Gas Nitrocarburizing Research Articles

Influence of oxygen in plasma nitriding

AbstractThe development of low temperature nitriding treatments to enhance wear resistance over the past 40 years is reviewed. The progress achieved is illustrated with evidence from extensive laboratory investigations and industrial trials on textile machine components and other parts. It is demonstrated that small additions of an oxidising component to gaseous nitrocarburising or plasma nitriding/nitrocarburising atmospheres have a beneficial effect on layer formation and on the wear properties of the treated components. The presence of oxygen increases layer formation rates and stabilises the ϵ-compound layer. The role of oxygen on surface activation is important for chromium containing steels but also on plain carbon or alloyed steels and to neutralise the passivating effect of finishing procedures. Applying a post-oxidation step after nitrocarburising can significantly increase the corrosion resistance of the surface but close control of gas composition and treatment parameters is necessary.

Nature of Surface Changes in Stamping Tools of Gray and Ductile Cast Iron During Gas and Plasma Nitrocarburizing

Two cast irons, pearlitic-ferritic gray and ferritic ductile, were plasma and gas nitrocarburized at the same temperature and for the same processing time to produce a compound zone of about 10-14 μm thick. It was demonstrated that both processes caused changes in the surface roughness of the irons, and the most dramatic increase of roughness was observed after gas nitrocarburizing of the gray cast iron. It was shown that the primary reason that the results were not the same is the difference in the nitriding mechanism. Significant penetration of the surface voids and imperfections between the graphite particles and the metallic matrix by ammonia molecules led to the formation of a locally thicker compound zone and a bulging of the metallic matrix above the surface. This phenomenon did not occur in the plasma process and as a result the surface changes were much smaller than in the gas process.

The Analysis and Control of Distortion in Carbonitrided and Nitrocarburized Thin-Shelled Plain Carbon Steel Automotive Powertrain Components

Abstract Gaseous ferritic nitrocarburizing was investigated as a replacement for the carbonitriding process currently used to improve the surface properties of thin-shelled, plain carbon steel automotive powertrain components. Nitrocarburizing can impart a hard, wear-resistant case suitable for light load applications at lower processing temperatures than carbonitriding, thereby potentially reducing size and shape distortions and the associated need for finish grinding. Carbonitriding and gaseous ferritic nitrocarburizing were compared with respect to size and shape distortion, retained austenite, and residual stress values. Testing was performed on a torque converter piston, an automotive powertrain component stamped from a sheet of 3-mm cold-worked SAE 1010 steel. A total of 53 pistons were used. While the heat treatment conditions for the carbonitrided pistons were similar to the current production schedule, conditions for the nitrocarburized pistons incorporated a range of processing times from 2–14 hours and temperatures from 510–605°C (950–1125°F). The results of this work were generally consistent with those from an earlier study involving the use of Navy C-ring specimens. For both the Navy C-rings and torque converter pistons, the nitrocarburizing process gave rise to lower part distortion, but resulted in overall lower compressive residual stress values than in the carbonitrided steel.

Plasma and gaseous nitrocarburizing of C60W steel for tribological applications

A comparison has been made between plasma and gas nitrocarburizing processes for C60W steel in terms of structural features of the layers and the tribological properties of the specimens. Gaseous treatments were performed in a sealed quench industrial furnace in optimized process cycles. A semi – industrial size unit was operated at a range of process parameters for plasma nitrocarburizing. Experimental results indicate that increasing the treatment time increases the thickness of the compound layer but lowers the wear resistance. Plasma treatment produced a more effective case depth compared with gaseous process, providing a better uniformity. Analysis of X-ray diffraction patterns indicated that the compound layer in gas nitrocarburizing and plasma nitrocarburizing consisted of both ɛ and γ′ phases.

Cementite-layer formation by ferritic nitrocarburising

Abstract Massive cementite surface-layers can be prepared upon gaseous nitrocarburising of α-iron substrates, where the ammonia contained in the gas atmosphere suppresses the otherwise occurring metal dusting and sooting reactions. The growth kinetics and properties of such cementite layers have been reviewed, in particular in view of possible deliberate generation of such layers in the course of commercial heat-treatment processes.

