l. Lack of fusion in the weld: Seemingly “sealed” but actually “hidden cracks
During workshop inspections, I often encounter apprentices holding up welded workpieces and asking, “Master, look, the surface is smooth, there must be no problem, right?” But ultrasonic testing reveals a “layer” hidden between the weld and the base material—this is lack of fusion, one of the most concealed and dangerous problems in welding.
This type of problem often stems from three misconceptions: Beginners tend to use “low heat and slow welding,” worrying about burn-through and setting the current too low, resulting in the metal not reaching its melting temperature and thus not adhering properly; experienced welders sometimes try to speed things up, moving the welding torch too quickly, causing the molten pool to cool before it can spread.
The solution is actually quite simple: Before welding, select the correct current based on the thickness of the base material. For low-carbon steel welding, the current is generally controlled at 100-150A, while thicker plates can be appropriately increased; maintain the welding torch angle at 30-45 degrees and move it at a uniform speed to ensure the molten pool covers both sides of the bevel; clean the bevel with an angle grinder before welding until the metal shines through—this step, no matter how troublesome, should not be skipped.
II. Porosity Defects: “Honeycombs” in Welds, “Explosion Points” Under Stress
“Master, why are my welds full of small holes?” This is one of the most common questions asked by beginners. These pores, distributed on or inside the weld surface, act like honeycombs, disrupting the weld’s density. This not only affects the appearance but also significantly reduces the weld’s strength. In pressure vessel welding, excessive porosity is directly considered a failure because under high pressure, these small holes become stress concentration points, ultimately leading to weld cracking.
To avoid porosity, implement “three levels of protection”: Pre-welding protection: Dry welding rods according to requirements (150-200℃ for acidic rods, 250-350℃ for basic rods), and store them in an insulated container for immediate use; Environmental protection: For outdoor welding, construct a windproof and moisture-proof shed; stop work when humidity exceeds 80%; Operational protection: Maintain a short arc welding position, keep the welding torch 3-5 mm away from the workpiece, and move the electrode at a uniform and steady speed to allow sufficient time for the molten pool to expel gases.
III. Welding Cracks: The Most Deadly “Structural Killer”
Of all welding defects, cracks are undoubtedly the “number one killer.” Like a lurking blade within metal, once formed, they continue to expand. Even a tiny crack can cause the entire piece of equipment to collapse under stress.
Cracks can be classified by their cause into cold cracks, hot cracks, and reheat cracks, with cold cracks being the most easily overlooked. Cold cracks often occur during the cooling process after welding, especially when welding high-strength steel and thick plates. Due to uneven shrinkage between the weld and the base metal, significant internal stress is generated. Combined with excessively high hydrogen content in the weld, cracks will form. Many beginners think, “If there are no cracks after welding, everything is fine,” but they don’t realize that cold cracks can appear hours or even days after welding—this is the terrifying aspect of “delayed cracks.”
Preventing cracks requires focusing on three key aspects: temperature control, hydrogen control, and stress control. Preheating should be performed according to the material and thickness of the base material before welding. For example, when welding Q345 steel thick plates, the preheating temperature should not be lower than 100℃. Low-hydrogen welding rods should be selected and strictly dried before welding to reduce the hydrogen content of the weld. Slow cooling measures should be taken after welding, such as covering the weld with asbestos cloth to allow it to cool slowly. For important workpieces, post-weld stress relief heat treatment is also required.
