The most commonly used surface heat treatment processes for hardware parts processing include induction heating and flame quenching, in addition to contact resistance heating quenching, electrolytic heating quenching, laser heat treatment and electron beam heat treatment. details as follows:
Contact resistance heating and quenching: A voltage of less than 5 volts is applied to the workpiece through the electrode, a large current flows in the contact between the electrode and the workpiece, and a large amount of resistance heat is generated, so that the surface of the workpiece is heated to the quenching temperature, and then the electrode is moved. To remove the heat, the heat is transferred to the inside of the workpiece and the surface is rapidly cooled, which is the purpose of quenching. When processing long workpieces, the electrode keeps moving forward, and the part left behind keeps hardening. The advantages of this method are simple equipment, convenient operation, easy automation, minimal distortion of the workpiece, no need for tempering, and can significantly improve the wear resistance and scratch resistance of the workpiece, but the hardened layer is thinner (0.15 ~ 0.35 mm ). The uniformity of microstructure and hardness is poor. This method is mostly used for surface quenching of machine tool guide rails made of cast iron, and its application range is not wide.
Electrolytic heating and quenching: The workpiece is placed in the electrolyte of acid, alkali or salt aqueous solution, the workpiece is connected to the cathode, and the electrolytic cell is connected to the anode. After the direct current is switched on, the electrolyte is electrolyzed, releasing oxygen on the anode and hydrogen on the workpiece. Hydrogen forms a gas film around the workpiece, which becomes a resistance body and generates heat. The surface of the workpiece is quickly heated to the quenching temperature, and then the power is turned off. The gas film disappears immediately, and the electrolyte becomes the quenching medium, so that the surface of the workpiece is rapidly cooled and hardened. The commonly used electrolyte is an aqueous solution containing 5-18% sodium carbonate. The electrolytic heating method is simple, the processing time is short, the heating time is only 5-10 seconds, the productivity is high, the quenching distortion is small, and it is suitable for mass production of small parts. It has been used for surface quenching of the end of the exhaust valve stem of the engine.
Laser heat treatment: The application research of laser in heat treatment began in the early 1970s, and then entered the production application stage from the laboratory research stage. When a focused high-energy density (106 W/cm2) laser irradiates the metal surface, the metal surface rises to the quenching temperature within a few hundredths of a second or even a few thousandths of a second. Because the irradiation spot heats up very quickly, and the heat is too late to transfer to the surrounding metal, when the laser irradiation is stopped, the metal around the irradiation spot acts as a quenching medium and absorbs a lot of heat, so that the irradiation spot is rapidly cooled and an extremely fine structure is obtained , Has high mechanical properties. Hardware parts processing manufacturers tell you that if the heating temperature is high enough to melt the metal surface, a smooth surface can be obtained after cooling. This operation is called glazing. Laser heating can also be used for local alloying treatment, that is, the parts that are easy to wear or need heat resistance are first coated with a layer of wear-resistant or heat-resistant metal, or coated with a layer of coating containing wear-resistant or heat-resistant metal, and then laser Irradiation makes it melt quickly, forming a wear-resistant or heat-resistant alloy layer. Plate a layer of chromium on the parts that need heat resistance, and then use a laser to quickly melt it to form a hard tempering and heat-resistant chromium-containing surface layer, which can greatly improve the service life and heat resistance of the workpiece.
Electron beam heat treatment: The hardware parts processing manufacturer tells you that when the electron beam bombards the metal surface, the bombardment point is quickly heated. The depth of the electron beam penetrating the material depends on the acceleration voltage and the material density. For example, the theoretical penetration depth of a 150 kilowatt electron beam on an iron surface is about 0.076 mm; on an aluminum surface it can reach 0.16 mm. The electron beam bombards the surface in a short time, and the surface temperature rises rapidly, while the substrate remains cold. When the electron beam stops bombarding, the heat is quickly transferred to the cold base metal, so that the heated surface is quenched by itself. In order to effectively carry out “self-cooling quenching”, the volume of the entire workpiece and the volume of the quenched surface must be maintained at a ratio of at least 5:1. The surface temperature and the depth of hardening are also related to the bombardment time. The electron beam heat treatment has a fast heating speed, and the austenitization time is only a few tenths of a second or less. Therefore, the surface of the workpiece has very fine grains, higher hardness than general heat treatment, and good mechanical properties.