What is normalizing?
Normalizing is a heat treatment that improves the toughness of steel. After heating the steel component to 30~50℃ above the Ac3 temperature, keep it warm for some time and then air cool it out of the furnace. The main feature is that the cooling speed is faster than annealing but lower than quenching. During normalizing, the crystal grains of the steel can be refined in a slightly faster cooling, which can not only obtain satisfactory strength but also significantly improve the toughness (AKV value) and reduce the cracking tendency of the component. After normalizing, the comprehensive mechanical properties of some low-alloy hot-rolled steel plates, low-alloy steel forgings, and castings can be greatly improved, and the cutting performance is also improved.
Normalizing has the following purposes and uses:
① For hypereutectoid steel, normalizing is used to eliminate the overheated coarse-grained structure and Widmanstatten structure of castings, forgings, and weldments, and the banded structure in rolled materials; refine the grains; and can be used as a pre-heat treatment before quenching.
② For hypereutectoid steel, normalizing can eliminate the network secondary cementite and refine the pearlite, which not only improves the mechanical properties but also facilitates the subsequent spheroidizing annealing.
③ For low-carbon deep-drawn thin steel plates, normalizing can eliminate the free cementite at the grain boundary to improve its deep-drawing performance.
④ For low-carbon steel and low-carbon low-alloy steel, normalizing can obtain more fine-flaky pearlite structures, increase the hardness to HB140-190, avoid the “sticking knife” phenomenon during cutting, and improve the machinability. For medium-carbon steel, in situations where both normalizing and annealing can be used, normalizing is more economical and convenient.
⑤ For ordinary medium-carbon structural steel, in situations where the mechanical properties are not required to be high, normalizing can be used instead of quenching and high-temperature tempering, which is not only simple to operate but also makes the structure and size of the steel stable.
⑥ High-temperature normalizing (150-200℃ above Ac3) can reduce the component segregation of castings and forgings due to the high diffusion rate at high temperatures. The coarse grains after high-temperature normalizing can be refined by a subsequent second normalizing at a lower temperature.
⑦ For some low- and medium-carbon alloy steels used in steam turbines and boilers, normalizing is often used to obtain bainite structure, and then high-temperature tempering, which has good creep resistance when used at 400-550℃.
⑧ In addition to steel parts and steel products, normalizing is also widely used in the heat treatment of ductile iron to obtain pearlite matrix and improve the strength of ductile iron. Since the characteristic of normalizing is air cooling, the ambient temperature, stacking method, airflow, and workpiece size have an impact on the structure and performance after normalizing. Normalizing structure can also be used as a classification method for alloy steel. Usually, alloy steel is divided into pearlite steel, bainite steel, martensite steel, and austenite steel according to the structure obtained by air cooling after heating a sample with a diameter of 25 mm to 900℃.
What is annealing?
Annealing is a metal heat treatment process in which the metal is slowly heated to a certain temperature, kept for a sufficient time, and then cooled at an appropriate rate. Annealing heat treatment is divided into complete annealing, incomplete annealing, and stress relief annealing. The mechanical properties of annealed materials can be tested by tensile test or hardness test. Many steels are supplied in the annealed heat treatment state. The hardness test of steel can be tested by Rockwell hardness tester to test HRB hardness. For thinner steel plates, steel strips, and thin-walled steel pipes, a surface Rockwell hardness tester can be used to test HRT hardness.
The purpose of annealing is:
① Improve or eliminate various structural defects and residual stresses caused by steel in the casting, forging, rolling, and welding process to prevent deformation and cracking of workpieces.
② Soften the workpiece for cutting.
③ Refine the grains and improve the structure to improve the mechanical properties of the workpiece.
④ Prepare the structure for the final heat treatment (quenching, tempering).
Common annealing processes are:
① Complete annealing. It is used to refine the coarse overheated structure with poor mechanical properties of medium and low carbon steel after casting, forging, and welding. Heat the workpiece to 30-50℃ above the temperature at which all ferrite is transformed into austenite, keep it warm for some time, and then slowly cool it with the furnace. During the cooling process, austenite will transform again, which can make the steel structure finer.
② Spheroidizing annealing. Used to reduce the high hardness of tool steel and bearing steel after forging. Heat the workpiece to 20-40℃ above the temperature at which steel begins to form austenite, keep it warm, and then cool it slowly. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thereby reducing the hardness.
③ Isothermal annealing. 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 relatively fast speed and kept warm for an appropriate time. Austenite is transformed into troostite or troostite, and the hardness can be reduced.
④ Recrystallization annealing. Used to eliminate the hardening phenomenon (increased hardness and decreased plasticity) of metal wires and thin plates during cold drawing and cold rolling. The heating temperature is generally 50-150℃ below the temperature at which steel begins to form austenite. Only in this way can the work-hardening effect be eliminated and the metal softened.
⑤ Graphitization annealing. It is used to turn cast iron containing a large amount of cementite into forgeable cast iron with good plasticity. The process operation is to heat the casting to about 950℃, keep it warm for a certain period, and then cool it appropriately to decompose the cementite to form flocculent graphite.
⑥ 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 keep it warm for a long time, and then slowly cool it after the various elements in the alloy diffuse and tend to be evenly distributed.
⑦ Stress relief annealing. It is used to eliminate the internal stress of steel castings and welded parts. For steel products, the temperature below 100-200℃ at which austenite begins to form after heating, and then cool in the air after keeping warm, the internal stress can be eliminated.
Post time: Jun-11-2024