4. ELECTRIC RAPID HEATING OF METALLIC MATERIALS

Transcription

1 4. ELECTRIC RAPID HEATING OF METALLIC MATERIALS Classification of chapters: 4.1. Basic principles of electric rapid heating 4.2. Basic technical data on electric rapid heating 4.3. Comparison of reached properties after electric rapid heating to the conventional heat treatment Summarization of chapter terms and questions Literature Time necessary for study: 70 minutes Aim: After study of this capture you get information about basic principles of electric rapid heating; you will understand the asset of given technique in comparison with conventional heat treatment; you will be able to consider and review application possibility of given heat treatment type in technique praxis Author: Eva Mazancová 58

2 Lecture 4.1. Basic principle of electric rapid heating This is one of progressive technical-technological variants of heat treatment of metal materials, which uses the Joule thermal effect of electric current flowing directly through a heat treated metal material. Holding times on temperatures for heat treatment are shorter than 30 s and heating rates up to 10 3 C.s -1 are achieved, especially at direct resistance heating. In comparison with conventional processing, thermal efficiency is enormously high at electric rapid heating it reaches up to 90 % Basic technical data on electric rapid heating This heat treatment method offers a variety of advantages. Compared to conventional technologies, long-time heating in a furnace with all its implications, which a conventional mode brings along, do not take place, as well as the very cooling process in various types of baths. Advantages of the electrical rapid heating can be summarized as follows: a) Higher qualitative parameters of processed materials b) Minimal variance of achieved mechanical-technological properties c) High thermal efficiency, which can be even higher than 90 % d) Relatively simple testing of used technical-technological parameters (current density, voltage, heating time etc.) e) Easy integrability into one technological line f) Reduced operation costs (power, additional materials) g) Reduced investment costs (by 2/3 in average) compared to the cost for installation of conventional equipment h) Decreased requirements for live work with higher utilization of workspace (i.e. higher effectiveness of utilization of a working area considering the processed material production level) Author: Eva Mazancová 59

3 Preferably, electric rapid heating can be used for optimization of properties of both low-carbon and medium-carbon steels or high-carbon steels, or even for optimization of properties of low-alloyed structural steels, but also e.g. for aluminum wire or two-phase titanium alloys. In preference, electric rapid heating is used for thinner materials (for example in a form of wires and thin strips), which are often in an initial strain cold hardened condition. Difficult measuring of the heating temperature is a drawback; the character of a relation of temperature to time - T(t) changes during the whole heat treatment cycle. A control of a heat treatment (heating) mode is usually performed through a regulation of decisive electrical values, whereas values of current I(t) and voltage U(t) (depending on the exposure time) are measured in the main. Then, an energy characteristic (transition of the supplied energy to heat) is determined from these data: W t 0 t. It. dt U (1) V where V is a volume of the heat treated material. Equation (1) expresses a thermal characteristic of the determined heat treatment mode related to a unit volume. Under the given conditions, this formulation of heat treatment parameters is much more advantageous than a heating temperature determinable with difficulties (where many inaccuracies and errors may occur) Comparison of achieved properties after electric rapid heating to the conventional heat treatment Compared to data after the conventional heat treatment, very favourable basic strength and plastic properties of the assessed metal materials (structural steels) have been observed after the electric rapid heating. These are higher than after the application of conventional heat treatment methods. Fig. 4.1 shows selected stress-strain characteristics determined for observed steels processed both by electric rapid heating and a conventional method. Electric rapid heating was performed at J.cm -3 power input for a period of approximately 3s and subsequent water-cooling As Fig. 4.1 implies, low-carbon steel A (after electric rapid heating) achieved better stress-strain characteristics than conventionally processed steel SAE 980X (with higher content of carbon, manganese, micro-alloyed with vanadium addition and with a higher aluminum content). For comparison, in Fig. 4.1 there is Author: Eva Mazancová 60

