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* E - experimental research; M - modeling research. Selected studies on the synergistic interplay of HE mechanisms in steels and iron. Table 2

Table 2 * E - experimental research; M - modeling research. Selected studies on the synergistic interplay of HE mechanisms in steels and iron. Table 2

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Fig. 1. Historical overview of approaches to HE study. disagreement and polarization of opinions among researchers on the inherent nature of the viable and active HE mechanisms and general HE models [8,9,17,23,24,26,28]. Particularly, their coexistence or a synergistic interplay in steels and iron is still not completely resolved [13,16,29-36]. Activation of the particular HE mechanism and its effects on the degradation of mechanical properties in different grades of steel, iron and structural materials depends on [2,10,13,22,23,27,29,36,37]:
Fig. 1. Historical overview of approaches to HE study. disagreement and polarization of opinions among researchers on the inherent nature of the viable and active HE mechanisms and general HE models [8,9,17,23,24,26,28]. Particularly, their coexistence or a synergistic interplay in steels and iron is still not completely resolved [13,16,29-36]. Activation of the particular HE mechanism and its effects on the degradation of mechanical properties in different grades of steel, iron and structural materials depends on [2,10,13,22,23,27,29,36,37]:
The aim of this critical review is to present all the most up-to-date experimental and computational modeling approaches and published results related to the synergistic interplay of HE mechanisms in steels and iron. This paper provides a critical overview of Fig. 2. A contemporary approach in evaluation, control and prediction of hydrogen-assisted mechanical degradation processes in steels and iron.
The aim of this critical review is to present all the most up-to-date experimental and computational modeling approaches and published results related to the synergistic interplay of HE mechanisms in steels and iron. This paper provides a critical overview of Fig. 2. A contemporary approach in evaluation, control and prediction of hydrogen-assisted mechanical degradation processes in steels and iron.
HE models, the coexistence of models and hydrogen-deformation interactions in steels. HELP - hydrogen-enhanced local plasticity; AIDE - adsorption-induced dislocation emission; HEDE - hydrogen-enhanced decohesion; HESIV - hydrogen-enhanced strain-induced vacancy; HIPT - hy- drogen-induced phase transformation.
HE models, the coexistence of models and hydrogen-deformation interactions in steels. HELP - hydrogen-enhanced local plasticity; AIDE - adsorption-induced dislocation emission; HEDE - hydrogen-enhanced decohesion; HESIV - hydrogen-enhanced strain-induced vacancy; HIPT - hy- drogen-induced phase transformation.
Fig. 3. Different hydrogen trapping sites in steels [25]. “Reprinted from Mater Sci Technol, 33/13, Koyama M, Rohwerder M, Tasan CC, Bashir A. Akiyama E, Takai K, et al., Recent progress in microstructural hydrogen mapping in steels: quantification, kinetic analysis, and multi-scale char. acterisation, 1481-1496, Copyright, The Authors (2017), Under a creative commons license CC BY-NC-ND 4.0”. a aa a I a, ie eee The last Section (3) provides a summary of published results and findings of the synergy of hydrogen-enhanced plasticity and decohesion in steels and iron. It also proposes a unified model for the synergistic interplay of HE mechanisms (HELP + HEDE and HELP mediated HEDE) in steels (HELP + HEDE model). The unified HELP + HEDE model represents the proposal for the specific microstructural features/hydrogen trapping sites together with the corresponding fracture modes induced due to an activity of a particular HE mechanism (plasticity-mediated HE models and/or HEDE). The synergistic action and interplay of HE mechanisms in total four different grades of steels were also analyzed and summarized with respect to the applied hydrogen charging methods and mechanical testing methods. Lastly, it outlines the major conclusions and recommendations for the future necessary research related to the synergistic action and interplay of HE mechanisms in steels and iron.
Fig. 3. Different hydrogen trapping sites in steels [25]. “Reprinted from Mater Sci Technol, 33/13, Koyama M, Rohwerder M, Tasan CC, Bashir A. Akiyama E, Takai K, et al., Recent progress in microstructural hydrogen mapping in steels: quantification, kinetic analysis, and multi-scale char. acterisation, 1481-1496, Copyright, The Authors (2017), Under a creative commons license CC BY-NC-ND 4.0”. a aa a I a, ie eee The last Section (3) provides a summary of published results and findings of the synergy of hydrogen-enhanced plasticity and decohesion in steels and iron. It also proposes a unified model for the synergistic interplay of HE mechanisms (HELP + HEDE and HELP mediated HEDE) in steels (HELP + HEDE model). The unified HELP + HEDE model represents the proposal for the specific microstructural features/hydrogen trapping sites together with the corresponding fracture modes induced due to an activity of a particular HE mechanism (plasticity-mediated HE models and/or HEDE). The synergistic action and interplay of HE mechanisms in total four different grades of steels were also analyzed and summarized with respect to the applied hydrogen charging methods and mechanical testing methods. Lastly, it outlines the major conclusions and recommendations for the future necessary research related to the synergistic action and interplay of HE mechanisms in steels and iron.
