PROFESSOR INDUCTION by Valery I. Rudnev, FASM, Inductoheat Group

Professor Induction welcomes comments, questions, and suggestions for future columns. Metallurgical insights for Since 1993, Dr. Rudnev has been on the staff of Inductoheat Group, where he currently serves as group induction heat treaters director – science and technology. In the past, he was an associate PART 1: TEMPERATURES professor at several universities. His expertise is in materials science, , , applied electromagnetics, This is the first part of a new series that will alternate with “Systematic computer modeling, and process development. Dr. Rudnev analysis of failures.” There have been 10 installments in the coil is a member of the editorial boards of several journals, failures series. Part 11 will be published in the July Heat Treating Progress. including Microstructure and Materials Properties and It will focus on failure modes of induction coils used for heating internal Materials and Product Technology. He has 28 years of surfaces (ID ). experience in induction heating. Credits include 16 patents and 128 scientific and engineering publications. ardening of and cast ness pattern is directly related to tem- represents the most popular perature distribution and is controlled Contact Dr. Rudnev at Inductoheat Group H application of induction heat by selection of frequency, time, power, 32251 North Avis Drive treatment. The three most common and workpiece/coil geometry. Madison Heights, MI 48071 forms of induction hardening are sur- When is alloyed with different tel: 248/629-5055; fax: 248/589-1062 face hardening, through hardening, percentages of carbon, the critical tem- e-mail: [email protected] and selective hardening.1 peratures are often determined by the Web: www.inductoheat.com hardening is conventionally iron-iron carbide phase transforma- described as involving heating the en- tion diagram (Fe-Fe3C diagram). The tire component, or a part of it, to the lower left portion of this diagram rep- austenitizing temperature, holding it if resents heat treating of steels, and is necessary for a period long enough to shown in Fig. 1. obtain a complete transformation to This widely used diagram is a homogeneous austenite, and then graph of temperature versus the rapidly cooling it to below the Ms tem- carbon content of the steel and shows perature where the martensitic trans- how heating to elevated temperatures formation begins. or cooling from an elevated tempera- The first step in designing an in- ture can cause a transformation in the duction hardening machine is to steel’s crystal structure. specify the required hardness pattern, Induction’s different: However, it including surface hardness, case is important to be aware that this depth, and transition zone. The hard- phase diagram might be misleading

Fig. 1 — The lower left portion of the Fe-Fe3C equilibrium phase transformation diagram. Note: A”3; A’3 and A’cm; and A3 and Acm at heating rates (°F/s or °C/s) V”, V’, and V, respectively (V”>V’>V). HEAT TREATING PROGRESS • MAY/JUNE 2007 15 PROFESSOR INDUCTIONcontinued

