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The present work evaluates hydrogen trapping and hydrogen induced ductility loss of four lab cast Fe-C-X martensitic Q&T alloys. Ti, Mo, Cr and V are used as ternary alloying elements. The materials are compared in two conditions: asQ and Q&T. Tensile tests are performed to evaluate H induced mechanical degradation. The hydrogen trapping capacity of the precipitates is investigated by TDS, while hot/melt extraction is done to determine the amount of hydrogen. A correlation between the degree of hydrogen induced mechanical degradation and the amount of hydrogen trapped by dislocations was observed. INTRODUCTION In steel alloy development, the role of carbides has been a relevant topic [1]. They are also often cited to be beneficial as efficient H trapping sites. Although TDS studies showed the H trapping ability of several carbides [2-9], literature relating their impact on the mechanical properties in the presence of H is limited [10, 11]. However, steels with an increased strength level are considered to be prone to HE [12-14]. The interaction of these high strength steels with H has been studied thoroughly during the last decade [15, 16]. Recently, our group presented some results on four industrial multiphase high strength steels [17-22]. The mechanical properties were investigated as well [14] and a significant H induced ductility loss was observed, except for the HSLA (high strength low alloy) steel, which was attributed to the presence of Tiand Nbcarbo-nitrides. Recently, more and more emphasis is put on the addition of carbides as H trapping sites. Generally, trapping diffusible H using nano-sized carbides as H traps is one of the main strategies to enhance the resistance against HE [10, 14, 23]. However, the complex microstructure of multiphase steels hinders interpreting H related observations. Recently, we discussed specifically designed Fe-C-X alloys containing carbide forming elements, i.e. X = Ti, Cr, Mo and V [24-27]. However, each paper focused on one specific type of carbide forming element. The present study aims at making comparison between the different carbides. 2 EXPERIMENTAL PROCEDURE Four different steel grades with a stoichiometric amount of a ternary alloying element X (cf. Table 1) were processed. The Fe-C-X alloys 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, also a tempering treatment of 1h was applied to introduce carbides in the martensitic microstructure. Secondary hardening due to the precipitation of carbides was optimal at 600°C for the Fe-C-Ti, Fe-C-Mo and Fe-C-V alloys and at 550°C for the Fe-C-Cr material [24-27]. This second condition will be referred to as Q&T. The microstructure were studied by optical microscopy, SEM and TEM and the degree of H induced mechanical degradation, the H content and TDS measurements were determined as described in [24-27]. The %HE index was determined as: with εch and εun being the elongation of the charged and uncharged sample, respectively. Hence, the %HE varies between 0 and 1, with 0 meaning that there is no ductility loss and the material is insensitive to HE. When an index of 1 is obtained, the ductility drop is 100% and HE is maximal. Table 1: Chemical compositions of the Fe-C-X materials. Alloy Fe-C-X wt.% C wt.% X Other elements Fe-C-Ti 0.313 1.34 Al: 200-300 wt. ppm Other elements: traces Fe-C-Cr 0.184 2.20 Fe-C-Mo 0.177 2.99 Fe-C-V 0.286 1.67 MATERIALS CHARACTERIZATION Optical microscopy showed a clear martensitic and Q&T microstructure. TEM bright field images are presented in Fig. 1, and displayed small carbides with sizes less than 10 nm for the Tiand V-alloy and coarser precipitates for the Moand Cr-alloy. More microstructural details can be found elsewhere [24-27]. %HE = 100 ∙ 1 − εch εun

Comparison of the Role of Four Different Carbides in the Hydrogen Induced Mechanical Degradation of Lab Cast Fe-C-X Alloys