What are the connections and differences between normalizing and annealing steel pipes

What is normalizing steel pipes?

Normalizing steel pipes is a heat treatment that improves the toughness of steel. After the steel component is heated to 30~50°C above the Ac3 temperature, it is kept warm for some time and then air-cooled. The main feature is that the cooling rate is faster than annealing and lower than quenching. When the steel pipe is normalized, the crystal grains of the steel can be refined during slightly faster cooling. Not only can satisfactory strength be obtained, but the toughness (AKV value) can be significantly improved. Reduce the tendency of components to crack. After normalizing some low-alloy hot-rolled steel plates, low-alloy steel forgings, and castings, the comprehensive mechanical properties of the materials can be greatly improved, and the cutting performance is also improved.

Normalizing steel pipes has the following purposes and uses:

① For hypereutectoid steel, normalizing of steel pipes is used to eliminate the overheated coarse grain structure and Widmanstatten structure of castings, forgings, and weldments, and the banded structure in rolled materials; to refine the grains; and can be used as a preliminary treatment before quenching. heat treatment.

② For hypereutectoid steel, normalizing the steel pipe can eliminate the reticular secondary cementite and refine the pearlite, which not only improves the mechanical properties but also facilitates subsequent spheroidizing annealing.

③ For low carbon deep drawing thin steel plates, normalizing the steel pipe can eliminate 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 be used to obtain more fine flake pearlite structures, increasing the hardness to HB140-190, avoiding the "sticking" phenomenon during cutting, and improving the cutting processability. For medium carbon steel, when both normalizing and annealing of steel pipes can be used, normalizing steel pipes is more economical and convenient.

⑤ For ordinary medium-carbon structural steel, when the mechanical properties are not high, normalizing the steel pipe can be used instead of quenching and high-temperature tempering, which is not only easy to operate but also stabilizes the structure and size of the steel.

⑥ High-temperature normalizing (150~200℃ above Ac3) can reduce component segregation of castings and forgings due to the high diffusion rate at high temperatures. Coarse grains after normalizing at high temperatures can be refined by subsequent normalizing at a lower temperature.

⑦ For some low- and medium-carbon alloy steels used in steam turbines and boilers, the steel pipe is often normalized to obtain a bainitic structure, and then tempered at high temperatures to have good creep resistance when used at 400 to 550°C.

⑧ In addition to steel parts and steel products, normalizing is also widely used in the heat treatment of ductile iron to obtain a pearlite matrix and improve the strength of ductile iron. Since normalizing is characterized by air cooling, the ambient temperature, stacking method, air flow, and workpiece size all have an impact on the structure and performance after normalizing. Normalized structure can also be used as a classification method for alloy steel. Usually, alloy steel is divided into pearlitic steel, bainitic steel, martensite steel, and austenitic steel based on the structure obtained by air cooling after heating a 25 mm diameter sample to 900°C.

What is the annealing of steel pipes?

Annealing of steel pipes is a metal heat treatment process that slowly heats the metal to a certain temperature, maintains it for a sufficient time, and then cools it at an appropriate speed. The annealing heat treatment of steel pipes is divided into complete annealing, incomplete annealing, and stress relief annealing. The mechanical properties of annealed materials can be tested by tensile testing or hardness testing. Many steel products are supplied in the annealed heat treatment state of steel pipes. The hardness of steel can be tested using a Rockwell hardness tester to test the 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 steel pipes is to:

① Improve or eliminate various structural defects and residual stress caused by steel casting, forging, rolling, and welding processes to prevent workpiece deformation and cracking.

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 final heat treatment (quenching and tempering).

Commonly used annealing processes for steel pipes are:

① Complete annealing. It is used to refine the coarse overheated structure with poor mechanical properties that appear after casting, forging, and welding of medium and low carbon steel. The workpiece is heated to 30 to 50°C above the temperature at which all ferrite is transformed into austenite, kept warm for some time, and then slowly cooled in the furnace. During the cooling process, the austenite transforms 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. The workpiece is heated to 20 to 40°C above the temperature at which the steel begins to form austenite, and is slowly cooled after heat preservation. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thereby reducing the hardness.

③ Isothermal annealing. It is used to reduce the high hardness of some alloy structural steels with high nickel and chromium content for cutting processing. Generally, it is first cooled to the most unstable temperature of austenite at a faster speed and then kept for an appropriate time. The austenite transforms into troostite or sorbite, and the hardness can be reduced.

④ Recrystallization annealing. It is used to eliminate the hardening phenomenon (hardness increases and plasticity decreases) of metal wires and sheets during cold drawing and cold rolling. The heating temperature is generally 50 to 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.

⑤ Graphitization 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, and then cool it appropriately to decompose the cementite to form flocculated graphite.

⑥Diffusion 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 slowly cool it after the various elements in the alloy have diffused and become evenly distributed.

⑦ Stress relief annealing. Used to eliminate internal stress in steel castings and welded parts. For steel products that are heated to a temperature 100 to 200°C below the temperature at which austenite begins to form, and then cooled in the air after heat preservation, the internal stress can be eliminated.