Steel pipe heat treatment process-Normalizing

Seamless steel pipe heat treatment process-Normalizing

What is normalizing?

Normalizing is a heat treatment that improves the toughness of steel. After the steel components are heated to 30-50°C above the Ac3 temperature, they are kept warm for a period of time and then air-cooled. The main feature is that the cooling rate is faster than annealing and lower than quenching. During normalizing, the crystal grains of the steel can be refined in a slightly faster cooling. Not only can satisfactory strength be obtained, but also the toughness (AKV value) can be significantly improved and reduced The tendency of the component to crack. -After normalizing treatment of 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 has the following purposes and uses:

① For hypoeutectoid steels, normalizing is used to eliminate the overheated coarse-grained structure and Widmanstatten structure of cast, forging, and weldments, and the band structure in rolled materials; refine grains; and can be used as a pre-heat treatment before quenching.

② For hypereutectoid steels, normalizing can eliminate the reticulated 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-drawing thin steel sheets, normalizing can eliminate the free cementite in the grain boundary to improve its deep-drawing performance.

④For low-carbon steel and low-carbon low-alloy steel, normalizing can obtain more flake pearlite structure, increase the hardness to HB140-190, avoid the phenomenon of "sticking knife" during cutting, and improve the machinability . For medium carbon steel, it is more economical and convenient to use normalizing when both normalizing and annealing are available.

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

⑥High temperature normalizing (150～200℃ above Ac3) can reduce the composition segregation of castings and forgings due to the high diffusion rate at high temperature. The coarse grains after high temperature normalization can be refined by a second lower temperature normalization.

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 after high temperature tempering, it has good creep resistance when used at 400-550℃.

⑧ In addition to steel parts and steel materials, 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 the characteristic of normalizing is air cooling, the ambient temperature, stacking method, airflow and workpiece size all affect the organization and performance after normalizing. The normalizing structure can also be used as a classification method for alloy steel. Generally, alloy steels are divided into pearlite steel, bainite steel, martensitic steel and austenitic steel based on the structure obtained by air cooling after a sample with a diameter of 25 mm is heated to 900°C.