Fig. 4. IG failure in iron due to hydrogen. The IG crack propagates in the vicinity and only follows the contour of the grain boundary [167]. “Reprinted from Eng. Fract. Mech., 68/6, Robertson IM, The effect of hydrogen on dislocation dynamics, 671-692, Copyright (2001), with per- mission from Elsevier”. The HE phenomena at a grain boundary in steels and effects on the crack nucleation, crack growth and plastic deformati mechanisms are strictly related to different but interrelated processes. The processes include: (i) interaction between the dislocati and the grain boundary including the slip transfer of dislocations across the grain boundary [188]; (ii) grain boundary slidir migration and twinning [189,190]; and (iii) ability for crack-tip dislocation emission, which leads to the localized plasticity, cra blunting and prevention of a completely cleavage-like separation [191]. Different mechanisms highlight the importance of plas deformation associated with hydrogen during the ductile to brittle fracture transition from the HELP-mediated ductile fracture to t macroscopically and seemingly completely brittle TG and IG fractures. They are based on the hydrogen-enhanced dislocation activ. and different hydrogen-dislocations interactions, as proposed by Robertson [167], Gavrilyuk et al. [192], Astafurova et al. [193] a others. One of the early research studies that provided a sound basis for some later formulation of the hydrogen-enhanced-plastici mediated decohesion model (HELP mediated HEDE model) is the paper by Novak et al. [32], Fig. 5. This paper on a statistical mic1 mechanical model of hydrogen-induced IG fracture in steels was published in 2010 and also listed in Table 2, together with oth
Fig. 4. IG failure in iron due to hydrogen. The IG crack propagates in the vicinity and only follows the contour of the grain boundary [167]. “Reprinted from Eng. Fract. Mech., 68/6, Robertson IM, The effect of hydrogen on dislocation dynamics, 671-692, Copyright (2001), with per- mission from Elsevier”. The HE phenomena at a grain boundary in steels and effects on the crack nucleation, crack growth and plastic deformati mechanisms are strictly related to different but interrelated processes. The processes include: (i) interaction between the dislocati and the grain boundary including the slip transfer of dislocations across the grain boundary [188]; (ii) grain boundary slidir migration and twinning [189,190]; and (iii) ability for crack-tip dislocation emission, which leads to the localized plasticity, cra blunting and prevention of a completely cleavage-like separation [191]. Different mechanisms highlight the importance of plas deformation associated with hydrogen during the ductile to brittle fracture transition from the HELP-mediated ductile fracture to t macroscopically and seemingly completely brittle TG and IG fractures. They are based on the hydrogen-enhanced dislocation activ. and different hydrogen-dislocations interactions, as proposed by Robertson [167], Gavrilyuk et al. [192], Astafurova et al. [193] a others. One of the early research studies that provided a sound basis for some later formulation of the hydrogen-enhanced-plastici mediated decohesion model (HELP mediated HEDE model) is the paper by Novak et al. [32], Fig. 5. This paper on a statistical mic1 mechanical model of hydrogen-induced IG fracture in steels was published in 2010 and also listed in Table 2, together with oth
Fig. 5. “Dislocation pile-up induced decohesion” in steels: (a) SEM micrograph - strain-controlled ductile fracture associated with MCV initiating at the notch in a non-charged sample; (b) SEM micrograph - stress-controlled brittle fracture associated with IG cracking initiating ahead of the notch in a 138 MPa hydrogen-charged sample; (c) Mechanistic steps in the generation of IG fracture in the presence of hydrogen and “dislocation pile-up induced decohesion” activity at a carbide/matrix interface (hydrogen atoms are denoted by colored circles: blue for hydrogen carried by the dislocations, green for hydrogen trapped at the carbide, red for hydrogen at normal interstitial lattice sites) [32]. “Reprinted and adapted from J. Mech. Phys. Solids, 58/2, Novak P, Yuan R, Somerday BP, Sofronis P, Ritchie RO, A statistical, physical-based, micro-mechanical model of hy- drogen-induced intergranular fracture in steel, 206-226, Copyright (2010), with permission from Elsevier” (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. “Dislocation pile-up induced decohesion” in steels: (a) SEM micrograph - strain-controlled ductile fracture associated with MCV initiating at the notch in a non-charged sample; (b) SEM micrograph - stress-controlled brittle fracture associated with IG cracking initiating ahead of the notch in a 138 MPa hydrogen-charged sample; (c) Mechanistic steps in the generation of IG fracture in the presence of hydrogen and “dislocation pile-up induced decohesion” activity at a carbide/matrix interface (hydrogen atoms are denoted by colored circles: blue for hydrogen carried by the dislocations, green for hydrogen trapped at the carbide, red for hydrogen at normal interstitial lattice sites) [32]. “Reprinted and adapted from J. Mech. Phys. Solids, 58/2, Novak P, Yuan R, Somerday BP, Sofronis P, Ritchie RO, A statistical, physical-based, micro-mechanical model of hy- drogen-induced intergranular fracture in steel, 206-226, Copyright (2010), with permission from Elsevier” (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. HELP mediated HEDE model in steels: (a) SEM micrograph of the fracture surface of the specimen: “flat” IG and “QC” features; (b, c) TEM micrographs of the microstructure immediately beneath the hydrogen-induced IG fracture surface; (d, e) TEM