The effect of carbon content on selected mechanical properties of model Mn-Cr-Mo alloy steels during tempering

Transcription

1 Rafał Dziurka, Jerzy Pacyna The effect of carbon content on selected mechanical properties of model Mn-Cr-Mo alloy steels during tempering Introduction Civilizational progress has a strong focus on improving properties of construction materials and their production technology. Due to the fact that so far, the steel is one of the basic construction materials the primary focus is exerted on the heavy industry. A phase transformation occurring during tempering of steels depends not only on the carbon content in the martensite and in retained austenite, but also on the content of various alloying elements. Tempering reduces the hardness, residual stress, but increases the ductility. The reduction of hardness is the inevitable consequence of improved strength. The structural changes that occur during the tempering of steels depend on the temperature, time of the process and the concentration of carbon. During tempering of steels, occurred two unfavorable effects of decrease the impact strength. The first in the temperature range of C (referred to as irreversible temper brittleness) and the second in the range of C (called the reversible temper brittleness). Both of the effects continue to inspire many researchers. The aim of this study is to explain influence of the kinetics of phase transformations during tempering on the fracture toughness of model steel with different carbon content. Optimum mechanical properties are achieved by proper design and careful implementation of heat treatment technology. Above all, it is necessary to avoid the temperature range C, in which the temper brittleness occurs. Decrease in fracture toughness of steel tempered at this temperature range may be due to the destabilization of retained austenite [1 5]. Also, due to the non-uniform dissolution of martensite (preferential along the primary austenite grain boundaries) [6, 7], or by growth of cementite precipitations, which formed easy way of cracking [8 15]. Another theory explains the temper brittleness by the cementite nucleation mechanism during the tempering [16 19]. After the dissolution of metastable ε carbide the carbon redistribution leads to its strong local enrichment in the matrix, resulting in increasing stress and, consequently, reducing the ductility. To expand the knowledge of the carbon effect on mechanical properties (hardness, fracture toughness) in this study the tempering temperature impact on the selected properties of the Cr-Mn-Mo model steels was studied. Experimental procedure The three steels used in this work are Cr-Mn-Mo model alloys with different carbon content. These steels melted and cast in the Institute of Ferrous Metallurgy in Gliwice then reforged in the INTECH- MET Company in Gliwice. It is necessary to apply an adequate heat treatment to have the material in a state near the equilibrium one. Therefore the first melt was undergoing the normalizing annealing, Mgr inż. Rafał Dziurka (dziurka@agh.edu.pl), prof. dr hab. inż. Jerzy Pacyna (pacyna@agh.edu.pl) AGH University of Science and Technology, Faculty of Metals Engineering and Industrial Computer Science, Kraków, Poland for the second melt the full annealing was proposed, and for the last one WIII the technology of soft annealing was applied. The chemical composition of these steels are given in Table 1. The photographs of microstructures of investigated steels after annealing are presented in Figure 1. WI alloy had the microstructure consisting mainly of ferrite and small areas of bainite (Fig. 1. Whereas WII steel was characterized by a microstructure consisting of bainite and pearlite, which carbides were partially coagulated (Fig. 1. A microstructure of WIII steel is characteristic for the hypereutectoid steel, in addition to which carbides both in pearlite and hypereutectoid ones (forming network) were partially coagulated (Fig. 1. After annealing the austenitising temperatures were assumed, in a standard way on the bases of data from [20], which means higher by 50 C then Ac 3 temperature for WI and WII steel and 50 C higher than Ac 1f temperature for WIII steel. The first WI steel due to the low carbon content is characterized