The relationship between induction case depth and load power for high frequency, high load power and short heating time
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HIGH f r e q u e n c y induction heating has
b e e n widely a p p l i e d for s u r f a c e , or c a s e , h a r d e n i n g of s t e e l in the p a s t 30 y e a r s . A g r e a t n u m b e r of p u b l i c a t i o n s can be found in t h i s field 1-3 which have c o n t r i b u t e d to the u n d e r s t a n d i n g of the u n d e r l y i n g p r i n c i p l e s that c o n t r o l the induction ease heating of m e t a l s , e s p e c i a l l y the i n duction e a s e h a r d e n i n g of s t e e l . The p u r p o s e of this p a p e r is to d e s c r i b e in detail the c o n t r i b u t i o n of induction load power to the induction heated and s u b s e q u e n t l y h a r d e n e d ease depth, or s i m p l y the ease depth. The r e l a t i o n s h i p b e t w e e n the two will be d e r i v e d and e x p e r i m e n t a l data c o m p a r e d with the t h e o r e t i c a l v a l u e s c a l c u l a t e d f r o m the d e r i v e d r e l a t i o n ship. In p r a c t i c e , t h e r e i s a r a n g e of load power d e n s i t i e s that p r o v i d e s p r o p e r m i c r o s t r u c t u r e within the heated and s u b s e q u e n t l y h a r d e n e d c a s e . Beyond a l i m i t i n g v a l u e of load power d e n s i t y g r a i n growth or b u r n i n g o c c u r s . The o p t i m u m v a l u e i s one that p r o v i d e s the b e s t m i c r o s t r u c t u r e with the a i m case depth. A c o m plete g r a s p of the r e l a t i o n s h i p between the induction load p o w e r d e n s i t y and the e a s e depth would f a c i l i t a t e the e v a l u a t i o n of such o p t i m u m load power d e n s i t y .
I) D E R I V A T I O N O F T H E R E L A T I O N S H I P BETWEEN THE INDUCTION CASE DEPTH AND THE INDUCTION POWER INPUT For a given frequency f (cps or Hertz) of oscillator, the skin depth depends on the resistivity p and the permeability p of the material. However, this skin depth should not be taken as the actual case depth that can be produced. The actual case depth is affected also by the load power density and the heating time. The skin depth D is given by A', McGraw-Hi11,New York, 1950. 5. A. B. Newman and A. H. Church: J. Appt. Mech., I935, vol. 2, p. A-96. 6. H. H. Macey: Proc. Phys. Soc., 1940, vol. 52, p. 625. 7. H. H. Macey: Proc. Phys. Soc., 1942, vol. 54, p. 128. 8. J. H. Awbery: Proc. Phys. Soc., 1943, vol. 55, p. 202. 9. J. C. Jaeger: Proc. Phys. Soc., 1944, vol. 56, p. 197. 10. P. H. Egli: Thermoelectricity, p. 279, John Wiley and Sons, 1960. METALLURGICAL TRANSACTIONS
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