Plasma-arc Remelting Research Articles

Influence of technological parameters on nitriding rate in obtaining metal powders by plasma centrifugal spraying

Increase in metal operational properties is ensured by introduction of a certain set and amount of alloying elements into it. These elements include nitrogen, the interest in which is constantly growing. As many works have shown, nitriding with gaseous nitrogen is used, among other things, in plasma-arc remelting. The data is given on metal alloying with nitrogen at the stage of obtaining granules and powders. However, the latter process requires further study. The process of obtaining nitrogen-containing metal microgranules from EP741NP alloy by plasma centrifugal spraying was studied in this work. Metal powders are obtained by melting the end of a rotating workpiece with a stream of ionized gas (mixture of gases). The technology makes it possible to obtain nitrogen-alloyed fine metal powders of multicomponent spherical alloys with a minimum number of satellites that do not differ in size and chemical composition. Investigation of the nitriding rate is of great interest, especially in the production of powder metal. One of the parameters that affect the degree of metal saturation with nitrogen is time spent by liquid melt under nitrogen-containing plasma and crystallization time of the metal drop. The paper presents a technique that allows one to quantify the role of these parameters on the absorption of nitrogen by the metal during powder production. It is known that the kinetic parameters of the nitriding process are determined by the contact area of ​​two metal – gas phases. In the case of obtaining a powder, this parameter depends on the size of a powder speak. In this connection, the paper presents a calculation method that allows estimating the average fractional composition of metal powders depending on a number of technological factors. The obtained values ​​were compared with the data of semi-industrial heats. It is shown that the fractional composition of microgranules depends on the rotation speed and diameter of the workpiece being remelted, the density of the alloy, and the force of surface tension. It was established that with an increase in the rotation frequency of the consumable electrode, it is possible to achieve a decrease in the fineness of metal powders.

Thermodynamics of nitrogen solubility in nickel-based alloys at plasma-arc remelting

Heat-resistant nickel-based alloys are widely used in the domestic aircraft industry, rocket engineering, and instrument making. Increasing the basic mechanical and operational characteristics of the metal is achieved mainly by alloying the base with various elements such as rhenium, ruthenium, hafnium, etc. However, increasing the operational properties can also be achieved by metal nitriding, as a result of which interstitial solid solutions are formed (finely dispersed nitrides), which increase the strength of the alloy. The work is devoted to the study of nitriding process of nickel-based complex alloys. Various options of the interaction of nitrogen with the melt are considered under conditions of open melting and processing with low-temperature nitrogen-containing plasma. The use of plasma-arc remelting allows one to obtain various types of gas in form of atoms, ions and molecules in plasma. The first two forms are much more active than molecular nitrogen, which leads to super-equilibrium concentrations in the alloy. A thermodynamic analysis of nitrogen solubility in a nickel-based melt was performed during open melting and under plasma-arc remelting. The nitrogen solubility in model EP741NP alloy was calculated depending on the partial pressure of nitrogen above the melt surface and in the plasma-forming gas. It was shown that treatment of the melt with nitrogen-containing plasma makes it possible to obtain higher content of nitrogen in the alloy. In this work, temperature of the metal in the zone of contact with plasma arc was estimated using the technique according to which the evaporation of the melt components from the arc spot area occurs when the metal boiling point is reached.

Melting of High-Nitrogen Austenitic Steels at Atmospheric Pressure

The article considers conditions for nitrogen bubble formation at the crystallization front for iron and stainless steels. Using the results obtained during plasma-arc remelting, a semi-empirical formula is proposed for calculating maximum permissible nitrogen concentrations. It is shown that during steel crystallization with austenite formation at atmospheric pressure it is possible to obtain dense steel ingots with a nitrogen content exceeding its solubility at the liquidus temperature by a factor of 1.4–1.5.

