Product News

Cooling Methods for Seamless Steel Pipes

Date:2024-04-28

Steel seamless pipes undergo various manufacturing processes such as casting, forging, and welding, each contributing to specific structural and mechanical characteristics. However, these processes often result in coarse and overheated microstructures, particularly affecting the mechanical properties of low to medium carbon steels. To refine these properties, several cooling methods are employed, each tailored to address specific structural nuances and enhance the overall performance of seamless steel pipes.


1. Full Annealing

Full annealing is deployed to refine the coarse and overheated microstructure prevalent in seamless steel pipes post-casting, forging, or welding, especially in low to medium carbon steels. The process involves heating the workpiece to a temperature 30 to 50°C higher than the critical transformation temperature from ferrite to austenite, holding it for a specified duration, and then gradually cooling it in a furnace. During the cooling phase, austenite regeneration occurs, facilitating phase transformation and resulting in a finer steel structure.


2. Spheroidizing Annealing

Spheroidizing annealing is instrumental in reducing the high hardness observed in tool steels and bearing steels post-forging. By heating the workpiece to a temperature 20 to 40°C higher than the onset of austenite formation and subsequently slow-cooling it after insulation, the lamellar carbides within the pearlite transform into spherical shapes, effectively lowering the material's hardness.


3. Isothermal Annealing

Isothermal annealing finds application in lowering the excessive hardness exhibited by certain alloy structural steels with high nickel and chromium contents, primarily for machining purposes. Typically, the material is rapidly cooled to the least stable temperature of austenite and held for an appropriate duration, allowing the transformation of austenite into either pearlite or martensite, consequently reducing hardness.


4. Recrystallization Annealing

Recrystallization annealing serves to eliminate the hardening effects of seamless steel tubes observed in metal wires and sheets during cold drawing and rolling processes, resulting in increased hardness and reduced ductility. The heating temperature is typically maintained 50 to 150°C below the onset of austenite formation, ensuring the elimination of work hardening effects and the subsequent softening of the metal.


5. Graphitization Annealing

Graphitization annealing is applied to transform cast iron, which contains a significant amount of cementite, into malleable cast iron with improved ductility. The technique involves heating the castings to approximately 950°C, followed by appropriate cooling after a specified duration to decompose the cementite and promote the formation of dispersed graphite.


6. Diffusion Annealing

Diffusion annealing aims to homogenize the chemical composition of homogeneous alloy castings and improve their performance. This method involves heating the castings to the highest possible temperature and maintaining the heat for an extended period without melting, allowing the various elements within the alloy to achieve uniform distribution before slow cooling.


7. Stress Relief Annealing

Stress relief annealing is employed to mitigate internal stresses within steel castings and weldments, ensuring structural integrity and dimensional stability.

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