EFFECT OF HEAT TREATMENT ON PROPERTIES OF HOT-WORK TOOL STEEL. Janusz KRAWCZYK, Piotr BAŁA

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1 EFFECT OF HEAT TREATMENT ON PROPERTIES OF HOT-WORK TOOL STEEL Janusz KRAWCZYK, Piotr BAŁA 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 Hot-work tool steels already at the stage of design of their chemical composition are anticipated to be subjected to medium- and high-temperature tempering in order to obtain stable microstructure and, as a result of it, stabilized properties during work. Desired properties may be achieved in these steels by properly designed heat treatment. Objective of this work was to compare fracture toughness of newly implemented steel for hot-forging tool dies depending on heat treatment applied. In order to do that the impact testing of investigated steel was performed on samples after heat treatment carried out in accordance with technology proposed by the producer of steels for different hardness (48 57 HRC). First variant of heat treatment consisted in first tempering at the temperature of 540 C, second tempering at the temperature of 560 C and third tempering at the temperature of 560 C as well. Second variant of heat treatment consisted in first tempering at the temperature of 560 C, second tempering at the temperature of 580 C and third tempering at the temperature of 560 C. Third variant of heat treatment consisted in first tempering at the temperature of 560 C, second tempering at the temperature of 600 C and third tempering at the temperature of 580 C. Fourth variant of heat treatment consisted in first tempering at the temperature of 560 C, second tempering at the temperature of 620 C and third tempering at the temperature of 580 C. Each tempering in above described heat treatment variants was performed for 3 hours. It was found that decrease of hardness alone as a result of heat treatment does not guarantee an increase of fracture toughness of the steel. 1. INTRODUCTION Hot working tool steels already at the stage of designing of their chemical composition are anticipated to be in a medium and high tempered state in order to obtain a stable microstructure and thus stabilized properties during work. Nowadays the hot working tool steels have complex chemical composition, contain between 0.25 and 0.6% of C [PACYNA 1997] and are characterized by certain kinetics of phase transformations during tempering [BAŁA 2007, PACYNA 1987]. Only then, it would be possible to find a suitable heat treatment which results in an optimal combination of mechanical properties and overall performance [BAŁA PACYNA 2008a,b]. It may seem that the decrease of hardness of steel for hot work by increasing the highest temperature of tempering will result in improvement of fracture toughness of such steels. However, the complexity of phase transformations occurring during tempering of such steels does not always allow to control their properties in such a simple way [BAŁA 2007, BAŁA PACYNA 2008a-c, PACYNA 1

2 1987]. In particular, this concerns the range and morphology of so called complex carbides [BAŁA 2007, BAŁA PACYNA 2008b,c]. The objective of this work was to determine the changes in impact resistance of the newly introduced tool steel for hot-forging in dies in relation to the heat treatment applied. Present work focuses on the heat treatment proposed by the manufacturer of the steel, which is consistent with the existing rules. However different temperatures of the highest tempering were applied. 2. TEST MATERIAL Tests were performed on tool steel for hot work 50CrMoV marked as W 360 by the manufacturer. 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. The steel in as-delivered condition provided by the manufacturer to the die producer and for heat treatment is soft annealed with divorced pearlite microstructure with evenly distributed alloy carbides in ferritic matrix (Fig. 1). Fig. 1. Microstructure of investigated steel (as-delivered condition). Etched with 2% nital 3. HEAT TREATMENT Heat treatment was designed according to popularized metal science [DAVIS J.R. et. all 1991, VÖGE H. 1992, WILMES BECKER KRUMPHOLZ VERDERBER 1992] on the basis of CCT diagram [BAŁA KRAWCZYK 2009, KRAWCZYK BAŁA 2009]. Applied austenitizing temperature was 1050 C for 50 minutes with cooling in air. First variant of heat treatment consisted in first tempering at the temperature of 540 C, second tempering at the temperature of 560 C and third tempering at the temperature of 560 C as well. Second variant of heat treatment consisted in first tempering at the temperature of 560 C, second tempering at the temperature of 580 C and third tempering at the temperature of 560 C. Third variant of heat treatment consisted in first tempering at the temperature of 560 C, second tempering at the temperature of 600 C and third tempering at the temperature of 580 C. Fourth variant of heat treatment consisted in first tempering at the temperature of 560 C, second tempering at the temperature of 620 C and third tempering at the 2

3 METAL 2009 temperature of 580 C. Each tempering in above described heat treatment variants was performed for 3 hours. Figure 2 presents microstructures of samples after above mentioned variants of heat treatment. a) b) c) d) Fig. 2. Microstructures of investigated steel after heat treatment: a) variant no. 1, b) variant no. 2, c) variant no. 3, d) variant no. 4. Etched with 2% nital As one may notice, the microstructures after tempering according to variants of heat treatment 1 and 2 are similar without clearly etched grain boundaries. This is a typical microstructure for tempered martensite. By contrast, the microstructure of samples heat treated in accordance with variants 3 and 4 is a high-temperature tempered martensite with clearly etched boundaries indicating too far advanced processes of tempering what should be manifested by lower hardness. It seems that the heat treatment variant no. 3 may be the most unfavorable due to the microstructure, since in this range of tempering temperature the precipitations of alloying carbides of MC and M2C type may already transform into a more complex carbides of MC6 and M23C6 type and achieve sufficiently large size to participate in the process of cracking. However, the matrix is not tempered enough yet to play a major role in the process of cracking. After heat treatment variant no. 4 a significant tempering of the matrix is already noticeable. 3

