Heat treatment for steel pipe

The role of heat treatment is to improve the mechanical properties of steel pipe materials, eliminate residual stress and improve the cutting performance of steel pipe metal.
According to different purposes of heat treatment, heat treatment process can be divided into two categories: preliminary heat treatment and final heat treatment
 

1. Preliminary heat treatment
The purpose of preliminary heat treatment is to improve the processing performance of steel pipe, eliminate internal stress and prepare good metallographic structure for final heat treatment. The heat treatment process includes annealing, normalizing, aging, tempering, etc.

(1) annealing and normalizing
Annealing and normalizing are used for hot worked blanks. For carbon steel pipes and alloy steel pipes with carbon content more than 0.5%, in order to reduce their hardness and be easy to cut, annealing treatment is often used; for carbon steel pipes and alloy steel pipes with carbon content less than 0.5%, normalizing treatment is used to avoid sticking the cutter when cutting with low hardness. Annealing and normalizing can refine the grain and even the structure, which can prepare for the later heat treatment. Annealing and normalizing are usually arranged after the rough manufacturing and before the rough machining.

(2) aging treatment
Aging treatment is mainly used to eliminate the internal stress produced in blank manufacturing and machining.
In order to avoid too much transportation work, for parts with general accuracy, it is enough to arrange an aging treatment before finishing. However, for parts with high accuracy requirements (such as the box of coordinate boring machine), two or more aging treatment procedures shall be arranged. Aging treatment is generally not required for simple parts.
In addition to castings, for some precision parts with poor rigidity (such as precision lead screw), in order to eliminate the internal stress generated in processing and stabilize the processing accuracy of parts, multiple aging treatments are often arranged between rough machining and semi finish machining. Some shaft parts processing, in the straightening process also need to arrange aging treatment.
 
(3) tempering
Quenching and tempering is to carry out high temperature tempering treatment after quenching. It can obtain uniform and fine tempered sorbite structure, which is prepared for reducing deformation during surface quenching and nitriding treatment. Therefore, quenching and tempering can also be used as preparatory heat treatment.
Due to the better comprehensive mechanical properties of the parts after quenching and tempering, some parts with low hardness and wear resistance requirements can also be used as the final heat treatment process.
 
 
2. Final heat treatment
The ultimate purpose of heat treatment is to improve the mechanical properties of steel pipe, such as hardness, wear resistance and strength.
 
(1) quenching
Quenching includes surface quenching and integral quenching. Surface quenching is widely used because of its small deformation, oxidation and decarburization, and it also has the advantages of high external strength, good wear resistance, and good internal toughness and strong impact resistance. In order to improve the mechanical properties of surface hardened parts, heat treatment, such as tempering or normalizing, is often required as a preliminary heat treatment. The general process is: blanking forging normalizing (annealing) - Roughing - tempering - semi finishing - surface quenching - finishing.
 
(2) carburizing and quenching
 Carburizing and quenching is suitable for low carbon steel and low alloy steel. First, the carbon content of the surface layer of the parts is increased. After quenching, the surface layer obtains high hardness, while the core still maintains certain strength and high toughness and plasticity. Carburizing consists of integral carburizing and local carburizing. In case of local carburization, anti-seepage measures (copper plating or anti-seepage material plating) shall be taken for the non carburized part. Due to the large deformation of carburizing and quenching, and the carburizing depth is generally between 0.5-2mm, so the carburizing process is generally arranged between semi finishing and finishing.
 
The process is generally as follows: blanking forging normalizing roughing semi finishing carburizing quenching finishing.
When the non carburized part of the local carburized part adopts the process plan of removing the extra carburized layer after increasing the allowance, the procedure of removing the extra carburized layer shall be arranged after carburizing and before quenching.
 
 
(3) nitriding treatment
Nitriding is a method to make nitrogen atoms penetrate into metal surface to obtain a layer of nitrogen compounds. The nitriding layer can improve the surface hardness, wear resistance, fatigue strength and corrosion resistance of parts. As the nitriding treatment temperature is low, the deformation is small, and the nitriding layer is thin (generally no more than 0.6 ~ 0.7mm), the nitriding process should be arranged as far back as possible. In order to reduce the deformation during nitriding, high temperature tempering for stress relief is generally required after cutting.