micrographs of the microstructure immediately beneath the hydrogen-induced “QC” fracture surface (Arrows and arrowheads in (b, d) indicate slip bands and lath boundaries, respectively; tracings in (c, e) include slip bands (- - -) and lath boundaries (-)) [196]. “Reprinted from Procedia Mater. Sci., 3, Nagao A, Smith CD, Dadfarnia M, Sofronis P, Robertson IM, Interpretation of hydrogen-induced fracture surface morphologies for lath martensitic steel, 1700-1705, Copyright, The Authors (2014), Open access funded by European Structural Integrity Society under a creative commons license CC BY-NC-ND 4.0”. These findings were interpreted as the synergistic interplay of the HELP and HEDE mechanisms. According to the proposed “dislocation pile-up induced decohesion” model, activation of the HEDE at the carbide-matrix interface and a brittle IG fracture has occurred after the impingement on the carbide-matrix interface of high-density dislocations in pile-ups [32]. The model is based on
Fig. 6. HELP mediated HEDE model in steels: (a) SEM micrograph of the fracture surface of the specimen: “flat” IG and “QC” features; (b, c) TEM micrographs of the microstructure immediately beneath the hydrogen-induced IG fracture surface; (d, e) TEM micrographs of the microstructure immediately beneath the hydrogen-induced “QC” fracture surface (Arrows and arrowheads in (b, d) indicate slip bands and lath boundaries, respectively; tracings in (c, e) include slip bands (- - -) and lath boundaries (-)) [196]. “Reprinted from Procedia Mater. Sci., 3, Nagao A, Smith CD, Dadfarnia M, Sofronis P, Robertson IM, Interpretation of hydrogen-induced fracture surface morphologies for lath martensitic steel, 1700-1705, Copyright, The Authors (2014), Open access funded by European Structural Integrity Society under a creative commons license CC BY-NC-ND 4.0”. These findings were interpreted as the synergistic interplay of the HELP and HEDE mechanisms. According to the proposed “dislocation pile-up induced decohesion” model, activation of the HEDE at the carbide-matrix interface and a brittle IG fracture has occurred after the impingement on the carbide-matrix interface of high-density dislocations in pile-ups [32]. The model is based on
Fig. 7. HELP mediated HEDE model in steels. The previous activity of hydrogen-enhanced localized plasticity (HELP) is responsible for the acti- vation of a decohesion (HEDE) mechanism and the generation of hydrogen-induced IG and “QC” fracture features in a lath martensitic steel [145]. “Reprinted from J. Mech. Phys. Solids, 112, Nagao A, Dadfarnia M, Somerday BP, Sofronis P, Ritchie RO, Hydrogen-enhanced-plasticity mediated decohesion for hydrogen-induced intergranular and ”quasi-cleavage“ fracture of lath martensitic steels, 403-430, Copyright (2018), with permission from Elsevier”. Me ee: CA Me, SES, VE ee Oe ee B ie td £4tsi’isn GLLivativile Numerous modern experiments also used the FIB microscope and machining [194]. They revealed the microstructure that has evolved beneath different types of the macroscopically ductile and brittle fracture modes (MVC, QC, C, and IG) [23,24]. These experiments have provided more insights about different fracture pathways found on the hydrogen-induced fracture surfaces in different metallic materials, including iron [154,195], low carbon pipeline steels [42,43,164] and lath martensitic steels [33,145]. The most recent fatigue crack propagation study in a-iron after gaseous hydrogen charging [173] and molecular dynamics calcu- lations of hydrogen effects on the dislocation-grain boundary (GB) interactions [188] provides additional insights. These authors observed that the brittle hydrogen-assisted fracture processes and transition in fracture modes from MVC to TG, QC, and IG are always followed by micro-plasticity behavior (plastic strain localization). Such observations seem to be in agreement with the hy- drogen-enhanced-plasticity-mediated decohesion model (HELP mediated HEDE model) [154,195]. Beneath the “QC” and IG fracture surfaces of lath martensitic steel specimens, Fig. 6a, an increased dislocation density was detected, Fig. 6b-e [196]. Specimens were fractured using a four-point bend test after hydrogen charging in a high-pressure gaseous hydrogen environment (138 MPa) at elevated temperature (250 °C) for 21 days [196]. The interpretation of this finding was linked to increased activity of plasticity- mediated processes (HELP mechanism activity) [33,42,43,196]. Nagao et al. [33] assumed that TEM micrographs revealed the previous activities of the HELP mechanism detected in the microstructure immediately beneath the IG and “QC” fractures. The results of the HELP mechanism activity is the refinement in the dislocation cell structure and extensive plasticity in the form of intense slip bands [33,42,43,196]. The slip bands are in the “IG” fracture case approximately parallel to the longitudinal direction of laths, Fig. 6b and c, while the intersection between the slip bands and the interface is inclined to the prior austenite grain boundary. In the case of “QC” fracture, slip bands are inclined to the longitudinal direction of laths and the intersection is inclined with respect to the lath boundaries, Fig. 6d and e [196]. However, Lynch [197] has questioned such experimental TEM observations of dislocation structures beneath transgranular cleavage-like fracture surfaces [43] and the overall conclusion about the activity of only HELP mechanism. He further pointed out that the relative importance of AIDE, HEDE and HELP is likely to depend on the fracture mode [9,17] and that the TEM observations make it impossible to distinguish between the AIDE and HELP mechanisms and their relative contributions [197].
