Structural and phase transformations and micromechanical properties of the high-nitrogen austenitic steel deformed by shear under pressure

Using transmission electron microscopy, X-ray diffraction analysis, Mössbauer spectroscopy, microdurometry, and microindentation, the effect of large plastic deformations (through shear under pressure in Bridgman anvils) on the structure, phase composition, and micromechanical properties of high-nitrogen (1.24 wt % N) 08Kh22GA1.24 steel has been investigated. The steel was obtained by the casting method with counterpressure of nitrogen and was subjected to different heat treatments (quenching from1180°С, aging at 450 and 550°С) that form an austenitic (FCC) structure of the metallic matrix with chromium nitrides. It has been established that deformation by shear under pressure at room temperature results in the dispersion and deformation-induced partial dissolution of primary nitrides Cr₂N in quenched and aged steel and in the complete (after aging at 450°С) and partial (after aging at 550°С) dissolution of secondary nitrides CrN. It has been noted that, for aged steel that contains finely dispersed secondary chromium nitrides upon shear deformation, as compared to the quenched state, the dispersion of the austenitic structure (down to nano- and submicrocrystalline states) is more intense and the enhancement in the microhardness and resistance to elastic–plastic deformations upon contact loading is more effective.     

Nitrogen distribution in austenitic high-nitrogen chromium-manganese steel under friction and high-pressure torsion

Mössbauer spectroscopy and electron microscopic analysis were used to investigate the precipitation of products of cellular decomposition and their dissolution in high-nitrogen chromium-manganese steel FeMn₂₂Cr₁₈N_0.8 under room-temperature severe deformation via dry sliding friction and high pressure torsion in Bridgman anvils. It has been established that the nitrogen content increases in interstitial positions in the quenched and pre-aged alloy due to the strain-induced dissolution of chromium nitrides, which are contained in the products of decomposition. Mössbauer analysis showed that the friction-induced dissolution of chromium nitrides occurs at a depth of more than 10 μm. Aging reduces the amount of nitrogen that occurred in the solid solution upon deformation. This is explained by the additional energy consumed in grinding the decomposition products.     

Effect of aluminum on mechanical solid-state alloying of iron with nitrogen in ball mill

M?ssbauer spectroscopy and X-ray diffraction analysis have been used to study mechanical solidstate alloying with nitrogen and chromium of iron and an Fe-3Al alloy in the process of the mechanical activation with chromium nitrides in a ball mill. It is shown that the deformation-induced dissolution of chromium nitrides in iron and Fe-3Al matrices results in the formation of substitutional chromium and interstitial nitrogen solid solutions. The alloying of iron with aluminum accelerates the process of the deformation-induced dissolution of chromium nitrides, but reduces the nitrogen content in the interstitial solid solution. Post-deformation annealing generally leads to the escape of aluminum from the matrix, the substitution of chromium for aluminum, and the formation of fine nitrides AlN.     

Dissolution of the Fe4N nitride in the nitrided layer of iron upon cold deformation by shear under pressure

Mössbauer spectroscopy, X-ray diffraction, and transmission electron microscopy were used to study structural and phase transformations upon cold (300 K) deformation by shear under pressure in thin layers of nitrides Fe₄N formed on the surface of bcc iron. Strain-induced dissolution of nitrides in bcc iron with the formation of bcc and fcc solid solutions supersaturated with nitrogen and secondary Fe₁₆N₂ and Fe₄N nitrides was found. The dispersiveness of nitride phases in the layers deposited on bcc iron determines the accelerated kinetics of cold mechanosynthesis and the possibility of the formation of nanocrystalline iron-based solid solutions supersaturated with nitrogen and containing secondary nitrides.     

Producing a gradient-composition nanocrystalline structure on nitrided surfaces of invar-type Fe-Ni alloys using megaplastic deformation

A nanocrystalline Fe-Ni matrix strengthened by dispersed CrN and TiN nitrides has been produced on the ion-plasma-nitrided surfaces of the austenitic Fe-Ni₃₈-Cr₁₅ and Fe-Ni₃₆-Ti₄ alloys using cyclic “nitride dissolution-nitride precipitation” phase transformations induced by megaplastic deformation. The high-pressure torsion of the nitrided alloys has led to the dissolution of the CrN nitrides and Ni₃Ti intermetallic compounds, which appeared in the matrix, in the surface layer and to the mechanical alloying of the nitrided subsurface layer and the unnitriderd bulk of the specimens. Subsequent annealing has resulted in the formation of secondary nitrides, which propagated to a depth substantially exceeding the thickness of the original nitrided layer.     

