The available experimental data and hypotheses concerning cementite decomposition during the cold work of pearlitic steels are reviewed. The results of studies performed using thermomagnetic ...analysis, Mössbauer spectroscopy, internal friction and APFIM are used to discuss the mechanism governing cementite decomposition. The following features of this phenomenon seem to be important: (i) the fraction of the decomposed cementite increases with the refining of the initial pearlitic structure, i.e. with the increase of the ferrite–cementite interfacial area; (ii) the decomposition effect saturates as strain increases; (iii) carbon–dislocation interaction in ferrite and MeC bonding in cementite have a strong influence on cementite decomposition. The conclusion is made that cementite decomposition is controlled by the transfer of carbon atoms from cementite to dislocations accumulated near the interface during deformation. This is because the binding enthalpy between carbon atoms and dislocations in ferrite exceeds the solution heat of cementite. Some relevant effects of cementite decomposition in practice are discussed.
▶ Low-temperature martensitic transformation is important for beneficial effect of DCT. ▶ Plastic deformation occurs in the course of low-temperature martensitic transformation. ▶ Carbon clouds ...around dislocations are formed due to the capture of immobile carbon atoms by gliding dislocations. ▶ Carbide phase is partially dissolved during DCT.
The tool steel X220CrVMo 13-4 (DIN 1.2380) containing (mass%) 2.2C, 13Cr, 4V, 1Mo and the binary alloy Fe–2.03
mass% C were studied using transmission electron microscopy, Mössbauer spectroscopy, X-ray diffraction and internal friction with the aim of shedding light on processes occurring during deep cryogenic treatment. It is shown that the carbon atoms are essentially immobile at temperatures below −50
°C, whereas carbon clustering in the virgin martensite occurs during heating above this temperature. An increase in the density of dislocations, the capture of immobile carbon atoms by moving dislocations, the strain-induced partial dissolution of the carbide phase, and the abnormally low tetragonality of the virgin martensite are found and interpreted in terms of plastic deformation that occurs during martensitic transformation at low temperatures where the virgin martensite is sufficiently ductile.
The low-temperature martensitic transformation in steel X153CrMoV12 containing (mass%) 1.55C, 11.90Cr, 0.70V, 0.86Mo is studied using dilatometry, Mössbauer spectroscopy, X-ray diffraction, ...mechanical spectroscopy and transmission electron microscopy. Some additional measurements were carried out on steel X220CrMoV13-4. It is shown that, in contrast to the widely known absence of martensitic transformation during deep cryogenic treatment, this transformation occurs with isothermal kinetics within the temperature range of −100 down to −170°C with its largest intensity near −150°C. No transformation is observed at −196°C. The remarkable features of the isothermal martensitic transformation are: (i) the plastic deformation, which is explained by the absence of ageing of martensite at low temperatures; and (ii) the abnormally low tetragonality of martensite. In contrast to existing interpretations, the abnormally low c/a ratio is interpreted in terms of the capture of immobile carbon atoms by gliding dislocations during plastic deformation at low temperatures. A recommendation is proposed for optimizing the deep cryogenic treatment of tool steels.
•Carbon in the iron decreases concentration of free electrons, nitrogen and hydrogen increase it.•Carbon in the iron decreases line tension of dislocations, whereas nitrogen and hydrogen increase ...it.•Effect of C, N and H on strength of S-K relaxation and ADIF is correlated with change in the electron structure.
Effect of interstitial elements on the Snoek-Köster relaxation and the amplitude-dependent internal friction is analyzed in terms of a change in the electron structure. The density of electron states in the bcc and fcc iron doped with carbon, nitrogen or hydrogen has been ab initio calculated paying due attention to that at the Fermi level. It is obtained that nitrogen and hydrogen increase density of states in the both iron phases, whereas the effect of carbon is opposite. In consistency, using electron spin resonance in the fcc iron alloys, the concentration of free electrons is found to be increased due to nitrogen and hydrogen and decreased by carbon. Based on the obtained results, a conclusion about corresponding change in the dislocation line tension controlling mobility of dislocations is derived. This correlation between electron structure and mobility of dislocations is confirmed for the Snoek-Köster relaxation in the bcc iron phase and for the internal friction background, ADIF, in the fcc phase.
The effect of grain boundaries (GBs) on the diffusion of interstitial and substitutional atoms in α-iron was studied experimentally and theoretically. Autoradiography and the sectioning method, ...applied to different grain size polycrystals, prove that a higher GB area leads to a reduction of the migration of the 14C isotope. An opposite effect is observed for the 60Co isotope. Theoretical calculations of hydrogen and carbon migration in selected GBs having different disorientation angles show that, for both interstitial elements, the activation enthalpy for migration is higher in the GBs than in the bulk. Both experimental and theoretical results lead to the conclusion that GBs in α-iron act as traps for interstitial atoms and retard their diffusion. To put these results into perspective, hydrogen-induced intergranular cracking is discussed in relation to the specificity of hydrogen diffusivity along the GBs.
