In-situ Characterization of Martensitic Transformation in High Carbon ...

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ScienceDirect Materials Today: Proceedings 2S (2015) S941 – S944

International Conference on Martensitic Transformations, ICOMAT-2014

In-situ characterization of martensitic transformation in high carbon steel under continuous-cooling condition H. Terasakia,*, Y. Shintomea, A. Takadaa, Y. Komizoa, S. Moritob a Joining and Welding Reserch Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Department of Physics and Materials Science , Shimane University, 1060 Nishikawatsu-cho, Matsue, Shimane 690-8504, Japan

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Abstract To inspect volume expansion effect on the morphological features of martensite under continuous cooling conditions in which the transformation area (i.e. austenite) steadily decreases, we observed the martensitic transformation behaviour of 0.9C steel insitu on free surfaces. The microstructural evolution of surface martensite with an abnormal grain size and that of coarse and fine lath martensites was observed, in the stated order, under continuous cooling conditions; each martensite exhibited a different morphology. The differences in martensite morphology are discussed from elastic strain viewpoint induced by incipient-formed martensite.. © 2014 The Authors. Published by Elsevier Ltd. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations (http://creativecommons.org/licenses/by-nc-nd/4.0/). an open access under the license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 2014. This Selection andisPeer-review underarticle responsibility ofCC the BY-NC-ND chairs of the International Conference on Martensitic Transformations 2014. Keywords: Ms-tempeature, In-situ characterization, lath martensite, contnuous-cooling, high carbon steel, self-accomodation

1. Introduction Martensite is an essential microstructure that improves the mechanical strength of steel. Understanding the nature of martensite, including its various morphologies and defect concentrations, is necessary for controlling the properties of steel [1].

* Corresponding author. Tel.: +81-6-6879-4377; fax: +81-6-6879-4377 . E-mail address: [email protected]

2214-7853 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. doi:10.1016/j.matpr.2015.07.437

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H. Terasaki et al. / Materials Today: Proceedings 2S (2015) S941 – S944

In the use of steel martensite in applications such as welding, martensite is formed under continuous cooling conditions [2]. Under such conditions, incipient-formed martensite produces an elastic strain field around the martensite, which may affect its transformation behaviour. However, there are few studies on elastic strain effect induced by incipient-formed martensite [3] on martensitic transformation under continuous cooling conditions, where the transformation area (i.e. austenite) steadily decreases. In this study, 0.9C steel was selected because its low martensite start (Ms) temperature results in a large volume expansion during martensitic transformation. We observed the martensitic transformation behaviour on the free surface in-situ during a continuous cooling cycle because the free surface provides drastic conditions for the first martensite formed during continuous cooling. The elastic strain effect induced by incipient-formed martensite on the martensite morphology, including the free surface effect, is discussed. 2. Experimental Procedure High-carbon steel with a chemical composition of Fe–0.91C–0.2Si–0.53Mn–0.01Ni–0.01Mo (mass%) was used. The specimen was heated at 16.7 K/s and maintained at 1573 K for 100 s. It was then cooled at −50 K/s to 603 K and further cooled at −1.67 K/s to 293 K. The slow cooling rate of the second cooling step enabled observation of the evolution of the martensitic transformation between the Ms and Mf temperatures. The microstructural evolutions during the thermal cycle were observed in situ using laser scanning confocal microscopy (LSCM) for hightemperature applications. To prevent oxidation during observation, argon of 99.99996% purity was used as the inert gas. Furthermore, a purifier was used with the shielding gas to remove H2O to a concentration of less than 10 ppb. Details of the system are described elsewhere [4]. For the analysis of the crystallographic characteristics of steel, electron backscattering diffraction (EBSD) measurements were performed in a scanning electron microscope equipped with a TSL EBSD system operated at an acceleration voltage of 15 keV and step sizes of 0.2 and 0.1 m. Specimens for EBSD measurements were prepared by mechanical polishing using colloidal silica. By combining LSCM observations and EBSD analysis [5], three types of martensite formation in time series were evaluated. 3. Results and Discussions Figure 1 shows the LSCM images of the phase evolution of 0.9C steel during the applied thermal cycle. In Fig. 1(a), ‘P’ denotes the perlite nodule formed during the fast cooling step of the applied thermal cycle, and ‘GB’ denotes austenite grain boundary. As indicated by the arrow in Fig. 1(a), significantly coarse martensite formed at the Ms temperature of 495.7 K. When the specimen was further cooled by 26.3 K, other plates of coarse martensite formed, as shown in Fig. 1(b). The significantly coarse martensite is referred to as ‘type (i) martensite’. When the temperature was decreased by an additional 13.3 K, martensite with intermediate dimensions formed, as shown in Fig. 1(c). This martensite is referred to as ‘type (ii) martensite’. A further decrease of 33 K resulted in fine lath martensite formation, referred to as ‘type (iii) martensite’, between type (ii) martensites or between type (i) and (ii) martensites. Incipient-formed types (i) and (ii) martensite induce an elastic strain in the untransformed austenite and require a stronger driving force for type (iii) martensite formation. Figure 2(a) shows the alpha-orientation map of a selected area where microstructural evolution was observed using LSCM; an image-quality map is superimposed on it. The white-dashed line shows prior austenite grain boundaries (PAGBs). Within the PAGBs, very coarse type (i) martensite microstructures form, as indicated by the arrows. Finer microstructures were observed between type (i) martensites. Figure 2(b) shows an image-quality map coloured as per the image-quality value corresponding to the area shown with a dashed line in Fig. 2(a). The image quality represents a measure of the crystal defect [6], and the image-quality map indicates that crystal defects are greatest in the case of type (iii) martensite. This result is consistent with the transformation order observed in Fig. 1. However, the type (i) martensite contains few defects. It penetrates the PAGBs and contains straight lines inside, as shown in Figs. 2(a) and (b). The martensite microstructures have faceted boundaries, and their morphology is similar to that of lenticular martensite in Fe–29Ni steel, as reported by Shibata et al. [7]. However, the plate size of martensite in our specimen is significantly coarser, and the Ms temperature is the highest among the three types of martensite. Furthermore, the midrib-like line in type (i) martensite was analysed using focussed ion beam/transmission electron microscope, as shown in Fig. 2(c). We unexpectedly observed a lack of a typical twin


