Cause analysis of hydrogen induced crack of LSAW steel pipe

LSAW (Longitudinal Submerged Arc Welding) steel pipes, subjected to varying conditions and environments over time, may experience damage due to intrinsic defects and changing stress factors. One of the most critical forms of damage is hydrogen-induced cracking (HIC). This occurs when welded steel pipes, containing hydrogen, crack under the influence of applied or residual stress. Hydrogen-induced cracking is caused by the diffusion of hydrogen into the metal matrix, where it either forms a solid solution or metal hydrides, leading to brittle fractures. This phenomenon poses a significant risk to the structural integrity of steel pipes.

Hydrogen-Induced Cracking Mechanism

The hydrogen-induced cracking process is primarily driven by hydrogen atoms entering the metal structure, which can happen during contact with hydrogen or through a cathodic corrosion reaction. Once inside the pipe’s metal tissue, these hydrogen atoms make the material brittle, although the material may not immediately lose its ductility or begin to crack. Proper heat treatment of the welded pipe, either online or offline, can expel hydrogen from the material, restoring the pipe’s original strength and reducing the risk of cracking.

Formation of Metal Hydrides

In LSAW steel pipes, hydrogen-induced cracking often occurs when hydrogen atoms diffuse into the pipe and form brittle hydrides. These hydrides significantly increase the sensitivity of the material to stress concentrations, making it more prone to cracking. Flaky hydrides, in particular, are highly susceptible to stress-induced failure, leading to hydrogen-induced cracking.

One characteristic of this type of cracking is the latency period before cracks appear under applied stress. Hydrogen-induced cracking does not occur immediately but manifests after some time. The length of this latency period is independent of the magnitude of the applied stress but decreases as the hydrogen concentration inside the pipe increases. The delay in cracking indicates the time required for hydrogen to diffuse to critical regions near the crack and reach the concentration needed to initiate cracking.

Crack Propagation and Stress Concentration

The interaction between hydrogen and dislocations in the steel structure leads to intermittent crack propagation. Initially, plastic deformation occurs, followed by hydrogen diffusion into defects caused by the deformation. This leads to partial hardening of the weld joints, which allows cracks to propagate further, ultimately resulting in hydrogen-induced cracking.

This process explains why hydrogen-induced cracks often appear after a period of stress exposure, and not immediately upon contact with hydrogen. The cycle of hydrogen diffusion, hydride formation, and crack propagation creates a delayed but inevitable failure of the pipe, particularly under high stress or pressure conditions, such as those found in underground applications. In some cases, cracks may appear within days or weeks after exposure.

Conclusion

Hydrogen-induced cracking in LSAW steel pipes is a complex process driven by hydrogen diffusion, stress concentration, and metal hydride formation. While proper heat treatment can mitigate this risk by removing hydrogen from the material, hydrogen-induced cracking remains a potential source of damage, particularly in high-stress environments. Understanding this process is crucial for preventing pipe failure and ensuring long-term reliability in critical infrastructure applications.