Fig. 7. HELP mediated HEDE model in steels. The previous activity of hydrogen-enhanced localized plasticity (HELP) is responsible for the acti- vation of a decohesion (HEDE) mechanism and the generation of hydrogen-induced IG and “QC” fracture features in a lath martensitic steel [145]. “Reprinted from J. Mech. Phys. Solids, 112, Nagao A, Dadfarnia M, Somerday BP, Sofronis P, Ritchie RO, Hydrogen-enhanced-plasticity mediated decohesion for hydrogen-induced intergranular and ”quasi-cleavage“ fracture of lath martensitic steels, 403-430, Copyright (2018), with permission from Elsevier”. Me ee: CA Me, SES, VE ee Oe ee B ie td £4tsi’isn GLLivativile Numerous modern experiments also used the FIB microscope and machining [194]. They revealed the microstructure that has evolved beneath different types of the macroscopically ductile and brittle fracture modes (MVC, QC, C, and IG) [23,24]. These experiments have provided more insights about different fracture pathways found on the hydrogen-induced fracture surfaces in different metallic materials, including iron [154,195], low carbon pipeline steels [42,43,164] and lath martensitic steels [33,145]. The most recent fatigue crack propagation study in a-iron after gaseous hydrogen charging [173] and molecular dynamics calcu- lations of hydrogen effects on the dislocation-grain boundary (GB) interactions [188] provides additional insights. These authors observed that the brittle hydrogen-assisted fracture processes and transition in fracture modes from MVC to TG, QC, and IG are always followed by micro-plasticity behavior (plastic strain localization). Such observations seem to be in agreement with the hy- drogen-enhanced-plasticity-mediated decohesion model (HELP mediated HEDE model) [154,195]. Beneath the “QC” and IG fracture surfaces of lath martensitic steel specimens, Fig. 6a, an increased dislocation density was detected, Fig. 6b-e [196]. Specimens were fractured using a four-point bend test after hydrogen charging in a high-pressure gaseous hydrogen environment (138 MPa) at elevated temperature (250 °C) for 21 days [196]. The interpretation of this finding was linked to increased activity of plasticity- mediated processes (HELP mechanism activity) [33,42,43,196]. Nagao et al. [33] assumed that TEM micrographs revealed the previous activities of the HELP mechanism detected in the microstructure immediately beneath the IG and “QC” fractures. The results of the HELP mechanism activity is the refinement in the dislocation cell structure and extensive plasticity in the form of intense slip bands [33,42,43,196]. The slip bands are in the “IG” fracture case approximately parallel to the longitudinal direction of laths, Fig. 6b and c, while the intersection between the slip bands and the interface is inclined to the prior austenite grain boundary. In the case of “QC” fracture, slip bands are inclined to the longitudinal direction of laths and the intersection is inclined with respect to the lath boundaries, Fig. 6d and e [196]. However, Lynch [197] has questioned such experimental TEM observations of dislocation structures beneath transgranular cleavage-like fracture surfaces [43] and the overall conclusion about the activity of only HELP mechanism. He further pointed out that the relative importance of AIDE, HEDE and HELP is likely to depend on the fracture mode [9,17] and that the TEM observations make it impossible to distinguish between the AIDE and HELP mechanisms and their relative contributions [197].
More recently, Wan et al. [166] have proposed a “hybrid” HE model based on the dislocation-grain boundary reaction by dis- location impingement/emission on the grain boundary. According to this model, a locally activated state of the GB is the trigger mechanism for the transition from the plasticity-mediated “QC” and IG fracture modes at lower hydrogen concentrations to the macroscopically pure brittle IG mode at higher hydrogen concentrations. The transition is followed by a negligible sign of previous dislocation-driven plasticity in a-iron. They further report that at lower hydrogen concentrations the dislocation activities nearby the grain boundary are required to trigger decohesion and “QC” and IG fractures. This conclusion at lower hydrogen concentrations complies with the HELP mediated HEDE model. Contrary, at a very high hydrogen concentration fewer dislocation activities nearby Fig. 8. HELP + HEDE model: Schematic of the transition of the HE mechanism, HELP (HESIV) - HEDE (HESIV), for different hydrogen con- centrations and applied stress conditions [127]. “Reprinted from ISIJ Int., 57/11, Taketomi S, Matsumoto R, Hagihara S, Molecular statics simu- lation of the effect of hydrogen concentration on {112} <1 11} edge dislocation mobility in alpha iron, 2058-2064, Copyright (2017), with permission from the Iron and Steel Institute of Japan”.
