Role of Nonmetallic Inclusions in Hydrogen Embrittlement of High-Strength Carbon Steels with Different Microalloying
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e mechanism of hydrogen embrittlement of highstrength carbon steels has been widely discussed during the previous decades.[1] The presence of a few parts per million weight of hydrogen in carbon steels may result in a dramatic reduction of elongation to fracture, result in a loss of ductility, or even initiate brittle fracture without macroscopic plastic strain.[2,3] Another effect of hydrogen on high-strength engineering materials is the so-called hydrogen-induced delayed fracture, during which the material cracks under constant load, which can be less than the load corresponding to the yield stress of the material. It is widely discussed today that vacancies enhanced by hydrogen play the major part in the hydrogen embrittlement of high-strength carbon steels.[4] Excessive vacancies, as assumed, are formed due to hydrogendislocation interactions and operate as effective trapping sites for hydrogen. Actually, the peaks of hydrogen thermal desorption observed in the high-strength carbon steels between room temperature and 600 K (327 °C) are often referred to as vacancy or vacancy-type trapping sites.[5] Hydrogen content and applied stress,
OLGA MADELEN INGRID TODOSHCHENKO, Postdoctoral Student, YURIY YAGODZINSKYY, Senior Research Scientist, TAPIO SAUKKONEN, Laboratory Manager, and HANNU HA¨NNINEN, Professor, are with the Department of Engineering Design and Production, Aalto University School of Engineering, 00076 Aalto, Finland. Contact e-mail: olga.todoshchenko@aalto.fi Manuscript submitted April 2, 2014. Article published online July 18, 2014 4742—VOLUME 45A, OCTOBER 2014
time of formation, and accumulation of vacancies have been concluded to be important factors causing hydrogen embrittlement.[4] On the other hand, while studying the hydrogeninduced delayed fracture in Mo-V martensitic steels tempered at 823 K and 923 K (550 and 650 °C), it was found that nonmetallic inclusions (NMI) act as a strong trapping site for hydrogen.[6] Hydrogen was also assumed to enhance NMI decohesion or cracking.[7] Hydrogen-induced fracture initiating at TiN and Al2O3Æ(CaO)x particles in steel is the subject of Reference 8. In that study, it was supposed that hydrogen is concentrated in voids, which form around inclusions. Under tensile loading, voids grow due to high hydrogen concentration, initiating crack. NMIs and precipitates play the major role in hydrogen embrittlement, acting as trapping sites for hydrogen in high-strength carbon steels, but the role of type and chemical composition of inclusions in their interaction with hydrogen have not been studied in detail. The aim of the present study is to clarify the role of NMIs of different types and chemical compositions in hydrogen embrittlement of high-strength carbon steels. Three high-strength steel grades with different contents of Mn, Cr, Ti, and Nb were chosen for this study. The chemical composition of the steels is shown in Table I. Steels in the form of sheets with thickness of 1.0 to 1.5 mm were supplied by ThyssenKrupp Steel Europe (Duisburg, Germany), Voestalpine Stahl
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