Today, Jinan Lehao metal will tell you what internal defects may appear in the production process of Jinan galvanized pipe.
White spot of internal defect of galvanized pipe. White spot, also known as crack, is the internal crack of galvanized pipe, which is not allowed to exist.
When large forgings are cooled to room temperature, white spots usually appear after several hours, dozens of hours or even longer. After etching, straight, thin and curved cracks can be observed on the transverse specimen. When the crack breaks along the direction perpendicular to the crack, silver white round or oval spots can be found on the fracture surface on one side of the crack.
It is better to check the white spot in the quenching state to avoid the white spot deformation caused by plastic deformation when the sample breaks.
Different schools of thought have different views on the generation mechanism of white dots. Some people think that hydrogen is the key factor to produce the white spot, and the huge internal stress is another important reason to produce the white spot. That is to say, when the hydrogen atoms in the ingot that have not escaped gather in the micro void and become the molecular state, the galvanized pipe will produce larger internal stress due to the volume expansion. Under the combined action of the deformation force in hot pressing process, the thermal stress in cooling process and the transformation stress in the process of microstructure transformation, the white spots of the local cracks in the material are shown as many tiny cracks on the macro level, which are distributed radially from the inside of the workpiece to the outside or along the deformation flow direction (usually called streamline distribution) of the pressure processing; in the longitudinal fracture , showing silver white oval spots.
Because white spots often appear in batches, some people also claim that white spots are the cancer of galvanized pipes. Some steels containing nickel and molybdenum are sensitive to the formation of white spots. They are called white spot sensitive steels, such as 5CrNiMo, 5CrMnMo, etc.
What's the harm of white dots?
The existence of white spots leads to the deterioration of the mechanical properties of the steel. The galvanized pipe with white spots is easy to crack in the process of heat treatment, or internal cracks suddenly appear in the process of processing and use, leading to the scrap of forgings. White spot will reduce the plastic toughness of galvanized pipe and the strength of parts. They are the focus of pressure. Under the action of alternating load, they are easy to become the source of fatigue cracks, leading to fatigue damage. Therefore, white spots are absolutely not allowed in forgings. Once white spots are found in forgings, they can also be re forged with a certain forging ratio to carry out white spot welding.