Induction heat treatment has become one of the main means of modern heat treatment because of its green, high efficiency, energy saving, and easy composition of the production line. The author has discussed the characteristics of modern induction heat treatment in previous articles, many of which are in line with the characteristics of modern cold processing, which is currently the only efficient heat treatment process similar to the cold processing production structure. Although China has become a worldwide manufacturing power, CNC/CAM technology is widely used in mechanical manufacturing, especially cold processing, and production organization and quality control have quite mature technology and mode, but compared with mechanical processing, induction heat treatment has a big gap in production organization and quality control technology.
In this paper, the similarities between inductive heat treatment technology and cold processing technology such as parts cutting are compared, and the methods of production organization and quality management are put forward to improve the application level of induction heat treatment.
1. Single piece processing
There are two kinds of heat treatment modes: single processing and batch processing. Induction heat treatment is a single piece processing mode, while other heat treatment processing using a heating furnace is a batch processing mode. For induction heat treatment, no matter how large the production capacity of the enterprise, the induction heat treatment of each part is carried out one by one. And the only way to increase productivity is to increase the number of processing units, for example, simplex machine tools to increase the number of machine tools; Multi-station machine tools increase the number of stations.
The advantage of batch processing is to increase production capacity, but it also brings the disadvantage of quality control. For example, you can’t control the status of any of the products, so you can’t control the quality of any of the products. In order to ensure the same batch of parts processing results is consistent, only improve the furnace state uniformity, such as temperature uniformity, uniformity of atmosphere, etc.; There is also a need to increase the number of controlled samples. But the furnace because has the structure on the uneven sex, it has the entrance and exit, corner to wait, want to achieve completely even is impossible.
Individual processing has unique advantages in quality control, which can monitor each piece and realize the traceability of product quality. After the completion of the processing can also be inspected piece by piece.
The cold machining of the parts adopts the single – piece machining mode completely. And the quality control of cold machining parts has been a very complete system and implementation measures, especially the application of CNC and CAM technology so that the on-line machining quality accuracy of parts has been greatly improved, the processing consistency of parts is also very good. Similarly, induction hardening is also increasingly using CNC and CAM technologies. Therefore, as long as we adopt the mature production organization mode of cold processing, the product quality consistency of induction heat treatment can also be greatly improved.
Another advantage of induction heat treatment is that the production organization is configured according to the beat, the man-hour is approved, and the cost is accurately controlled.
2. Limit of machining deviation (tolerance)
In order to control the consistency of single machining results, tolerance is used in cold machining. This is also applicable to induction heat treatment processing, which can be seen from the technical drawing marks of the parts products.
FIG. 1 The appearance of a certain type of flange is known to be marked with detailed tolerances for both axial and radial dimensions in machined drawings. The processing pattern of induction hardening is similar to that of induction hardening (see Figure 2). In the figure, not only is the contour line of the hardening layer shown, but also the corresponding deviation. The term “deviation” is used here to distinguish “tolerances”, not only because of differences in cold and hot machining but also because of the great difference in their magnitude. For example, the effective depth (SHD) of the hardened layer at is 2.6+0.8mm; Among them, 2.6mm is the basic required depth of quenching and hardening layer (nominal value, lower difference), while +0.8mm is the allowable upper difference. Similarly, the effective depth (SHD) of the hardening layer is 1.1+1.8mm; Among them, 1.1mm is the nominal depth (lower difference) of the hardening layer required, while +1.8mm is the allowable upper difference. The hardening layer is 1.71+3mm at the starting point in the axial direction; The hardening layer is 26+5mm at the radial starting point.
Figure 1. Example flange
FIG. 2 Example of quenched-hardening layer distribution and deviation of flanges
The dimensional tolerance of the parts ensures the assembly consistency on the assembly; The machining deviation of the induction hardening layer ensures the consistency of the bearing capacity and even the consistency of the deformation. We know that when the hardened layer depth is greater than the stress penetration depth, the bearing capacity of the parts is no problem and the service process is safe. Therefore, only the minimum value of the hardened layer is stipulated in the previous extensive induction quenching process pattern requirements in China. However, with the increase of the hardened layer depth, the internal stress and deformation increase, and even crack in special cases. Therefore, the requirements of modern induction heat treatment fine processing not only stipulate the lower error but also stipulate the permissible upper error. Although the magnitude of the deviation (mm) is much larger than that of cold working (m).