More recently, Wan et al. [166] have proposed a “hybrid” HE model based on the dislocation-grain boundary reaction by dis- location impingement/emission on the grain boundary. According to this model, a locally activated state of the GB is the trigger mechanism for the transition from the plasticity-mediated “QC” and IG fracture modes at lower hydrogen concentrations to the macroscopically pure brittle IG mode at higher hydrogen concentrations. The transition is followed by a negligible sign of previous dislocation-driven plasticity in a-iron. They further report that at lower hydrogen concentrations the dislocation activities nearby the grain boundary are required to trigger decohesion and “QC” and IG fractures. This conclusion at lower hydrogen concentrations complies with the HELP mediated HEDE model. Contrary, at a very high hydrogen concentration fewer dislocation activities nearby Fig. 8. HELP + HEDE model: Schematic of the transition of the HE mechanism, HELP (HESIV) - HEDE (HESIV), for different hydrogen con- centrations and applied stress conditions [127]. “Reprinted from ISIJ Int., 57/11, Taketomi S, Matsumoto R, Hagihara S, Molecular statics simu- lation of the effect of hydrogen concentration on {112} <1 11} edge dislocation mobility in alpha iron, 2058-2064, Copyright (2017), with permission from the Iron and Steel Institute of Japan”.
Fig. 9. The sharp drop in various mechanical characteristics and ductility in steels upon local reaching a critical hydrogen concentration an activation of the HEDE mechanism of HE. It is important to make distinctions between the critical hydrogen concentration defined in different HE studies as (i) a global (total) or average hydrogen concentration in the specimen; (ii) local hydrogen concentration at the trap site that causes cracking; or (ii) local hydrogen concentration at a crack tip or a notch root. The measurements of the local critical hydrogen at a crack tip are very difficult to conduct [26]. In most of the mentioned HE studies the term “critical hydrogen concentration” is related to the lattice (global) hydrogen concentration or the calculated (local) hydrogen content at the trap site. In both cases, the critical hydrogen concentration depends strongly on the material system, experimental conditions, as well as the stress/strain level [204-208,224-231]. It seems that the critical hydrogen concentration is mostly related to a hydrogen content at the crack initiation site [231]. However, there is still disagreement about the true meaning of the critical hydrogen concentration value at the global and local levels [13,36,112,224—231]. It is still very difficult to theoretically-numerically calculate or experimentally measure the local critical hydrogen concentration, particularly at the crack tip [231]. Also, the same applies to the modeling of hydrogen diffusion, hydrogen distribution at a crack tip [141] and validation of crack-tip mechanics under the simultaneous action of HE mechanisms (plasticity-enhanced models + HEDE). Wang et al. [231] pointed out that it is necessary to understand the stress-driven hydrogen
Fig. 9. The sharp drop in various mechanical characteristics and ductility in steels upon local reaching a critical hydrogen concentration an activation of the HEDE mechanism of HE. It is important to make distinctions between the critical hydrogen concentration defined in different HE studies as (i) a global (total) or average hydrogen concentration in the specimen; (ii) local hydrogen concentration at the trap site that causes cracking; or (ii) local hydrogen concentration at a crack tip or a notch root. The measurements of the local critical hydrogen at a crack tip are very difficult to conduct [26]. In most of the mentioned HE studies the term “critical hydrogen concentration” is related to the lattice (global) hydrogen concentration or the calculated (local) hydrogen content at the trap site. In both cases, the critical hydrogen concentration depends strongly on the material system, experimental conditions, as well as the stress/strain level [204-208,224-231]. It seems that the critical hydrogen concentration is mostly related to a hydrogen content at the crack initiation site [231]. However, there is still disagreement about the true meaning of the critical hydrogen concentration value at the global and local levels [13,36,112,224—231]. It is still very difficult to theoretically-numerically calculate or experimentally measure the local critical hydrogen concentration, particularly at the crack tip [231]. Also, the same applies to the modeling of hydrogen diffusion, hydrogen distribution at a crack tip [141] and validation of crack-tip mechanics under the simultaneous action of HE mechanisms (plasticity-enhanced models + HEDE). Wang et al. [231] pointed out that it is necessary to understand the stress-driven hydrogen
