TRANSFORMATIONS DURING QUENCHING AND TEMPERING OF HOT-WORK TOOL STEEL. Piotr BAŁA, Janusz KRAWCZYK

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1 TRANSFORMATIONS DURING QUENCHING AND TEMPERING OF HOT-WORK TOOL STEEL Piotr BAŁA, Janusz KRAWCZYK AGH University of Science and Technology Faculty of Metals Engineering and Industrial Computer Science Department of Physical and Powder Metallurgy A. Mickiewicza Av. 30, Krakow, Poland ABSTRACT The paper presents a description of phase transformation kinetics of under-cooled austenite in form of CCT diagram and kinetics of phase transformations during heating from as-quenched state in form of CHT diagram of newly implemented tool steel for hot-forging dies. The knowledge of transformations taking place during quenching and tempering of test steel will make possible to design a precise heat treatment process allowing to make of test steel a die that would be characterized by good mechanical properties and high fracture toughness. In order to determine correct quenching temperature an analysis of critical points determined on the way of dilatometric tests and on basis of so called hardening series. It was found that optimal temperature for quenching of test steel is 1050 C. Investigated steel should be cooled at highest possible rate during quenching (higher than 5 C/s) yet safe in order to avoid quenching cracks. On the basis of above mentioned kinetics of phase transformations during heating from as quenched state (tempering) one may match suitable tempering temperatures of test steel. Based on said above results and present knowledge investigated steel should be tempered in three stages. First is tempering at such temperature that retained austenite is destabilized. Second tempering is performed in order to obtain hardness at certain level and possibly further transformation of retained austenite and tempering the products of its transformation. Third tempering to stabilize the structure of transformations products from second tempering. Possible nitriding process, performed after heat treatment, should be carried out at the temperature lower by 30 C from the temperature of last tempering 1. INTRODUCTION Microstructure and properties of tools have strong influence on their throughput and reliability what favors development of mechanization and automation of technological lines. Hot working tool steels are used for tools working at wide range of temperatures. For example, working temperature of some forging dies is about 200 C, while dies for extrusion and pressure die casting work at C. Therefore very important properties of these steels are: strength, hardness, wear resistance at working temperatures and resistance to sudden temperature changes (thermal fatigue). The properties mentioned above are obtained in these steels on the way of suitable chemical composition and properly designed heat treatment (PACYNA 1997, DAVIS 1991, BROOKES 1999). Better properties are obtained in steels with complex chemical composition if compared to steels containing high amount of one or two alloying elements (PACYNA 1997, DĄBROWSKI 2002, PACYNA JĘDRZEJEWSKA-STRACH STRACH 1997, PACYNA 1987) 1

2 Hot-working tool steels already at the stage of design of their chemical composition are designated to medium or high-temperature tempering in order to obtain stable microstructure, and thus stabilized properties while working. Recently designed tool steels for hot-working should have complex chemical composition, contain between 0.25 and 0.6 %C and be characterized by given kinetics of phase transformations of under-cooled austenite (PACYNA 1997) and kinetics of phase transformations during tempering (BAŁA 2007, BAŁA PACYNA 2008, BAŁA PACYNA KRAWCZYK 2007). Only at that time it is possible to choose appropriate heat treatment in result of which one receives optimal combination of mechanical and plastic properties. Main purpose of study in this paper was a description of phase transformations kinetics of under-cooled austenite in form of CCT diagram (continuous cooling transformations) and kinetics of phase transformations during heating from quenched state in form of CHT diagram (continuous heating transformations) of newly introduced tool steel for hot work in dies. In order to determine appropriate quenching temperature also the analysis of critical temperatures and hardening series was made. 2. TEST MATERIAL Tests were performed on tool steel for hot work 50CrMoV marked as W 360 (BOHLER 2007) by the producer. Chemical composition of investigated steel is presented in table 1. Table 1. Chemical composition (weight %) of the investigated steel C Mn Si Cr Mo V Fe bal. 3. HEAT TREATMENT Dilatometric tests were performed using DT 1000 dilatometer by French company Adamel. Tests were performed on samples with dimensions of 2x12 mm. Critical points and CCT diagram were determined. In order to make a CCT diagram of under-cooled austenite the samples were heated at rate of 5 C/s to the temperature of 1050 C, maintained for 20 minutes and after that were cooled at different rates ( C/s) to the temperature of 20 C. Digitally recorded dilatograms were differentiated in order to receive more precise characteristic temperatures. In order to make a CHT diagram of phase transformations kinetics during continuous heating from as quenched state (tempering) the previously hardened samples (T A = 1050 C, t A = 20 min, cooling rate = 10 C/s) were heated at rates of 0.05; 0.10; 0.5; 1; 5; 10; 15; 35 o C/s to the temperature of 700 o C, recording the changes of samples elongation in relation to the temperature. Also in this case the digitally recorded dilatograms were differentiated in order to make more precise read out of the characteristic temperatures. Recording of cooling dilatograms after tempering allowed to make a diagram of retained austenite temperature M S change in relation to previous heating rate during continuous tempering. 2