by low hardenability. Therefore it was necessary to applied for Charpy impact test sample a water hardening. To compare the reliable results for each of the steel, the samples from WII and WIII were also hardened in water. Austenitizing time was 20 minutes. The temperature of the water was 20 C. The theoretical cooling rate in such conditions is about 400 C/s. Samples immediately after quenching were subjected to tempering in preheated oven at selected temperature for 2 hours. To determine the largest drop of impact strength related to the irreversible and reversible temper brittleness, the tempering was performed at temperatures range of 100 to 600 C in steps of 50 C. Results and discussion Figures 2, 3 and 4 shows the results of the tempering temperature effect on impact strength and hardness of investigated steels (with marked standard deviation values). While the Figure 5, 6 and 7 contains photographs of the selected fracture of the samples after impact strength tests. As it was expected, due to the corresponding carbon content in investigated steels, the highest toughness has WI steel and the lowest WIII. The highest impact strength for the WI steel was obtained after tempering at 250 C and it was 185 J/cm 2. While for the WII and WIII steels the maximum impact strength was obtained after tempering at 600 C and it was respectively 150 J/cm 2 for WII and 16 J/cm 2 for WIII. As shown in Figures 2, 3 and 4, in each of the Table 1. The chemical composition of the investigated alloys (wt %) Tabela 1. Skład chemiczny badanych stali (% mas.) C Mn Si P S Cr Ni Mo V WI WII WIII NR 3/2013 INŻYNIERIA MATERIAŁOWA 157

2 Fig. 1. Microstructure of the investigated steels: WI, WII, WIII after annealing. Etched by 2% nital Rys. 1. Mikrostruktura badanych stali: WI, WII, WIII po wyżarzaniu. Trawiono 2% nitalem Fig. 2. Effect of tempering temperature on impact strength and hardness of WI steel Rys. 2. Wpływ temperatury odpuszczania na udarność i twardość stali WI steels the clear irreversible and reversible temper brittleness effect occurred after tempering. Occurring effects during tempering have been associated with the CHT diagrams for these steels [21]. Thanks to that the changes in the mechanical properties during tempering were associated with phase transformations. For the WI steel the first drop in toughness occurred after tempering at 300 C, which is associated with irreversible temper brittleness effect. Within the lowest toughness was after tempering at 350 C and it was 20 J/cm2. Interesting is the fact that up to 250 C hardness increases. This ought to be explained that this steel have very low carbon content (0.05% C) and the resulting martensite after quenching is not very hard (330 HV). The increase in hardness causes cementite precipitation. At lower temperatures, it precipitated dispersely within the martensite lath, in morphology reminding lower bainite. Therefore, after tempering at 250 C there is an increase of hardness and toughness. Cementite precipitated inside the martensite lath improved toughness and hardness by blocking the dislocations movement inside the lath. At higher temperatures, hardness slightly decreases and toughness rapidly decreases. This should be explained by the fact that the cementite still precipitated but this time higher temperature allows him to be precipitated in more preferred places like borders of primary austenite or martensite lath boundaries. Cementite precipitation at the boundaries creates an easy way of cracking, therefore he effects more on the toughness than on the hardness. Another drop in impact strength occurred for the tempering temperature of 500 C and is probably associated with the reversible temper brittleness effect. As it can see drop in impact strength is smaller than that for irreversible temper brittleness. It can be also 158 Fig. 3. Effect of tempering temperature on impact strength and hardness of WII steel Rys. 3. Wpływ temperatury odpuszczania na udarność i twardość stali WII Fig. 4. Effect of tempering temperature on impact strength and hardness of WIII steel Rys. 4. Wpływ temperatury odpuszczania na udarność i twardość stali WIII noted a slight increase in hardness at this temperature. Most likely this changes is caused by the diffusion of the Mo and V atoms to the phase boundaries. This causes a slight increase in hardness by blocking the dislocation move, but at the same time reducing boundaries cohesion. For the WII steel the first drop in toughness occurred after tempering at 250 C, which is associated with irreversible temper brittleness effect. Similarly as in the case of steel WI the lowest toughness was after tempering at 350 C and was 38 J/cm2. Within the area of decrease in toughness, associated with the irreversible temper brittleness is an exemption in hardness dropping. INŻYNIERIA MATERIAŁOWA ROK XXXIV

