| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 97 === To improve the disadvantages of the apparatus in the cryogenic treatment, a design of buoyancy-drive and a PID temperature control apparatus were applied for cryogenic treatment in this research. With the purpose of extending the service life, workpieces were cooled down slowly and steadily from room temperature to the temperature of liquid nitrogen, kept for an appropriate period of time for the relaxation of inner stress and the stabilization of microstructures. Compared with the cryogenic equipment used in industries, the buoyancy-drive cryogenic apparatus designed in this research has the following features: lower temperatures could be reached, thus resulting in better effects of treatment; the temperature variation rates were steadier and smoother, without the occurrences of abrupt rises or falls in temperatures, thus, no thermal stress would be induced in the workpiece. Besides, the consumption rate of liquid nitrogen was relatively low, and the cost of cryogenic treatment could be reduced.
There are many methods in industry for metals to attain coated layers, including electroplating, physical vapor deposition, ion coating, chemical coating, thermal immersed coating, etc. Among these the most widely used method is electroplating. Electroplating is an oxidation-reduction reaction, in which the workpiece needed for coating is placed at the cathode, with the desired coated metal layer at the anode, and then placed into the metallic liquid of the desired coating layer. When power source is applied, the surface will attain the desired coating layer. During electroplating process, hydrogen generated at the cathode will penetrate into the interiors of the products and decrease the toughness of products. This is the so-called hydrogen embrittlement. To solve this problem, the products are reheated to an appropriate temperature and kept for a period of time. Thus, the hydrogen atoms dissolved in the products will be released, and the toughness and ductility of the products will be recovered. But, this method tends to lower the hardness of the products.
In this study, cryogenic dehydrogenation was accomplished by adapting the principle that the solubility of hydrogen in steels reduces with temperature. Electroplated sewing needles were cooled down to the temperature of liquid nitrogen (-196℃), and kept for a period of time to release the hydrogen atoms dissolved in them. Then, temperatures were raised back to or above the room temperature with appropriate rates. In the experimental process, electroplated sewing needles were baked under various conditions in advance to the cryogenic treatment. The hardness, bending angle, hydrogen amount of the specimens were then tested respectively to understand the effect of dehydrogenation. Thus, optimized dehydrogenation treatment conditions could be attained. Compared to traditional dehydrogenation treatment, in this research, besides altering different baking parameters, cryogenic treatments were also applied to the baked sewing needles in order to promote the effect of dehydrogenation. Moreover, cryogenic treatment also has the effect to stabilize the microstructure of the sewing needle and extend its service life.
|
|---|
