Issue 52

A. Laureys et alii, Frattura ed Integrità Strutturale, 52 (2020) 113-127; DOI: 10.3221/IGF-ESIS.52.10 114 containing environment, atomic hydrogen can be absorbed in the metal and hence diffuses inside the metal. Its movement can be interrupted or hindered by microstructural discontinuities, such as voids, dislocations, second phase particles (such as MnS [17] inclusions), grain boundaries, and microcracks, which act as trap sites [18]. At such sites, atomic hydrogen can recombine to form molecular gaseous hydrogen, which is incapable of further migration and locally creates a high internal pressure [14, 19]. A large amount of hydrogen can hence diffuse with time into the traps resulting in a continuously increasing hydrogen pressure. This process is active until an equilibrium in the chemical potential is achieved between the molecular hydrogen in the trap and the hydrogen in the lattice [20]. The result is the formation of overpressurized gas-filled cavities, which cause plastic deformation of the surrounding lattice and promote crack formation. If the internal pressure rises to levels which exceed the tensile strength, crack propagation occurs, even in the absence of externally applied loads. Additionally, lattice hydrogen could facilitate crack propagation at stresses lower than the actual tensile stress of the material by several mechanisms, such as hydrogen-enhanced decohesion (HEDE) [21], hydrogen-enhanced localized plasticity (HELP) [22] or hydrogen-enhanced vacancy formation [23]. The internal pressure is temporarily relieved due to crack propagation, which therefore occurs discontinuously. As proof, some wavy lines perpendicular to the crack propagating directions were observed on the fracture surface of a blister [24, 25]. In a next stage microcracks propagate further and connect, creating a series of stepwise cracks through the material [26]. When the phenomenon takes place close to the sample surface, it is referred to as blistering. The high pressure then pushes material upwards, resulting in a surface blister [5]. Different blister initiation sites and initiation mechanisms have been proposed in literature for alloys charged in a high fugacity hydrogen environment. Numerous studies [5, 6, 24, 25, 27, 28] revealed that non-metallic inclusions and second phase particles, such as MnS and Al 2 O 3 , in steels act as major nucleation sites for blisters. Such inclusions typically serve as irreversible trap sites with high binding energies for hydrogen [29, 30], where hydrogen can accumulate. The non-metallic inclusions in steel are usually formed during the steel manufacturing, deoxidation and desulfurization process. Cracking occurs if locally a critical hydrogen concentration necessary for crack initiation is reached [31, 32]. Huang et al. [33] stated that the more hydrogen entrapment occurs in the steel, the more the steel is susceptible to HIC. Elboujdaini and Revie found that a linear relationship exists between the threshold hydrogen concentration in a material and the quantity of inclusions in the steel. Domizzi et al. [34] studied different grades of pipeline steel with various inclusion contents and degrees of microstructural banding through thickness and concluded that the presence of hard bands is more critical than inclusions for HIC. Multiple authors [1, 35, 36] state that the presence of a second phase is not a prerequisite for blister formation and claim that vacancy-hydrogen interaction plays a role in the initiation process as well. Garofalo et al. [37] stated that the hydrogen induced propagation of internal cracks in steel is promoted by hydrogen gas in voids or microcracks which may be formed by plastic deformation or are present as porosities from the casting process. Griesche et al. [1] visualized small pores with diameters of ~1 µm all over the hydrogen induced crack surfaces. These pores were located along grain boundaries, which are strong hydrogen traps. In summary, crack nucleation during hydrogen charging has been related to a localized concentration and subsequent recombination of hydrogen at suitable heterogeneities such as grain boundaries, second phase particles, microvoids and tangled dislocations [38]. This investigation gives an overview of previous work [6, 9, 39, 40] in order to bring together results obtained on different materials, compare the initiation mechanism for hydrogen induced cracking and as such give new insights on an important topic. The study compares the blistering behavior of four different types of materials, i.e. ultra low carbon (ULC) steel [6], TRIP (transformation induced plasticity)-assisted steel [40], generic Fe-C-Ti alloy [39], and pressure vessel steels [9]. These materials exhibit strongly varying microstructural features, such as different type of inclusions, precipitates and phases. All these microstructural components interact differently with hydrogen and exhibit a specific critical hydrogen concentration [6]. The aim of this work is to describe a general mechanism for hydrogen induced crack initiation in steels. It is essential to understand the microstructure dependence of HIC to help guide future microstructural design and alloy applications. The materials were cathodically charged as such that damage formed and cross sectional analysis allowed to estimate the initiating internal damage. A thorough microstructural characterization of blisters and internal cracks was performed by optical microscopy, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). E XPERIMENTAL PROCEDURE Materials he hydrogen induced cracking behavior of five different steel alloys was compared during the current investigation, i.e. ultra-low carbon (ULC) steel, TRIP (transformation induced plasticity)-assisted steel, generic Fe-C-Ti alloy and two types of pressure vessel steels. The chemical compositions of these materials are given in Tab. 1 and 2. T