Nitriding of High-Alloy Steel During Plasma Arc Remelting

An experimental unit is described for effective alloying of steel with nitrogen by plasma arc remelting. Production regimes are developed for unit operation making it possible to obtain a super-equilibrium content of nitrogen (up to 0.22%) in remelted metal with action of a plasma flow at a temperature of ≈ 7000 K in molten metal with the metal temperature in the arc-binding zone of up to ≈ 2700 K. The technology of remelting of 55Kh20G9N4 steel facilitates grain refinement, an increase in strength properties, ductility, and overall energy for failure, wear resistance under dry friction conditions and a reduction in the corrosion rate.

Influences of plasma arc remelting on microstructure and service performance of Cr3C2-NiCr/NiCrAl composite coating

This work studies the influences of plasma arc remelting on the microstructure and properties of thermal sprayed Cr3C2-NiCr/NiCrAl composite coating. The microstructure and composition are analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD). The microstructure and density of the coating are modified after remelting. It is found that the amounts of the Cr23C6 and Cr2O3 phases of the remelted coating are increased compared with the as-sprayed coating. Based on the experimental results, the coating adhesion, microhardness and wear resistance are all enhanced through plasma arc remelting with limited power (below 42 kW). Specifically, the adhesion is improved due to the enhancement of the element diffusion near the interface and metallurgical bonding between the functional and bonding layer. In addition, the increase of microhardness is attributed to the decrease of porosity and the formation of secondary carbides and eta phase. However, remelting with excessive powers (above 42 kW) results in the delamination of the mechanical joint between the bonding layer and substrate, which has adverse effect on coating adhesion as well as the wear resistance. Therefore, the proper usage of plasma arc remelting is an effective approach to enhance the properties of the thermal sprayed Cr3C2-NiCr/NiCrAl composite coating.

Quantification of heat transfer phenomena within the melt pool during the plasma arc re-melting of titanium alloys

This paper summarizes the development and application of a mathematical model, based on the commercial Computational Fluid Dynamics code ANSYS-CFX, to describe the transport processes occurring during the heating of a cylindrical slice of Ti-6 wt%Al-4 wt%V (Ti64) ingot with a plasma torch. The model simulates the conditions observed in a test (trial) that was conducted within an industrial scale plasma arc furnace. The test conditions, data, analysis methodology and results, in terms of characterization of the surface heat flux, have previously been published and are only presented in this manuscript in a summary form. This work focuses on the development of a thermal-fluid model of the ingot slice in order to better describe the heat transport processes occurring in the molten pool, which was identified as a shortcoming of the previous work. The model includes heat and fluid transport by diffusion and advection, thermal buoyancy, surface shear resulting from both thermal Marangoni forces and the plasma gas and Darcy damping within the two-phase (solid/liquid) region. A preliminary thermal-fluid model of the plasma plume was also developed as part of the research (not presented) and used to help guide development of a suitable gas shear stress distribution, which was implement using a log-normal approximation. The model was found to be able to reproduce thermocouple and pool profile data collected during the trial. In particular, the prediction of the pool profile was found to be considerably more accurate than had previously been the case with a diffusion-only heat transfer model. A sensitivity analysis conducted with the model showed the liquid pool profile to be highly dependent on the magnitude of the gas shear force and the input power efficiency and distribution. The gas shear was found to have a significant impact on near-surface fluid flow, helping to enhance radial heat transport. In comparison to the Marangoni induced shear, the gas shear was found to be dominant.

Development of a Commercial Technology for Plasma Arc Remelting of a Nitrogen-Bearing 05Kh21AG15N8MF Steel

Steel 05Kh21AG15N8MF forgings 250 mm in diameter with a nitrogen content higher than 0.60%, which is necessary for a high-strength austenitic structure, are fabricated by plasma arc remelting. An experimental batch and strips are made of this steel. The chemical composition, the macrostructure, the mechanical properties, the intergranular corrosion resistance, and the hardness of the forgings are studied.

Influence of Nitrogen in Plasma-Arc Remelting on the Structure and Properties of Stainless Steel

Low-temperature plasma is used in the plasma-arc remelting of 55Kh20G9N4 steel. The plasmaforming gases employed are nitrogen and argon. The influence of the remelting parameters on the structure, the mechanical properties, the flexural strength, and the corrosion and wear resistance of the stainless steel is studied. When using nitrogen to form the plasma, its content in the steel increases. That increases the strength, crack resistance, and corrosion and wear resistance of the steel.