3 d) e) f) Fig. 5. Fracture of the WI steel samples after impact strength test after tempering at temperature: 100, 200, 300, d) 400, e) 500, f) 600 C Rys. 5. Przełomy próbek udarnościowych ze stali WI po odpuszczaniu w temperaturze: 100, 200, 300, d) 400, e) 500, f) 600 C d) e) f) Fig. 6. Fracture of the WII steel samples after impact strength test after tempering at temperature: 100, 200, 300, d) 400, e) 500, f) 600 C Rys. 6. Przełomy próbek udarnościowych ze stali WII po odpuszczaniu w temperaturze: 100, 200, 300, d) 400, e) 500, f) 600 C NR 3/2013 INŻYNIERIA MATERIAŁOWA 159

4 d) e) f) Fig. 7. Fracture of the VIII steel samples after impact strength test after tempering at temperatures: 100, 200, 300, d) 400, e) 500, f) 600 C Rys. 7. Przełomy próbek udarnościowych ze stali WIII po odpuszczaniu w temperaturze: 100, 200, 300, d) 400, e) 500, f) 600 C It is related to the cementite precipitation and of retained austenite transformation. Retained austenite transform into fresh martensite ends at temperature around 350 C. As you can see above this temperature there is a decrease in hardness but still low impact strength. Only above 450 C toughness increased when exactly cementite precipitation ends. The effect of reversible brittleness is associated with a drop in toughness, which was marked during tempering at 550 C. Impact strength decreased to 57 J/cm 2. As in the case of WI steel, decrease in toughness associated with reversible temper brittleness is less than in the case of irreversible temper brittleness. In this area is an exemption in hardness dropping probably associated with the precipitation of MC-type carbides. For the WIII, the first slight drop in impact strength was marked at 250 C while the minimum occurred during tempering at 300 C. This drop in impact strength is most likely associated with the irreversible temper brittleness. The second minimum in impact strength associated with reversible brittleness is marked similarly as in the case of the WI steel at 500 C. While, if it comes to hardness changes of the WIII steel during tempering can be seen that these changes are very similar to those for WII steel. Only the second drop in hardness is smoother, probably due to early precipitation of MC carbides. Based on these conclusions, it seems that the main cause of irreversible temper brittleness is cementite precipitation on the grain boundary. Can be assumed on the basis of the fact that irreversible temper brittleness occurs in the steel in which due to the low carbon content only cementite precipitation occurs during tempering. Furthermore this temper brittleness increase occurs at tempering temperature dominated by the precipitation of the cementite in each of the examined steels. In the case of reversible temper brittleness, it can be concluded that responsible for decrease in toughness is the elements Mo and V. The drop of impact strength occurs in the temperature range in which they begin to diffuse. In the case of low-carbon steel, they diffuse to the phases boundaries. In steel, medium and high carbon, these elements formed MC carbides on the border weakening their cohesion. Fractures of the samples from WI steel had clear plastic character with plastically deformed areas. For samples in which a decrease in toughness was found the nature of the fracture changed to more transcrystalline. For WII steel the fractures of the samples are intercrystalline, while for WIII steel fractures of the samples are transcrystallize character. Conclusions The results allow formulating the following conclusions: due to the corresponding carbon content in investigated steels, the highest toughness has WI steel and the lowest WIII, the highest impact strength 185 J/cm 2 was obtained for the WI steel after tempering at 250 C. While for the WII and WIII steels the maximum impact strength, respectively 150 J/cm 2 for WII and 16 J/cm 2 for WIII, were obtained after tempering at 600 C, for the WI steel the first drop in toughness occurred after tempering at 300 C, which is associated with irreversible temper brittleness effect. The lowest toughness was after tempering at 350 C and was 20 J/cm 2. another drop in impact strength occurred after tempering at 500 C is associated with the reversible temper brittleness effect, 160 INŻYNIERIA MATERIAŁOWA ROK XXXIV

