Insufficient hardness and uneven hardness of seamless steel pipe after quenching

What is the normalizing of seamless steel pipes

In the production process of seamless steel pipes, in order to remove the internal stress of the material, improve the hardness of the material, slightly reduce the plasticity, reduce the cost, and improve the efficiency, normalizing is sometimes used. Normalizing is a kind of heat treatment that improves the toughness of steel. The heat treatment process of heating the steel to 30-50 ℃ above Ac3 (or Acm), holding it for an appropriate time, and cooling it in still air. The normalizing that heats the steel to 100-150 ℃ above Ac3 is called high-temperature normalizing. For castings and forgings of medium and low carbon steel.The main purpose of normalizing is to refine the tissue.

Normalizing has the following purposes and uses:

① For hypoeutectoid steel, normalizing is used to eliminate overheated coarse-grained structure and Widmanners structure of cast, forged and welded parts, and banded structure in rolled material; to refine grains; it can also be used as a pre-heat treatment before quenching.

For hypereutectoid steel, normalizing can eliminate the reticulated 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 free cementite at grain boundaries to improve its deep-drawing properties.

④ For low carbon steel and low carbon low alloy

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

⑤ For ordinary medium carbon structural steel, when 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 makes the structure and size of the steel stable.

⑥ High 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 normalizing can be refined by the second normalizing at 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 tempered at high temperature, it has good creep resistance when used at 400-550 °C.

⑧ In addition to steel and steel, normalizing is also widely used in heat treatment of ductile iron to obtain pearlite matrix and improve the strength of ductile iron.

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