Powder Metallurgy Progress, Vol.15 (2015), special issue 73

HYDROGEN SINGS IN THE FRACTURE OF T/P92 MARTENSITIC STEEL AFTER THE TENSILE AND NOTCH TOUGHNESS TESTS

A. Výrostková, L. Falat, P. Ševc

Abstract This contribution describes in detail the specific features of the fracture of T/P92 steel after hydrogen electrolytic charging. Static tensile test at room temperature and notch toughness test at temperatures 20, -30, -60°C were used to achieve the fractures. Reference samples without hydrogen were tested simultaneously. Signs of the brittle trans- and inter-crystalline fracture typical of T/P92 steel are even highlighted by mutual influence of the hydrogen and both kinds of testing methods. High measure of local plastic deformation is particularly expressive, what also supports the so called HELP hydrogen embrittlement mechanism. Keywords: 9Cr-Mo steel, hydrogen embrittlement, fracture, mechanical testing

INTRODUCTION Hydrogen in steels is known as a detrimental element leading to premature

fractures. From the time when the flakes in the large forgings and castings were recognized as a consequence of hydrogen presence in the steels, its behavior in materials generally is far better understood.

During the last century more mechanisms of hydrogen behaviour in materials were suggested, four of which are discussed as the most probable at present if we omit the creation of hydrides, blisters and flakes. These are: HEDE-hydrogen enhanced decohesion [1], HELP-hydrogen enhanced localized plasticity [1-3], AIDE- adsorption induced dislocation emission [4-7], and HESIVE- hydrogen enhanced strain induced vacancy [8, 9]. The last mentioned model supposes the voids formation as a consequence of vacation agglomeration during plastic deformation and not their nucleation at secondary particles as it is supposed in the case of AIDE mechanism. The voids either create clusters -microvoids or destabilise the area at the front of a growing crack. The HELP model is one of the most general and best elaborated mechanisms describing influence of hydrogen on the failure of materials. It supposes the hydrogen-dislocation interaction supporting the dislocation movement resulting in the reduction of stress at the yield point and dislocation slip at stresses lower than those required in case without hydrogen influence. This behaviour leads to the increased plasticity, however also to macroscopically brittle fracture. It should however be said, that in some cases more mechanisms together are needed to explain the observed embrittlement phenomena.

The paper describes some special features found out in the fractures after the Charpy impact and uni-axial tensile tests of the notched samples after the hydrogen charging in comparison with the not charged samples.

Anna Výrostková, Ladislav Falat, Peter Ševc, Institute of Material Research, Slovak Academy of Sciences, Košice, Slovakia

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MATERIAL AND METHODS The experimental material investigated in this study, P/T92 martensitic steel

(0.11C-0.38Si-0.49Mn-9Cr-1.6W-0.3Mo-0.33Ni-0.2V-0.056N-0.023B), belongs among the newer materials for power industry, where individual components are joined by means of fusion welding and could be endangered by hydrogen embrittlement (HE). To simulate the exploitation conditions, the material was annealed at 565°C up to 3000 h. In spite of the fact that this steel should be resistant to the HE, we decided to check this statement, because any material could in some conditions be prone to the HE.

Tensile test of the notched samples was used to judge the HE of the steel. The method used is based on the comparison of the properties with and without hydrogen, when the samples after electrolytic hydrogen charging and without charging are tested by uni-axial tensile test at the rate of the order of 10-5.s-1. Detailed information on the methods and results achieved can be found in the work [10]. In this paper we focus mainly on the special features observed in the fractures after the above mentioned tensile test and Charpy impact test of the half-size sampels with V notch performed in the AR state at room temperature, -30, and -60°C.

RESULTS AND DISCUSSION

Mechanical properties The results of the mechanical tests are plotted in Fig.1 and Fig.2.

Fig.1. Dependence of the yield stress and ultimate tensile stress on aging time.

Fig.2. Impact toughness of the samples with and without hydrogen in the AR state.

Mechanical properties decrease slowly with the time of annealing till about 1000 hours, when their sharp increasing starts in both states- with and without hydrogen. The first tendency could be explained by the secondary particle growth and substructure recovery processes, while the onset of intensive precipitation of Laves phase is responsible for the following strengthening effect. While the yield stress after hydrogen charging is higher than without hydrogen, the UTS is almost not influenced by hydrogen. Impact toughness values decrease with testing temperature, and after the hydrogen charging the values are lower in comparison to the samples without hydrogen. This can be described as a typical behaviour when the higher the strength the lower the toughness.

Fractography Macrographs of the samples after uni-axial tensile test at room temperature of the

as received and annealed states and the samples after Charpy -V notch test at 20, -30, and -

Powder Metallurgy Progress, Vol.15 (2015), special issue 75 60°C with and without hydrogen charging are given in Fig.3 and Fig.4. The fractures of the tensile test samples without hydrogen are ductile, with higher amount of plastic deformation. The influence of hydrogen is evident, the fractures are flat with brittle features.

