High Zinc Coil Welding: Why Thicker Galvanization Leads to Pipe Cracking

High-zinc-coil (e.g., Z180, Z275, Z350) is more difficult to process during pipe making and welding compared to low-zinc-coil (e.g., Z60, Z80), and can lead to a higher tendency for the welded pipe to crack in subsequent processing (such as bending or flattening), especially in the weld zone.

Primary Cause Analysis:

1. Zinc Volatilization and Welding Defects

• During welding (especially high-frequency welding), high temperatures cause the zinc coating to vaporize (zinc boils at ~907°C, lower than steel's melting point).

• A high zinc coating means more zinc per unit area needs to vaporize. The vaporized zinc can contaminate the weld pool, leading to defects like porosity, slag inclusions, and cracks within the weld. These defects are internal "flaws" that directly reduce the weld's strength and toughness.

2. Zinc Vapor Disrupts the Protective Atmosphere

• Large amounts of zinc vapor severely interfere with the protective environment of the weld zone, making the molten weld metal more susceptible to reacting with oxygen and nitrogen from the air, forming brittle phases and degrading weld quality.

3. Formation of "Low-Melting-Point Brittle Phases"

• Zinc infiltrates the molten steel and can form brittle iron-zinc intermetallic compounds (e.g., FeZn13). These compounds aggregate at grain boundaries, acting like impurities in glue, severely weakening the bonds between grains and making the weld and its heat-affected zone (HAZ) brittle.

4. "Zinc Burn-Off" Leading to Edge Depletion and Stress

• During welding, the zinc coating on both sides of the weld seam is completely burned off, creating a zinc-free, bare steel edge. The bonding layer between the substrate and the thick zinc coating in high-zinc material can be more complex. After welding, the abrupt transition from the zinc-rich area to the zinc-free area creates inhomogeneity in chemical composition and microstructure. During bending, this transition zone easily becomes a stress concentration point and the origin of cracking.

⚙️ Impact on Pipe Making and Subsequent Processing:

• On Pipe Making (Welding): Weldability deteriorates. More precise control of welding power, speed, and squeeze pressure is required. Improper parameters easily lead to lack of fusion, burn-through, or the aforementioned defects.

• On Subsequent Processing (e.g., Bending): Bend formability decreases, particularly at the weld.

• The weld seam itself is a cast structure and is inherently more brittle than the base metal.

• The presence of welding defects and brittle phases prevents the weld zone from deforming plastically and uniformly like the base metal during bending. Stress concentrates at these defects, causing cracking initiating from the weld or HAZ. This phenomenon is especially pronounced when the bending axis is perpendicular to the weld direction.

✅ How to Address This? (Industry Practices)

1. Select the Appropriate Zinc Coating: Do not blindly pursue ultra-high zinc coatings unless in extremely corrosive environments. Prioritize lower zinc coatings (e.g., Z120-Z180 instead of Z275) that meet corrosion life requirements, as this significantly improves weldability and formability.

2. Optimize Welding Process: For high-zinc materials, pipe mills employ special techniques, such as:

• Zinc Removal (Wiping/Shaving): Before welding, remove the zinc coating from the strip edges (milling or wiping) to achieve "zinc-free welding."

• Parameter Adjustment: Increase welding speed, optimize HF frequency, precisely control the V-angle and squeeze pressure to minimize zinc vapor effects.

• Post-Weld Heat Treatment: In-line annealing of the weld seam to relieve stress and improve microstructure.

3. Control Processing Methods: When bending high-zinc welded pipes:

• Try to position the weld at the neutral axis (midway between the outer and inner bend radii) to avoid subjecting it to maximum tensile or compressive stress.

• Reduce bending speed and use larger die radii.

• Perform flaring or flattening tests on finished pipes to verify weld quality meets standards.

Summary:

The statement that "high-zinc-coil is harder to weld, and welded pipes made from it are more prone to cracking during bending" is true to fact. The root cause is the deterioration of weld quality due to zinc volatilization and brittle phase formation during welding. This is a classic example of the conflict between material properties, welding processes, and subsequent processing. Therefore, a balance must be struck between corrosion resistance and formability during product design and material selection.