Cold Cracking Tests on X80 Pipeline Steel

Characteristics and Welding Challenges of X80 Pipeline Steel: 

X80 pipeline steel is manufactured using TMCP (Thermomechanical Control Process) and microalloying technology, possessing high strength and toughness. It is also an ultra-fine-grained, high-purity steel. Therefore, this steel grade places special requirements on the welding process, mainly in preventing grain coarsening, local softening, and embrittlement in the heat-affected zone (HAZ), ensuring the purity and grain refinement of the weld metal, and selecting appropriate welding methods and improving welding processes.

Grain Coarsening Problem and Countermeasures:

Grain coarsening in the HAZ is an unavoidable phenomenon, and grain growth reduces the performance of the welded joint. To control grain growth in the HAZ of pipeline steel under high heat input, low heat input welding or high-energy beam welding methods are typically used to reduce the coarse-grained zone, thus not affecting the service performance of the welded joint. However, under low heat input welding conditions, cold cracking is prone to occur at the weld. Therefore, preheating of the weld is necessary before welding, and the effect of preheating on the cold cracking susceptibility of X80 pipeline steel is studied by adjusting the preheating temperature.

Crack Rate Variation at Different Preheating Temperatures: 

The crack rate of the welded joint of X80 pipeline steel with a wall thickness of 7.9 mm and an outer diameter of 610 mm varies significantly under different preheating temperatures. Under preheating conditions of room temperature and 50℃, the cross-sectional crack rate is approximately 30%. When the preheating temperature reaches 100℃, the cross-sectional crack rate decreases to 3.55%; when the preheating temperature rises to 150℃, the cross-sectional crack rate is only 0.85%. This indicates that the crack rate decreases significantly with increasing preheating temperature; when the preheating temperature reaches 100℃, the cross-sectional crack rate is far below 20%, and the crack susceptibility is greatly reduced.

Metallographic Structure Changes and Crack Resistance Mechanism:

The metallographic structure of the welded joint of X80 pipeline steel differs under different preheating temperatures. As the preheating temperature increases and the cooling rate decreases, the granular bainite structure in the fusion zone increases, and the bainite laths become refined. Bainite lamellars split, with lamellae of the same orientation gradually thinning, and the lamellar boundaries acting as grain boundaries. When the crack propagates to the slab boundary, fracture occurs, effectively hindering crack propagation during low-temperature fracture and enhancing weld toughness.

Causes of Cold Cracking:

Cold cracking is the result of the combined effects of hardening tendency, hydrogen content, and confinement. X80 pipeline steel has a low carbon content and a low hardening tendency, possessing a certain ability to suppress cold cracking. However, with increasing strength grade and plate thickness, a certain tendency for cold cracking still exists. In field welding, due to the use of high-hydrogen welding materials such as cellulose electrodes and self-shielded flux-cored wires, the welding line energy is relatively low, and the cooling rate is relatively fast, further increasing the risk of cold cracking.

Mechanism of Preheating on Hydrogen Diffusion:

Hydrogen is one of the main factors causing cold cracking during welding. Hydrogen diffusion and accumulation require a certain amount of time, which leads to a certain delay in the initiation of cold cracks. Under preheating conditions, the diffusion rate of hydrogen in the weld is significantly accelerated, and the hydrogen concentration in the weld metal decreases rapidly. Simultaneously, preheating also slows down the cooling rate of the weld. Higher preheating temperatures result in slower cooling rates and longer time for diffusing hydrogen to escape. A large amount of diffusing hydrogen escapes through accelerated diffusion, thus reducing the diffusing hydrogen content in the heat-affected zone. This is the main reason why increasing the preheating temperature can significantly reduce the crack propagation rate.

Conclusion:

1. X80 pipeline steel has a low carbon content and low hardening tendency. Generally, no heat treatment or other insulation measures are required after welding.
2. When performing oblique Y-groove tests on X80 pipeline steel, metallographic images of cracks near the fusion zone show transgranular crack characteristics, consistent with the typical morphology of cold cracks.
3. Increasing the preheating temperature from room temperature to 150℃ reduces the crack rate from approximately 30% to 0.85%, indicating that preheating significantly reduces the susceptibility to cold cracking.

4. The mechanisms by which preheating reduces cold cracking sensitivity include: accelerating hydrogen diffusion and escape, reducing cooling rate, and refining bainitic lath structure.

Read more: X70 vs. X80 Pipeline Steel or API 5L PSL1/PSL2 Pipe Specification