Cooling process of Large diameter LSAW welded pipe

The cooling process of large-diameter LSAW (Longitudinal Submerged Arc Welding) welded pipes involves several factors that influence the magnitude of tissue stress. These factors include the cooling rate, shape, and chemical composition of the workpiece, particularly in the martensitic transformation zone.

Key Factors Influencing Tissue Stress

- Cooling Rate:

The faster the cooling rate, the greater the thermal stress. Rapid cooling leads to uneven plastic deformation, contributing to higher residual stress.

- Chemical Composition:

Higher carbon content and alloy compositions in the steel increase the magnitude of residual stress. This is due to the higher thermal and tissue stresses during the cooling phase.

- Shape of the Workpiece:

The shape of the pipe also plays a role in how stress is distributed and relieved during the cooling process.

Transformation and Stress Formation

During the heat treatment process, when steel transforms from austenite to martensite, there is an increase in specific volume. This transformation causes the workpiece to expand, leading to inconsistent volume growth and the formation of tissue stress. The resulting tissue stress manifests as tensile stress on the surface layer and compressive stress on the core, which is opposite to the thermal stress.

Final Cooling and Residual Stress

At the end of the cooling process, the core portion of the pipe does not contract freely due to the contraction of the final cooling volume of the core. This results in the surface of the workpiece being compressed while the core is pulled. This phenomenon is influenced by several factors:

- Cooling Rate:

A faster cooling rate can intensify the thermal stress, leading to greater compressive stress on the surface and tensile stress in the core.

- Material Composition:

The specific composition of the steel affects how the stresses are distributed. Higher alloy content can lead to increased residual stress.

- Heat Treatment Process:

The specific heat treatment methods applied, including the duration and temperature of the heat treatment, also impact the residual stress.

Conclusion

Understanding the cooling process and the factors that influence tissue stress is crucial for ensuring the structural integrity of large-diameter LSAW welded pipes. Proper management of the cooling rate, material composition, and heat treatment process can help minimize residual stress, thereby improving the quality and performance of the welded pipes.