MICROSTRUCTURAL CHARACTERIZATION OF MEDIUM CARBON DUAL PHASE STEELS AFTER INTERMEDIATE QUENCHING

Transcription

1 MICROSTRUCTURAL CHARACTERIZATION OF MEDIUM CARBON DUAL PHASE STEELS AFTER INTERMEDIATE QUENCHING Ersoy ERİŞİR 1, Serap GÜMÜŞ 1, Oğuz Gürkan BİLİR 1 1Department of Metallurgical and Materials Engineering, Kocaeli University, Kocaeli-Turkey Abstract Multi-phase microstructure gain importance increasingly in new generation steels. Dual phase (DP) steels can be produced by the quenching from a temperature in intercritical temperatures in the phase region of austenite and ferrite. The final microstructure is composed of martensite and ferrite. Microstructure before the annealing at intercritical temperature can include martensite, ferrite, pearlite or austenite. The initial microstructure affects the final morphology strongly and the annealing temperature affects the amount of ferrite and martensite phases. Thus, the mechanical properties of dual phase steel depend on the morphology and phase distribution. In the intermediate quenching method, the initial microstructure is fully martensitic and final microstructure is fine and randomly scattered at former martensite boundaries in a ferritic matrix. This type of microstructure is called fibrous microstructure. There are some difficulties in selecting the correct etching solutions to obtain a sharply different contrast for the microstructure like this. On this study, hot rolled medium carbon steel was employed and intermediate quenching method was applied. Dual phase steel showed randomly distributed martensite laths in ferritic matrix in the microstructure. On the other hand, not only martensite and ferrite but also bainite, and pearlite were observed. The work presented here demonstrates the effect of the different etchings on the microstructure. Nital and Le Pera solutions were applied in order to overcome the difficulties in distinguishing the different phases. Light microscope and scanning electron microscope were carried out observing the microstructures. The Le Pera etchant was more effective to distinguish the martensite and ferrite. On the other hand, distinguishing the bainite, retained austenite and pearlite from martensite, same etchant was not favorable. Keywords: microstructural characterization, dual phase steel, medium carbon steel, intermediate quenching, phase transformation 1. INTRODUCTION Dual phase microstructures have enormous advantages over the conventional steels with respect to good combination of high strength, good machinability and high toughness [1, 2]. Dual phase microstructures consist of ferrite and martensite phases as a ferrite matrix together with distributed martensite phase commonly up to %15 volume fractions. The main application area of dual phase steels is in general limited to the hot rolled sheets for automotive industry. Thus, many research papers are related to the low carbon steels containing relative small amounts of martensite. In the automotive industry, carbon amount is restricted by the machinability and weldability properties which inversely proportional to the carbon content and amount of martensite [1-15]. However, recent studies about dual phase steels with increased amount of martensite gained importance to strength and wear resistance. Medium carbon dual phase steels can be used for applications in mineral and mining processes which do not require any welding operation [16, 17]. Even the obtaining a dual phase microstructure from medium carbon steels is an important challenge, there are some difficulties. Calculating volume fractions of different phases is not as easy as low carbon steels and also it made increase possibility of forming retained austenite, pearlite and bainite. It may be sometimes difficult separating this phases. Therefore, because of the expectations about the differences between microstructures of low carbon and medium carbon steels, etching solutions may differ with respect to

2 characterizing the different microstructures. In this study, different etching solutions applied to medium carbon hot rolled steel to analyze the microstructure of medium carbon dual phase steels. 2. MATERIAL AND EXPERIMENTAL Chemical composition of investigated steel is given in Table 1. The intermediate quenching experiments were carried out in a Bähr DIL805 plastodilatometer in order to investigate the phase transformations in the investigated steel. The standard specimen size of 5 mm diameter and 10 mm length was used for the dilatometer experiments. The samples were annealed at 1100 ºC for 5 min and followed by gas quenching with a cooling rate of 130 K/s to obtain fully martensitic initial microstructure. After martensitic transformation, specimens were annealed respectively at 725, 730, 740 and 750 C for 15 min and finally gas quenched again. The samples were metallographically prepared. After sample preparation, samples were finally etched with Nital and Le Pera. Nital etchant contains 3% nitric acid in ethyl alcohol. Le Pera etchant is prepared as a mixture of 1% metabisulfide in distilled water and 4% picric acid in ethyl alcohol. Light microscope and scanning electron microscope investigations were carried out. Table 1 Chemical composition of steel. Chemical composition (%weight) C Si Mn P S Cr Mo Ni V W Cu 0,368 0,279 0,865 0,0195 0,0063 0,0413 0,0168 0,0769 0,0021 0,0310 0,0597 Al Co Nb Ti B Zr As Pb Sn Ca 0,0472 0,0076 0,0033 0,0034 0,0012 0,0025 0,0133 0,0050 0,0055 0, RESULTS Dual phase microstructure of annealed sample is given in Fig. 1. The samples for SEM are Nital etched in order to etching ferrite, pearlite and bainite while it leaves martensite unetched state. In Fig. 1, SEM micrograph reveals the martensite islands in a ferritic matrix as well as a third phase (mixture of bainite/pearlite) at ferrite-martensite phase boundaries. It was very hard to identify the nano-sized F/P with light microscope as dark areas in Fig. 2. However, martensite islands (brown) and ferrite (white) can be distinguished using light microscope. Fig. 1 SEM micrograph of steel annealed at 730 C and intermediate quenched, M: martensite, F: ferrite, B/P: bainite/pearlite.