Quantification of the heat transfer during the plasma arc re-melting of titanium alloys

This paper summarizes the development and application of an Inverse Heat Conduction Code (IHCC) to determine the surface heat flux distribution from an industrial plasma torch applied to heat a sample of Ti-6wt%Al-4wt%V (Ti64) alloy. The test (trial) was conducted within an industrial scale plasma arc furnace and the sample was instrumented with 15 thermocouples embedded below the top surface. The sample was heated for 278 s, which allowed sufficient time for a liquid pool to form within the sample. Following the trial, the sample was sectioned to obtain the liquid pool profile. The IHCC analysis method described in the paper is based on the future time-step approach and uses the commercial finite element code ABAQUS™ as the forward conduction engine. The IHCC analysis was conducted with both isotropic thermal conductivity and anisotropic thermal conductive in the liquid. The later, was used to approximate the effect of fluid flow on heat transport in the liquid. Additionally, the method used linear interpolation to vary the estimated heat flux between the discrete heat flux evaluation points associated with the thermocouple positions. The results indicated that it is critical to account for fluid flow and suggest that the heat flux distribution can be accurately described by two over-lapping Gaussian distributions: one narrow distribution associated with convective heat transfer; and a second, broader distribution, associated with radiation. An overall heat transfer efficiency of 28% was estimated from the heat flux distribution.

A Study of Process factors on the Recycling of Reactive Metal Scraps in Plasma Arc Remelting

A Study of Process factors on the Recycling of Reactive Metal Scraps in Plasma Arc Remelting

Laws of nitriding of alloy steel in a plasma–arc remelting furnace

Metal drop samples are taken during plasma–arc remelting of 12Kh25N16G7AR steel billets. An analysis of the drops shows that the fraction of steel nitriding in the drops accounts for 20–40% of the total steel nitriding at a nitrogen content of 0.06% in a billet. An equation is derived to estimate the nitriding of a liquid Cr–Ni–Mn steel from the nitrogen concentration in the billet to be remelted.

TiB2 and TiC single crystals by plasma-arc remelting of SHS products

The growth of TiB2 and TiC single crystals by plasma-arc remelting of SHS products was explored. The process was markedly improved by using special stabilization systems. The synthesized crystals exhibited a cellular structure and a strong dependence of the longitudinal distribution of impurity elements on a growth rate.

Influence of the electric field strength and hydrogen on the rate of removal of iron atom impurities in refining remelting of tantalum

A comparison of the results of electron beam remelting (EBR) of tantalum and plasma arc remelting (PAR) of tantalum in a hydrogen atmosphere shows that some impurity elements are removed better during PAR with hydrogen than during EBR. The refining mechanism, the role of hydrogen, and the change in the electric field strength during the removal of impurity elements from tantalum are analyzed by molecular dynamic simulation.

Microstructure and corrosion properties of plasma-sprayed NiCr–Cr 3C 2 coatings comparison with different post treatment

The effects of laser and plasma arc remelting on the microstructure and properties of plasma-sprayed NiCr–Cr 3C 2 coatings on steel substrates have been investigated. The microstructure of the coatings has been analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM). It is found that the Cr 3C 2, δ-(Cr,Ni), Cr 7C 3 and Cr 23C 6 phases were obtained for both coatings, before and after remelting treatment. The laser remelting was operated in a continuous way with 800 W power in different scan speed, while the plasma arc remelting was operated with a plasma cladding machine under different scan currents. However, the denser microstructure of both remelted coatings can be obtained, especially for the plasma arc remelted coating. The Vickers microhardness measurement showed certain enhancement values for both remelted coatings. The corrosion behavior was evaluated through salt spray corrosion (SSC) method. Energy-dispersive spectroscopy (EDS) showed that the chloride was produced during SSC process. The higher corrosion resistance for plasma arc remelted coating may be due to the more compact microstructure, less porosity rate and tensile residual stress. Compared with laser remelting method, plasma arc remelting is a cheap, convenient and effective remelting method.