High temperature oxidation of chromium nitrides

Chromium nitride powders oxidized to Cr₂O₃ noticeably above 400°C. Bulk chromium nitrides that were manufactured by sintering were oxidized between 900 and 1100°C in atmospheric air. The Cr₂O₃ layer that formed on bulk chromium nitrides having pores was relatively dense, and grew primarily via the inward transport of oxygen. The Cr₂O₃ layer to some degree deterred the nitrogen evolution from bulk chromium nitrides.     

Effect of chromium additions on the mechanical and physical properties and microstructure of Fe-Co-Ni-Cr-Mo-C ultra-high strength steel: Part I

The effect of chromium additions to an Fe-14Co-10Ni-0.1Mo-0.16C (AF1410 based) secondary hardening steel was evaluated by mechanical and physical properties and by microstructural examination. This unique behavior was extended to encompass a large range of aging temperatures and times that may be encountered during commercial thermal treatment and/or welding. In the aging range of 482 to 550 °C, an increase in chromium from 2 to 3% in the AF1410 based steel resulted in a substantial strength decrease concomitant with an increase in toughness. This behavior is related to a peak hardening shift, early M₂C carbide coarsening, and an increase in reverted austenite for the 1 wt% Cr increase. The increased aging kinetics resulting from the 3Cr steel caused a faster dissolution of Fe₃C and rapid changes in chromium partitioning in the (Mo,Cr)₂C carbide resulting in a coherency loss with a corresponding decrease in lattice parameter. The kinetics of the secondary hardening reaction, for the two steels, was determined by resistivity data for changes in aging parameters (time/temperature).     

Corrosion of low-temperature nitrided molybdenum-bearing stainless steels

Austenitic stainless steels with up to 6.1 wt.% Mo were nitrided at 425 °C and examined in 0.1 M Na₂SO₄ without and with chlorides at pH 3.0 and 6.5. Nitrided steels exhibited an increased resistance to pitting, but at pH 3.0 they had a decreased resistance to general corrosion. After corrosion at pH 3.0 surface films contained chromium nitrides and oxides of Mo, Cr and Fe. It is proposed that the improved pitting resistance of nitrided steels is associated with the initially accelerated dissolution which leads to the accumulation of corrosion resistant CrN and of oxidised steel components.     

Effect of the rate of cold plastic deformation on the kinetics of mechanical alloying of the Fe-35Ni-5Al alloy

The strain-induced dissolution of Ni₃Al intermetallic particles in the matrix of the aging Fe-35Ni-5Al alloy has been found to become more intense with increasing rate of deformation in rotating Bridgman anvils. This regularity has been presumably explained by a disbalance in the kinetics of the dual process of strain-induced dissolution and formation of phases owing to an increase in the number of interstitial atoms and a retardation of the development of an alternative process of the precipitation of second phases with increasing rate of deformation.     

Einfluß der Legierungszusammensetzung auf das Oxidations- und innere Nitrierungsverhalten von Nickelbasis-Superlegierungen

Influence of the alloy composition on the oxidation and internal-nitridation behaviour of nickel-base superalloys Internal nitridation of nickel-base superalloys takes place as a consequence of the failure of protecting oxide scales (Al₂O₃ and Cr₂O₃, respectively) and leads to a deterioration of the material properties due to near-surface embrittlement caused by the nitrides precipitated (TiN and AlN, respectively) and due to near-surface dissolution of the γ′ phase. By using thermogravimetric methods in a temperature range between 800 °C and 1100 °C supplemented by microstructural examinations, the failure potential due to internal nitridation could be documented. A quantification was carried out by extending the experimental program to thermogravimetric studies in a nearly oxygen-free nitrogen atmosphere which was also applied to various model alloys of the system Ni-Cr-Al-Ti. It could be shown that the nitrogen diffusivity and solubility in nickel-base alloys is influenced particularly by the chromium concentration. An increasing chromium content leads to an enhanced internal-nitridation attack.     