3 Fig. 2 LM micrograph of steel annealed at 730 C and intermediate quenched, M: martensite, F: ferrite, B/P: bainite/perlite. 3.1 Le Pera Etching Le Pera etchant allow the distinction of ferrite (brown/blue), bainite (dark brown/black), martensite and retained austenite (white). Fig. 3 shows the microstructures of steels annealed at 730 and 740 C after etching with Le Pera. The third phase (dark brown/black) can be noticed in the microstructure which can be pearlite and/or bainite. There are two possibilities that can explain the formation of pearlite/bainite. First thought may be that pearlite/ bainite is formed before decomposition of austenite and ferrite to dual phase microstructure. Second is the pearlite/bainite is formed during quenching from annealing temperature. According to first thought, one may think that pearlite/bainite is already existed in martensitic initial structure or it was formed as a result of annealing the initial martensite structure. Fig. 3 LaPera etched microstructure of steel annealed at (a) 730 C and (b) 740 C, and followed by intermediate quenching.

4 3.2 Nital Etching Nital etchant can be alternative where Le Pera etchant cannot produce desired tinted effect in microstructural constituents. For example, SEM investigations are required the topographic contrast for low alloyed steels in which phases do not have enough difference in the chemical compositions to produce the material contrast in SEM. In such a case, Nital can etch ferrite, pearlite and bainite while martensite remains unetched condition. Figs. 4 and 5 show micrographs of LM and SEM after Nital etching. In Fig. 4, dual phase microstructure can be seen with the third phase as in Le Pera etching in Fig. 3. However, it can be said that Nital is better to recognize the Widmanstätten type ferrite (white). Monotype coaxial ferrite is seen in Le Pera etched samples in Fig. 3. However, according to intermediate quenching, there must be fibrous microstructure consist of martensite and ferrite which can be seen in Nital etched samples. This is due to Nital has a better performance on obtaining grain boundaries. However, in point of the calculating phase distributions, separating the Widmanstätten ferrite and martensite from fibrous complex microstructure is not easy. La Pera etching may be useful to fulfill this difficulty for the image analysis studies. SEM micrographs are given in Fig 5. Ferrite seems darker than martensite on the contrary LM micrographs after Nital etching. a) b) c) d) Fig. 4 LM micrographs of steels annealed at a) 725, b) 730, c) 740, and d) 750 C, and followed by intermediate quenching; etchant: Nital.

5 a) b) c) d) Fig. 5 LM Micrographs of steels annealed at a) 725, b) 730, c) 740, and d) 750 C, and followed by intermediate quenching; etchant: Nital. 4. DISCUSSION AND CONCLUSION The advantages and disadvantages of applying Le Pera and Nital etchants to medium carbon dual phase steels were demonstrated. Martensite seemed lighter (light brown) and ferrite seemed darker colors (brown/blue) in Le Pera etched samples, while martensite is brown and ferrite is white in Nital etched samples. A third phase (bainite/pearlite) is also seen in black color and nearly same contrast in Le Pera and Nital. The third phase may be obtained due to quenching product from annealing temperature. As a result of microstructural investigations with different etchants, Nital is favorable to understand all microstructure, while Le Pera is not as good as Nital for obtaining grain boundaries and Widmanstätten ferrite. Therefore, it is concluded that Le Pera is better for using in image analysis for phase distributions for medium carbon dual phase steels. ACKNOWLEDGEMENT The authors are grateful to Yapı-Tek Çelik Sanayi A.Ş., Kocaeli, Turkey for the supplement of the alloy and to Dr. U. Prahl for access to a precision dilatometer.