3 4. RESEARCH RESULTS AND DISCUSSION 4.1. The kinetics of phase transformations of under-cooled austenite Figure 1 presents a dilatometric curve of investigated steel with determined critical temperatures. Although, the test steel contains 0.5% of C, one should include it among hypereutectoid steels for the sake of concentration of other alloying elements (Cr, Mo, V). Characteristic points determined for investigated steel are: Ac 1s = 835 C, Ac 1f = 895 C and Ac m = 1075 C. On the basis of hardening series (Fig. 2) and the analysis of microstructures presented in Figure 3 one should state that optimal (safe) austenitizing temperature is 1050 C. After quenching from this temperature there are still sparse undissolved carbides in the structure and significant grain coarsening did not take place. Applying lower quenching temperature it is not possible to obtain high values of hardness (compare Fig. 2). Applying higher quenching temperatures causes grain over coarsening what would result in lower fracture toughness of the steel. This is why the temperature of 1050 C is considered as optimal in the rest of this study. Fig. 1. Characteristic points determined on the basis of dilatometric measurements together with corresponding differential curve Hardness HV Austenitizing temperature, ºC Fig. 2. Hardening series of investigated steel. Quenching in oil a) b) 3

4 METAL 2009 c) d) Fig. 3. Microstructures after quenching from: a) 1030ºC, b) 1050ºC, c) 1070ºC, d) 1100ºC. Etched with 2% nital Figure 4 presents a CCT diagram of investigated steel. Metallographic documentation for the diagram is presented in Figure 5. It is a diagram from IVth group of Wever and Rose classification (WEVER ROSE 1954) with separated range of pearlite transformation (diffusive) from the range of bainitic transformation (semidiffusive). Applying a temperature criterion of 350 C in accordance with (PACYNA 1997) bainite range was split into range of upper (grey) and lower bainite. The steel under investigation in spite of complex chemical composition is characterized by high temperature MS = 295 C (for a steel for hot work) what shall allow to avoid quenching cracks even if final product of quenching would be martensite itself. The range of diffusive transformations is strongly shifted to the right while bainite range to the right and down. In consequence almost whole bainite that is produced is low bainite. Only applying of low cooling rate (<0.33 C/s) may result in that upper bainite would be present in the microstructure. As one may notice, applying during quenching of cooling rate from the range of 10 1 C/s allow to receive close microstructure. It suggests that in order to avoid hardening cracks one could cool the investigated steel at the rate of 1 C/s. However, if the hardness measured is investigated one may notice a significant decrease of hardness at cooling rates lower than 5 C/s. During slow cooling the bainitic transformation is preceded by intensive precipitation of carbides from austenite and, in result of that, the supersaturation of matrix with alloying elements and carbon is lower. Obtaining high value of hardness after hardening and tempering would not be possible. Therefore the steel of that type should be cooled at possibly high rate but safe enough in order to avoid generation of hardening cracks. 4

5 METAL 2009 Fig. 4. CCT diagram for investigated steel a) Cooling rate = 10 C/s, 775HV10 b) Cooling rate. = 5 C/s, 775HV10 c) Cooling rate = 1 C/s, 721HV10 d) Cooling rate = 0,03 C/s, 456HV10 Fig. 5. Microstructures and hardness corresponding to CCT diagram. Etched with 2% nital 4.2. The kinetics of phase transformations during continuous heating from asquenched state (tempering) Figure 6 presents an example of heating dilatogram at the rate of 0.05 C/s for the sample from investigated steel, previously hardened from 1050 C, along with corresponding differential curve, illustrating the way of interpretation the dilatograms on which the CHT diagrams were based (Fig. 7). 5

6 During the first stage of tempering the investigated steel demonstrates a contraction related to precipitation of ε carbides. The contraction begins at ε s temperature and ends at ε f temperature. Because as soon as the temperature reaches the temperature ε f almost immediately begins the transformation of retained austenite, which is accompanied by volume increase, it is assumed that the temperature of ε carbide precipitation end (ε f ) equals the temperature of the beginning of retained austenite transformation RA S. This effect is evident within the temperature range between RA S RA f. This positive dilatation effect is accompanied by the second contraction beginning at the temperature (M 3 C) S and is related to precipitation of cementite (alloying). End of cementite precipitation takes place at temperature (M 3 C) f. Increase of volume within the range of temperatures MC S MC f is related to precipitation of independently nucleating carbides of MC type. Subsequently, at the temperature (M 2 C) s begins the independent precipitation of alloying carbides of M 2 C type. Figure 7 presents full CHT diagram of investigated steel. There are marked the ranges of ε carbide precipitation, cementite precipitation, retained austenite transformation and precipitation of independently nucleating carbides of both MC and M 2 C type. The temperature of the beginning of MC type alloying carbides precipitation is strongly dependent on heating rate. For recorded heating rates higher than 1 C/s it is above 700 C. Similarly, the temperature (M 2 C) s was determined only for the two lowest heating rates 0.05 C/s and 0.01 C/s, for the other applied heating rates the temperature is above 700 C. One may notice that along with the increase of heating rate from 0.05 to 35 C/s the temperatures of beginning and end of individual transformations increase as well. Fig. 6. Dilatogram of investigated steel heated from as-quenched state at the rate of 0.05 C/s together with corresponding differential curve Fig. 7. CHT (Continuous Heating Transformations) diagram for investigated steel 6