Radiation-induced dissolution of Ni3M intermetallic particles (M = Ti, Al, Zr) in displacement cascades in Fe-Ni-M alloys irradiated by neutrons at 340 K

The method of M?ssbauer spectroscopy has been used to study the dissolution of dispersed Ni₃ M intermetallic particles (M = Ti, Al, Zr) in matrices of fcc Fe-Ni-M alloys at 340 K in displacement cascades produced by neutrons. It has been found that the rate of dissolution of intermetallic particles is affected by preliminary aging, size of particles, and competing radiation-accelerated processes. The processes of dissolution of particles in neutron displacement cascades and under severe deformation have been compared.     

Effect of ultrasonic surface treatment of steel 40Kh13 on the microstructure of nitrided layer formed by high-intensity low-energy implantation with nitrogen ions

X-ray diffraction and transmission electron microscopy were used to study the microstructure and phase composition of surface layers of steel 40Kh13 subjected to irradiation with high-intensity low-energy ion beams, ultrasonic surface modification, and combined treatment including ultrasonic surface modification and ion implantation. It was found that the ultrasonic modification of steel surface leads to changes in the structure of tempered martensite and the formation of grain structure with a grain size of 0.3 μm and nanosized special carbides Cr₂₃C₆ in the martensite lamellae. The ion implantation into this steel results in the formation of a nitrided layer consisting of a nitride region, which represents a mixture of several phases (α-, γ′-, ?, and ultrafine chromium nitrides), and a zone of internal nitriding (α″ and nitrogen-containing martensite α_N). The preliminary ultrasonic modification causes an increase in the nanohardness and in the thickness of the nitrided layer, which is due to the more intense penetration of nitrogen atoms into the surface layer and an increase in the volume fraction of iron nitrides and density of ultrafine chromium nitrides in this layer.     

Dynamic aging in an Fe–Ni–Al alloy upon megaplastic deformation. Effect of the temperature and deformation rate

The method of Mössbauer spectroscopy has been used to investigate the effect of the temperature and the rate of megaplastic deformation on the processes of dissolution–precipitation of intermetallic compounds in aging austenitic alloy with a composition of Fe–36Ni–9Al. It has been established that, upon deformation in revolving Bridgman anvils, in the temperature range of cryogenic temperatures (liquid nitrogen) up to 573 K, a change occurs in the character of phase transitions from atomic disordering and the dissolution of intermetallic compounds to their additional accelerated precipitation. The factor that affects the kinetics of the processes of dissolution–precipitation of intermetallic compounds in the metallic matrix is dynamic aging. Dynamic aging is activated with an increase in the temperature and a decrease in the deformation rate.     

Structure of chromium-zirconium bronze subjected to dynamic channel-angular pressing and aging

Structure changes in chromium-zirconium bronze upon high-speed deformation and subsequent annealing have been studied using the methods of metallography and electron microscopy and microhardness measurements. Deformation was performed by the method of dynamic channel-angular pressing in one and three passes. The deformation creates a submicrocrystalline structure and increases the microhardness by 2.4 times. Aging additionally increases the microhardness by 10%. During annealing at temperatures of 400–700°C, aging and recrystallization take place. At early stages of aging, nanosized particles pin dislocations, thus hampering the formation of recrystallization centers. At subsequent stages, chromium particles impede the migration of large-angle boundaries, thereby blocking the development of recrystallization. At the beginning of aging process in the deformed structure, chromium particles are coherent with the copper matrix and have an fcc structure. Upon the coarsening of the particles in the recrystallized structure, they acquire a bcc structure that is typical of chromium.     

The dissolution behaviour of iron,chromium, molybdenum and copper from pure metals and from ferritic stainless steels

Dissolution rates of iron, chromium, molybdenum and copper as pure metals and as components of three ferritic stainless steels in neutral solutions (distilled H₂O and 0.5 M NaCl) and acid solutions (1 M and 9.2 M of HNO₃ and H₂SO₄) are presented. The relative position of dissolution rates of the elements is in general the same in pure metal and alloy dissolution. Selective dissolution of iron and simultaneous surface enrichment of chromium is observed under all the investigated conditions. A slight surface enrichment of molybdenum is found upon exposure in the neutral and sulfuric acid solutions but not in nitric acid solution. Large amounts of copper are found in the surface of the copper-containing steel exposed to 1 MH₂SO₄. Evidence is presented which shows that an increase in electrolyte volume decreases the thickness and changes the composition of the passive film. It is believed that this hardly ever mentioned effect is due to an increased amount of dissolution of the film components upon increased volume of electrolyte.     

