September AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 1 8 4 8 structions for providing 360 o access for spray quenching. In some cases, quenching is done from one side, covering only 90 o -120 o of the austenized surface. This could result in ap- preciable circumferential spray-quench nonuniformity and correspondent hardness deviation. Solution: Redesign the quenching device, making an attempt to cover as much of the workpiece surface as possible. In addition, try to change the rotation speed during spray quenching (for example, a 25% increase and then a 25% decrease) and compare re- sults. Both insufficient rotation and excessive rotation could result in circumferential quench nonuniformity [1] . We are planning to use induction to heat composite me- tallic materials prior to diffusion bonding. Is there any concern with respect to excessive noise? Have you found that certain frequencies are noisier than others? If so, do you have a solu- tion for containing the noise level? Answer: In the vast majority of induction applications, noise does not reach an appreciable level. Therefore, there is no reason to be concerned about excessive noise, although there are a few exceptions. Lower frequencies typically re- sult in higher coil current thus increasing electromagnetic forces and coil turn vibration. So, it is reasonable to expect that a system applying line frequency would be associated with a greater magnitude of industrial noise compared to 30 kHz. In addition, systems with a greater amount of kW most likelywould also be associatedwithmore noise. There- fore, assuming all other factors are identical, 500 kWcan pro- duce greater noise than 100 kW. When discussing audible noise, we must bear in mind the two main factors that impact how humans are affect- ed by industrial noise—magnitude and unpleasantness/ discomfort level. For example, low frequency audible noise (e.g., 60 Hz) could have a higher magnitude, but it might not be as unpleasant to the human ear compared to a lower magnitude noise at an elevated frequency (e.g., 1 kHz). Following are the fourmain sources of noise generation during induction heating: • Noise generated by the power supply . Numerous power supplies are available on the market. For induction heating, power and frequency combinations that use single-module inverters vary from line frequency to several hundred kHz and power levels exceeding 1000 kW even for a frequency in the 800 kHz range. Design features of modern power sources based on semi-conductor technology result in nearly silent operation. Therefore, noise stemming from the power supply is not usually a concern. • Noise generated from vibration of copper coil turns . The two main approaches to building induction coils for hot working can be categorized as either open- wound or refractory-encased [Ref. 1]. The open-wound method provides more convenient repair in the event of failure, but coil turns must be secured using studs and proper fixtures to eliminate or minimize vibration and noise. An encased coil using a castable refractory (for example, special grades of cement) offers durabili- ty and longer life, eliminating or dramatically reducing the vibration of coil copper turns. Properly designed coils do not exhibit problematic noise levels. • Noise generated by workpiece vibration or resonant sound waves (amplifying effect) . If a workpiece con- sists of some loose parts, then they may vibrate and produce noise. To assess this possibility, the specific workpiece geometry would need to be reviewed. Pressure can be applied in certain diffusion bonding applications to minimize the possibility of individual component vibrations. When induction heating hollow workpieces (e.g., relatively thin-walled pipes or tubes positioned inside other tubes), certain frequencies in combination with sufficiently high power densities could emit resonant sound waves of an appreciable magnitude, exceeding the audible limit. In cases like this, audible noise can also be a dominant factor that greatly affects the selection of frequency. Each tube has its own structural resonant frequency (SRF), which 16 Fig. 3 — A nonuniform circumferential temperature pattern occurring at the time of quenching.