4 Stress MPa a stress-strain relation observed in steel SAE 980X (see Table 1) after the electric rapid heating, from which a favourable effect of this heat treatment mode on achieved mechanical properties, particularly plastic characteristics, implies RO = rapid heating, KZ = conventional heat treatment, uhl. ocel = carbon steel Strain % Fig. 4.1 Stress strain plotting Table 4.1 Chemical composition of chosen steel types [wt. %] Steel C Mn Si P S Mo V N Al low carbon steel A SAE 980X SAE950X It is also interesting that plastic characteristics of the steel A after the rapid heating are higher than in conventionally processed steel SAE 980X, although for example contents of sulfur and phosphorus are relatively high in the steel A (0.020, %), while in the compared steel SAE 980X, these are % S and % P, as shown in Table 4.1. Plotting presented in Fig. 4.1 serves for an analysis of the above mentioned results. This represents carbon distribution around a cementite particle depending on temperature and time in heat treatment of the cold deformed steel. If the temperature corresponding to GOS in the binary iron-carbon diagram was exceeded, a complete ferrite to austenite phase transformation occurs. At the temperature corresponding to this equilibrium relation, ferrite is unstable not depending on carbon content. For origination of austenite, carbon diffusion is also necessary however, upon achieving temperature of 910 C, the ferrite-austenite phase transformation can occur without the supplementing carbon diffusion. However, the formed Author: Eva Mazancová 61

5 Tensile stress MPa microstructure depends on an amount of dissolved carbon and austenite to a large extent. At voltage about 24 V (i.e. c. W = J.cm -3 ), the detected cementite part has been dissolved in martensite (at U = 22 V there are still only cementite traces] and a ferritic matrix recrystallization process prevails). A heating energy increase (at a constant acting time) leads to a more intense process of dissolving cementite particles in a basic austenitic matrix. In rapid heating conditions corresponding to a level W = J.cm -3, a very complicated microstructure can be formed. In the middle of martensitic regions (formed after the heating-induced reverse phase transformation of austenite), fine cementite particles can be observed, which have been dissolved during the short heat cycle. On the contrary, in the edge regions of the formed martensite, bainite occurrence is often found. This apparently relates to a partial dissolving of cementite (in accordance with a diagram shown in Fig. 4.1). Depending on a distance from a cementite particle, different austenite enrichment with carbon occurred and thus its different hardenability has been achieved. The achieved mechanical properties in medium-carbon steel (0.57 wt. % C) depending on the acting energy are shown in Fig This figure implies that possibilities for electric rapid heating application are quite wide. For instance, at TS (tensile stress) around 750 MPa, ductility (A5) of approximately 15 %, or 20 %, can be achieved. These values can be achieved in one technological cycle (adhering to geometrical requirements) lasting circa 2 s, while quenching and tempering and substantially longer times of heat treatment modes need to be applied in conventional processing. TS Fig. 4.2 Mechanical properties in dependence on applied energy J 2 Author: Eva Mazancová 62

6 Um Summarization of chapter terms In the end of chapter main terms are recapitulated that you should master and understand their sense, resp. mutual connections. Jaul s heat, electrical rapid heating, current density, exposition time Questions: 1. Explain the principle of electric rapid heating. 2. What the advantages offers the electric rapid heating? 3. What the material types are suitable for application of electric rapid heating? 4. How carbon content and current density influence the hardness and microstructures under electric rapid heating process? Literature: VOCHVEST, P. Electric rapid heating application for heat treatment. Diploma work, TU-Liberec, (1983). PROHASZKA, J. Neue Hütte, 30 (1985) 387. MAZANCOVÁ, E., MAZANEC, K. Modern methods of heat treatment (Chosen captures). VŠB-TU Ostrava (1987). YANG, Yu-P., PETERSON, W., GOULD, J. Accurate spot weld testing for automotive applications. Advanced Mater. Process., (11-12) (2014) 19. Author: Eva Mazancová 63