Impulse Excitation Internal Friction Study of Retained Austenite in Ferrous Martensite
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ear model describing the interaction between solute C atoms and dislocations is needed to understand the strengthening mechanism of ferrous martensite by solute C. The internal friction (IF) technique is one of the few methods which makes it possible to probe the interaction between point defects, such as interstitial solutes and dislocations in steels.[1] The IF technique has been applied to several ferrous alloys with a martensitic microstructure such as Fe-Ni-C alloys,[1,2] Fe-C alloys,[3,4] and the 22MnB5 grade press hardened steel (PHS).[5,6] The IF spectrum of these martensitic steels has been reported to contain a maximum of five distinct relaxation peaks.[3,7] Sulistiyo et al.[6] have made a detailed review of the IF peaks reported for martensitic steels in the published literature. After comparing the reference peaks with their own experimental results for PHS, the relaxation peaks were characterized by related parameters. Table I shows the parameters of the
JI HOON KIM, YU NA JU, and BRUNO C. DE COOMAN are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang 37673, Republic of Korea. SINGON KANG is with the Department of Materials Science and Engineering, Dong-A University, 37, Nakdong-daero 550, Saha-gu, Busan 49315, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted June 15, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
martensite relaxation peaks[6,8] and parameters of the austenitic IF peaks which contain interstitial carbon[9–11] for further discussion. The notation of the five peaks as P1, P2, P3, P4, and P5, first used by Tkalcec and Mari,[3] was adopted for the martensite peak designation. Note that the P1 peak, which has a peak maximum temperature far below room temperature (RT), was not included in Table I. The consensus about the origin of the main martensite IF peaks relevant to the present work is as follows. The martensite P3 peak, also known as the dislocation-enhanced snoek peak (DESP), is affected by the transition carbides precipitation during the first stage of tempering. The P3 peak is almost coincident with the Snoek peak at 393 K (120 C) which is observed for steels which have a low dislocation density and which contain C in supersaturation. The Snoek peak is due to the rearrangement of solute C atoms between the three equivalent tetragonal interstitial sites in cyclic straining.[12] As noticed previously by Tkalcec et al.,[7] the P4 peak is part of the P5 peak. The P5 peak of martensite was identified as the SnoekKeˆ-Ko¨ster (SKK) peak reported previously for ferritic steels.[13] The SKK peak, which is due to the motion of kinks on screw dislocations dragging solute C atoms, is the main martensite IF peak. A distinct feature of the publications discussed in the previous paragraphs is that the authors did not address the possible effect of retained austenite on the IF spectrum of martensitic steels. Retained austenite is a major secondary constituent in several martensitic steels and it influe
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