Problems after improper heat treatment of seamless steel pipes

Improper heat treatment of seamless steel pipes can lead to a range of production issues, significantly diminishing product quality and resulting in waste. Avoiding common mistakes during heat treatment is essential for cost savings. What problems should we focus on during heat treatment? Let’s explore some of the common issues in the heat treatment of seamless steel pipes.

Unqualified Steel Pipe Structure and Performance:
Improper heat treatment can result in structural and performance deficiencies, typically caused by three factors: temperature (T), time (t), and cooling method.

Coarse Grain Structure (A): Formed when steel undergoes excessive heating, leading to the formation of a lamellar ferrite (F) that becomes distributed on the pearlite (P) during cooling. This overheated structure weakens the comprehensive performance of the steel pipe, particularly by reducing its room temperature strength and increasing brittleness.

Normalization: The coarse grain structure with a lighter degree can be eliminated by normalizing at an appropriate temperature, while a more severe case may require secondary normalization at a higher temperature. The cooling process should be carefully controlled to refine the grains.

Equilibrium Diagrams: The F-C equilibrium diagram plays a crucial role in determining the heating temperature for heat treatment. Additionally, transformation diagrams, such as supercooling A, provide important guidelines for formulating appropriate cooling temperatures.

Unqualified Steel Pipe Dimensions:
Issues such as changes in the outer diameter, ovality, and curvature of the steel pipe often arise during the heat treatment process.

Outer Diameter Changes: These occur primarily during quenching due to volume changes from structural transformations. The outer diameter of the pipe may increase as a result.

Ovality and Curvature Changes: Ovality changes are common, particularly in large-diameter thin-walled pipes. Steel pipe bending, caused by uneven heating or cooling, can be corrected by straightening. For pipes with special requirements, a warm straightening process (around 550°C) should be employed.

Surface Cracks:
Cracks on the surface of steel pipes are often caused by rapid heating or cooling, leading to excessive thermal stress. To minimize heat treatment cracks:

Tailor the heating and cooling systems to the specific steel type.

Choose the appropriate quenching medium.

Ensure timely tempering or annealing of the quenched steel pipes to relieve stress.

Surface Scratches or Hard Scratches:
These are typically caused by relative motion between steel pipes, workpieces, tools, and rollers. Proper handling and maintenance of equipment can help prevent these scratches.

Oxidation, Decarbonization, Overheating, or Overburning:
Excessive temperature (T↑) and time (t↑) during heat treatment can lead to oxidation, decarbonization, and even overheating or overburning of the steel pipes. Strict control over temperature and time parameters is crucial to prevent these issues.

Oxidation Due to Protective Gas Failure:
If the heating furnace is not properly sealed, air may enter the furnace, causing oxidation of the steel pipe surface. This can also happen if the composition of the furnace gas is unstable. Strengthening quality control at each stage of the heating process is essential to prevent this issue.

By paying close attention to these factors during heat treatment, we can ensure the production of high-quality seamless steel pipes and avoid unnecessary waste and cost.