Development history and tempering technology of API 5L pipeline steel pipes

API 5L pipeline pipe belongs to the United States petroleum standard pipeline pipe, pipeline pipe is to take out the oil, steam, water from the ground, through the pipeline pipe to the oil and natural gas industry enterprises.The pipe comprises seamless steel pipes and a welded steel pipes, with plain ends, threaded ends and socket ends.The connection mode is end welding, collar connection, socket connection, etc.


Development history of API 5L steel pipeline:

API released API 5L standard in 1926. At first, it only included A25, A and B steel grades, with the minimum yield values of 172MPa, 207MPa and 241MPa respectively.

API released API 5LX standard in 1947, which added X42, X46 and X52 steel grades, with the minimum yield values of 289MPa, 317MPa and 358MPa respectively.

Since 1966, four steel grades X56, X60, X65 and X70 have been issued successively, with the minimum yield values of 386MPa, 413MPa, 448MPa and 482MPa respectively.

In 1972, API released U80 and U100 standards, with the minimum yield values of 551MPa and 691Mpa respectively. Later, API changed U80 and U100 into X80 and X100.

Development history and tempering technology of API 5L pipeline steel pipes

Tempering of pipeline steel pipe

API 5L pipeline steel pipe annealing: Heat the API 5L pipeline steel pipe whose structure deviates from the equilibrium state to the appropriate temperature, keep it for a certain time, and then cool it slowly (with the furnace cooling) to obtain the heat treatment process of the structure close to the equilibrium state.

There are many kinds of annealing processes for API 5L pipeline steel tubes, which can be divided into two categories according to the heating temperature: one is annealing above the critical temperature (Ac1 or Ac3), also known as phase change recrystallization annealing, including complete annealing, incomplete annealing, spheroidizing annealing and diffusion annealing, etc.The other is annealing below critical temperature, including recrystallization annealing and stress relieving annealing.

  1. Complete annealing and isothermal annealing of API 5L pipeline steel pipes:Complete annealing and weighing crystallization annealing, generally referred to as annealing, this annealing is mainly used for the casting of various carbon steel and alloy steel of subeutectoid composition, API 5L pipeline steel and hot rolled profiles, and sometimes also used for welding structures.It is usually used as the final heat treatment for some API 5L pipeline steels, or as a pre-heat treatment for some API 5L pipeline steels.
  2. Spheroidizing annealing:Spheroidizing annealing is mainly used for eutectoid API 5L pipeline steel and alloy tool steel (such as steel used in manufacturing cutting tools, measuring tools and moulds).Its main purpose is to reduce hardness, improve machinability and prepare for future quenching.
  3. Stress relief annealing:Also known as low temperature annealing (or high temperature tempering), this annealing is mainly used to eliminate the residual stress of API 5L pipeline steel, forgings, welding parts, hot rolled parts, cold drawn parts and so on.If these stresses are not eliminated, they may cause deformation or cracks in the steel after a certain period of time or during subsequent machining.


Quenching: When quenching, the most commonly used cooling medium is salt water, water and oil.Brine quenching workpiece, easy to get high hardness and smooth surface, not easy to quench not hard soft point, but easy to make the workpiece deformation serious, even cracking.However, oil as quenching medium is only suitable for the quenching of some alloy steels with high stability of supercooled austenite or API 5L pipeline steel pipes of small size.


Purposes of tempering API 5L pipeline steel pipes:

  1. Reduce brittleness and eliminate or reduce internal stress. After quenching, API 5L pipeline steel pipe has great internal stress and brittleness.
  2. To obtain the mechanical properties required by the workpiece, API 5L pipeline steel pipe has high hardness and great brittleness after quenching. In order to meet the requirements of different properties of API 5L pipeline steelpipes, hardness can be adjusted through appropriate tempering to reduce brittleness and obtain the required toughness and plasticity.
  3. Keep workpiece size stable.
  4. For some alloy steels that cannot be softened by annealing, high temperature tempering is often used after quenching (or normalizing) to make the carbides in API 5L pipeline steel pipe gather properly and reduce the hardness for cutting.
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