Contact resistance heating and quenching: at the beginning and the end of the electrode, the voltage less than 5 V is applied to the workpiece, and a large amount of resistance heat flows through the contact between the electrode and the workpiece, so that the workpiece surface is heated to the quenching temperature, and then the electrode is removed, and the heat is transferred into the workpiece interior and the surface is cooled rapidly, that is to say, the quenching target is reached. When the long workpiece is processed, the electrode moves forward continuously, and the part left behind is hardened continuously. The advantages of this method are simple equipment, simple operation, easy automation, small distortion of the workpiece, no need for tempering, which can significantly improve the wear resistance and scratch resistance of the workpiece, but the hardening layer is relatively thin (0.15-0.35mm). The average of microstructure and hardness is poor. This method is mainly used for surface hardening of machine tool guide rail made of cast iron, and the use boundary is not wide.
Electrolytic heating and quenching: place the workpiece in the electrolyte of acid, alkali or salt water solution, connect the workpiece to the cathode, and connect the electrolytic cell to the anode. After the direct current is connected, the electrolyte is electrolyzed, oxygen is released on the anode, and hydrogen is released on the workpiece. Hydrogen around the workpiece causes gas film, which becomes a resistance body and generates heat. The surface of the workpiece is heated to the quenching temperature quickly, and then the power is cut off. The gas film dissipates immediately, and the electrolyte becomes the quenching medium, which makes the surface of the workpiece cool and harden rapidly. 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 production rate is high, the quenching distortion is small, it is suitable for a large number of small parts, and it has been used for the surface quenching of the exhaust valve rod end of the engine.
Laser heat treatment: the use of laser in heat treatment began at the beginning of the 70’s, and then entered the stage of production and use from the stage of laboratory discussion. When the laser with high energy density (106 w / cm2) is focused on the metal surface, the metal surface rises to the quenching temperature in a few percent seconds or even a few thousandths of a second. Due to the unique fast temperature rise of the mapping point, the heat is not enough to transmit to the surrounding metal, so when the laser mapping is stopped, the metal around the mapping point will play the role of quenching medium and absorb a large amount of heat, so that the mapping point will be cooled rapidly, and get very fine structure, with high mechanical function. If the heating temperature is high enough to melt the metal surface, a smooth surface may be obtained after cooling. This operation is called polishing. Laser heating can also be used for one-sided alloying, that is to say, plating a layer of wear-resistant or heat-resistant metal on the parts that are easy to wear or need heat resistance, or coating a layer of coating containing wear-resistant or heat-resistant metal, and then using laser mapping to make it melt rapidly, resulting in wear-resistant or heat-resistant alloy layer. A layer of chromium is first plated on the heat-resistant parts, and then the laser is used to melt it quickly, resulting in a hard tempering resistant chromium containing heat-resistant surface layer, which may greatly increase the service life and heat resistance of the workpiece.
Electron beam heat treatment: the beginning of the 70’s research and use. In the early stage, it is used for continuous annealing of thin steel strip and steel wire, with energy density up to 108W / cm2. Electron beam surface quenching should be carried out in vacuum, and other characteristics are the same as laser. When the electron beam bombards the metal surface, the bombardment point is heated rapidly. The depth of the electron beam through the raw material depends on the accelerating voltage and the density of the raw material. For example, a 150 kilowatt electron beam has a critical penetration depth of 0.076 mm on the iron surface and 0.16 mm on the aluminum surface. The electron beam bombards the surface in a short time, the surface temperature rises rapidly, while the substrate remains cold. When the electron beam stops bombarding, the heat transfers rapidly to the cold base metal, which makes the heated surface self quench. In order to effectively carry out “self cooling quenching”, a ratio of 5:1 should be maintained between the volume of all workpieces and the volume of quenching surface. The surface temperature and quenching depth are also related to the bombardment time. The heating speed of electron beam heat treatment is fast, and the time of austenitizing is only a few seconds or even shorter. Therefore, the surface grain of workpiece is very fine, the hardness is higher than that of normal heat treatment, and it has excellent mechanical function.
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