According to national and industry standards, the porosity of welded joints should generally not exceed 3%. If the porosity exceeds 3%, the mechanical properties and corrosion resistance of the welded joint will be affected, resulting in fatigue fracture and damage to the weld. Therefore, when conducting welding quality inspections, the porosity of welded joints needs to be strictly controlled to ensure that it meets the standard requirements.

1. Improper quenching temperature
The quenching temperature has a very important influence on the structure of steel parts. Too low or too high a quenching temperature will lead to the presence of residual austenite in the quenched steel. The too-low quenching temperature will lead to the incomplete transformation of martensite, while the too-high quenching temperature will lead to the re-precipitation of austenite.
2. During the quenching process, the martensite that has been transformed first hinders the transformation of the untransformed austenite to martensite.

Square tube welding can be performed on four sides or only on two sides. These two methods are called four-sided welding and two-sided welding respectively.

Two-sided welding: When the welding quality is not high, just lay the pipe and the parts connecting the pipe flat on the ground and use a handheld arc welding machine to perform meat welding. After welding, the weld cannot be seen, the appearance is not worse than butt welding, and the reliability is also OK. It is suitable for some pipe connections and installations with low-quality requirements, such as some outdoor iron art, etc.

Four-sided welding: For some square tube welding situations that require higher strength or better appearance, skilled welders should use four-sided welding, that is, all four sides except the bottom are welded. It is suitable for some industries, ships, and other buildings that require higher strength. At the same time, the appearance of four-sided welding is better and more beautiful.