3. Induction coils and knives
The inductor is the “tool” of induction heat treatment, relative to the cutting tool of the part. In order to achieve the consistency of the cutting process, the technologist has a very high understanding of the knife. And we don’t know enough about the height of the sensors. In addition to some international companies, many Chinese enterprises do not even order the sensors from professional companies, and still, stay in the “self-sufficient” hand-made mode of master with apprentices. However, even in professional companies, the design and production standards of sensors vary, and there is no interchangeability. Because we don’t have an industry standard for the design and manufacture of sensors.
We feel that sensors can also, like knives, have a number of common characteristics, including:
(1) Generally manufactured with special materials, special design for processing parts, and processing technology. Just as cutting tools are made of special materials, sensors must be made of special materials. The main function of the inductor of induction heat treatment is to generate a certain shape of the electromagnetic field, coupled with the part of the workpiece being heated to generate a certain shape of the distribution of eddy current, and finally achieve quenching. Therefore, the inductor is required to work with the good conductivity of the coil. At the same time, the electromagnetic force in the induction heat treatment process requires the inductor to have a certain stiffness. At the same time, considering the economic nature of the materials, combined, pure copper has become the industry recognized as the first choice. The commonly used pure bronze number is T2. In some special applications, oxygen-free copper is chosen because it can greatly improve the conductivity.
(2) High manufacturing accuracy and process accuracy. With the development of the manufacturing industry, the manual manufacturing of cutting tools has been replaced by specialized mechanized batch production. However, most of the sensor manufacturing in China’s induction heat treatment industry is still at the stage of manual manufacturing, which is highly dependent on the producer and obviously unable to meet the requirements of modern processing. In recent years, more and more mechanism sensors have become the first choice of mass production enterprises.
(3) The “tool base” of the machine tool is another feature of the efficient processing of modern machine tools. To realize efficient and reliable production, the sensor library of the machine tool can be used for reference. Although the sensor is not replaced as often as the machining center, it is perfectly correct to manage the sensor library according to the requirements of managing the tool library. This requires the sensor to shape, size consistency, design and manufacturing standardization, good interchangeability. A custom sensor or mechanism sensor of a professional company is easy to meet the requirements.
(4) High-precision connection and quick exchange function. One of the typical features of tool standardization is the standardization of tool hilts (see Figure 3). This has formed several standard series internationally. It is the core of fast tool replacement, reliable connection, and high precision positioning. Sensor connectivity has not been standardized to this extent, and there is no uniform standard. At present, there have been some international induction heat treatment professional companies working on this aspect, and there are also standardized products launched in China. For example, our patented product “Sensor Companion” (see Figure 4) is an effective choice for fast sensor replacement and standardized production.
(5) Life is related to processing quantity. When a certain number of tools have been machined, the “tool” has reached its life. This is the same for both cutting tools and sensors. Although there is no contact between the inductor and the workpiece during the processing, repeated on-off and off-off as well as cold and hot fatigue also cause its failure, which must be replaced with a new one. This again requires the sensor’s manufacturing accuracy to be as good as the tool’s substitutability.
Figure 3. Example of a standard handle
Figure 4. Example of a standard sensor boot
4. The machine tool
Whether the induction heat treatment processing or cutting (cold) processing, the implementation of the processing is through the machine. Induction quenching machine tools and cold machining machine tools have a lot of commonness, also have respective particularity. Comparing the typical characteristics of the two types of machine tools, the two types of machine tools in addition to the sensor and the workpiece does not contact, no cutting force, so that the induction quenching machine tool tonnage is relatively light, the other features of the machine host is very similar. The high and new technology applied in the field of cold working machine tools has been more and more applied to induction hardening machine tools. For example, numerical control technology, network management, flexible machining center, and so on. Linear motion unit, standard indexing rotary unit, centralized lubrication, and so on are more used in the structure of machine tools. In addition, the induction quenching processing area is also corroded by the quenching liquid sputtering, so the area to which the quenching liquid can sputter is generally made of stainless materials.
