Tap shank crack at the square tail
Machine M10 taps are made of two materials for welding. The shank material is 45 steel and the blade material is W6Mo5Cr4V2 steel. Handle technical requirements are: hardness (35 ~ 45) HRC, and maintain (35 ~ 45) HRC within 1/2 ~ 2 / 3 handle length. When the shank is heated in a 850°C salt bath and quenched in a conventional process where the 8% CaCl 2 (mass fraction) aqueous solution is cooled, quench cracks occur at the square tail of the shank (see the right figure), and the average batch crack occurrence rate is 2^% seriously affected normal production. For this reason, the quench cracks appearing when the tap shank is quenched are analyzed to propose a technological measure for preventing the occurrence of quench cracks, so as to solve the production problem. 1 Analysis of Causes of Cracking The chemical composition analysis of the tap shank has an average carbon content of 0.48% and a manganese content of 0.79%. The rest of Si, P, and S are normal. Since both the carbon and manganese contents are at the upper limit of 45 steel, the Ms point is reduced, and the tendency of quenching and cracking of the steel is increased. In particular, the manganese content is in the upper limit range, and the transformation point Ac3 and Ar3 of the steel abruptly decrease. If the conventional temperature quenching, austenite grains must be coarse, hardenability increased, it is easy to produce quenching cracks. According to the literature, the Ac3 and Ar3 points of medium-carbon steel with a carbon content of about 0.48% have a minimum value of 750°C and 720°C, respectively. If they are quenched by conventional techniques, the materials may be overheated. So that quenching cracking. Secondly, quenching cracking is closely related to the shape of the workpiece. The gaps, corners, sharp corners, grooves, and sharply changing parts of the workpiece are places where quenching stress concentrates, and are dangerous parts of quenching cracks, and the tap square tail appears. The crack is in this area. The sharp corners, corners and other parts of the tap are first cooled in the quenching agent to obtain a martensitic structure, and when the later cooled core also forms martensite, the volume expands, so that the sharp corners and corner parts are subjected to a large tensile stress. And three-dimensional volume stress. With the stress concentration factor, the stress at the sharp corners and corners can be 10 times that of the smooth part, and cracks are easily generated. According to the literature, 45 steel easily quenched and cracked size range of 5 ~ 11mm, cross-sectional size is too small or too large are not easy to quench crack. Since the tap shank size 6.3mm×φ8mm is within the critical quench-cracking size range of 45 steel, the square tail is also provided with edges and corners, sharp corners, and the carbon and manganese content are within the specified upper limit, the quenching temperature is high and the coolant is high. In the case of a CaCl2 aqueous solution, quenching cracks are easily generated at the corners and sharp corners of the square tail when the tap shank portion is quenched. In order to prevent the occurrence of quenching cracks at the tangs of the shank, multiple quenching tests were conducted and effective measures were taken to successfully solve the problem of quenching cracks at the tangs of the shank. The two-liquid quenching test considers the upper limits of carbon and manganese content and the steep drop of Ac3 point, and reduces the quenching heating temperature from 850°C to 820°C. Process: 820 °C heating, heat preservation (30 ~ 40) s / mm, brine - oil cooling. The hardness after quenching is maintained above 50HRC. With this process, the batch crack occurrence rate is 5%. Mastering the residence time in water is the key to this process, otherwise quench cracks still occur at the square tail of the handle. Martensite austempering test process: 820 °C heating, heat (30 ~ 40) s / mm, 150 °C alkaline bath and then air-cooled. The hardness after quenching is maintained above 50HRC. No quench cracking occurs at the square tail of this handle. Zero-insulation martensite isothermal quenching test process: 820°C heating, zero heat retention, 150°C alkali bath isothermal, followed by air cooling. The hardness after quenching is maintained above 50HRC. With this process, quench cracks do not occur at the square ends of the shank. The Yawen martensitic isothermal quenching test process: 780 °C heating, heat preservation (30 ~ 40) s / mm, 150 °C alkaline bath temperature, followed by air cooling. The hardness after quenching is maintained above 50HRC. With this process, no quench cracks appear at the square tails of the shank. Analyze the above four kinds of tests. Test (1) Rapid cooling of brine in the high temperature zone suppresses the decomposition of supercooled austenite. When the temperature is lower than 400°C, it is immediately transferred to the oil and slowly cooled to reduce the quenching internal stress and prevent quenching. crack. Strictly controlling the residence time of the part in salt water is the key to this process. If the residence time is too long, it is a single-liquid quenching, which will lead to quench cracking. In tests (2), (3) and (4), despite the fact that the heating temperature and holding time were not the same, coarsening of grains was avoided during austenitization and the tendency to quench cracking was reduced. At the same time, the quenching heating temperature is low, and the cooling is all stopped in the alkali bath at 150° C. and air-cooled, which greatly reduces the quenching internal stress and prevents the occurrence of quenching cracks. In the 150 °C alkaline bath at an isothermal stop, in the elimination of thermal stress, the occurrence of martensite transformation of supercooled austenite, martensite was tempered and then transformed into tempered martensite, eliminating part of the phase change stress. The longer the residence time, the more phase transformation stress is eliminated. After air cooling, the retained austenite has a small amount of martensite transformation and a small phase transformation stress. Thus, the residual quenching stress of the shank is small enough to cause quench cracking. Therefore, the martensite austempering can effectively prevent the quenching of the tap shank. In the test (4), although the determined heating temperature was close to the Ac3 line, the upper limit of the manganese content caused the Ac3 line to move downward, so that a single austenite structure was still obtained during the quenching and heating. 2 Conclusions Production practice has proved that the use of martensitic austempering or zero-insulation martensitic austempering or sub-temperature martensitic austempering can effectively prevent the cracking of M10 tap shank quenching and can meet the hardness requirements. From the viewpoint of energy saving, the zero-insulation martensite has the best austempering effect. Since the process was adopted, the shank has no quench cracking phenomenon, and the production is normal and the output is increased.
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