Microstructures and properties of high Cr content coatings on inner surfaces of carbon steel tubular components prepared by a novel mechanical alloying method

In the present work, a novel mechanical alloying method was developed to prepare high Cr content coatings on the inner surface of carbon steel tubular components using a planetary ball mill. The microstructure and elemental and phase composition of mechanically alloyed coatings at different processing conditions were studied using SEM, XRD, and EDX. It showed that a proper increase in the applied milling time and the disc rotation speed favored the improvement in the thickness, surface smoothness, densification level, and microstructural homogeneity of the deposited coatings. With processing conditions optimized (rotation speed of 500 rpm and milling time of 10 h), a fully dense, 120 μm thick, high Cr content coating, consisting of metal Cr and Fe-20Cr solid solution alloy, was metallurgically bonded to the inner substrate. Comparative studies on the microhardness, corrosion resistance, and anti-oxidation capability of carbon steel substrates with and without coatings were performed. It was found that the maximum microhardness of the coating reached HV_0.1667, showing a threefold improvement upon the substrate. The coated surfaces exhibited favorable resistance against corrosion and thermal oxidation as compared with the bare substrate. Based on two important action mechanisms (i.e., friction effect and impact effect) associated with a planetary ball mill, a reasonable mechanism was presented for the formation of mechanically alloyed coatings on inner surfaces of tubular components.     

Laser surface treatment of tool steels

Laser surface treatment is a promising technique for improving the wear and corrosion resistance of materials. In the case of tool steels, laser surface treatment is preferably carried out in the liquid state to allow for complete dissolution of carbides. This paper concerns the application of laser melting to the surface treatment of AISI 420 and 440C martensitic stainless steels and sintered AISI T15 high-speed steel. Usually, laser-melted tool steels contain martensite, retained austenite and carbides. In steels containing large proportions of ferrite-forming alloying elements, -ferrite may also be observed. When applied to sintered steels, laser treatment leads to the elimination of residual porosity. The proportion of retained austenite in laser-melted steels is much higher than in conventionally treated steels. However, the hardness is high because austenite is strengthened by solid solution, dislocations and small grain size. The high volume fraction of retained austenite usually prohibits the application of tool steels in the laser-treated condition. Austenite may be eliminated by multiple tempering treatments at temperatures in the range 550–650°C. During tempering, carbides precipitate within austenite and martensite, and austenite transforms to martensite on cooling or isothermally to ferrite. Strong secondary hardening is often observed and the temperature of the secondary hardening peak of laser-surface-melted steels is higher than after conventional heat treatment.     

Structural changes in austenitic steel in contact with solid oxide electrolyte at 950°C

Changes in the structure and redistribution of alloying elements in 10Kh23N18 austenitic steel after operating at 950°C for 8800 h in contact with La-Sr-Mn-O electrolyte are investigated to establish the reasons for the degrading of the surface layer of a current collector in a solid-oxide fuel cell over long periods of service. It is established that the degradation of the surface layer of steel is associated with the formation of a network of silicon and aluminum oxides along grain boundaries and the considerable discrepancy between the linear thermal expansion coefficients of steel and the solid electrolyte.     

Thermal conductivity of laser-produced plasma corona

Various methods of theoretically describing the thermal conductivity of a of plasma corona are considered. The processes of laser heating and ablating a spherical-shell target in the TRITON program (Inst. of Appl. Mech., USSR Acad. Sci.) are computer-simulated. Numerical and analytic methods are used to investigate the influence of heat-transport suppression on the principal hydrodynamic characteristics of the plasma. It is shown that the most sensitive to a reduction of the heat transport is the electron-density distribution in space and in time. The requirements imposed on experimental measurements capable of determining, in comparison with numerical computations, the degree of heat-transport suppression, are analyzed for a large range of flux densities. It is shown that when the flux density is decreased to 10¹³ W/cm² the present accuracy of measuring the position of the criticaldensity region in the corona, as well as the rate of evaporation of the material, becomes inadequate to determinethe deviation of the thermal conductivity from the classical value. Reliable conclusions concerning the transport coefficients can be drawn in this case from a comparison of high-speed interferometry data on the dynamics of a low-density corona (/_cr = 10^-3–10^-1) with the results of computer simulation.Division of Quantum Physics, Lebedev Physics Institute, Academy of Sciences of the USSR. Translated from Preprint No. 188 of the Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow (1988).     