4 4. RESEARCH RESULTS AND DISCUSSION Heat treatments subjected to the analysis in present study were chosen so that the changes of temperature, at which the second tempering was conducted characterized by the highest tempering temperature among the three tempering temperatures applied for each variant of heat treatment, would affect the hardness of the investigated steel. One may notice on the diagram in Figure 3 that mentioned goal was achieved i.e. along with the increase of temperature of the highest (second) tempering the hardness decreases Hardness, HRC The higest tempering temperature, C Fig. 3. Dependence of hardness on the highest tempering temperature Because the die works in percussive conditions a critical parameter for its susceptibility to damage is impact resistance. It turns out that increasing the temperature of the second tempering is not beneficial to impact resistance in any case (Fig. 4). KCU, KCV [J/cm 2 ]; KU, KV [J] KCU5 KU5 26 KCV 24 KV The higest tempering temperature, C Fig. 4. Dependence of impact resistance on the highest tempering temperature 4

5 METAL 2009 In order to explain the changes in microstructure caused by the increase of tempering temperature resulting in impact resistance change, a fractographic analysis is important. Figure 5 presents images of fractures of V notched impact strength test samples obtained using scanning electron microscope. Whereas, Figure 6 presents fractographic images of fractures of above samples obtained using a confocal microscope. a) b) c) d) Fig. 5. Images of fractures from Charpy V samples of heat treated steels according to particular variant: a) variant no. 1, b) variant no. 2, c) variant no. 3, d) variant no. 4. SEM One may see that after tempering at 560ºC (Fig. 5a) there are trans-crystally distributed dispersive alloying carbides observed in the fracture. In this case plastic deformation zone was small (Fig. 6a) and fracture nucleation took place on dispersive carbides precipitated within whole volume. Above mentioned small zone of plastic deformations is a result of strong supersaturation of the matrix. A similar character of the fracture is present after tempering at 580ºC but with a difference that its development is greater (Fig. 5b and 6b). Sparse larger precipitates of alloying carbides were observed on grain boundaries. Further increase of tempering 5

6 temperature (600ºC) results in increase of fraction of brittle inter-crystalline fracture (Fig. 5c and 6c). Despite that sharp V notch favours trans-crystalline fracture, a fraction of trans-crystalline (ductile with dimples) fracture is small. Applying the highest tempering temperature resulted again in the increase of fraction of transcrystalline fracture what indicates that plastic matrix plays significant role in cracking process (Fig. 5d and 6d). a) b) c) d) Fig. 6. Fracture images of heat treated samples according to particular variant: a) variant no. 1, b) variant no. 2, c) variant no. 3, d) variant no. 4. Confocal microscope The above presented results of the research allow to explain the influence of individual heat treatment on hardness and fracture toughness expressed by impact resistance on the basis of both the metal science concerning phase transformations occurring in such type of steels [BAŁA 2007] during tempering and the metallographic studies presented in Figure 2. One may state that heat treatment according to variant no. 1 caused the precipitation of independently nucleating carbides of MC and M 2 C type only. Coherent with matrix MC and M 2 C carbides allow to obtain high hardness. These carbides are precipitating within the whole volume of former austenite grain not causing the decrease of impact resistance. The temperature of second tempering 560 C, however, is low enough not to cause the decrease of the ferrite supersaturation and this is why the matrix would not be strongly tempered. 6

7 The increase of tempering temperature to the temperature of 580 C causes more intensive, if compared to previous case, precipitation of carbides of MC and M 2 C type. At this temperature and with the tempering periods applied they are not subjected to significant extent to transformation into carbides of M 6 C and M 23 C 6 type. However, in result of higher number of carbide precipitates of MC and M 2 C type and the initiated precipitation process of M 6 C and M 23 C 6 carbides along grain boundaries the supersaturation of ferrite was reduced what results in lower hardness with accompanying insignificant increase of KCV impact resistance (as well as KV) and decrease of KCU5 impact resistance (KU5). In case of samples with U notch the fraction of cracking along grain boundaries is greater than in case of V notch, when trans-crystalline cracking is forced (due to notch sharpness) and during which the complex carbides have significantly lower fraction, while the role of matrix increases. Further increase of second tempering temperature to 600 C causes that part of independently precipitated alloying carbides of MC and M 2 C type lost coherence with the matrix and could be subjected to partial transformation into carbides of M 23 C 6 and M 6 C type. Most probably the dimensions of these precipitates became large enough to have a significant part in the process of cracking of the investigated steel. The loss of coherency with matrix of previously precipitated carbides of MC and M 2 C type resulted in significant hardness decrease of investigated steel. In connection with still small degree of matrix tempering it causes that impact resistance of investigated steel tempered within this range significantly decreased. Further increase of the highest tempering temperature (second) to the temperature of 620 C causes complete loss of coherency of MC and M 2 C carbides with the matrix and their transformation into carbides of M 6 C and M 23 C 6 type precipitating on grain boundaries. However, in this case the matrix is subjected to such strong tempering (strongly decreases the level of supersaturation of ferrite) that impact resistance strongly increases. Major role is played by the matrix in this case. 5. CONCLUSIONS On the basis of the research presented in this study the following conclusions formulated: 1. Increasing the temperature of second tempering one obtains a hardness decrease of investigated steel, but one may also obtain a dramatic reduction of impact resistance. 2. Low fracture toughness of investigated steel is a result of carbides precipitations of M 6 C and M 23 C 6 type on grain boundaries. Optimal microstructure from hardness and fracture toughness point of view may be obtained when the microstructure contains a lot of coherent with matrix precipitates of carbides of MC and M 2 C type in connection with such ferrite tempering that guarantees its plasticity at suitable level. 3. In case of tool steel for hot work containing large amount of Mo one should avoid the tempering range between ºC due to susceptibility of Mo to segregation to grain boundaries. Tempering within this range would result in sudden decrease of both hardness and impact resistance. ACKNOWLEDGEMENTS The authors would like to thank Adam Syrek, Michał Szczebak 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

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