5 for the WII steel, the first drop in toughness occurred after tempering at 250 C, which is associated with irreversible temper brittleness effect. Similarly, as in the case of WI steel the lowest toughness 38 J/cm 2 was observed after tempering at 350 C, the effect of reversible brittleness is associated with a drop in toughness, which was marked during tempering at 550 C. Impact strength decreased to 57 J/cm 2, for the WIII steel, the first slight drop in impact strength was marked at 250 C, while the minimum occurred during tempering at 300 C. This drop in impact strength is most likely associated with the irreversible temper brittleness. The second minimum in impact strength associated with reversible brittleness was observed similarly as in the case of the WI steel at 500 C. Acknowledgements The project was financed by the National Science Centre, granted by a decision number DEC-2011/01/N/ST8/ references [1] Narasimha-Rao B. V., Thomas G.: Structure-property relations and the design of Fe-4Cr-C base structural steels for high strength and toughness. Met. Trans. 11A (1980) [2] Horn R. M., Ritchie R. O.: Mechanisms of tempered martensite embrittlement in low alloy steels. Met. Trans. 9A (1978) [3] Materkowski J. P., Krauss G.: Tempered martensite embrittlement in SAE 4340 steel. Met. Trans. 10A (1979) [4] Sarikaya M., Jhingan A. K., Thomas G.: Retained austenite and tempered martensite embrittlement in medium carbon steels. Metall Mater Trans A, 14 (1983) [5] Wesołowski K.: Physical metallurgy and heat treatment. WNT, Warszawa (1972). [6] Przybyłowicz K.: Physical metallurgy. WNT Warszawa (1997). [7] Gulajew A. P.: Physical metallurgy. Wyd. Śląsk, Katowice (1967). [8] Malkiewicz T.: Ferrous alloys. Physical metallurgy. WNT, Kraków (1976). [9] Blicharski M.: Wstęp do inżynierii materiałowej. WNT, Warszawa (1998). [10] Bhadeshia H. K. D. H., Edmonds D. V.: Tempered martensite embrittlement: Role of retained austenite and cementite. Met. Sci. 13 (1979) [11] Krawczyk J., Bała P., Pacyna J.: The effect of carbide precipitate morphology on fracture toughness on low-tempered steels containing Ni. Journal of Microscopy 237 (2010) [12] Bała P., Pacyna J., Krawczyk J.: The microstructure changes in high-speed steels during continuous heating from the as-quenched state. Kovové Materiály 49 (2011) [13] Peters J. A., Bee J. V., Kolk B., Garrett G. G.: On the mechanisms of tempered martensite embrittlement. Acta Metall. 37 (1989) [14] Zając G.: Struktura i własności stali z niklem do ulepszania cieplnego. Praca doktorska AGH, Kraków (2006). [15] Węgrzyn R.: Nieodwracalna kruchość odpuszczania w stopach modelowych z krzemem. Praca magisterska AGH, Kraków (2000). [16] Pacyna J., Pawłowski B.: The effect of the tempering temperature on 30HGSNA steel toughness. Metallurgy and Foundry 10 (4) (1984) [17] Dubiel M.: Przemiany fazowe w stopach modelowych z chro mem imitujących osnowę zahartowanych stali konstrukcyjnych i nierdzewiejących. Praca doktorska, Kraków (1996). [18] Dobrzański L. A.: Metaloznawstwo i obróbka cieplna stopów metali. Gliwice (1993). [19] Pietikäinen J.: Considerations about tempered martensite embrittlement Mater. Sci. Eng. A (1999) [20] Dziurka R., Pacyna J.: The influence of carbon content on the kinetics of phase transformations of undercooled austenite of the Cr-Mn-Mo model alloys. Archives of Materials Science and Engineering 47 (2011) [21] Dziurka R., Pacyna J.: Influence of the carbon content on the kinetics of phase transformations during continuous heating from as-quenched state in a Cr-Mn-Mo model alloys. Archives of Metallurgy and Materials 57 (2012) NR 3/2013 INŻYNIERIA MATERIAŁOWA 161