Annealing conditions of carbon steel seamless pipes

Carbon Steel (CS) Seamless Pipes are widely used due to their high strength and stable mechanical properties. They are commonly used in industries such as petrochemicals, construction, aerospace and manufacturing. In order to further improve the performance and quality of carbon steel seamless pipes, annealing is often required. Annealing is a heat treatment process that involves gradually heating the material to a specific temperature and then cooling it to change its crystal structure and improve its mechanical properties.

In the production of carbon steel seamless pipes, annealing conditions are a key issue that directly affects product quality and performance. The key annealing parameters are as follows:

1. Temperature conditions

(1) Full annealing

Full annealing is heating the carbon steel seamless pipe to a temperature 30-50°C above the critical temperature (Ac3). For carbon steel with a carbon content of 0.3%-0.6%, the Ac3 temperature is about 750-800°C, which means that the full annealing temperature is usually set between 800-850°C. At this temperature, all ferrite and cementite in the steel will recrystallize, refine the grain structure and eliminate structural defects. This process reduces the hardness of the steel pipe, thereby improving its machinability.

(2) Incomplete annealing
Incomplete annealing is carried out at a temperature between the critical temperatures Ac1 and Ac3. For example, for hypoeutectoid carbon steel, the Ac1 temperature is around 727°C, so the incomplete annealing temperature is generally set at 740-770°C. This method refines the grain structure, reduces the hardness while maintaining a certain strength and toughness, strengthens the internal structure of the steel pipe, and lays the foundation for subsequent processing.

(3) Spheroidizing annealing
Spheroidizing annealing is mainly used for eutectoid and hypereutectoid carbon steels. The annealing temperature is generally set at 20-30°C above Ac1. For example, T10 steel with a carbon content of about 1.0%, the Ac1 temperature is around 730°C, and the spheroidizing annealing temperature is controlled at 750-760°C. The purpose of this process is to spheroidize the cementite, reduce the hardness, improve the machinability, and prepare the material for subsequent heat treatment (such as quenching).

2. Holding time

The holding time refers to the time that the steel pipe maintains uniform grain size at the annealing temperature. The specific holding time depends on the material composition, wall thickness, processing requirements, etc. The general holding time is about 2-4 hours to ensure that the microstructure transformation is complete and uniform.

3. Cooling method

(1) Complete annealing cooling
After complete annealing, the cooling rate should be slow. Generally, it is cooled to 500-600℃ in the furnace and then air-cooled. This slow cooling process allows the austenite to fully decompose into stable ferrite and pearlite structures to prevent excessive internal stress. During this cooling stage, the residual stress is gradually released to ensure the excellent quality of the steel pipe.

(2) Incomplete annealing cooling
The cooling rate after incomplete annealing is also relatively slow. Usually furnace cooling or sand cooling is used, and the cooling rate is controlled at 30-50℃/h until the temperature drops to a lower level. This gradual cooling ensures uniform microstructure transformation and reduces internal stress, thereby stabilizing the internal structure for further processing.

(3) Spheroidizing annealing cooling
The cooling rate after spheroidizing annealing must be strictly controlled. The temperature is usually reduced at a rate of 10-20℃/h until it reaches 500-600℃, and then air-cooled outside the furnace. This controlled cooling process helps to maintain the spheroidized cementite structure, ensuring excellent mechanical properties and improved machinability.

4. Furnace atmosphere control

The furnace atmosphere plays a vital role in the annealing process and affects the surface quality and overall performance of the steel pipe. A neutral or slightly oxidizing atmosphere is usually used to prevent surface decarburization, which will reduce the quality of the material. For example, in gas furnace annealing, the ratio of gas to air must be strictly controlled to prevent the generation of excessive reducing gas, which will lead to decarburization. On the contrary, an over-oxidizing atmosphere may lead to surface oxidation and the formation of oxide scale, so the furnace atmosphere needs to be properly adjusted.

5. Furnace loading method

Proper charging of carbon steel seamless pipes in the furnace is essential for uniform heating. The pipes should be arranged evenly, not too densely, and avoid mutual squeezing to avoid uneven temperature distribution and affect the annealing effect. Usually, they are arranged in layers or intervals, and the loading position is reasonably arranged according to the size and shape of the steel pipe so that each steel pipe can be fully heated and cooled.

Conclusion:

Choosing appropriate annealing conditions for carbon steel seamless pipes is crucial to ensure product quality and performance. Parameters such as temperature, holding time, cooling method, furnace atmosphere, etc. must be reasonably selected according to specific product requirements to achieve the required mechanical properties and overall quality.

Hunan Standard Steel Co., Ltd is professional carbon steel seamless pipe manufacturer, for more details, please contact: sales@hu-steel.com

Read more: Types of seamless carbon steel pipes or 5 Common heat treatment processes for seamless pipes