Effect of Mo addition on the corrosion resistance of Zr-based alloy in water containing LiOH

As a sequel to the previous works on the mechanical properties and texture, the corrosion resistance of Mo-containing Zr-base alloys in LiOH-added water was studied. Experimental alloys of Zr–1Nb–1Sn–0.1Fe and Zr–1Nb–1Sn–0.1Fe– xMo ( x=0, 0.2, 0.5 wt%), prepared by plasma arc remelting, showed lower weight gains than those of the commercial alloys including Zircaloy-4 and ZIRLO. Mo-addition resulted in a higher corrosion rate in the experimental alloys. Increased corrosion rate in these alloys was associated with lumpy microstructure of oxides with a higher portion of the monoclinic phase. The microstructural characteristic of oxides was attributed to the uneven distribution of β-Zr that was stabilized by Mo. Incomplete recrystallization and decreased normal basal texture also adversely affected the corrosion resistance of the Mo-containing alloys. It was shown, however, that the corrosion resistance of Mo-containing alloys could be improved to the level of the Mo-free alloy by a proper heat treatment to remove the undesirable microstructural feature.

Fabrication of titanium carbide and diboride specimens by the method of self-propagating high-temperature synthesis for plasma-arc growth of single crystals

Fabrication of long cylindrical specimens of TiB2, and TiC by burning homogeneous pressings of Ti + 2B and Ti + C mixtures with an additive of final combustion products (up to 30%) in quartz shells with the aim of growing single crystals by the method of plasma-arc remelting is considered. Dependences of the ultimate bending strength of rods of TiB2 and TiC on the percentage of the ballast, the dispersity of the components, and the density of the specimen are presented. Problems of shrinkage of the rods in the process of synthesis, contamination of the internal surface of the shells, repeated use of quartz molds, and withdrawal of SHS rods from the shells and the distribution of impurities over their length are discussed. It is shown that self-cleaning in the process of SHS using the shells reduces the content of some impurities in the rods by a factor of 2–3.

Special Features Of Stabilization Of The Growth Process In Plasma-arc Growth Of Single Crystals Of Refractory Compounds From SHS Products

Problems of the stability of the growth process in growing single crystals of refractory compounds (carbides and bondes of transition metals) by the method of plasma-arc remelting are considered. The main reasons behind the loss of stability of the spatial configuration of the melt (a shift of the anode lock-on of the arc from the center of the molten pool, disturbance of the symmetry of the plasma jet, superheating of the melt) are considered. The effect of cyclic perturbations in the crystallization system that appear upon replenishment on the fluctuation of the energy characteristics of the arc, the temperature of the melt, and the displacements of the crystallization surface is described. It is established that the crystallization rate and the process of coarsening of the crystallites in the formation of a columnar structure in the ingot depend on the frequency of feeding new portions and on their mass.

Plasma arc remelting of steels and nickel-based superalloys

The IRC Plasma Arc Cold Hearth Remelting (PACHR) furnace has been used to produce 100mm and 150mm diameter ingots of alloy 718, M50 bearing steel, and G110 maraging steel from VIM/VAR feedstocks. These trials highlighted the importance of accuratelycontrolling the level of the melt pool surface in the crucible in order to produce an acceptable ingot finish. Chemical analysis of the ingots produced indicated no significant changes in the level of any element. Cleanness of the ingots has been assessed using the Electron Beam Button Melt (EBBM) technique. The results obtained are encouraging but highlight the necessity for careful control of the plasma remelting process

Plasma-arc Remelting of Steels and Superalloys

Plasma-arc Remelting of Steels and Superalloys

Development of Melting and Thermomechanical-Processing Parameters for a High-Nitrogen Stainless Steel Prepared by Plasma-Arc Remelting

Development of Melting and Thermomechanical-Processing Parameters for a High-Nitrogen Stainless Steel Prepared by Plasma-Arc Remelting