Fig. 10. HE susceptibility of advanced high-strength steels and the effect of critical hydrogen concentration: (a) so-called embrittlement curve, i.e. the strength vs hydrogen concentration for the advanced high-strength steel (the critical hydrogen concentration is about 4 wppm); Micrographs of fracture surface of: (b) uncharged specimen; (c) sample charged at 2.6 wppm (lower than the critical hydrogen concentration); (d) sample charged at 7.1 wppm (higher than the critical hydrogen concentration) [230]. “Reprinted from Metall. Mater. Trans. A, 43/11, Lovicu G, Bottazzi M, D’Aiuto F, De Sanctis M, Dimatteo A, Santus C, et al., Hydrogen embrittlement of automotive advanced high-strength steels, 4075-4087, Copyright (2012), with permission from Springer”. OS Se ee ee ee ee EL lhe eee eee Se ee ee ee ee The initial hydrogen content of 1.5 ppm exhibits a fully ductile (D) - MVC fracture mode with very fine dimple sizes. It change: after charging with 5 ppm hydrogen to the mixed TG and IG mode with a few ductile MVC islands, the cracks propagating through the prior austenite grain boundaries (PAGB), as well as the martensite lath. However, with a further increase of the hydrogen conten above the critical concentration, the TG and IG fractures are more prominent and the decohesion (HEDE) occurs mostly along the PAGBs, Fig. 11b [161]. In the previously explained examples (see Figs. 10a and 11a), the phenomenon of decohesion and activatior of the HEDE mechanism of HE occurs when the critical value of hydrogen content is reached. However, the possible previous effect: of the HELP mechanism at a lower hydrogen content, or the synergy between HELP and HEDE mechanism (HELP mediated HEDE anc HELP + HEDE models) have not been considered and further analyzed. The correlations between mixed fracture modes and active
Fig. 10. HE susceptibility of advanced high-strength steels and the effect of critical hydrogen concentration: (a) so-called embrittlement curve, i.e. the strength vs hydrogen concentration for the advanced high-strength steel (the critical hydrogen concentration is about 4 wppm); Micrographs of fracture surface of: (b) uncharged specimen; (c) sample charged at 2.6 wppm (lower than the critical hydrogen concentration); (d) sample charged at 7.1 wppm (higher than the critical hydrogen concentration) [230]. “Reprinted from Metall. Mater. Trans. A, 43/11, Lovicu G, Bottazzi M, D’Aiuto F, De Sanctis M, Dimatteo A, Santus C, et al., Hydrogen embrittlement of automotive advanced high-strength steels, 4075-4087, Copyright (2012), with permission from Springer”. OS Se ee ee ee ee EL lhe eee eee Se ee ee ee ee The initial hydrogen content of 1.5 ppm exhibits a fully ductile (D) - MVC fracture mode with very fine dimple sizes. It change: after charging with 5 ppm hydrogen to the mixed TG and IG mode with a few ductile MVC islands, the cracks propagating through the prior austenite grain boundaries (PAGB), as well as the martensite lath. However, with a further increase of the hydrogen conten above the critical concentration, the TG and IG fractures are more prominent and the decohesion (HEDE) occurs mostly along the PAGBs, Fig. 11b [161]. In the previously explained examples (see Figs. 10a and 11a), the phenomenon of decohesion and activatior of the HEDE mechanism of HE occurs when the critical value of hydrogen content is reached. However, the possible previous effect: of the HELP mechanism at a lower hydrogen content, or the synergy between HELP and HEDE mechanism (HELP mediated HEDE anc HELP + HEDE models) have not been considered and further analyzed. The correlations between mixed fracture modes and active
Fig. 11. Hydrogen-induced cracking in a martensitic stainless steel: (a) evolution of hydrogen contents and the fracture strength according to the hydrogen charging time determined by the SSRT test; (b) fracture surfaces of specimens with different amounts of pre-charged hydrogen [161]. “Reprinted from Metals, 3, Hiiter C, Shanthraj P, McEniry E, Spatschek R, Hickel T, Tehranchi A, et al., Multiscale modelling of hydrogen transport and segregation in polycrystalline steels, 430., Copyright, The Authors (2018), Open access under a creative commons license CC BY 4.0”.
Fig. 11. Hydrogen-induced cracking in a martensitic stainless steel: (a) evolution of hydrogen contents and the fracture strength according to the hydrogen charging time determined by the SSRT test; (b) fracture surfaces of specimens with different amounts of pre-charged hydrogen [161]. “Reprinted from Metals, 3, Hiiter C, Shanthraj P, McEniry E, Spatschek R, Hickel T, Tehranchi A, et al., Multiscale modelling of hydrogen transport and segregation in polycrystalline steels, 430., Copyright, The Authors (2018), Open access under a creative commons license CC BY 4.0”.
Legend: HELP/(+)HEDE’, HELP mediated HEDE? - see text for explanation; EI - electrochemical in-situ (during operation) charging; EC - electrochemical charging; G - gasous charging; I - immersion charging method; MnS - manganese sulfide inclusions; Ma - martensite/martensitic; PAGB - prior austenite grain boundaries; T - tensile test; IT - Charpy impact test; FG - fatigue crack growth test; FT- fracture toughness test; SSRT - slow strain rate tensile test; FPB - four-point bending test; IPB - in-situ bending tests; TeS - tearing and shearing fracture; MVC, QC, C, TG, IG - see text for explanations; PRHIC - plasticity related hydrogen induced cracking. HELP + HEDE and HELP mediated HEDE models, microstructural features, hydrogen trapping sites and corresponding fracture modes.