Evolution of carbides and chromium depletion profiles during oxidation of Alloy 602 CA

The microstructure evolution in carbide strengthened Alloy 602 CA during exposure to a synthetic flue gas (N₂–2.5%O₂–8.6%H₂O–16.4%CO₂) at 1100 °C has been studied. The chromium and aluminium loss resulted in a chromium depleted alloy subsurface area and the dissolution of the carbides within this area. An increase of the carbide fraction in the sample core was observed and quantified. Phase equilibria calculations revealed that the depletion of aluminium as well as that of chromium triggers carbon to leave the depleted area. The overall carbon depletion in that area corresponded to the observed increase in carbide fraction in the sample core.     

Deformation-induced dissolution of carbides of the Me(V, Mo)-C Type in high-manganese steels upon the friction effect

By the method of M?ssbauer spectroscopy, it is shown that the aging at 923 K (for ten hours) of quenched stable austenite steels, FeC_0.8Mn₂₀V₂Mo₂ and FeC_0.8Mn₂V₂, which leads to a depletion of austenite in carbon, causes the enhancement of steel magnetism owing to the development of antiferromagnetic ordering in these steels. A severe plastic deformation under conditions of dry sliding friction reduces the Néel temperature and the mean effective magnetic field of aged steels because of the deformation-induced dissolution of Me-C carbides and the transition of part of carbon atoms to a ?? solid solution. The deformation-induced dissolution of the carbide phase proceeds most vigorously in the FeC_0.8Mn₂V₂ steel, which is characterized by the minimum size of carbide particles. It is concluded that the deformation-induced dissolution of the carbide phase is a factor that has a substantial positive effect on the wear resistance of austenite steels dispersion-hardened by aging.     

Deformation-induced cyclic phase transitions of dissolution-precipitation of nitrides in surface layers of Fe-Cr-(Ni)-N alloys

Methods of M?ssbauer spectroscopy, transmission electron microscopy, and X-ray diffraction have been used to study structural and phase transformations in surface layers of iron and Fe-Cr-(Ni) alloys subjected to ion-plasma nitriding and subsequent severe cold plastic deformation by shear under pressure in Bridgman anvils. It has been shown that the cyclic phase transformations of dissolution-precipitation of nitrides in the alloys result in the formation of nitrogen-supersaturated solid solutions, precipitation of secondary nitrides, and the nanostructurization of the metallic matrix. It has been established that the introduction of chromium into iron alloys accelerates the formation of nitrides upon nitriding and makes it possible to obtain solid solutions strongly supersaturated with nitrogen (more than 10 at % N in the fcc lattice) during subsequent deformation.     

Corrosion behavior of martensitic and precipitation hardening stainless steels treated by plasma nitriding

Plasma nitriding is a well established technology to improve wear and corrosion properties of austenitic stainless steels. Nevertheless, in the case of martensitic stainless steels, it continues being a problem mainly from the corrosion resistance viewpoint.In this work, three high chromium stainless steels (M340, N695 and Corrax) were hardened by ion nitriding at low temperature, intending to preserve their corrosion resistance.Corrosion behavior was evaluated by CuSO₄ spot, salt spray fog and potentiodynamic polarization in NaCl solution. Microstructure was analyzed by optical microscopy, SEM (EDS) and glancing angle X-ray diffraction. All the samples showed an acceptable corrosion resistance in experiments with CuSO₄, but in salt spray fog and electrochemical tests, only Corrax showed good behavior. The poor corrosion performance could be explained by chromium carbides formed in thermal treatment stage in martensitic steels and chromium nitrides formed during nitriding, even though the process was carried out at low temperature.     

The polarization behaviour of chromium in acidic sulphate solutions

The polarization behaviour of chromium was studied in solutions of various pH, with particular regard to the hydrogen evolution reaction and chromium dissolution. Hydrogen evolution occurred on active and passive chromium and the exchange current density was ca. 10^?6 A/cm² at pH 1.0 on both electrodes. The rate of active dissolution of chromium did not depend on pH, an observation that indicated the absence of a hydroxyl-containing intermediate in the rate determining step. The transpassive dissolution of chromium was strongly dependent on pH, and it was suggested that the rate was controlled by a base-accelerated anodic dissolution of an oxide film.