With high-frequency induction heating equipment, you no longer have to worry about black tissue defects caused by self-tapping screws.

Self-tapping screws are one of the commonly used fasteners and are made of 20 steel finely drawn round steel. The process flow is: wire drawing - heading - thread rolling - edge milling - heat treatment - galvanizing - finished product. The heat treatment is strengthened by carbonitriding surface treatment, the depth of the carburizing layer is 0.15-0.28mm, the surface hardness is 480-780HVO.3, and the core hardness is 290-380HVO.3. During production, it was found that after carbonitriding treatment at 880°C in a continuous mesh belt furnace, the hardness of the product was qualified, but black tissue was found on the surface of the screw, where the hardness was low, causing some products to be unqualified. After inspection and analysis, the dotted black tissue here is a hole defect. Research has found that using high-frequency induction heating equipment for heating and heat treatment can effectively avoid the occurrence of such defects.

Metallographic examination found that after the workpiece was processed, the structure of the carburized layer was hidden pin tempered martensite + carbonitride + retained austenite, and the core structure was tempered low carbon martensite + ferrite, with some parts having low hardness. The screws were found to have black tissue defects. Analysis believes that the main reason for the low hardness of the workpiece is black tissue defects. After scanning electron microscopy observation, holes with a diameter of about 2-3 cm were found under the surface of the infiltration layer, which were black tissues when observed under an optical metallographic microscope. Inspection and analysis believe that the nitrogen potential of the workpiece is too high in the early stage of carbonitriding, and a large amount of nitrogen is absorbed into the carburizing layer. In the later stage of the process, the carbon concentration is high, resulting in nitride decomposition and denitrification. The atomic nitrogen changes into molecular nitrogen to form holes, and the surface layer of the workpiece is A "balloon" is formed inside, resulting in point-like black tissue observed metallographically.

Black tissue defects significantly reduce the contact fatigue strength of the screw, reduce the wear resistance, and the permeable layer is easy to peel off during workpiece work, and even cause the cutting edge to become blunt and the workpiece to break; at the same time, early fatigue fracture failure of the screw is prone to occur.

High-frequency induction heating equipment is used for rapid heating, shortening the heating time of the workpiece, increasing the heating temperature by 40°C, shortening the process cycle, and further reducing the appearance of point-like black tissue on the workpiece.

After adopting the above process improvement measures, obvious results have been achieved. Dot-like black tissue defects have been effectively prevented and eliminated during production. The quality of self-tapping screws is excellent and production runs well.