in the majority of induction hardening cooling transformation (CCT) applications because it is valid only for curve to the “left” and an in- the equilibrium condition of plain crease in the Ms temperature. carbon steels. Nonequilibrium condi- The CCT curve for regions tions, appreciable amounts of alloying having excessive amounts of elements, pressure, and certain prior carbon will be shifted in the op- treatments can noticeably shift the crit- posite direction with a corre- 2 ical temperatures. sponding decrease in the Ms One of the major requirements of temperature. The degree of het- an equilibrium condition is enough erogeneity in the microstructure time at temperature. Ideally, in the of the as-quenched part can be case of a sufficiently slow heating/ reduced by increasing the hard- cooling, transformation temperatures ening temperature. should be approximately the same for Observation of “ghost pearl- Fig. 2 — Effect of initial microstructure and both heating and cooling; in other ite” or an “excessive amount of free heating rate on the A3 critical temperature for words, there should be no appreciable ferrite” during a metallographic eval- AISI 1042 carbon steel. (From Ref. 3 and 4.) difference between Ac and Ar critical uation of as-hardened specimens can temperatures. also indicate the presence of hetero- amount of time the part is required to Any observed difference between geneous austenite. be held at that temperature. Ac and Ar represents thermal hys- Figure 2 shows the effect of heating As can be seen in Fig. 2, the re- teresis that is a function of several fac- rate on the A3 critical temperature of quired induction hardening temper- tors including the metal’s chemical steel.3,4 The inability of the classical Fe- ature range, even for AISI 1042 plain composition and the heating/cooling Fe3C diagram to take into account carbon steel, depends on not only the rate. The greater the rate of heating intensity limits its use for pre- heat intensity but also the material’s heating/cooling the greater will be the dicting the temperatures required for prior microstructure: difference between the two critical induction hardening applications. • 1620 to 2000°F (880 to 1095°C), for temperatures. Practically speaking, the Benchmark study: Probably the annealed prior microstructures equilibrium condition simply does not most comprehensive study regarding • 1550 to 1830°F (840 to 1000°C), for exist in induction hardening. the correlation of heat intensity with normalized prior microstructures the ability to obtain homogeneous • 1510 to 1710°F (820 to 930°C), for Rapid heating defines induction austenite was conducted by J. Orlich, steels having quenched and tempered Induction hardening is a very fast A. Rose, and colleagues at the Max- prior microstructures process. The intensity of heating or Planck-Institut für Eisenforschung Q&T structure best: The effect of heating rate often exceeds 100°C/s GmbH, Düsseldorf, Germany.5,6 They prior microstructure can be explained (180°F/s), and in some cases reaches developed atlases that consist of more as follows:1,7 1000°C/s (1800°F/s) and even higher. than 500 pages of nonequilibrium A quenched and tempered prior Therefore, the phase transformation time-temperature-austenitizing dia- structure is the most favorable.1 It con- cannot by any means be considered at grams for a variety of steels after they sists of fine pearlite, and ensures rapid equilibrium, and the phenomenon of had been induction heated using heat transformation, which allows a re- thermal hysteresis will always be intensities ranging from 0.05 to duction in the temperature required pronounced. 2400°C/s (0.09 to 4320°F/s). Those di- for austenite formation. This results in Rapid heating drastically affects the agrams should be used by induction a fast, consistent response of the metal kinetics of austenite formation, shifting hardening practitioners instead of the to induction hardening with min- it toward higher temperatures in order conventional Fe-Fe3C diagram in de- imum amounts of grain growth, to create conditions conducive to the termining the most suitable hardening shape/size distortion, and surface ox- required diffusion-based processes temperatures of steels. idation; a minimum required heating and resulting in a homogeneous energy; and a well-defined — “crisp” austenitic structure with a uniform Initial structure is important — hardness pattern having a narrow distribution of carbon. The microstructure of steel prior to transition zone. This type of initial The presence of heterogeneous heat treatment (also referred to as the structure can also result in higher austenite can result in an as-quenched initial structure, structure of the parent hardness and a deeper hardened case part having an unacceptable mi- material, or structure of the “green” depth compared with other prior crostructure. Upon quenching, de- part) also has a pronounced effect on structures. composition of heterogeneous the results of the heat treatment and the Unfavorable structures: If the ini- austenite first begins in regions of required process parameters. These pa- tial microstructure of a steel compo- lower carbon concentration. This re- rameters include but are not limited to nent has a significant amount of coarse sults in a shift of the continuous the austenitizing temperature and the pearlite and, most importantly, coarse 16 HEAT TREATING PROGRESS • MAY/JUNE 2007 ferrites or clusters or bands of ferrites, have poor response to induction hard- Rudnev, D. Loveless, R. Cook, and M. Black: then the structure cannot be consid- ening and also result in the need for Marcel Dekker Inc., New York, 2003, 800 p. ered “favorable.” Ferrite is practically prolonged heating and significantly 2. “Can the Fe-Fe3C phase transformation a pure iron and does not contain the higher temperatures to complete diagram be directly applied in induction carbon required for martensitic trans- austenitization. Longer heating times hardening of steel?” by V. Rudnev: Heat formation. Pure ferrite consists of less lead to grain growth, the formation of Treating Progress, Vol. 3, No. 4, June/July 2003, p. 27. than 0.025% C. Large areas (clusters coarse martensite, a larger transition 3. Induction Heat Treatment of Steel, by S.L. or bands) of ferrite require a long time zone, surface oxidation/decarburiza- Semiatin and D.E. Stutz: ASM Interna- for carbon to diffuse into carbon-poor tion, and increased shape distortion. tional, Materials Park, Ohio, 1986, 308 p. areas. Those ferrite clusters or bands Coarse martensite has a negative ef- 4. “Elevation of Critical Temperatures in could act as one very large grain of fer- fect on such important properties as Steel by High Heating Rates,” by W.J. rite and will often be retained in the toughness, impact strength, and Feuerstein and W.K. Smith: Transactions austenite upon rapid heating.1 After bending fatigue strength, and is sus- ASM, Vol. 46, 1954, p. 1270-1284. quenching, a complex ferritic-marten- ceptible to cracking. 5. Atlas zur Warmebehandling der Stahle, Vol. sitic microstructure can form. Scattered Therefore, when determining ap- 3, “Zeit-Temperatur-Austenitisierung- soft and hard spots and poor me- propriate induction hardening tem- Schaubilder,” J. Orlich, A. Rose, and P. chanical properties characterize this peratures for a carbon steel compo- Wiest (Eds.): Verlag Stahleisen M.B.H., structure. Appreciably higher tem- nent it is imperative to bear in mind Düsseldorf, Germany, 1973, 264 p. peratures and longer heating times are the limitations of the equilibrium Fe- 6. Atlas zur Warmebehandling der Stahle, Vol. 4, “Zeit-Temperatur-Austenitisierung- required to “austenitize” steels having Fe C phase transformation diagram, 3 Schaubilder,” J. Orlich and H.-J. Pietrze- these structures. It is strongly recom- and the need to take into account niuk (Eds).: Verlag Stahleisen M.B.H., Düs- mended to avoid segregated and heating intensity and the mi- seldorf, Germany, 1976, 282 p. banded initial microstructures in crostructure of the “green” part. 7. “Be aware of the ‘Fine Print’ in the sci- “green” parts. ence of metallurgy of induction hardening: Steels with large stable carbides References Part 1,” by V. Rudnev: Industrial Heating, (spheroidized microstructures) also 1. Handbook of Induction Heating, by V. March 2005, p. 37–42.

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