Laser-induced local modification of the magnetic properties of 301 stainless steel

The martensitic phase of 301 stainless steel has been locally transformed into the austenitic phase by irradiating it with a focused Ar-ion laser beam in order to investigate the local change of magnetic properties accompanying the phase transformation. The intensity of the magnetic signal was found to locally reduce at a laser power of 350 mW and almost extinguish at 550 mW with a beam spot size of 13.2 m (at 1/e intensity), indicating the local austenitic phase transformation of the irradiated stainless steel.On leave from Ltd., Shimoteno, Himeji, Hyogo 670, Japan     

Structural and phase transitions in nitrided layers of iron alloys during severe cold deformation

A nanostructuring procedure similar to that proposed previously for iron alloys with carbides, nitrides (γ′-Fe₄N, TiN), and oxides, was implemented for X22 fcc alloy and X18H8 austenitic stainless steel. The procedure is based on the deformation-induced dissolution of disperse CrN nitride particles in the alloy matrices and the formation of supersaturated solid solutions of nitrogen, followed by the precipitation of secondary nanonitrides inhibiting the grain growth in the matrix during heating.     

Stationary and quasistationary models of carbon redistribution in austenitic steel weldments

In this IInd part of our paper (Czech. J. Phys. B35 (1985) 1355) the analysis of carbon uphill diffusion data is presented. The analysed data were measured in the polycomponent steel weldments. All of the data satisfy well the conditions for stationary model application. On the basis of the present analysis the carbon diffusivities (D ₁ ^* ) appertaining to a non-alloyed austenite, the activity ( _C ^s ) and diffusion ( _C ^s ) interaction coefficients are evaluated. A Si anomaly in Darken's experiments is observed and discussed. On the contrary to the other substitutional elements Mn, Cr and Mo, which decrease simultaneously C-activity and C-diffusivity, silicon increases the carbon activity and, at the same time, decreases its diffusivity in the Fe-C-X_s austenitic solid solutions.     

Stationary and quasistationary models of carbon redistribution in austenitic steel weldments

In this paper models describing carbon redistribution in austenitic steel weldments are presented. The stationary model (SM) is based on the assumption that the substitutional atoms do not diffuse, they are stationary. The quasistationary model (QSM) describes the C-redistribution even in the case in which the substitutional atoms diffuse in a narrow interval which is situated in the neighbourhood of the weldment interface. In developing the models the following assumptions were used: The driving force of carbon diffusion is the gradient of the chemical potential in which the C-C interaction is not taken into account ( _C ^C = 0); the molar volumes of the solid solutions under consideration are the same and constant in both parts of the weldment; the Kirkendall effect does not occur.     

Strain Hardening and Fracture of Austenitic Steel Single Crystals with High Concentration of Interstitial Atoms

Stages in the flow curves, mechanisms of deformation (slip or twinning), evolution of the dislocation structure and fracture are studied in austenitic stainless steel single crystals alloyed with nitrogen (C _N = 0–0.7 wt. %) and Hadfield steel in relation to the orientation of the crystal axis of tension, test temperature, and atomic concentrations of nitrogen and carbon. The dislocation-structure pattern (cellular or planar) and deformation mechanisms (slip or twinning) are shown to depend on the matrix stacking-fault energy _sf, friction forces due to solid-solution hardening by interstitial atoms, and crystal orientation. An interrelation between the stages in the flow curves and the type of dislocation structure is found. The contribution of mechanical twinning to the plastic flow of steel crystals is shown to increase with increase in nitrogen and carbon concentrations. The mechanical twinning develops in the early stages of deformation and determines the strain-hardening coefficient and fracture of crystals in high-strength states for interstitial atomic concentration C 0.5–0.7 wt. %. High deforming stresses due to solid-solution strain hardening by interstitial atoms of nitrogen and carbon in combination with low _sf are found to result in twinning in the <001> orientations. The values of _sf in Hadfield steel single crystals and in austenitic stainless steel single crystals are found experimentally depending on the concentration of nitrogen atoms and test temperature.     

