Hydrogen induced phase transformations in austenitic-ferritic steel A. Głowacka 1, a , M.J. Woźniak 1,b , G. Nolze 2,c and W.A. Świątnicki 1,d 1 Warsaw University of Technology, Faculty of Materials Science& Engineering, 141 Woloska St., 02-507 Warsaw, Poland 2 Federal Institute for Materials Research and Testing (BAM), Unter den Eichen 87, 12205 Berlin, Germany a annabur@inmat.pw.edu.pl, b wozmi@inmat.pw.edu.pl, c gert.nolze@bam.de, d wiesl@inmat.pw.edu.pl Keywords: Duplex steel, hydrogen embrittlement, TEM, AFM, EBSD. Abstract. The hydrogen influence on the microstructure of the austenitic-ferritic Cr22-Ni5-Mo3 stainless steel was investigated. Cathodic hydrogen charging was performed electrochemically from aqueous solution of 0,1M H 2 SO 4 with hydrogen entry promoter addition. The aim of this study was to reveal microstructural changes appearing during the hydrogen charging and particularly to clarify the occurrence of phase transformations induced by hydrogen. The specific changes in the phases of both types of steel were observed. In the ferritic phase, strong increase of dislocation density was noticed. Longer time of hydrogen charging leaded also to the strips and twin plates formation in ferrite phase. On the other hand, in the austenitic phase, the generation of stacking faults, followed by the formation of ' martensite was remarked. Introduction Hydrogen is well known element, which in certain condition may be absorbed into the material leading to the various changes of its microstructure, and in consequence, of its mechanical properties []. Even though the austenitic-ferritic duplex stainless steels have a good resistance to corrosion cracking, they may suffer from so-called hydrogen embrittlement (HE) [1,2,3,4,5,6,7]. Hydrogen cathodic charging provokes significant changes in both austenite () and ferrite () phases. In ferrite, one can observe a strong increase of dislocation density, whereas in austenite, a growth of stacking fault density [8,9]. The microstructural changes are also connected with a loss of steel ductility and steel hardening [10], but the mechanism of this phenomenon is not clear. The hypothesis of hydrogen induced phase transformations should be verified. Therefore, the aim of our study was to describe the microstructural changes involved by hydrogen in ferrite () and austenite () phases of a duplex steel, including hydrogen induced phase transformations. Experimental The material used in this study was Cr23-Ni5-Mo3 austenitic-ferritic duplex stainless steel (chemical composition in wt %: 0.026C, 0.35Si, 1.57Mn, 5.43NI, 22.94Cr, 2.75Mo, 0.164N, 0.001S, 0.021P, 0.012Al), containing the same volume fraction of austenite and ferrite, i.e. about 50%. The samples were annealed for 1 hour at 1000 o C in argon atmosphere and subsequently quenched into water. The microstructure of the steel is shown in Fig. 1. Cathodic hydrogen charging of steel specimens was carried out electrolytically in a 0.1M H 2 SO 4 aqueous solution, with addition of hydrogen entry promoter, at a room temperature. Samples were observed using various microscopic techniques. Atomic Force Microscopy (AFM).