Release time：2020-01-16 18:44 Browse：
In the fastener industry, high strength screws of grade 10.9 and above have attracted much attention for their excellent mechanical properties and a wide range of application scenarios. These screws not only carry the important connection tasks of mechanical equipment, but also need to maintain long-term stability and durability in harsh environments. Therefore, for screws above grade 10.9, the dehydrogen treatment in the galvanizing process is particularly important. In this paper, the heat treatment of screws, the phenomenon of hydrogen embrittlement, the principle of galvanizing and the necessity of dehydrogenation are analyzed in depth.
First, heat treatment and hydrogen embrittlement of screws
In the production process of screws, heat treatment is a crucial link. Through the high-temperature heating and cooling process, the internal organizational structure of the screw can be changed, thereby improving its hardness, strength and toughness. However, during heat treatment, a certain amount of hydrogen may be absorbed inside the screw. These hydrogen mainly comes from the high temperature environment during heat treatment, pickling activation treatment and electrolytic reaction during electroplating.
The presence of hydrogen inside a metal causes a number of problems, the most significant of which is hydrogen embrittlement. Hydrogen embrittlement means that after the metal absorbs hydrogen, its mechanical properties are significantly reduced, which is manifested by a decrease in strength and toughness, and even fracture during use. This is because hydrogen atoms can penetrate into the metal lattice, causing lattice distortion and internal stress, which leads to the formation and expansion of micro-cracks. For class 10.9 screws bearing high strength and high stress, hydrogen embrittlement is undoubtedly a serious safety hazard.
Second, the anti-corrosion principle and potential problems of galvanizing
Galvanizing is a common metal anti-corrosion technology, by covering the screw surface with a layer of zinc, you can effectively isolate the direct contact between the screw and the external environment, so as to prevent corrosion. The zinc layer has good electrochemical stability and sacrificial anode protection, when the zinc layer on the screw surface reacts with oxygen and water vapor in the air, it will be preferentially oxidized, thus protecting the internal matrix metal from corrosion.
However, the electrogalvanizing process itself also has certain risks. During electroplating, electrolysis causes water molecules to break down, releasing hydrogen ions. These hydrogen ions are reduced to hydrogen at the cathode (i.e., the plated screw), and some of the hydrogen escapes as bubbles, but some of the hydrogen atoms may still penetrate into the screw's lattice. These infiltrated hydrogen atoms will not only affect the quality of the galvanized layer, such as pinholes, pitting, bubbles and other defects, but also may further aggravate the occurrence of hydrogen embrittlement.
Third, the necessity and principle of dehydrogen treatment
In view of the above problems, for high strength screws 10.9 and above, the dehydrogenation treatment before galvanizing is particularly important. Dehydrogenation, also known as dehydrogenation annealing, is the process of placing a heat-treated screw in a high temperature oven for a period of time, so that the hydrogen atoms that have penetrated into the screw are diffused back to the surface and released from the metal.
The principle of dehydrogenation is to use the thermal diffusion effect at high temperatures to accelerate the migration rate of hydrogen atoms inside the metal, so that it is easier to diffuse to the metal surface and escape. At the same time, the high temperature can also promote the release of the internal stress of the screw and the homogenization of the organization, which helps to further improve the mechanical properties of the screw.
In the specific implementation process, the temperature and time of dehydrogen treatment should be comprehensively considered according to the material of the screw, the heat treatment process and the use environment. In general, the hydrogen drive temperature is usually set between 190-230 ° C, and the hydrogen drive time is 6-8 hours. In addition, in order to ensure the maximum dehydrogenation effect, the hydrogen drive treatment should be carried out as soon as possible before passivation after electroplating, and the residence time of the screw in a high temperature environment should be minimized.
Fourth, the impact of dehydrogen treatment on screw performance
Dehydrogenation has a significant positive effect on the performance of high strength screws of grade 10.9 and above. First of all, by removing the hydrogen inside the screw, the occurrence of hydrogen embrittlements can be effectively avoided and the strength and toughness of the screw can be improved. Secondly, dehydrogen treatment can also promote the homogenization of the internal organization of the screw and the release of stress, which helps to further improve the mechanical properties and stability of the screw. Finally, the dehydrogenation treatment can also improve the quality of the galvanized layer, so that the zinc layer is more uniform and dense attached to the screw surface, thereby improving the corrosion resistance and service life of the screw.
In summary, for high strength screws of grade 10.9 and above, dehydrogenation before galvanizing is a key step to ensure that they have stable performance and excellent corrosion resistance during use. This step not only tests the technical level and production experience of fastener manufacturers, but also an important guarantee for screw performance and use safety.
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