Fig.3. Macrographs, tensile test, states in the order: AR, 565 °C/1000 h , 565 °C/3000 h

after hydrogen charging –upper row, and without hydrogen charging-lower row.

In case of impact test the fractures are more brittle at lower temperatures. Hydrogen influence on the impact test at room temperature did not manifest itself much, evident is a little bit smaller lateral expansion.

Fig.4. Macrographs of Charpy-V impact test at +20, -30, -60 °C, a,b,c - states after

hydrogen charging, d,e,f - without hydrogen charging.

Fig.5. Ductile fracture, AR state, a-tensile test, b, tensile test, H2, c-impact test, H2.

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Ductile part of the fractures consists of dimple mixture of bimodal distribution, Fig.5. In comparison to the fractures without hydrogen (Fig.5a), the dimples after hydrogen charging are larger, deeper, and dimples often engulf other smaller dimples to form the larger ones, Figs.5b, c. Macroscopically “wavy” ductile fracture contains also localities of intercrystalline decohesion, mainly in the form of the open cracks perpendicular to the fracture surface, Fig.5b, deep tearing crack, Fig.6a and/or uncovered intercrystalline cracks and facets, Fig.6b.

Fig.6. Tearing and intercrystalline cracks in samples without hydrogen charging.

After the hydrogen charging the fractures after the tensile tests are a mixture of ductile and brittle elements, i.e. dimples, and intercrystalline and quasicleavage facets, Fig.7. The size and number of intercrystalline facets increase with annealing time. Brittle quasi-cleavage fracture is typical for impact testing at lower temperatures, Fig.8. In spite of lower tempera-ture, the fracture in Fig.8b after hydrogen charging shows evidently higher amount of deformation than not charged sample at -30°C only. More detailed view shows that the mixture of quasicleavage and inter-crystalline facets is often interconnected with dimple ductile ridges, and very deep tearing and/or intercrystalline cracks Fig.9.

Fig.7. The fractures after the tensile test of hydrogen charged samples.

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Fig.8. Mixture of brittle elements in fractures after impact testing, a) -30°C, b) -60°C, H2.

Fig.9. Cracks and dimple ridges in the fractures after impact testing, charged samples.

There is a dense population of particles embedded at the grain boundaries, Fig.10. These were identified as large oval M23C6 carbides and Laves pase, that is visualized using the back scattered electron mode in Fig.10b. The particles grow with aging time leading to the weake-ning of grain boundaries and their opening at loading with and without charging, Fig.11.

Fig.10. Particles at the grain boundaries after 3000 h aging, a-SEI, b-BSEI.

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Fig.11. Intercrystalline fracture after the tensile test, a) 3000 h aging, H2, b) detail.

Fig.12. “Fish eyes” after a) 100 h aging and in b) AR state, charged samples.

Fig.13. Crack propagation from centre of a fish eye by, a) cleavage, b) intercrystalline

decohesion mechanisms.

Large BN particles and/or small intercrystalline facets perpedicular to the fracture surface are often the nucleli of so called “fish eyes”, Fig.12a crack propagation is evident in

Powder Metallurgy Progress, Vol.15 (2015), special issue 79 Fig.13, documenting quasicleavage and decohesion fracture mechanisms, Fig.13a and b, respectivelly.

Fig.14. Special signs in the quasicleavage fracture after the tensile tests, charged samples.

Also cleavage fractures of the charged samples show specific signs, Fig.14. These are mainly the fracture fragmentation (a), deep holes (b), viusualization of parallel lines marking slip planes (b, c), and large amount of plastic deformation (d). After impact testing the facets are larger, more straight, with more pronounced presence of deformation twins, Fig.15. The speed of the test does not allow formation of a fish eye around the large inclusion in Fig.15a. However the deep dimples and uncovered particles are visible similar to the tensile test.

Fig.15. Cleavage in fractures of impact test samples, a, b- charged, c- not charged.

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CONCLUSION Comparison of the fractures with and without hydrogen charging after two kinds

of testing can be summarised as follows: 1. Presence of hydrogen at testing results in the more or less flat fractures containig brittle

cleavage and intercrystalline facets combined with ductile dimple fracture. The amount of brittle part increases with aging time and temperature decreasing. The fractures are frag-men-ted, with high content of microscopical plastic deformation, secondary cracks, deep intercrystalline and tear cracks, and “fish eyes”. Typical for ductile part is a mixture of fine and rather flat large dimples, created by merging of more dimples, with a deep cen-tral hole. Quasi-cleavage facets often contain deep dimples and/or embedded particles, visible slip lines, and much of local plastic deformation, that is typical also for intercrys-talline facets with plenty of embedded particles and pulled holes. In not charged samples a lot of signs of deformation twins are present in cleavage facets after impact testing.

2. Yield stress is reduced in the presence of hydrogen by about 100 MPa up to 3000h of aging, while impact toughness at RT is almost not influenced by hydrogen.

Acknowledgement The work was done in the frame of the VEGA project no. 2/0116/13

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