As medium-strength steels are promising candidates for the hydrogen economy, it is important to understand their interaction with hydrogen. However, there are only a limited number of investigations on the behavior of medium-strength steels in hydrogen. The existing literature indicates that the influences of hydrogen on the tensile properties of medium-strength steels are mainly the following: (i) the steel can be hardened by hydrogen, as demonstrated by an increase in the yield stress or ultimate tensile stress; (ii) some steels can be embrittled by hydrogen, as revealed by lower yield stress or ultimate tensile stress; (iii) in most cases, these steels may experience hydrogen embrittlement (HE), as indicated by a reduction in ductility. The degree of HE mainly depends on the test conditions and the steel. The embrittlement can lead to catastrophic brittle fracture in service. The influence of hydrogen on the fatigue properties of medium-strength steels is dependent on many factors such as the stress ratio, temperature, yield stress of the steel, and test frequency. Generally, the hydrogen influence on fatigue limit is small, whereas hydrogen can accelerate the fatigue crack growth rate, leading to a shorter fatigue life. Inclusions are an important factor influencing the properties of medium-strength steels in the presence of hydrogen. However, it is not possible to predict the influence of hydrogen for any particular steel that has not been experimentally evaluated or to predict service performance. It is not known why similar steels can have different behavior, ranging from good resistance to significant embrittlement. A better understanding of the microstructural characteristics is needed.

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A critical review of the influence of hydrogen on the mechanical properties of medium-strength steels

An integrated overview is presented of a viewpoint on the present understanding of nucleation and growth mechanisms in both diffusional and shear (martensitic) transformations. Special emphasis is placed on the roles played by the anisotropy of interphase boundary structure and energy and also upon elastic shear strain energy in both types of transformation. Even though diffusional nucleation is based on random statistical fluctuations, use of the time reversal principle shows that interfacial energy anisotropy leads to accurately reproducible orientation relationships and hence to partially or fully coherent boundaries, even when nucleation at a grain boundary requires an irrational orientation relationship to obtain. Since the fully coherent boundary areas separating most linear misfit compensating defects are wholly immobile during diffusional growth because of the improbability of moving substitutional atoms even temporarily into interstitial sites under conditions normally encountered, partially and fully coherent interphase boundaries should be immovable without the intervention of growth ledges. These ledges, however, must be heavily kinked and usually irregular in both spacing and path if they, too, are not to be similarly trapped. On the other hand, the large shear strain energy usually associated with martensite requires that its formation be initiated through a process which avoids the activation barrier associated with nucleation, perhaps by the Olson-Cohen matrix dislocation rearrangement mechanism. During growth, certain ledges on martensite plates serve as transformation dislocations and perform the crystal structure change (Bain strain). However, the terraces between these ledges in martensite (unlike those present during diffusional growth) are also mobile during non-fcc/hcp transformations; glissile dislocations on these terraces perform the lattice invariant deformation. Growth ledges operative during both diffusional and shear growth probably migrate by means of kink mechanisms. However, diffusional kinks appear to be nonconservative and sessile (and therefore resist immediate transmission of elastic shear strain energy), whereas those associated with martensitic growth must be conservative and glissile (and fully transmit such strain energy). The broad faces of both diffusionally and martensitically formed plates contain an invariant line, as emphasized by Dahmen and Weatherly. However, in the diffusional case, minimization of growth ledge formation kinetics seems to be the main role thereby played, whereas in martensitic growth, the main purpose of such an interface is to minimize elastic shear strain energy. The latter minimization requires that martensite forms as plates (or perhaps as laths) enclosed by a pair of invariant line-containing interfaces. During diffusional transformations, on the other hand, other interfaces at which growth ledge formation kinetics are not too much faster than those at the invariant line interface can also comprise a significant portion of the interfacial area, thereby leading to the formation of other, quite different morphologies, such as intragranular idiomorphs and grain boundary allotriomorphs. Critical problems remaining unsolved in diffusional transformations include calculation of critical nucleus shapes when the crystal structures of the two phases are significantly different, highly accurate calculation of the energies of the interphase boundaries thus formed, and direct observation of atomic scale kinks on the risers of growth ledges by means of a yet-to-be-invented three-dimensional (3-D) atomic-resolution form of transmission electron microscopy. Experimental identification and characterization of transformation dislocations and experimental testing of “nucleation” mechanisms are now of special importance in fundamental studies of martensitic transformations.