Carbon redistribution in austenitic steel weldments

In parts I and II of our papers (Czech. J. Phys.35 (1985) 1355 and36 (1986) 514) concerning carbon redistribution in austenitic steel weldments the stationary (SM) and quasistationary (QSM) models have been developed and applied in the analysis of Darken's and our own data. It was necessary, in deriving the quoted SM and QSM, to introduce some simplifying presumptions. The present part III gives the general solution (GS) of the problem. It is based on the theory of thermodynamics irreversible processes, and the resulting system of differential equations is integrated numerically. The properties and chances of the GS are demonstrated in comparison with experimental data and by different examples of the prediction of carbon redistribution. The events of the non-zero thermodynamic (skCi) and diffusion (skCi) interaction coefficients, of the Kirkendall effect, of the non-constant atomic volume effect and of the different Wagner's and Zupp-Stevenson expressions of carbon activity coefficient _C are discussed.     

Laser Nitriding of Iron,Stainless Steel,and Plain Carbon Steel Investigated by Mössbauer Spectroscopy

Nitriding is a common method for improving the hardness, mechanical properties, wear and corrosion resistance of metals. Laser nitriding of metals is an efficient process, where the irradiation of surfaces in air or nitrogen atmospheres with short laser pulses leads to a fast take-up of nitrogen into the irradiated surfaces. This process has been extensively investigated for pure iron, but usually, no tools or functional parts are made of pure iron. Mainly steel or cast iron is used as a base material. Therefore, when looking for technical applicability, also the influence of alloying elements on the laser nitriding process is of great interest. Besides the pure iron various carbon steels and an austenitic stainless steel were studied in laser nitriding experiments in order to investigate the influence of the material itself. Here, the process is investigated via Conversion Electron and X-ray Mössbauer Spectroscopy (CEMS and CXMS), Resonant Nuclear Reaction Analysis (RNRA), and X-Ray Diffraction (XRD). It appears that carbon steels are even better suited for the laser nitriding process than pure iron, and the laser nitriding also works efficiently for the stainless steel which is normally difficult to be nitrided.     

The Influence of Deformation and Short-Term Hightemperature Annealing on the Microstructure and Mechanical Properties of Austenitic Steel 17Cr-14Ni-3Mo (316 Type)

Russian Physics Journal - The deformation and thermal action on the microstructure and mechanical properties of a stable austenitic Cr-Ni stainless steel is investigated. It is shown that under...     

On the formation of periodic structures on solid surfaces

A model is proposed to explain the formation of periodic structures produced on solid surfaces by laser radiation. The model gives rise to a system of two linear integrodifferential equations with difference kernels for temperature correction due to the specific absorption of electromagnetic energy at a certain solid surface profile and at a surface profile formed due to heat expansion resulting from temperature correction. The solution of this system reveals, that, first, periodic structures are formed as a result of the propagation of periodic profiles generated from a certain original non-periodic profile over the body surface. Second, the amplitudes of these waves grow with time only for a laser density exceeding certain critical value, i.e. the formation of periodic structures is a threshold effect relative to the laser density.     

Pulsed laser-target effects and high-resolution process diagnostics

In pulsed laser-target interaction at fluences and fluxes in a range of interest close to the air-breakdown threshold limits, the processes of heating, boiling, melting, chemical decomposition or vaporization occur quasi-explosively. Nonlinear effects due to plasma formation then largely determine the response of the materials. Investigations were carried out at various IR wavelengths (at 2.7 m, 3.8 m and 10.6 m) using a multigas laser (at more than 10 J/pulse) which could be operated as a chemical HF or DF laser and as CO₂ laser in the single-pulse mode. Studies in the repetitively pulsed mode, providing both high peak power densities and rather high average power densities simultaneously, were carried out at =10.6 m with a specially designed multi-kW CO₂ laser system. Both thermal and mechanical effects were studied on dielectric, semiconducting and metallic targets in a large fluence range from several up to hundreds of joules per cm².Valuable insight into the transient phenomena was obtained using fast optical, optronic and electronic techniques. The methods applied, including high-speed photography, videography and laser diagnostics, have proved to be particularly useful for process monitoring or control in industrial laser materials processing applications in order to improve energy transfer rates for optimization purposes.     

