Elsevier

Acta Materialia

Volume 186, March 2020, Pages 133-148
Acta Materialia

Full length article
The role of plasticity and hydrogen flux in the fracture of a tempered martensitic steel: A new design of mechanical test until fracture to separate the influence of mobile from deeply trapped hydrogen

https://doi.org/10.1016/j.actamat.2019.12.045Get rights and content

Abstract

The design of an electrochemical permeation device on a tensile machine has allowed to control the hydrogen flux and to isolate the effects of trapped and mobile hydrogen on the hydrogen embrittlement of a martensitic steel. Based on a local approach of fracture, tensile tests on several notched specimens were completed in order to investigate the impact of hydrostatic stress, equivalent plastic strain, hydrogen concentration and flux on the damage processes. Analysis of the fracture surfaces revealed that trapped hydrogen favors ductile fracture, enhancing nucleation and growth of voids by reducing the interface energy between precipitates/inclusions and matrix. Mobile hydrogen leads to quasi-cleavage along the substructure (lath and/or blocks) boundaries at mainly the {101} planes. For both mechanisms, the mutual interaction between hydrogen and dislocations (drag process increasing hydrogen diffusion and hydrogen favoring dislocations mobility) has a large contribution to the hydrogen embrittlement of the martensitic steel.

Introduction

Hydrogen embrittlement (HE) is a phenomenon that is increasingly mentioned during premature fracture of industrial components in service. For example, oil and gas extraction is often accompanied by the presence of hydrogen sulfide, which favors the absorption and diffusion of hydrogen into metallic alloys [1,2]. Therefore, HE is a major concern for the development of tubular products for oil and gas production.

On a macroscopic scale, the consequences of hydrogen can be characterized by a decrease of the mechanical properties including fracture strain, fatigue life and changes of the fracture mode [3], [4], [5]. These effects have been reported by many studies on several alloys such as nickel base alloys [6], [7], [8], stainless [9], [10] and martensitic steels [11], [12], [13]. From a phenomenological point of view, without considering the case of the formation of hydrides, HE can be explained by three models. The first one is based on the assumption that hydrogen promotes plasticity, which is divided into two mechanisms: Hydrogen-Enhanced Localized Plasticity (HELP) [14] and Adsorption-Induced Dislocation Emission (AIDE) [15]. The second model is related to the reduction of cohesion energy (Hydrogen-Enhanced Decohesion, HEDE) [16]. Finally, the third model states that hydrogen promotes the formation of vacancies (Superabundant Vacancies, SAV) at high temperatures and hydrogen partial pressures [17,18], under severe cathodic charging [19] or as a result of electrodeposition reactions [20]. The possible combination of more than one of these models has also been proposed in order to understand the HE experimental results [21].

These HE models focus on the interactions between hydrogen and metallurgical heterogeneities and are primarily the result of the decrease of defect energies due to solute segregation according to the “defactant” concept proposed by Kirchheim [22,23]. Hydrogen seems to decrease the energies for emission and mobility of dislocations, vacancies formation and cohesion of interfaces, which may result in fracture modes with a strong contribution of plasticity and/or interfaces decohesion. It is, therefore, necessary to understand the nature of the interactions of hydrogen with structural defects. In terms of the impact of hydrogen on plasticity, the hydrogen-dislocation interactions have been the subject of numerous studies [24], [25], [26], [27], [28], [29], [30], [31]. These works show that the elastic field at the vicinity of edge dislocations is a reversible trap for hydrogen, whereas the edge dislocation core is an irreversible trap. The trapping of hydrogen by dislocations can decrease its diffusivity, but also influence the interactions between dislocations by shielding the elastic field and promoting the emission and mobility of dislocations (HELP). According to some authors [9,10,[32], [33], [34], [35]], these effects can lead to plasticity localization and brittle transgranular quasi-cleavage fracture. The impact of hydrogen on the cohesion energy of interfaces can also contribute to hydrogen-assisted damage. Evidence of plasticity and decohesion mechanisms were previously observed for several metallic alloys [7,9,[35], [36], [37]].

From a metallurgical point of view, the nature, density and distribution of trap sites, such as vacancies, inclusions, precipitates and dislocations are key parameters to understand the HE [38], [39], [40], [41], [42], [43], [44], [45]. In addition to the structure, the service mechanical request also determines the susceptibility to hydrogen-assisted cracking. Mechanical states present two components: hydrostatic and deviatoric stresses. Both affect damage and are classically formalized in terms of stress triaxiality. Notched specimens are often used to introduce variability of stress triaxiality. For instance, several authors used notched specimens to study the influence of triaxiality on nucleation and growth of the cavities in ductile fractures of steels and titanium alloys. They assume that there is a critical local stress to cavity formation, which is dependent on the hydrostatic stress and the local plastic strain [46], [47], [48].