As for the machine tool auxiliary machine, the difference is larger but does not affect the operation mode. The auxiliary machine of cutting machine plays a role in ensuring the effective operation of the main machine, which needs to be maintained according to the rules and generally does not participate in the quality management of products. The auxiliary machine tools for induction heat treatment, including induction heating power supply and quenching fluid circulating cooling system, are directly related to product quality management, which requires monitoring and control. The control accuracy affects the processing quality of induction heat treatment. Therefore, the investment budget and production operation cost budget should be separately guaranteed.
5. Machine tool working procedure capacity Cpk
In the process of production, the fluctuation of the product processing index is inevitable. At present, more CNC machine tools are used in cold machining and induction heat treatment, and the index to evaluate the processing capacity of CNC machine tools is also introduced in the evaluation of induction quenching machine tools.
Process capability is the ratio of the allowable maximum variation range of the process performance of a process to the normal deviation of the process. The process capability study is to confirm the degree of conformity of these characteristics to the specification, so as to ensure that the defective rate of finished products not conforming to the specification is above the required level, as the basis for continuous improvement of the processing.
Modern quality management system, especially for enterprises to carry out ISO9000, QS9000, and other quality management certification requirements, Cpk to carry out a production process capacity evaluation. Cpk is the abbreviation of the Complex Process Capability Index, indicating the ability of the Process to guarantee the target accuracy. The attached table is an example of the Cpk value requirements of an enterprise on the processing capacity of induction quenching machine tools before they are put into production. Cpk is the neutralization of accuracy and precision, the larger the better.
At present, Cpk is used to measure the machining capacity of induction heat treatment machine tools on continuous production lines. Especially for the automobile industry parts heat treatment of induction quenching processing equipment. Since the detection of heat treatment indicators (regardless of hardness and depth of quenched layer) of the workpiece is much more complicated than the detection of geometric dimensions, most of the time, sample cutting and metallographic preparation are required before detection. Therefore, the Cpk value calculation for a nominal size requires much more work and testing cost than cold working for the same number of samples, which requires special attention.
6. Learn from the cold processing mode to improve the application level of induction heat treatment
For a long time, China’s machinery manufacturing enterprises in pursuit of large and complete, product variety. The boundaries of cold and hot processing in enterprises are clear, and heat treatment processing often runs separately offline. This is mainly because most heat treatment processes must be carried out offline. Such as resistance furnace, atmosphere furnace, vacuum furnace, such as heat treatment processing. Therefore, we have been used to heat treatment extensive processing, and for cold processing production management technology understanding opportunities are not much.
Foreign enterprises have a high degree of specialization and a high degree of product refinement. Some enterprises organize production around a class of parts, so the production line automation degree is high, the induction heat treatment application level is high. Induction heat treatment has been able to achieve fine machining. For example, TRW Langfang company specializes in valve production; Holbege Changzhou company specialized in the production of a synchronizer ring; Nebo gear Tianjin company specializing in the production of slewing bearing gear ring. Induction heat treatment processing in these enterprises has been the same as other cold processing line organization and management, according to the production program, process beat, fixed hours of efficient and high quality production.
Induction heat treatment has similar characteristics of production organization as cold processing, which enables us to get rid of the extensive management mode of conventional heat treatment and use the similar production organization concept of cold processing for more reference, so as to achieve the goal of fine induction heat treatment.
To this end, we suggest the induction heat treatment, production organizer:
(1) self-learning cold processing quality management system.
(2) try to use CNC induction quenching machine tools.
(3)Sensor professional customization, try to use the standard connection and fast change, to ensure the interchangeability and installation accuracy.
(4) Consciously implement ISO9000 in the evaluation of process capacity, continue to improve the processing capacity of induction heat treatment.