Development Kinetics of the Plastic Wave Front at the Metal Interface

Russian Physics Journal - The paper considers an inhomogeneous development of plastic strain at the Chernov–Lüders band front in the bimetal consisting of carbon and austenitic steel...     

Phenomenon of Mechanoinduced Atomic Stratification in Alloys under Cold Plastic Strain

Low-temperature (24°C) redistribution of nickel in K12N30 and K12N40 stable austenitic steels under strong ( > 9) cold shear strain at a pressure of 8 GPa is investigated. The strain-induced atomic stratification of FCC alloys is identified from the enhanced alloy magnetization and the occurrence of ferromagnetic microregions with vastly higher Curie temperatures. Quantitative estimates demonstrate that the mechanoinduced redistribution of alloying elements can be described by the system of equations for diffusion fluxes of arising point defects on sinks. As in the case of irradiation, this leads to the formation of radiation-induced segregations on intergrain boundaries.     

Impact of subthreshold laser defect accumulation on the optical stability of crystals

Optoacoustic means are used to study the kinetics of laser damage accumulation in potassium chloride crystals. This approach allows us to observe the accumulation of changes in KCl crystals when it is irradiated by a series of laser pulses of subthreshold intensity in a variety of actions (alloying, applying an external electric field, UV illumination). Number of laser pulses N _cr required to destroy a specimen at the same density of radiation power depends on density power I as N ~ I ^?4. Results are explained by the formation and accumulation of structural defects during photochemical reactions under the action of laser radiation.     

Characteristics of laser supersonic heating method for producing micro metallic particles

In this article, the authors analyzed the process characteristics of laser supersonic heating method for producing metallic particles and predicted the in-flight tracks and shapes of micro-particles. A pulse Nd–YAG laser was used to heat the carbon steel target placed within an air nozzle. The high-pressure air with supersonic velocity was used to carry out carbon steel particles in the nozzle. The shock wave structures at the nozzle exit were visualized by the shadowgraph method. The carbon steel particles produced by laser supersonic heating method were grabbed and the spraying angles of the particle tracks were visualized. The velocity of the in-flight particles was measured by a photodiode sensor and compared with the numerical result. The solidification of carbon steel particles with diameters of 1–50 μm in compressible flow fields were investigated. The result shows that there is no significant difference in the dimension of solid carbon steel particles produced at shock wave fields under various entrance pressures (3–7 bar) with a constant laser energy radiation.     

Ultraviolet laser surface treatment for biomedical applications of β titanium alloys: morphological and structural characterization

To improve the surface properties of titanium alloys developed for biomedical applications we have recently suggested a methodology involving laser-assisted nanostructuration. This strategy would benefit from superficial laser heat treatment since laser annealing displays many advantages as compared to the conventional methods: high resolution, high operating speed, low cost and retaining the initial bulk properties. Therefore, this paper reports results concerning the laser treatment of titanium alloys under vacuum. Our interest has been focused on the excimer laser single-pulse irradiation (=248 nm) of a model titanium alloy (Ti6.8Mo4.5Fe1.5Al). The threshold laser fluences corresponding to transus, melting and ablation temperatures as well as the resulting modification depth were first approached theoretically. KrF laser annealings were then carried out in vacuum, varying the fluence conditions from submelting heating to ablation regimes. Subsequent atomic force microscopy and scanning electron microscopy observations were performed to follow the structural and topographical modifications of as-treated specimens and were then discussed as regards the above-mentioned theoretical parameters . PACS 81.05.-t; 68.37.Ps; 68.55.Jk