Microstructure and crystallography of massive cementite layers on ferrite substrates

Abstract Upon gaseous nitrocarburizing, massive cementite layers were grown on ferrite substrates. By means of electron backscatter diffraction, it was shown that the Bagaryatsky orientation relationship holds between the cementite grains of the compound layer and the ferrite grains of the substrate. The experimental data exhibit a preference for specific variants of this orientation relationship: those variants corresponding to relatively low cementite–ferrite misfit-strain energy are favoured. Furthermore, the orientation relationship of neighboring cementite grains grown on the same ferrite grain allows the establishment of low-energy grain boundaries between these cementite grains. In contrast, at ferrite-grain boundaries intersecting the substrate surface, high-energy cementite-grain boundaries occur between cementite grains on adjacent ferrite grains. The latter cementite-grain boundaries promote carbon grain-boundary diffusion through the cementite, which is experimentally supported by the observation of relatively large cementite-layer thicknesses at these locations.

Selective Laser Sintering and Subsequent Gas Nitrocarburizing of Low Carbon Steel Powder

Optimum conditions for laser irradiation to achieve fully dense SLS (Selective Laser Sintering) specimens have been investigated for S15C steel, in order to check the industrial feasibility of the SLS process. The mechanical properties of SLS processed S15C steel as well as the surface hardenability have also been investigated. The volume fraction of pores in S15C steel decreases as the scan speed, scan spacing and layer thickness decrease and as the laser power increases, so that the density of SLS specimens increases as the energy density increases. The specimens sintered at an energy density of 800 J/mm3 exhibit very low porosity and, hence, very high density, comparable to those of the corresponding wrought material. Complete elimination of pores is indispensable for the ductility and strength of SLS specimens. Excellent wear resistance can be provided with the surface of S15C steel produced by the SLS process by gas nitrocarburizing because of the formation of Fe2-3N nitride with a very high value of microhardness on the surface.

Examination of phase transformations in the system Fe–N–C by means of nitrocarburising reactions and secondary annealing experiments; the α + ∊ two-phase equilibrium

Abstract The until now controversially discussed existence of the two-phase equilibrium α-Fe + ∊-Fe3(N, C)1+x in the ternary system Fe – N – C was proven by the investigation (optical microscopy, X-ray diffraction, Electron Backscatter Diffraction) of compound-layer microstructures generated by gaseous nitrocarburising of pure iron and subsequent secondary annealing experiments under static inert-gas atmosphere. The invariant transition reaction γ-Fe4N + Fe3C → α-Fe + ∊-Fe3(N, C)1+x could be shown to proceed at a (single) temperature (at constant pressure of 1 atm) between 833 K and 843 K.

Short-Term Gas Nitrocarburizing of Tools for Cutting and Die Forging

The results of using gas nitrocarburizing of cutting and stamping tools in an atmosphere of exothermal gas and ammonia in a vacuum furnace instead of the traditional saturation in molten cyanides are presented. The process of nitrocarburizing has been put into large-scale production.

The effect of thermochemical treatments on case properties and impact behaviour of Astaloy CrM

Ion nitriding, ion nitrocarburizing, and gas nitrocarburizing were carried out on Astaloy CrM samples at a range of conditions. The influence of each treatment on case properties (including the composition of compound layer, hardness profile, and microstructure) and impact energy were investigated. Characterisation was performed by microhardness measurement, optical and scanning electron microscopy, and X-ray diffraction. XRD analysis indicated that the compound layers produced by different processes included ɛ and γ′ phases. In ion nitriding, the amount of ɛ phase, maximum microhardness, and the depth of diffusion zone increased with the nitrogen content of the atmosphere. Compared with ion nitriding, both nitrocarburizing processes showed lower maximum microhardness and thicker diffusion zone. All treatments reduced the impact energy of the samples; however, the effect of gas nitrocarburizing was more pronounced. Fractography observations confirmed that for ion-treated samples, cleavage was the prominent morphology from surface to a limited depth. Meanwhile, for gas-nitrocarburized sample, cleavage morphology was extended from the surface to a deeper distance.

Liquid, gaseous and plasma nitrocarburizing using three different stainless steel types as a substrate (austenite, ferritic and duplex): a comparative study

Samples of three different stainless steel types have been nitrocarburized under typical industrial conditions: an autenitic Cr-Ni-Mo steel, a 28% Cr-Ni-Mo super-ferritic steel and a duplex ferritic-austenitic steel. The obtained nitrocarburized layers were studied by optical, electron scanning microscopy, energy-dispersion analysis (EDS), glow discharge optical spectroscopy (GDOS), X-ray diffraction and hardness measurements. The properties of these layers are compared and the results are discussed.

