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Characteristics of modern induction heat treatment processing

When a part is made from a blank, we know that it has to go through a series of processes, such as turning, milling, grinding of the blank, and of course, heat treatment. In these processes, the former is called cold working and the latter is called hot working. The control object of the cold working process is the physical quantity of a single piece, such as geometric dimension, precision, and so on. These are easy to measure and even monitor online. With the development of modern digital technology, the degree of automation has been greatly improved, and now the production efficiency and quality stability of cold processing have been given full play, with distinctive modern characteristics.

Heat treatment and thermal processing, on the other hand, take advantage of the changes in the structure and shape of materials at a certain temperature to improve their properties, such as strength and toughness. These goals generally cannot online monitoring, and the general organization change and material composition, reaction temperature, time and other process related, therefore, heat treatment and processing a variety of process model of heating furnace is adopted to such as overall quenching tempering, carburizing and quenching, nitride, etc., it is for a batch of parts on machining, and the difference in cold working.

But there are also heat treatment processes for individual parts, such as induction heat treatment. It is a fast, efficient, energy-saving, less pollution green processing method, and an easy and cold processing line. Therefore, modern induction heat treatment must have some modern characteristics similar to cold processing.

1. Production organization

The goal of modern enterprise production management is efficiency and benefit. Modern induction heat treatment production is also based on a certain batch of parts, according to the automation system, through the scientific organization, so as to achieve efficiency and benefit.

First of all, the requirements of the production program, that is, the target of the batch processing and allowable processing hours. For example, for a product, the annual production program is 1 million pieces, the annual working time is 250 days, 1 shift per day, each shift is 8 hours, then the allowable theoretical production beat value of each product is (250×1×8×3600) /1000000 = 7.2s/ piece. In fact, in production, non-productive time consumption such as equipment maintenance, production preparation, operator turnover, machine failure, and waste is generally reserved, which is translated into the operating rate of the equipment, such as 95%, and the allowable beat demand of each product is 6.84s/ piece.

According to the above production program, carry out equipment configuration and production organization. If the tempo of the configured equipment is less than 6.84s/ piece, then one equipment can meet the requirements of production in one shift. If the configured equipment beats more than 6.84s/ piece, you can consider extended working hours such as 2 or 3 shifts per day; Or increase the number of equipment or the number of work stations, from one to two equipment processing or two work stations simultaneously processing.

Due to the high efficiency of induction heat treatment, especially some induction heat treatment equipment, and cold processing production line, therefore, the workshop logistics system also put forward corresponding requirements for induction quenching equipment. Modern logistics units such as automatic feeding machines, manipulators,s, and robots also appear in the induction heat treatment system and are interlocked with the control system of the machine through the standard data interface. Figure 1 is the induction hardening site of a workshop, including a transfer gantry manipulator, through a collinear manipulator transfer workpiece.

Induction quenching machine tool and cold machine tool are arranged in parallel

FIG. 1 Parallel arrangement of induction hardening machine and cold working machine

2. Automatic induction quenching machine tool, flexible processing center

Induction heat treatment is achieved by the induction heating of the inductor and the surface of the workpiece with appropriate cooling. The processing process requires the sensor relative to the workpiece positioning, scanning, and moving, in which the workpiece also needs to rotate, indexing, and other corresponding actions. With the application of NUMERICAL control technology, the induction heat treatment machine tool can realize the process automation as a cold machine tool.

Like a cold flexible machining center, an induction heat treatment machine can not only carry out single process processing but also can carry out multi-process combined processing. For example, some induction heat treatment systems, a card to complete the quenching, tempering compound processing; There are also some induction heat treatment systems that can complete the quenching and tempering of multiple parts; Even in the processing process, the parts are turned around and loaded with CARDS and turned over for quenching, etc. Figure 2 shows the auto balance shaft parts and their induction heat treatment equipment. This part needs to quench and temper several step shaft diameters. Due to the large change of section, this system adopts a duplex inductor to quench its different coaxial necks, an inductor to quench the gear surface, and an inductor to temper the whole part. Different from cold machining, the combined different processing procedures are not completed by automatic knife change, but by transferring to different stations for processing. The whole system consists of 3 high-frequency power sources, which are controlled by PLC.

Balance shaft induction quenching machining center A


Balance shaft induction quenching machining center B


FIG. 2 Balance shaft induction quenching machining center

To use automatic quenching machine tools for processing, it must have: machine tools have high reliability; The machine tool has high position accuracy and motion accuracy; High precision and good interchangeability of cutting tools (process tools); The processing technology is determined in advance.