•Hydrogen in α-iron increases velocity of grain boundaries.•GB mobility increases due to H-enhanced metallic character of interatomic bonds.•The obtained results explain the H-decreased temperature ...of recrystallization.
Using molecular dynamics, it is shown that, be located in the vicinity of grain boundaries, hydrogen in the α-iron increases their mobility. This effect is attributed to the hydrogen-increased concentration of free electrons, which decreases surface tension at grain boundaries. The obtained results allow to explain the early start of recrystallization in the hydrogenated cold-worked iron-based alloys.
Hydrogen effect on spatial distribution of valence electrons in the alpha-iron.
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•Hydrogen increases density of electron states at the Fermi level in the α-iron.•Hydrogen increases ...electron density in the interstitial sites.•Bonding between Fe and H atoms is formed by H-1s and Fe-3d + Fe-4s electrons.
The hydrogen-caused change in the electron structure of the α-iron has been studied using the first-principles atomic calculations. It is shown that hydrogen entry into the α-iron crystal lattice increases the density of electron states at the Fermi level, which suggests the increase in the concentration of free electrons. The studied spatial distribution of electron density gives the evidence for corresponding enhancement of the metallic character of interatomic bonds. A strong affinity of hydrogen atoms to the grain boundaries allows to suggest a non-trivial change in their mobility.
A new austenitic steel alloyed with carbon
+
nitrogen is developed based on ab initio calculations of the electronic structure, which were carried out using the local density functional theory, and ...on experimental studies by means of conduction electron spin resonance. It is shown that alloying of CrMn austenite with carbon
+
nitrogen increases the concentration of conduction electrons and assists their more homogeneous spatial distribution, which results in short-range atomic ordering and, consequently, in a higher thermodynamic stability of iron-based face-centered cubic solid solutions. The developed CrMnCN steel is characterized by a yield and ultimate strength of 600 and 1000
MPa, respectively, in combination with an extremely high fracture energy. The notch-impact toughness exceeds 400
J. The resistance to impact wear is comparable with that of Hadfield steel. Corrosive properties are in some corrosive media similar to those of austenitic Cr18Ni10 steel. Possible applications of the developed steel are discussed.
This study aims to shed light on some uncertainty in the effect of hydrogen-induced ε-martensite on mechanical behavior of hydrogen charged austenitic steels. It is shown that cathodic charging of ...austenitic steels is accompanied by plastic deformation, which creates the crystallographic texture and decreases plasticity during subsequent tensile tests. This deformation is the main reason for the hydrogen-induced γ→ε transformation. The ε-martensite is formed in the hydrogen-rich areas of the austenite, which decreases the hydrogen content in the retained austenite. In case of a small fraction of the hydrogen-induced ε-martensite, it can be additionally formed during subsequent mechanical tests due to trip effect, which diminishes the hydrogen loss of plasticity.
The experimental data on the concentration of free electrons in fcc iron-based alloys, results of theoretical calculations on the electronic structure and experimental data of atomic distribution are ...analysed. The electron structure of iron-based substitutional solid solutions and CrNi austenitic steels alloyed by Mn, Mo, Cu, Si, Al and C, N was studied by means of the measurement of conduction electron spin resonance. The electron exchange in binary fcc Fe–N and Fe–C alloys was also calculated using an
ab initio norm-conserving pseudopotential method. It is shown that Ni, Cu, Si and Al increase the concentration of free electrons, whereas Cr, Mn and Mo decrease it. Theoretical calculations as well as experimental data show that nitrogen in fcc iron and iron-based solid solutions increases the state density at the Fermi surface, whereas carbon contributes its electrons to the states below the Fermi surface. Mössbauer spectroscopy was used to study the distribution of carbon and nitrogen in binary fcc Fe–C and Fe–N alloys, while the data on the distribution of
d-solutes in multicomponent solid solutions were obtained from the analysis of the contributions of different electronic subsystems, namely free electrons, isolated localized
d-electrons (single solute
d-atoms) and superparamagnetic clusters (clusters of
d-atoms), to the temperature dependence of the magnetic susceptibility. The results of studies concerning the atomic distribution are consistent with the available data on the short range order in iron-based alloys. The following correlation is found: an increase in the concentration of free electrons assists the short range atomic ordering in iron-based alloys, whereas the localization of electrons promotes clustering of solute atoms. The state of atomic order influences properties like austenite stability, corrosion resistance and strength.
