Heat treatment process of carbon steel pipe

There are various types of seamless steel pipes, each requiring different steel grades. Even within the same type of pipe, the chemical composition can vary. However, through heat treatment, these pipes can be made to meet the relevant technical standards or the specific requirements of the purchaser. The heat treatment processes for carbon steel pipes, as outlined in product standards, typically fall into the following five categories.

Quenching + High-Temperature Tempering (Q+T, also known as Quenching and Tempering Treatment)

In this process, the steel pipe is heated to a quenching temperature to transform its internal structure into austenite. The pipe is then rapidly cooled at a rate higher than the critical quenching speed, converting the structure into martensite. Afterward, it is tempered at a high temperature to achieve a uniformly tempered sorbite structure. This treatment enhances the pipe's strength and hardness while balancing its strength, plasticity, and toughness to meet performance requirements.

Normalization (N, also known as Normalizing)

The steel pipe is heated to a normalizing temperature, transforming its internal structure completely into austenite. It is then cooled in air. Normalizing results in various metal structures such as pearlite, bainite, martensite, or a mixture of these. This process refines the grains, homogenizes the composition, eliminates stress, increases hardness, and improves cutting performance.

Normalizing + Tempering (N+T)

After normalizing the steel pipe, the tempering process is applied. The final structure can be tempered ferrite + pearlite, ferrite + bainite, tempered bainite, tempered martensite, or tempered sorbite. This combination stabilizes the internal structure and enhances the pipe's plasticity and toughness.

Annealing

Annealing involves heating the steel pipe to a specific temperature, maintaining it for a set period, and then cooling it slowly within the furnace before final cooling outside the furnace. The types of annealing include spheroidizing, full annealing, and stress relief annealing. The primary purposes of annealing are:

Reducing the hardness of the steel pipe and increasing its plasticity to facilitate cutting or cold deformation processing.

Refining the grain structure, eliminating defects, and preparing the pipe for subsequent processes.

Eliminating internal stress to prevent deformation or cracking.

Solid Solution Treatment (Primarily for 18-8 Austenitic Stainless Steel)

In this process, the steel pipe is heated to a solid solution temperature to dissolve carbides and alloying elements uniformly in the austenite. It is then rapidly cooled to prevent the precipitation of carbon and alloying elements, resulting in a single austenite structure. The main functions of solid solution treatment include:

Homogenizing the internal structure and composition.

Eliminating the hardening that occurs during processing, facilitating subsequent cold deformation.

Restoring the corrosion resistance of stainless steel.

These heat treatment processes are crucial for tailoring the properties of seamless steel pipes to meet specific performance and technical requirements.