Atomic mechanisms of diffusional nucleation and growth and comparisons with their counterparts in shear transformations

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fEnvironmental degradation of structural materials in liquid lead- and lead-bismuth eutectic-cooled reactors

Effect of crack closure and evaluation of the cyclic crack resistance of constructional alloys

On the occasion of the centenary of the Ukrainian National Academy of Sciences and 90th birthday of O. M. Romaniv, prominent Ukrainian scientist and active organizer of science and public figure, we analyze his contribution to the physicochemical mechanics of materials based on the structural fracture mechanics and mechanics of corrosion fracture of metals developed by Romaniv.

Structural and Corrosion Fracture Mechanics as Components of the Physicochemical Mechanics of Materials

Effect of structural factors on the kinetics of fatigue cracks in constructional steels

(230-mm diameter) with a center crack, rectangular (width 25 mm) with an edge crack for axial tensile testing, and rectangular compact (width 81 mm) for off-center tensile testing~ The line of action of the forces always coincided with the rolling direction and the through crack developed perpendicular to it. To obtain different structures, the disk samples were normalized or hardened in oil from 810~ and tempered at 180, 320, 380, 440 and 5800C. After heat treatment the samples were ground to a thickness of 3 mm. Data on the microstructure and the mechanical properties in short-term static tension of standard flat samples is shown on Table i. To study crack growth in steel of a given structure from one to four samples were used and in individual cases up to seven rectangular samples, which were cut from test disks. The tests were made using the known method [2, 3] in a machine with an inertial load exciter [4] in laboratory air with a relative humidity of 40-60% using an almost from-zero (R = 0.05) sinusoidal tensile cycle with a loading frequency of 16-20 Hz and at high crack growth rate (more than i0-6 m/cycle), because of the difficulty in measuring its length, also at a frequency of 0.017 Hz on a machine with rigid loading [5]. In the i0"-i0 -" m/cycles range of rates the tests were repeated at the two frequencies in order to be convinced of their weak influence on the results. The rate of development of the original crack in growth of it to an initial length of 2 mm from an initiating notch was always greater than 10-9 m/cycle despite the low loads and the use of various methods easing origin of it [6]. Therefore, for investigations of crack resistance at lower rates the load was reduced in steps with observance of existing recommendations [2, 7]. The sample was tested in stages during which all parameters of the load cycle were maintained constant. In each stage the range of the stress intensity factor ~K = KmaX-Kminin the disk samples was constant if the crack length did not exceed 95 mm and in the remalning cases increased with crack length. The rate of crack development v in the stages with a constant AK was calculated from five to eight successive measurements of its length using the method of least squares [2] while with an increasing AK the value of v was determined as the ratio of the increase in the crack between two successive measurements to the corresondlng number of cycles. For determination of the characteristics of cyclic crack resistance and an analytical description of the full fatigue failure curves, use was made of the equation [8]

Influence of structure on the resistance of 65G steel to fatigue crack growth

Kinetics and mechanism of corrosion-fatigue crack growth in ferritic-pearlitic class steels: Report I. High-strength steels with a martensitic structure

The authors study the rules of the influence of specimen thickness on fatigue crack kinetics and the threshold range of the stress intensity factor in plastic steels. Steel 10GN2MFA was used in the investigations.

Influence of the scale factor on the cyclic crack resistance of plastic steels in the low-amplitude area of loading

Techniques and results are presented for the rules of change in acoustic emission in fatigue crack propagation in hardened and low-temperature tempered 40Kh steel intended, in this study, for use as a reactor material. The results of the acoustic measurements are compared with data of quantitative aimed microfractography conducted during mechanical fracture tests.

Yu. V. Zima

Papers

Effect of structural factors on the kinetics of fatigue cracks in constructional steels

Influence of structure on the resistance of 65G steel to fatigue crack growth

Kinetics and mechanism of corrosion-fatigue crack growth in ferritic-pearlitic class steels: Report I. High-strength steels with a martensitic structure

Influence of the scale factor on the cyclic crack resistance of plastic steels in the low-amplitude area of loading

Relationship of acoustic emission to the kinetics and micromechanism of fatigue failure of high-strength steel with a martensitic structure