrease in plasticity) of metal wires and thin plates during cold drawing and cold rolling. The heating temperature is generally 50-150°C below the temperature at which the steel begins to form austenite. Only in this way can the work hardening effect be eliminated and the metal softened.

●Graphite annealing:

It is used to turn cast iron containing a large amount of cementite into malleable cast iron with good plasticity. The process operation is to heat the casting to about 950°C, keep it warm for a certain period of time and then cool it properly to decompose the cementite to form a group of flocculent graphite.

●Diffusion annealing:

It is used to homogenize the chemical composition of alloy castings and improve their performance. The method is to heat the casting to the highest possible temperature without melting, and keep it warm for a long time, and then cool slowly after the diffusion of various elements in the alloy tends to be evenly distributed.

●Stress relief annealing:

Used to eliminate the internal stress of steel castings and weldments. For iron and steel products heated to 100-200°C below the temperature at which austenite begins to form, cooling in air after heat preservation can eliminate internal stress.

●Incomplete annealing:

The heating temperature is between Ac1~Accm, the cooling rate: when it is above 500~600℃, the carbon steel is 100~200℃/h, the alloy steel is 50~100℃/h, and the high alloy steel is 20~60℃/h , mainly used for hypereutectoid steel.

●Annealing after welding:

Pure Fe was selected as the filler metal for TIG welding tests on YG30 cemented carbide and 45 steel. The morphology of the YG30/weld interface area before and after annealing was analyzed by scanning electron microscope. The results show that when commercially pure Fe is used as the filler metal, the η phase in the as-welded state does not change after annealing at 1050°C; after annealing at 1150°C, a new η phase begins to form; the η phase increases with the increase of annealing temperature and the extension of holding time. During annealing, the new η phase nucleates at the WC-γ phase boundary, engulfs the WC grains and grows up, and distributes at the boundaries of the WC grains.

Annealing Influence:

LiF and 2,9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP) were used as poly-3-hexylthiophene (P3HT)/[6,6]-phenyl-C61 – Butyric acid methyl ester (PCBM) system cathode interface layer of polymer photovoltaic cells, the effect of high temperature post-annealing treatment on the device performance of different interface layers was studied. The study found that the introduction of the LiF interface layer formed a strong dipole interaction between the active layer and the cathode interface, thereby improving the performance of the battery. After further high-temperature thermal annealing treatment, the good interface interaction can still be maintained, so that the device’s The energy conversion efficiency has been further improved. However, the introduction of the BCP interface layer blocked the electron transfer from the metal electrode Al to the PCBM, resulting in reduced recombination and increased the open circuit voltage of the device, but after further high-temperature post-annealing, the integrity of the BCP interface layer was destroyed. The energy conversion efficiency of the device is reduced.