Legend: HELP/(+)HEDE’, HELP mediated HEDE? - see text for explanation; EI - electrochemical in-situ (during operation) charging; EC - electrochemical charging; G - gasous charging; I - immersion charging method; MnS - manganese sulfide inclusions; Ma - martensite/martensitic; PAGB - prior austenite grain boundaries; T - tensile test; IT - Charpy impact test; FG - fatigue crack growth test; FT- fracture toughness test; SSRT - slow strain rate tensile test; FPB - four-point bending test; IPB - in-situ bending tests; TeS - tearing and shearing fracture; MVC, QC, C, TG, IG - see text for explanations; PRHIC - plasticity related hydrogen induced cracking. HELP + HEDE and HELP mediated HEDE models, microstructural features, hydrogen trapping sites and corresponding fracture modes.
Legend: H - hydrogen; EI - electrochemical in-situ (during operation) charging; EC - electrochemical charging; G - gaseous charging; I - immersion charging; MnS - manganese sulfide inclusions; F - ferrite; P - pearlite; F/P - ferrite/pearlite; B - bainite; A - austenite; Ma - martensite; Ce - cementite; PAGB - prior austenite grain boundaries; GBs - grain boundaries; T - tensile test; IT - Charpy impact test; FG - fatigue crack growth test; FT- fracture toughness test; SSRT - slow strain rate tensile test; FPB - four-point bending test; IPB - in-situ bending tests; TeS - tearing and shearing fracture; MVC, QC, C, TG, IG - see text for explanations; PRHIC - plasticity related hydrogen induced. The background for a unified model of synergistic interplay of HE mechanisms in steels: localized plasticity and decohesion (HELP + HEDE and HELP mediated HEDE models).
Legend: H - hydrogen; EI - electrochemical in-situ (during operation) charging; EC - electrochemical charging; G - gaseous charging; I - immersion charging; MnS - manganese sulfide inclusions; F - ferrite; P - pearlite; F/P - ferrite/pearlite; B - bainite; A - austenite; Ma - martensite; Ce - cementite; PAGB - prior austenite grain boundaries; GBs - grain boundaries; T - tensile test; IT - Charpy impact test; FG - fatigue crack growth test; FT- fracture toughness test; SSRT - slow strain rate tensile test; FPB - four-point bending test; IPB - in-situ bending tests; TeS - tearing and shearing fracture; MVC, QC, C, TG, IG - see text for explanations; PRHIC - plasticity related hydrogen induced. The background for a unified model of synergistic interplay of HE mechanisms in steels: localized plasticity and decohesion (HELP + HEDE and HELP mediated HEDE models).
Fig. 12. The special multiscale approach for investigation of the coexistence of the HELP and HEDE mechanisms (HELP + HEDE model [34]): (1) Industrial component, thermal power plant -TPP boiler tube; (2) Hydrogen damage of boiler evaporator tube unevenly saturated with hydrogen during exploitation; (3) Application of a structural integrity model-correlation between material macromechanical behavior and simultaneously active HE micro-mechanisms; (4) Assessment of hydrogen critical concentrations; (5) Predictive maintenance activities [36,112]. “Reprinted from Proc. Struct. Integr., 2, Djukic MB, Bakic GM, Sijacki Zeravcic V, Rajicic B, Sedmak A, Mitrovic R, et al., Towards a unified and practical industrial model for prediction of hydrogen embrittlement and damage in steels, 604-611, Copyright, The Authors (2016), Open access funded by European Structural Integrity Society under a creative commons license CC BY-NC-ND 4.0”.
Fig. 12. The special multiscale approach for investigation of the coexistence of the HELP and HEDE mechanisms (HELP + HEDE model [34]): (1) Industrial component, thermal power plant -TPP boiler tube; (2) Hydrogen damage of boiler evaporator tube unevenly saturated with hydrogen during exploitation; (3) Application of a structural integrity model-correlation between material macromechanical behavior and simultaneously active HE micro-mechanisms; (4) Assessment of hydrogen critical concentrations; (5) Predictive maintenance activities [36,112]. “Reprinted from Proc. Struct. Integr., 2, Djukic MB, Bakic GM, Sijacki Zeravcic V, Rajicic B, Sedmak A, Mitrovic R, et al., Towards a unified and practical industrial model for prediction of hydrogen embrittlement and damage in steels, 604-611, Copyright, The Authors (2016), Open access funded by European Structural Integrity Society under a creative commons license CC BY-NC-ND 4.0”.
Fig. 13. The special model for a structural integrity analysis and prediction of HE in industrial components based on the HELP + HEDE model, HELP (HELP > HEDE) and HEDE (HEDE > HELP) dominance [13,36]: Variation of impact strength (KCVyor.), and its crack propagation energy component (KCVp), and crack intiation energy component (KCV)) as a function of the specimen hardness (hydrogen concentration). Zone 1 - “Safety” (KCVror.qeasured) > KCVror.oy), Zone 2 - “Critical” (KCVror.@) > KCVror.measurea) > KCVror.criticay), and Zone 3 - KCVror.cmeasurea) < KCVrorcritica) and/or KCVp <= KCVj. Ductile to brittle fracture (DBT) transition; two characteristic values of hydrogen concentrations in a steel are Cryo) and Crcriticar- Adapted from [36]. “Reprinted and adapted from Eng. Fail. Anal., 58, Djukic MB, Sijacki Zeravcic V, Bakic GM, Sedmak A, Rajicic B., Hydrogen damage of steels: A case study and hydrogen embrittlement model, 485-498, Copyright (2015), with permission from Else- vier”.
