In machining, taps are the most common type of cutting tool used to machine internal threaded fasteners, consisting of a cutting portion and a shank. In addition to a variety of stresses during the cutting process, it is also subjected to strong friction. Therefore, the tap is required to have high strength, hardness, wear resistance, and small distortion after heat treatment. According to the above requirements, the materials for manufacturing taps are generally T12A, T10A carbon tool steel, CrWMn steel, 9CrSi and W18Cr4V high speed steel, W6Mo5Cr4V2 steel and so on.
In the heat treatment production process of the tap, because some of the details are often neglected, the heat treatment distortion of the tap is caused, and even the crack is scrapped, and a high scrap rate occurs, resulting in a large economic loss. To this end, the following issues should be noted in production:
1) The material of the tap is to be inspected before machining. The microstructure of the material should be spheroidized, and the carbides are fine and evenly distributed. If the microstructure of the material is flaky pearlite and the carbide size is too large, the unevenness is too high, or even the reticulated carbide is present, the plasticity of the tap is lowered, the quenching distortion cracking tendency is increased, and the brittleness of the tip is increased. Crushing, reducing the life of the tap.
2) In order to reduce the tendency of distortion cracking during quenching before tapping, especially for taps with high precision requirements, the machining stress generated in the previous process should be eliminated.
3) Before the quenching, the tap should be preheated to reduce the temperature difference to reduce the thermal stress and reduce the distortion tendency of the tap. Especially for W18Cr4V high-speed steel, because the quenching heating temperature is high, if the cold tap is directly heated in a high-temperature salt bath furnace up to 1280 °C, because of the faster heating speed, it will cause great thermal stress and increase. Distortion cracking tendency. Therefore, the tap should be preheated before quenching to reduce the temperature difference and reduce the thermal stress.
4) When the tap is heat treated, it is possible to quench only the blade and the shank, while the intermediate transition portion is not quenched. This will make the transition part tough enough to facilitate future calibration. The quenching heating temperature should be as low as possible to prevent coarse grains and reduce the strength, ductility and toughness of the tap. Especially small-diameter taps, it is better to reduce some hardness, but also must maintain a certain toughness, absolutely avoid high-temperature quenching. At the same time, the quenching heating time should not be too long. Otherwise, too high heating temperature and holding time will lead to coarse, decarburization, overheating and even over-burning of the tap, affecting its performance and service life, and even scrapping.
5) The tap after quenching should be tempered immediately to prevent long-term placement. The tempering temperature should be determined according to the hardness requirements of the tap, and the following two points should be noted: 1 adjust the tempering temperature according to the quenching condition; 2 avoid the first type of temper brittle zone of steel, such as the tempering temperature of CrWMn steel It is from 250 ° C to 300 ° C. The tempering time should be as long as possible. The hardness of carbon steel and alloy steel wire cone after heat treatment is 61-63HRC, and high-speed steel is 63-66HRC.
Practice has proved that "details determine success or failure", if you pay attention to the above details in the production process of taps, it will greatly reduce the heat treatment defects of taps and distortion and cracking, improve the pass rate and service life of taps, reduce production costs and increase economic benefits.
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