Seamless steel pipe heat treatment process and purpose

1 The purpose of heat treatment for seamless steel pipes

First, ensure that the product performance meets the requirements. For example, petroleum oil pipes, petroleum casing pipes, drill rods, and high-pressure boiler seamless steel pipes that have undergone heat treatment can be directly used in projects. Second, comply with deep processing regulations. For instance, bearing steel pipes and mechanical pipes, users can perform deep processing according to the usage direction. Third, enhance the performance index of the steel grade. With this technology, some steel grades with lower strength can be upgraded to meet the requirements. Through this method, the usage of alloy materials can be significantly reduced, and costs can be decreased.

2 Common heat treatment processes for seamless steel pipes

There are a wide variety of steel pipes on the market, and the types of steel used also vary greatly. Even for the same type of steel pipes, their chemical compositions have obvious differences. However, after heat treatment, their technical standards all comply with regulations. Specifically, according to the product production standards, the current heat treatment processes include the following types.

(1) Quenching + high-temperature tempering (also known as quenching and tempering): Heat the steel pipe until it meets the temperature required for quenching. At this time, the structure of the steel pipe will become austenite. Then, it is rapidly cooled at the set temperature. At this point, the structure of the steel pipe will become martensite. Through high-temperature tempering, its structure will eventually become the required tempered bainite structure. The advantages of this process are very obvious. It can not only significantly improve the strength of the material, but also increase hardness. At the same time, it can perfectly integrate the strength and toughness of the material, meeting the production regulations of the enterprise.

(2) Normalizing (also known as annealing): Specifically, this process refers to heating the steel pipe until it meets the annealing temperature. At this time, its structure becomes austenite, and then it is cooled by air. After normalizing treatment, we will obtain various types of metal structures, such as bainite, pearlite, or their mixtures. Its advantage is that it can refine the grain size and ensure consistent composition. Most importantly, it can increase its hardness, enhancing the cutting ability.

(3) Normalizing + tempering: Specifically, this process is to heat the steel pipe until it meets the normalizing temperature, then cool it by air, and finally perform tempering treatment. Its structure types are diverse, such as tempered ferrite + pearlite, or ferrite + bainite, or tempered bainite, or tempered martensite, or tempered sorbite. Its advantage is that it can enhance the stability of the structure, and also increase plasticity and toughness.

(4) Annealing:

This process refers to heating the steel pipe until it meets the annealing temperature, then appropriately holding it for a period of time, and then gradually cooling to the required temperature before being removed from the furnace. The steel pipe annealing process can be divided into: spheroidizing annealing, complete annealing, and stress relief annealing, etc. Its advantage is that it can reduce the hardness of the material and increase its plasticity index. At the same time, it can make the grain size smaller, ensure uniform composition, and improve material performance. The most crucial point is that it can eliminate stress and avoid deformation.

(5) Solution treatment (mainly used in the production of 18-8 type austenitic stainless steel):

This process refers to heating the steel pipe to the temperature required for solution treatment. At this time, carbides and other elements will dissolve, and then it is cooled to ensure that the elements do not precipitate. At this point, we will obtain the required single austenite structure. Its advantage is that it can ensure uniform structure, avoid hardening, and most importantly, has strong corrosion resistance. Currently, there are many types of seamless steel pipes on the market. Different grades have completely different requirements for heat treatment. To ensure that the new production line meets all quality production regulations, it is necessary to carefully study the heat treatment system.