Fig. 13. The special model for a structural integrity analysis and prediction of HE in industrial components based on the HELP + HEDE model, HELP (HELP > HEDE) and HEDE (HEDE > HELP) dominance [13,36]: Variation of impact strength (KCVyor.), and its crack propagation energy component (KCVp), and crack intiation energy component (KCV)) as a function of the specimen hardness (hydrogen concentration). Zone 1 - “Safety” (KCVror.qeasured) > KCVror.oy), Zone 2 - “Critical” (KCVror.@) > KCVror.measurea) > KCVror.criticay), and Zone 3 - KCVror.cmeasurea) < KCVrorcritica) and/or KCVp <= KCVj. Ductile to brittle fracture (DBT) transition; two characteristic values of hydrogen concentrations in a steel are Cryo) and Crcriticar- Adapted from [36]. “Reprinted and adapted from Eng. Fail. Anal., 58, Djukic MB, Sijacki Zeravcic V, Bakic GM, Sedmak A, Rajicic B., Hydrogen damage of steels: A case study and hydrogen embrittlement model, 485-498, Copyright (2015), with permission from Else- vier”.
Fig. 14. HELP + HEDE model in steels: Variation of impact strength (KCVyor,) and its crack propagation component (KCVp) and crack initiation component (KCV)) of Charpy specimens (S1-S6), as a function of the specimens hardness (hydrogen concentration); (b-f) SEM fractographs of the fracture surfaces of Charpy specimens (S2-S5) [36]. “Reprinted and adapted from Eng. Fail. Anal., 58, Djukic MB, Sijacki Zeravcic V, Bakic GM, Sedmak A, Rajicic B., Hydrogen damage of steels: A case study and hydrogen embrittlement model, 485-498, Copyright (2015), with permission from Elsevier”.
Fig. 14. HELP + HEDE model in steels: Variation of impact strength (KCVyor,) and its crack propagation component (KCVp) and crack initiation component (KCV)) of Charpy specimens (S1-S6), as a function of the specimens hardness (hydrogen concentration); (b-f) SEM fractographs of the fracture surfaces of Charpy specimens (S2-S5) [36]. “Reprinted and adapted from Eng. Fail. Anal., 58, Djukic MB, Sijacki Zeravcic V, Bakic GM, Sedmak A, Rajicic B., Hydrogen damage of steels: A case study and hydrogen embrittlement model, 485-498, Copyright (2015), with permission from Elsevier”.
Fig. 15. Summary of the unified model for synergistic interplay of HE mechanisms in steels: localized plasticity and decohesion (HELP + HED model). nee see a, © eee a ee The indicative example of a specific microstructure and trapping sites and fracture modes affected by the dominance of HELP or HEDE or both mechanisms, presented in Table 3, looks like: “The microstructural features and H trapping sites”: (pearlite/ferrite, MnsS, pearlite-ferrite interfaces)! ° 7 “Fracture modes due to”: (MVC/MVG, QC, C, cracks)! ° 7. The explanation for this indicative example is that findings are in accordance with the ' - HELP + HEDE model and the ” - HELP mediated HEDE model. Further, the microstructure affected by HELP is pearlite; microstructures affected by HEDE are ferrite, MnS, and pearlite-ferrite interfaces; the fracture mode due to the HELP activity is MVC; and the fracture modes due to the HEDE activity are MVC, QC, C, and cracks.
Fig. 15. Summary of the unified model for synergistic interplay of HE mechanisms in steels: localized plasticity and decohesion (HELP + HED model). nee see a, © eee a ee The indicative example of a specific microstructure and trapping sites and fracture modes affected by the dominance of HELP or HEDE or both mechanisms, presented in Table 3, looks like: “The microstructural features and H trapping sites”: (pearlite/ferrite, MnsS, pearlite-ferrite interfaces)! ° 7 “Fracture modes due to”: (MVC/MVG, QC, C, cracks)! ° 7. The explanation for this indicative example is that findings are in accordance with the ' - HELP + HEDE model and the ” - HELP mediated HEDE model. Further, the microstructure affected by HELP is pearlite; microstructures affected by HEDE are ferrite, MnS, and pearlite-ferrite interfaces; the fracture mode due to the HELP activity is MVC; and the fracture modes due to the HEDE activity are MVC, QC, C, and cracks.
Related topics:
Mechanical EngineeringMaterials ScienceMechanical Behavior Of MaterialsFracture MechanicsMetallurgy and Materials EngineeringMechanical Properties of MaterialsCast IronMetallurgical and Materials EngineeringMechanical Testing of MaterialsMaterials Characterization
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