Once hydrogen assisted-cracking happens in lath martensitic steels, intergranular and quasi-cleavage fracture surfaces are often observed [[11], [12], [13],33,35]. The structure immediately beneath both fracture morphologies reveals an extensive plasticity with intense slip bands and disturbed lath boundaries [11]. A synergetic action of HELP and HEDE mechanisms has been proposed to explain HE in these steels [35], [36], [37]. Hydrogen associated with dislocations would promote their activity and their pile-up at the laths, blocks, packets and prior austenite grain boundaries. The hydrogen deposited by the dislocations at these interfaces (or carbides/matrix interfaces) would then lead to local decohesion.

Despite the numerous studies on HE of martensitic steels, there are still gaps in the current knowledge. The contribution of diffusive and trapped hydrogen on the damage processes is not illucidated. It is also still not clear how hydrogen promotes the quasi-cleavage fracture and the role of plasticity in this process. The interactions between hydrogen and dislocations have not been fully understood yet. Additionally, it is still unknown if the hydrogen concentration or the flux of hydrogen impact the most the hydrogen-induced fracture. We have investigated the influence of hydrostatic stress on the hydrogen solubility, diffusivity, trapping and flux in tempered martensitic steels [49], [50], [51]. However, we have not identified the role of this hydrostatic stress component in the fracture process. Continuing these first studies, the present work aims to clarify the contributions of hydrogen flux, hydrostatic stress and plasticity to the damage mechanisms. To achieve this objective, tensile test with specimens that present several notch geometries were performed in pre-charging conditions or in an electrochemical permeation device combined with a tensile machine. Based on a local approach of fracture, the two observed damage modes (ductile and quasi-cleavage) were formalized in terms of hydrostatic stress-equivalent plastic strain (σm, εpeq) maps which allowed to demonstrate at a macroscopic scale the contribution of plastic deformation to both damage processes. In order to elucidate the meaning of this contribution, we evaluated firstly the impact of plastic strain on hydrogen flux and secondly the hydrogen impact on dislocations mobility using stress relaxation tests, which provide the activation volume and barrier energy to mobile dislocations. Both analysis gave further insights about the hydrogen-dislocation interactions and their effects on the development of fracture. Based on the tests results, new considerations are raised regarding the respective contributions of trapped and diffusible hydrogen to damage.

Section snippets

Material and characterization

The present work studies the Fe-0.3C-0.4Si-0.5Mn-1.0Cr-0.8Mo-0.05V-0.04Nb (weight%) martensitic steel with small amounts (less than 0.04 wt.%) of Al, S, Cu, Co and Ca. Samples of 180 × 120 × 15 mm were austenitized at 910 °C for 10 min and then water quenched. Thereafter, tempering was conducted at 710 °C for 30 min. Structural characterization was performed using SEM coupled with EBSD and TEM following a similar methodology previously employed [40]. The steel presents a fully lath tempered

Mechanical behavior

The mechanical tests provide information on the impact of hydrogen content (CH) and stress concentration (Kt) on the damage processes. We investigated the effect of notch geometry and hydrogen state (mobile or trapped) on the mechanical behavior of cylindrical in Fig. 2(a) and plate specimens in Figs. 2(b, c). The experimental details of the testing are listed in Table 1.

As can be seen in the tensile curves of unnotched axisymmetric and plate specimens (SWH), the steel has a yield strength of

Discussion

In this work, mechanical testing with unnotched and notched specimens at three different conditions (without hydrogen (SWH), pre-charged (PCD, UFD) and under flux (SUF)) were performed for plate and cylindrical design of specimens. These experiments aimed to investigate the hydrogen impact in the damage process for a quenched and tempered martensitic steel. Ductile and quasi-cleavage fractures are analyzed in terms of the contributions of hydrostatic stress and plasticity. The mechanical tests

Conclusions

In order to understand the origin of hydrogen embrittlement in the studied martensitic steel, we have separately investigated the effects of trapped and mobile hydrogen. For the mobile hydrogen impact, we have designed an electrochemical permeation device on a tensile machine allowing to control the hydrogen flux until fracture. When only trapped hydrogen is present, the fracture remains entirely ductile and depends on hydrostatic stress, plastic strain and trapped concentration. The hydrogen

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank E. Conforto for her contributions at the microscopy center facilities of the laboratory LaSIE.

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