Undercut defects in laser-welded stainless steel pipes: A comprehensive analysis from problem to sol
Posted by： Hangao Time：11/3/2025 10:09:52 AM
In a stainless steel welded pipe production workshop, laser welding machines operate at high speed, the weld surface gleaming with a silvery-white luster under the lights. However, quality inspectors, examining the weld under a microscope, discover tiny grooves on both sides. While this defect is difficult to detect with the naked eye, it can become a fatal flaw in subsequent pressure tests or long-term service. Engineers call it—undercut.
Undercut not only affects the appearance of the welded pipe but also reduces the strength and durability of the welded joint. So, how exactly does undercut occur? And how can it be prevented? The following is a question-and-answer format.
Q1: What is an undercut defect?
A1: Undercut refers to a groove or depression formed at the junction of the weld and the base material because the molten pool metal fails to fully fill the area. It disrupts the smooth transition between the weld and the base material, reducing the overall performance of the welded joint.
Q2: What are the main causes of undercut defects?
A2:
Welding speed is too fast: The molten metal does not have enough time to flow back and fill the weld toe area. Inappropriate energy input: Excessive laser power or a rapid decrease in power can cause pinhole collapse.
Excessive assembly gap: Insufficient molten metal to fill the joint edges.
Unstable shielding gas: Gas flow disturbs the molten pool, causing uneven metal distribution.
Q3: What are the hazards of undercut defects?
A3:
Stress concentration: Grooves become crack initiation sites, reducing fatigue life.
Reduced strength: Poor bonding between the weld and base metal weakens load-bearing capacity.
Poor appearance: Affects the surface quality of the welded pipe and customer satisfaction.
Q4: How to prevent undercut defects?
A4:
Optimize process parameters: Properly match laser power and welding speed, avoiding excessive speed or slowness.
Control energy input: Gradually reduce power at the end of welding to prevent pinhole collapse.
Improve assembly accuracy: Reduce joint gap to ensure sufficient weld metal.
Stable shielding gas: Maintain uniform gas flow and avoid disturbing the molten pool. Post-treatment repair: Existing undercut can be repaired through grinding, welding, etc.
Q5: How does the industry standard evaluate undercut?
A5: According to international welding standards (such as ISO 13919 2:2021), undercut is a welding defect and must be strictly controlled within the allowable range. For high-end applications (such as pressure vessels and automotive parts), undercut is generally considered an unacceptable defect and must be completely avoided through process optimization.
Summary: Although undercut defects are small, they can become a hidden danger for welded pipe failure. Through process optimization, assembly precision control, and gas protection improvements, undercut can be effectively avoided, ensuring weld quality and service life. For manufacturing enterprises pursuing the high-end market, understanding the mechanism and countermeasures of undercut defects is not only key to quality assurance but also an important step in winning customer trust and enhancing brand value.
