University Of Gezira Journals Of University Of Gezira Gezira Journal of Engineering and Applied Sciences Vol3 No2 Developing an Induction Heating & hardening System Sulieman M.S. Zobly 1, Abdu I. Omer 2 1. Dep. of Physics & Medical Instrumentation, Institute of Nuclear Medicine, Molecular Biology & oncology, University of Gezira, Wad Medani, Sudan. 2. Dep. of Applied Physics & Electronics & Instrumentation, Faculty of Engineering & technology, University of Gezira, Wad Medani, Sudan. ABSTRACT The aim of this paper is to discuss the developing of a new induction heating & hardening system to harden a shallow depth of stainless steel that used to manufacture surgical blades. The system has been designed to satisfy the requirement for this application by using radio frequency generator, current transformer and inductor coil. These three parts represent the main part of the system which is known as induction heating system. The system is connected to a temperature measurement device to control the system by limiting the temperature during the process of heating up the material to very high temperature and cool it rapidly, to satisfy the required characteristic for quenched material. Using this method, the process time is very short, yielding high quality. The results of the experiment show that the designed system is faster than all other types of induction heating systems that can harden only one side of the material. This design was conducted at University of Applied Science, Soest Division Germany. INTRODUCTION Surface hardening is a widespread technique to improve the mechanical characteristics of different workpieces, to provide a treated layer (called martensite layer) on specific area of the material (Brown et al., 1947). The purpose of this work is to develop such a system that fit the specification provided in Figure (1) below (heat up the material to around 1100 C. The system has to reach the maximum temperature within 2-3 seconds, hold on for a bout 10 seconds and then cool down the material within 3-4 seconds); other specifications related to selective
hardening the material in only one side (along the surface) to specific depth (about 2mm) ( Norna & Nacke, 2003). Figure 1 - Hardening process specification (Norna & Nacke, 2003) Hardening materials methods are used to change the properties of the material, hardening the material pass through two steps. During the first step, the material is heated up to degrees above the critical point, then the temperature is kept at that high to change the structure of the material. The second step is to cool the material to low temperatures by using coolants like water, oil, air or chemicals. Treatment process: Most factories use furnace system to treat steel for manufacturing surgical blades. The block diagram of the system is shown in Figure (2).The system consists of four heating units that were arranged in a series with maximum reachable temperature for each stage. The work pieces transferred mechanically from one stage to the next. The first stage concerned to heat up the material to 950 C, while the second stage heats it up to 1080 C and the third stage heats up the material up to 1125 C however, the fourth stage heats up the material up to 1065 C (IQ technologies, 2002). The temperature of the workpieces developed via heat that radiated to surrounding heating elements. The Hardening process specification diagram described during the ramp-up phase then the temperature was held on for a while. This allows the energy radiated to the workpiece
surface to conduct throughout the workpiece. Such approach can achieve equilibrium temperature and facilitate metallurgical changes which are time dependent. After the high temperature bath, the workpiece temperature is rapidly decreased in a process phase known as quenching. A wide range of the metallurgical properties of the metal can be achieved with small variation in temperature-time profiles. The cooling time is very critical to achieve the desired properties of strength and hardness while avoiding brittleness. The lifetime of the intensively quenched workpieces made of plain carbon steel proved to be 50% - 80% longer than the parts made of alloy steel and quenched in conventional ways (IQ technologies, 2002). After quenching is achieved, a second cycle of heating and cooling at lower temperatures are used to temper the workpieces. Tempering is used to adjust the toughness against strength parameters of the workpieces. Figure 2 - Block diagram of the furnace system The developed system: It s not possible to achieve the customer requirement by using furnaces system, thus a new technology has been adopted to fulfil the requirements of the customer. Comparing the old system (Figure 2) with the new one, it is realised that the new system is faster and easy to control so as to meet the requirement. This can be done by using radio frequency (RF) generator, current transformer, designing special coil and temperature sensor. This system is known as induction heating system. Figure (3) shows the block diagram of the system.
Figure 3 - Induction heating system block diagram Induction heating is a production of an electric charge, magnetism, or electromotive force in an object without contact of a similarly energized body or by the variation of a magnetic flux. Induction heating is accomplished by eddy currents induced by a varying electromagnetic field (Norna & Nacke, 2003). The current is caused to flow through the material to be heated by the electromagnetic induction. MATERIALS AND METHODS The basic component of the system were an AC power supply, induction coil, workpiece, workpiece handling unit, temperature measurement and control system. When steel or cast iron is hardened, it quenched. The power supply provides alternating current through the coil, generating a magnetic field. When the workpiece is placed in the coil and cut the magnetic field, eddy currents are induced within the workpiece, generating precise amount of heat without a physical contact between the coil and the workpiece. The temperature of the material is sensed by infrared sensor (IR). The sensor is connected to the computer so as to control the system, monitor and record the data. Figure (4) shows the developed system.
Figure 4 - The developed system RESULTS AND DISCUSSION After fixing the system, the material is heated up by applying different frequencies. The suitable range of the frequencies (400 450 khz) is used to harden the material. Due to the fact that some experiments have been conducted by using other range of frequencies, however, the results were not reliable, this because the heated material has a width of 8.2 mm, while the latter range of frequencies is capable only to harden a narrow part of then material to a depth less than 2 mm Figure (5) shows the material in the coil during the heating process. Figure 5 - Heating up the material to a round 1100 C, using 450 khz
Heating up the material and cool down should be within a few seconds. The temperature is recorded during the process so as to plot it to know how the process is going. Figure (6) shows the temperature vs. the time. From Figure (6), the temperatures begin to increase from 0 C to 1000 C within 3 second. It remains at this temperature for 10 seconds. Then it sharply decreases to 400 C for 3 seconds. in Figure (7). Figure 6 - Temperature during hardening the material From the process in figure (5) and figure (6), the hardened part of metal is shown Figure 7 - Hardened part using 450 khz
Frequency (KHz) Table 1 - hardened depth in for different frequencies Hardened depth (mm) 390 1.12 400 1.04 450 0.98 CONCLUSION Using induction heating, the material can be heated up within a short time to a very high temperature and cool it down rapidly. Thus the induction heating is the most suitable method in manufacturing application which needs to change the material property. For surface heating or shallow case depth of penetration, it required high frequencies. The depth of heated material depends not only on the frequency but also there are many other parameters affect it such as the position of the material inside the coil and changing the coupling distance of the coil. By adjusting all the parameters in the system it possible to get good results for quenched materials to the specific depth needed in the application. The developed system is simple, cost-effective and power consumption during the whole phases of processing. Also heating the material takes very short time comparing to the other types of heating applications used in industries.
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