PSI - Issue 13

T. Depover et al. / Procedia Structural Integrity 13 (2018) 1414–1420 Author name / Structural Integrity Procedia 00 (2018) 000–000

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The role of carbides has been a relevant subject in recent steel alloy development. Precipitates may induce material strenghtening due to precipitation hardening and are also often cited to be beneficial as potential H traps. As such highly diffusible H, which is supposed to be the most harmful one, is removed from the microstructure. Nevertheless, steels with increased strength appear to be more prone to H assisted failure (Hilditch et al. (2003) and Koyama et al. (2017)). The interaction of high strength steels with H has been considered thoroughly in the last decade (Ronevich et al. (2010), Venezuela et al. (2015) and Yu et al. (2016)). Recently, we also thoroughly studied these high strength steels, e.g. Pérez Escobar et al. (2012), Depover et al. (2014, 2016 (a)) and Laureys et al. (2015, 2016). The detrimental H effect was less outspoken for steels containing Ti- and Nb- carbo-nitrides, indicating their potential in this context. Hence, trapping diffusible H using nano-sized particles is generally assumed to be one of the main strategies to decrease the sensitivity to H related failure (Wei et al. (2006), Frappart et al. (2010) and Nagao et al. (2014)). However, multiphase steels contain a complex microstructure, complicating the interpretation of the H related data. Consequently, the study of H in simplified microstructures allows an improved understanding of the mechanisms (Barnoush et al. (2015), Di Stefano et al. (2016) and Hajilou et al. (2017)). In this perspective, Fe-C-X alloys were recently thoroughly examined by our group. Carbide forming elements, i.e. Ti, Cr, Mo, W and V, were added as ternary alloying element X. The findings for each carbide forming element were published separately by Depover et al. (2016 (b), 2016 (c), 2016 (d), 2016 (e), 2018 (a), 2018 (b), 2018 (c)). The present study, however, aims at comparing the carbides, which showed the most outspoken H trapping capacity, i.e. TiC and V 4 C 3 . 2. Experimental procedure 2.1. Materials Fe-C-Ti and Fe-C-V were chosen as materials of study, containing 0.313 wt% C - 1.34 wt% Ti and 0.286 wt% C - 1.670 wt% V, respectively. The Fe-C-X materials were cast, hot rolled and subsequently austenitized at 1250°C for 10 minutes followed by a brine water quench. This first condition will be referred to as “as-Q”. Next, a tempering treatment of 1h at 600°C was done to induce carbides. This condition will be referred to as “Q&T”. 2.2. Thermal desorption spectroscopy TDS measurements were done on samples which were H saturated by pre-charging the materials for 1 h in a 1g/L thiourea, 0.5 M H 2 SO 4 solution at 0.8 mA/cm 2 . TDS allowed identifying both the H traps and their corresponding activation energy (E a ). Hence, 3 different heating rates ( ) were used (200, 600 and 1200°C/h). The applied procedure required 1 h between the end of H charging and the start of the TDS measurement as sufficient vacuum needs to be created in the analysis chamber. To determine the E a of H traps from the peaks in the TDS spectra, the Kissinger (1957) method was used, with T max (K) the TDS peak temperature and R (J∙K -1 ∙mol -1 ) the universal gas constant: � ��� �� �� � � � 1 ��� � � � � 2.3. Permeation experiments Permeation experiments were done based on the Devanathan and Stachurski technique (1962) to determine the H diffusion coefficient. The electrolyte (0.1 M NaOH) was stirred in both cell compartments using a nitrogen flow to minimize the amount of dissolved oxygen, while the ambient temperature was kept constant at 25°C. The sample was polarized cathodically by applying a constant cathodic current density of 3 mA/cm². The absorbed H diffused through the sample to the anodic cell. There, H was oxidized, producing an external current recorded by a potentiostat. For this purpose, the sample was anodically polarized at a constant potential of -500 mV with respect to the reference electrode (Hg/Hg 2 SO 4 ,+650mV vs. SHE). D app was then calculated using the H oxidation current with following formula: ��� � �.� � 7 .7 where is the time (s) when the normalized steady-state value has reached 0.1, is the specimen thickness, equal 1 mm.