Thickness limit of heat treatment for carbon steel pipes

When the thickness of a carbon steel pipe (CS Pipe) exceeds 25mm, heat treatment becomes necessary. During the manufacturing process of carbon steel pipes, heat treatment is a crucial step involving heating, insulation, and cooling to adjust and enhance the material's structure and performance. This process aims to improve the pipe's durability, strength, and service life by making the material's structure more uniform, reducing grain size, increasing hardness and strength, and enhancing plasticity and toughness. It also helps prevent cracks and deformation due to stress.

Heat Treatment Types and Requirements

Heat treatment for carbon steel pipes is generally categorized into four types: annealing, normalizing, tempering, and quenching. The specific requirements and parameters depend on the material, thickness, and shape of the steel pipe.

Annealing: Softens the material for improved machinability.

Normalizing: Refines grain size and enhances mechanical properties.

Tempering: Reduces brittleness and increases toughness.

Quenching: Increases hardness by rapid cooling.

For carbon steel pipes, heat treatment is generally required when the thickness exceeds 25mm. The hardness requirements after heat treatment vary based on the diameter of the pipe:

Diameter less than 20mm: Hardness should be between 85-100 HBW.

Diameter between 20-60mm: Hardness should be between 92-130 HBW.

Diameter greater than 60mm: Heat treatment parameters and requirements depend on specific usage conditions.

Preheating Requirements for Welding

Wall Thickness ≥ 26mm: Preheating is required before welding.

Preheating Method and Temperature: Electric heating can be used with a preheating temperature of 100-200℃. The interlayer temperature during welding should not be lower than the preheating temperature.

Preheating Range: For butt welds, the preheating range should cover at least three times the wall thickness on both sides of the groove, ensuring even heat distribution inside and outside to prevent local overheating. The area within a 100mm range outside the heating zone should be kept warm.

Heat Treatment Steps

Pretreatment: Clean, degrease, and descale the pipe before heat treatment.

Heating: Heat the pipe to the specified temperature and maintain it for a certain period.

Cooling: Rapidly cool the pipe from the high-temperature state to achieve the desired structure and performance.

Quenching: Quickly cool the pipe by immersing it in a quenching medium to harden it.

Tempering: Reheat the quenched pipe at an appropriate temperature to eliminate brittleness and improve toughness and strength.

Heat treatment is essential for carbon steel pipes to enhance their performance and lifespan. Proper heat treatment methods and parameters must be selected based on the specific circumstances, and normal operating procedures and precautions should be followed to ensure maximum heat treatment quality and effectiveness.