7 Figure 8 presents dilatograms of cooling the samples (at the rate of 1 C/s) after previous heating from as-quenched state at the rate of 0.1 C/s (Fig. 8a) and 35 C/s (Fig. 8b) to 700 C, along with the corresponding differential curves with marked temperatures M S of the beginning of retained austenite transformation during cooling. One may notice that during heating prior to tempering at the higher rate (Fig. 8b) the dilatational effect, related to retained austenite transformation during cooling, is greater than during slow heating what indicates that the transformation of its larger amount occurs not earlier than during cooling after tempering. The increase of heating rate also resulted in decrease of retained austenite M S temperature from 410 C for 0.1 C/s to 350 C/s for 35 C/s (by 60 C). This effect is related to the fact that during heating at low rate the process of carbon diffusion to the austenitemartensite boundaries, where cementite nucleates, took place to the higher extent. In consequence, carbon impoverished retained austenite reduced its stability what found its reflection, during cooling after tempering, in form of higher M S temperature. As it is commonly known, the carbon dissolved in austenite influences the temperature M S the most, lowering it (PACYNA 1997). In case of the lowest heating rate 0.05 C/s during cooling (after such tempering) only a minimal dilatational effect was observed from the transformation of retained austenite, what leads to a conclusion that it almost all transformed during tempering. a) b) Fig. 8. Dilatograms of cooling at the rate of 1 C/s of investigated steel, together with corresponding differential curves. First samples were heated from as-quenched state to 700 C at the rate of: a) 0.1 C/s, b) 35 C/s 5. CONCLUSIONS On the basis of the research carried out the following conclusions were stated: 1. correct temperature from which the parts made of investigated steel should be quenched is 1050 C, 2. during hardening the steel under investigation should be cooled at possible high rate (higher than 5 C/s) but safe enough to avoid creation of hardening cracks, 3. CCT diagram of investigated steel should be included in IVth group of Wever and Rose classification with separated range of pearlite transformation from the range of bainitic transformation, 4. despite of complex chemical composition this steel is characterized by high temperature M S = 295 C what allows to avoid hardening cracks, 5. the following sequence of phase transformations during continuous heating from as-quenched state was found: precipitation of ε carbides, precipitation of cementite within of which precipitation range also the retained austenite transforms. After that the independent carbides of MC type and next carbides of M 2 C type nucleate, 7

8 6. increase of heating rate from as-quenched state (from 0.05 to 35 C/s) results in increase of temperatures of the beginning and the end of individual transformations and causes reduction of accompanying dilatational effects, 7. heating rate from as-quenched state affects the stability of retained austenite which increases with the increase of heating rate, 8. technology of heat treatment of investigated material should consist in quenching from the temperature of 1050 C (cooling at the rate of 5 10 C/s) and subsequent triple tempering. ACKNOWLEDGEMENTS The authors would like to thank Michał Szczebak, Adam Syrek and Szymon Derlatka for help in this research. Project financed by the Ministry of Science and Higher Education, completed under AGH-UST s own research activities no REFERENCES BAŁA P., 2007, The kinetics of phase transformations during tempering and its influence on the mechanical properties, PhD thesis, AGH University of Science and Technology, Krakow. Promotor: J. Pacyna (in Polish). BAŁA P., PACYNA J., 2008, The kinetics of phase transformations during tempering of the new hot working tool steel designed for a large size forging dies. Steel Research International Special edition Metal forming 2008 volume 2, pp BAŁA P., PACYNA J., KRAWCZY J., 2007, The influence of the kinetics of phase transformations during tempering on the structure development in a high carbon steel. Archives of Metallurgy and Materials. I, pp BOHLER, 2007, 20 February. BROOKES C. R., 1999, Principles of the heat treatment of plan carbon and low alloy steels, Materials Park, ASM International. DAVIS J. R. and others, 1991, ASM Handbook, Vol. 4. Heat Treating, ASM international. DĄBROWSKI R., 2002, The effect of vanadium on the structure and properties of quenched and tempered model alloy steels. PhD thesis, AGH University of Science and Technology, Krakow. Promotor: J. Pacyna (in Polish). PACYNA J., 1997, Design the chemical composition of steels, AGH University of Science and Technology, Krakow (in Polish). PACYNA J., JĘDRZEJEWSKA-STRACH A., STRACH M., 1997, The effect of manganese and silicon on the kinetics of phase transformations during tempering Continuous Heating Transformation (CHT) curves, Journal of Materials Processing Technology 64, pp PACYNA J., 1987, The effect of retained austenite on the fracture toughness of high speed steels, Steel Research, vol. 58 no 2, pp WEVER F., ROSE A., 1954, Atlas zur Wärmebehandlung der Stähle mit dem Werkstoffausschus des Verains Deutsch, T.L. Max-Planc Institut fűr Eisenforschung in Zsarb. 8