region observed in midrib of lenticular martensite in the type (i) martensite; we did, however, observe fine lath martensite in the midrib-like line. These observations suggest that the type (i) martensite observed is not lenticular martensite; it is a surface martensite formed under special stress conditions, i.e. surface conditions. In contrast, the morphology of type (i) martensite was not observed inside the test piece, as shown in Fig. 3. The first type (i) martensite formed suffered no effects from incipient-formed martensite. Furthermore, the low Ms temperature of 0.9C steel caused a large volume expansion because of the difference in thermal expansion coefficients between ferrite and austenite [7]. These conditions provided an opportunity to form the significantly coarse dimensions of lath martensite such as type (i). The dislocation accommodation in the midrib-like line in type (i) martensite is also attributed to the surface condition. Figure 4(a) shows variant boundaries in the Kurdjumov–Sachs orientation relation visualized using map property function in TSL OIM ANALYSIS software. The variant number (V) follows the notation of Morito et al. [8]. As evident in the figure, type (ii) and type (iii) martensites contain all pairs of V1–V2, V1–V3 and V1–V6; these are variant pairs in the common habit plane, and their combination enables self-accommodation [8, 9]. A higher elastic strain field decreases the Ms temperature, which results in greater volume expansion during martensitic transformation. Consequently, the variant of the self-accommodation pairs is mainly rich in type (iii) martensite.

Fig. 1. Snapshots of the phase evolution of 0.9C steel during continuous cooling: (a) first evolution of type (i) martensite, (b) evolution of type (i) martensite, (c) evolution of type (ii) martensite, and (d) evolution of type (iii) martensite.

Fig. 2. (a) Alpha-orientation map corresponding to LSCM images shown in Fig. 1, (b) Image quality map with colour-key corresponding to the area shown with the dashed line in Fig. 1 (a), and (c) Bright-field TEM micrograph corresponding to the white square shown in Fig. 2 (b).

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Fig. 3. OM observation results inside the specimen.

Fig. 4. Variant boundaries of K-S OR of type (i), (ii) and (iii) martensite.

4. Conclusions Using 0.9C steel, we demonstrated elastic strain effect induced by incipient-formed martensite on the morphological features of martensite under continuous cooling conditions, including free-surface conditions. A three-step evolution of surface martensite, significantly coarse martensite and fine lath martensite was observed insitu under cooling conditions. The elastic strain in the transformation area (i.e. austenite) affected the phase transformation order and the morphology of the three types of martensite. These observations were confirmed in-situ using LSCM. Although the in-situ characterization of phase evolution using LSCM is a unique and useful technique, the surface effects on the microstructural dimensions must be considered when the transformation temperature is low. Acknowledgements The authors acknowledge the financial support extended by an ISIJ Research Promotion Grant,, Japan. References [1] G. Krauss, Mater. Sci. Eng. A A273̄275 (1999) 40–57. [2] H. Terasaki, Y. Komizo, Scr. Mater. 64 (2011) 29–32. [3] S. Morito, S. Yoshida, R. Hayamizu, T. Hayashi, T. Ohba, H. Terasaki, Y. Komizo, Marter. Sci. Forum 783-786 (2014) 916–919. [4] H. Chikama, H. Shibata, T. Emi, M. Suzuki, Mater. Trans. 37 (1996) 620–626. [5] H. Terasaki, Y. Komizo, Metall. Mater. Trans. A 44 (2013) 5289–5293. [6] S.I. Wright, M.M. Nowell, Microsc. Microanal. 12 (2006) 72–84. [7] A. Shibata, T. Murakami, S. Morito, T. Furuhara, T. Maki, Mater. Trans. 49 (2008) 1242–1248. [8] S. Morito, H. Tanaka, R. Konishi, T. Furuhara, T. Maki, Acta Mater. 51 (2003) 1789–1799. [9] A. Stormvinter, G. Miyamoto, T. Furuhara, P. Hedstrom, A. Borgenstam, Acta Mater. 60 (2012) 7265–7274.