Since the early 1970s, induction heating has been used to quench steel files in our country. After decades of technological test and production, we have mastered the whole production process of 100-350mm sharp flat files, square files and triangular files, as shown in Figure 3-6. It has been proved that induction heating is completely suitable for quenching steel files, and the effect is very good, which is consistent with the strategy of energy saving and emission reduction.
1- inlet 2- spray ring 3- induction ring 4- rubber roller 5- steel file movement direction 6- steel file
1) the choice of induction heating frequency. It is well known that induction heating uses the principle of alternating current induction and skin effect to achieve the purpose of surface heating. Its current penetration depth (?) = 500 / F (f is the current frequency), that is, the penetration depth is inversely proportional to the square root of the current frequency, so changing the current frequency can get different depth of the heating layer. When choosing higher current frequency, because of the shallow penetration depth of electric current and the deep quenching depth, it is easy to cause the phenomenon that the root heating temperature is appropriate and the tip has already been overheated, or the root heating is not enough when the tip heating temperature is appropriate. When choosing the lower current frequency, because of the deep penetration depth of the current, it is easy to appear that the heating depth is too deep, the heating time is long, and the heating power is large. Therefore, it is very important to choose the appropriate current frequency to get the uniform heating temperature of the tooth tip and root under the premise of ensuring the depth of the hardened layer.
以典型的300mm粗纹尖扁锉为例，齿尖凸起部位大致为0.50~0.55mm，质量要求齿根下0. 30mm必须淬透，即钢锉表面的整个淬硬层深度δ要在0.80~0. 85mm以上，且要求齿尖不能出现粗针马氏体。
Taking a typical 300 mm thick-grained flat file as an example, the protruding part of the tooth tip is approximately 0.50-0.55 mm. The quality requires that 0.30 mm below the tooth root must be hardened, that is, the whole hardening depth of the steel file surface should be above 0.80-0.85 mm, and the tooth tip should not appear thick-needle martensite.
The production practice has proved that the thermal efficiency of delta is smaller than that of delta, and the thermal efficiency is the highest when the delta =1/2 delta is hot. In this way, when the 0. 80~0.85mm hardening layer is required, the delta heat should be 28=1.6~1.7mm, so the theoretical value of the current frequency is
f=( 500/1. 7)2～( 500/1.6)2Hz= 86.5～97. 7kHz
F= (500/1. 7) ~ (500/1.6) x Hz= 86.5 ~ 97. 7kHz
However, the current frequency used before was 250kHz. As a result, the thick needle martensite appeared on the tooth tip, and the quenching quality was difficult to control. Later, the equipment was modified to reduce the current frequency to 95 kHz (in the range of 97.7-86.5 kHz), and finally the thick needle martensite was controlled, and the quality fully met the technical requirements.
2) several conditions to ensure quenching quality. In order to ensure stable and high-quality products in the induction heating of steel files, the following 4 conditions should be met.
Select suitable power equipment. Practical experience is: 100 ~ 250mm sharp flat file, choose 60 kW, 250 kHz high frequency induction heating equipment, 300 mm or more sharp flat file, choose 100 kW, 95 kHz or 68 kHz ultrasonic induction heating equipment.
Second, formulate reasonable heat treatment process. Taking 300 mm thick-grained flat file as an example, the technological parameters are: screen voltage 12-13 kV, screen current 8-9 A, gate current 1.2-1.5 A, feeding time 12 s, cooling water pressure 17.64 MPa, current frequency 95 kHz.
Design suitable induction coil. Because of the high current frequency and sharp angle effect of the flat file below 250mm, the inductor coil is designed as shown in Figure 3-7. For large flat files of 300 mm or more, the induction coils shown in Fig. 3-8 are more suitable because they are equipped with ultrasonic induction heating equipment with uniform heating temperature and can make the steel files quenched.
250mm below the filing ring.
Fig. 3-7 induction coil below 250mm
300mm flat Keng induction coil
Fig. 3-8 300mm induction ring
4. The sprinkler should have strong jet cooling capability. The spray holes on the sprinkler should be evenly distributed, and the aperture angle is suitable, which can withstand water pressure of more than 19. 60MPa. In addition, it is also requested that no leakage should be allowed in the welds and no burrs inside and outside the spray holes.
3) experience of induction heating. Induction heating for steel files has the following advantages:
Small deformation. Because the steel file is not heated as a whole, and there are 16 rubber drums on both sides of the steel file during quenching, which ensures that the longitudinal deformation of the steel file is small. Even if a steel file with small deformation is produced, it is very convenient to straighten it with a wooden hammer because the surface is strengthened by induction heating and the outside is hard and soft.
Metallographic structure is good. Due to the rapid heating rate during induction heating, the substructures in austenite have no time to recrystallize, resulting in very fine martensite grains after quenching. In addition, due to the short heating time, the decarburization of the tooth tip is avoided.
Surface hardness is high. Because the steel file is only a rapid heating quenching of the surface layer, there is often a high stress distribution and high carbon martensite region on the surface of the hardened layer, so the additional high hardness (hardening phenomenon) can be obtained. The hardness of the technology is more than 62HRC, and the hardness value obtained by induction quenching is always over 65 HRC.
Labour productivity is high. Due to the rapid heating of induction heating system and the special semi-automatic quenching machine equipped with quenching operation, the output per shift is 2-3 times higher than that of salt bath quenching. Up to 4 times more.
(5) improving the working environment. Adopting induction heating technology, only need to insert the file into the stipulated conveyor cage, can complete the feeding, heating, cooling, straightening process, and then through the conveyor belt into the temper box automatically, after tempering automatically into the disk. The whole process is completed continuously. If there is a problem, the operator only needs to press the button. The labor intensity is small and the working environment is improved obviously. More importantly, energy conservation and environmental protection, in line with the development direction of cleaner production.
After decades of production and market test, induction hardening technology of steel file is more advanced, reliable and stable process, heat treatment quality is better than salt bath quenching, induction hardening to get finer structure, higher hardness
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