Due to the different working conditions and processing modes, the induction heat treatment automation system has more requirements than:

(1) As quenching requires heating and cooling, the working area is polluted by water, quenching fluid, steam, electromagnetic and other pollutants. These factors should be taken into account in the reliability design of machine tools, and the structure and component materials used should avoid or not be damaged by this pollution.

(2) The “tool” used in the processing includes not only sensors but also quenching firearms. In foreign countries, it is generally called process tools. As with cold working tools, this is usually customized by a professional manufacturer. Process tools should have repeatable assembly precision to meet the requirements of fast card mounting.

(3) Pre-determination of processing technology means that the process development process in advance must fix parts materials, dimensional precision, sensor geometric dimension and precision, power supply parameters, cooling parameters, and correct heat treatment results.

(4) Therefore, the reliability of the machine tool is not only the reliability and accuracy of structure and movement but also the controllability and reliability of power supply and load, cooling, and quenching spray system.

What we need to pay attention to here is that the main gap between our domestic products and the international advanced similar equipment is that these characteristics have not been solved. For example, in process tools, we often only think of sensors, but do not mention the accuracy and reliability of the “tool” to understand, the degree of specialization is not enough. Domestic “sensor companion” provides assistance for the high-precision and interchangeability specialization of induction heat treatment process tools (see Figure 3).

Sensor companion with automatic positioning and quick change functions

Figure 3 Sensor companion with automatic positioning and quick change functions

3. Digital quality control system and remote monitoring

The efficiency and batch quantity of modern processing require more comprehensive quality control, more timely, more close to the processing process because any quality accident is not found and dealt with in time will cause the mass scrap, the loss is magnified. The increase of control points does not affect the efficiency, which benefits from the digital quality control system.

A modern digital quality control system for induction heat treatment has been built in from the beginning of logistics. Including workpiece identification, counting, equipment, and “process tools” confirmation and production of finished products, identification, energy consumption statistics, online monitoring, and recording, allowing online or offline data call.

In addition to the basic functions of cold NC machine tools, the on-line quality control of the machine tool system also includes the monitoring and recording of the process parameters of heating and cooling by induction heat treatment. This requires the power supply to have a digital control interface, temperature, flow, and other media data using digital instruments. The process prefabrication of induction heat treatment is to set these quality control signals and allowable fluctuation interval in the system through the program to form a quality control package, as shown in Figure 4. The operation parameters and process parameters are recorded.

Induction quenching machine interface information

FIG. 4 Interface information of induction quenching machine tool

With the widespread adoption of the network, the real-time data of these quality control packages can be remotely retrieved and recorded, which not only facilitates the quality tracking of the superior data management system but also enables remote experts to diagnose and recover the data when the machine breaks down, greatly reducing the downtime and improving the equipment utilization rate.

4. Good process reliability, replacing (carburizing, nitriding, etc.) a variety of conventional heat treatment processes

Most heat treatment processes use a heating furnace, heat treatment process mode is generally long, often use off-line processing, so it is very different from the modern cold processing production mode. Traditionally, induction heat treatment due to the local processing the individual parts can be and thus is regarded as the supplementary means of conventional heat treatment only for less important parts or small batch parts by using this technique, and process reliability due to various reasons, poor repeatability, which may lead to a narrow range, efficiency is poor.

Modern induction heat treatment because of the improvement of process reliability, bearing capacity is confirmed by designers, so in more and more occasions instead of carburizing, quenching, nitriding, and other mainstream processes, has achieved efficiency, the benefit of the double harvest. Not only under the condition of small and medium loads such as automobiles, but also in some heavy loads such as wind power and locomotives. For example, the wind power large-diameter gear products produced by a Chinese enterprise, with modules of 10 ~ 26 and diameters of 2000 ~ 3500mm, were originally all carburized, quenching and nitriding processes, and then changed to induction heat treatment, the efficiency was significantly improved, the material cost and processing cost were significantly reduced, and the benefits were very good.

5. Conclusion

The advantages of rapid, efficient, energy saving, and less pollution of induction heat treatment have been brought into full play under the condition of modern technology. In China, many foreign-funded enterprises and a few high-quality domestic enterprises have directly adopted foreign modern induction heat treatment equipment and thus gained benefits in efficiency and efficiency. Domestic induction heat treatment equipment has also made great progress in the degree of automation in recent years, but there are still some gaps in reliability and process stability and the international level. Only by filling up these gaps, it is believed that induction heat treatment technology will be widely used.

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