6 REFERENCES [1] OLIVER, S., JONES, T.B. and FOURLARIS, G. Dual Phase versus Trip Strip Steels: Microstructural Changes as a Consequence of Quasi-Static and Dynamic Tensile Testing. Materials Characterization, 2007, Vol. 58, Issue: 4, page [2] SPEICH, G. R. Dual Phase Steel ASM Handbook Volume 1: Properties and Selection: Irons, Steels and High Performance Alloys section. ASM International, 2005, page [3] OBENCHAIN, W. A., SCHAIK, M. V., PETERSON, P. Green Engineering Case Study: Reducing Automobile Emissions and Saving Energy. ULSAB Advanced Vehicle Concepts, American Iron and Steel Institute, 2012 page [4] DAVIES, R. G. Influence of Martensite Composition Content on the Properties of Dual Phase Steels. Metallurgical and Materials Transactions A, 1978, Vol. 9, page [5] DEMIR, B., ERDOGAN, M. The Hardenability of Austenite with Different Alloy Content and Dispersion in Dual- Phase Steels. Journal of Materials Processing Technology, 2008, Vol. 208, Issue: 1-3, page [6] SARWAR, M., PRIESTNER, R. Hardenability of Austenite in a Dual-Phase Steel. Journal of Materials Engineering and Performance, 1999, Vol. 8, Nr. 3, [7] ERDOGAN, M., TEKELI, S. The Effect of Martensite Particle Size on Tensile Fracture of Surface Carburised AISI 8620 Steel with Dual Phase Core Microstructure. Materials and Design, 2002, Vol. 23, Nr. 7, page [8] BAYRAM, A., UGUZ, A., ULA, M. Effects of Microstructure and Notches on the Mechanical Properties of Dual- Phase Steel. Materials Characterization, 1999, Vol. 43, Nr. 4, page [9] AHMAD, E., MANZOOR, T., ZIAI, M.M.A., HUSSAIN, N. Effect of Martensite Morphology on Tensile Deformation of Dual-Phase Steel. Journal of Materials Engineering and Performance, 2011, Vol. 21, Nr. 3. [10] MODI, O.P., PANDIT, P., MONDAL, D.P., PRASAD, B.K., YEGNESWARAN, A.H., CHRSANTHOU, A. Highstress abrasive wear response of 0.2% carbon dual phase steel: Effects of microstructural features and experimental conditions. Materials Science and Engineering A, 2007, Vol. 458, page [11] KORZEKWA, D.A., MATLOCK, D. K., KRAUSS, G. Dislocation Substructure as a Function of Strain in a Dual- Phase Steel. Metallurgical Transactions A, 1984, Vol. 15, Nr. 6, page [12] BAYRAM, A., UGUZ, A. Effect of Microstructure on the Wear Behaviour of a Dual Phase Steel. Materialwissenschaft und Werkstofftechnik, 2001, Vol. 32, Nr. 3, page [13] MOHAVED, P., KOLAHGARA, S., MARASHIA, S.P.H., POURANVARI, M., PARVINA, N. The Effect of Intercritical Heat Treatment Temperature on The Tensile Properties and Work Hardening Behavior of Ferrite Martensite Dual Phase Steel Sheets. Materials Science and Engineering A, 2009, Vol. 518, Nr. 1, page 1-6. [14] KELESTEMUR, O., YILDIZ, S. Effect of various dual-phase heat treatments on the corrosion behavior of reinforcing steel used in the reinforced concrete structures. Construction and Building Materials, 2009, Vol.23, page [15] KUZIAK, R., KAWALLA, R., WAENGLER, S. Advanced high strength steels for automotive industry. Archives of Civil and Mechanical Engineering, 2008, Vol. 8, Nr. 2, page [16] TYAGI, R., NATH, S. K., RAY, S. Effect of Martensite Content on Friction and Oxidative Wear Behavior of 0.42 Pct Carbon Dual-Phase Steel. Metallurgical and Materials Transactions A, 2002, Vol. 33, page [17] TYAGI, R., NATH, S. K., RAY, S. Development of wear resistant medium carbon dual phase steels and their mechanical properties. Materials Science and Technology,2004, Vol. 20, page