Comparison of ferritic nitrocarburising technologies

Three steels of commercial grade AISI 1020, AISI 4140 and AISI 4340 have been subjected to ferritic nitrocarburising using a variety of heat treatments, such as a salt bath, fluidised bed reactor, and plasma and gaseous nitrocarburising. The effect of post-treatments on the white layer composition has been also studied. It has been found that for a given processing time, both salt bath and fluidised bed processing formed the thickest white layer. The formation of an ε monophase white layer was achieved only in the salt bath and gaseous process. It has been established that the oxidation treatment used to improve corrosion resistance of the white layer has the effect of transforming ε carbonitride to γ′.

Microstructural and compositional evolution of compound layers during gaseous nitrocarburizing

Microstructural and compositional evolution of compound layers during gaseous nitrocarburizing

Microstructural and compositional evolution of compound layers during gaseous nitrocarburizing

Compound layers developed at 848 K during gaseous nitrocarburizing of iron and iron-carbon specimens were investigated for several combinations of N and C activities imposed at the specimen surface by gas mixtures of NH3, N2, CO2, and CO. The microstructural evolution of the compound layer was studied by light microscopy and by X-ray diffraction analysis. Composition-depth profiles were determined by electron probe (X-ray) microanalysis. Layer growth kinetics was investigated by layer thickness measurements. The influence of the N and C activities on the microstructural and compositional evolution and the growth kinetics of the compound layers formed is discussed for the iron substrate. The results indicate that the microstructure is governed by a fast C and a slow N absorption at the surface in an early stage of gaseous nitrocarburizing. The influence of carbon in the substrate on the microstructural and compositional evolutions and on the growth kinetics was evaluated from comparing the results obtained for a normalized Fe-0.8C alloy with those for iron under identical nitrocarburizing conditions.

Überwachung und Regelung von Nitrier- und Nitrocarburieratmosphären mit dem HydroNit-Sensor

In many technical processes the value of the hydrogen content of a gaseous atmosphere is a suitable feature for the characterization of a reaction within the atmosphere. This is for instance the case in gas nitriding and gas nitrocarburizing. If during these heat treatment processes the hydrogen content is measured, and the amount of other gas volumes fed into the heat treatment installation is known, so-called atmosphere potentials can be calculated on the basis of the ammonium dissociation and the watergas equilibrium normally established. The value of these atmosphere potentials gives information about the nitriding or nitriding and carburizing ability of the atmosphere. This paper presents a newly developed sensor for in-situ measurement of the hydrogen partial pressure, which enables the monitoring and control of the atmosphere avoiding the known disadvantages of analysers and solid electrolyte gas sensors. The procedure of calculating the atmosphere composition during nitrocarburizing conditions, based on the measurement of the hydrogen partial pressure and the calculation of atmosphere potentials, is shown. Finally, the measurement principle as well as measurements showing the response and measuring accuracy are presented.

Direct simulation of compaction and consolidation of steel powder by eulerian analysis: J.Kihara et al. (University of Tokyo,Tokyo, Japan.)

Various thermochemical treatments, including duplex treatments, were carried out to improve the three-body abrasive wear resistance of plain carbon steels. The treatments under investigation are gas carburizing, gas carbonitriding, gas nitrocarburizing, ion nitrocarburizing, ion nitriding and stainless steel wire flame sprayed coating. The results after dry-sand-rubber-wheel (DSRW) abrasive wear tests show that all the thermochemical treatments exhibit better wear performance than the stainless steel sprayed coatings. Gas carburized and ion nitrocarburized specimens show the best wear resistance. Duplex treatments under investigation do not result in improved wear performance compared with single gas carburizing treatment.

Surface morphology and compound layer pores of plasma nitrocarburized low carbon steel

This article deals with various aspects of nitrocarburizing of AISI 1010 steel by DC plasma at 600 °C. The initial stages of nitride formation and the effects of surface bombardment by hydrogen plasma species, as well as the effect of treatment current density on the surface morphology, were investigated in detail. The results show that in the pearlitic region carbides are removed during treatment in hydrogen plasma and that the current density influences the surface morphology significantly. A porous surface formed in a 1-minute treatment under high current density (8 mA/cm2), whereas a lower current density (2.4 mA/cm2) resulted in a highly stressed surface attributed to uneven thermal gradients. Iron nitrides were identified on the surface in either set of conditions. A pure epsilon compound layer was obtained by plasma nitrocarburizing for 6 hours and quenching. The pores in this layer are discussed in view of the 1-minute treatment results and are also compared to those observed in a nitriding treatment. The lack of carbon species in the nitriding plasma results in different mechanisms for pore formation and growth during the treatment. These results are discussed in view of current models for plasma nitriding and gas nitrocarburizing.

Exploratory research on creep and fatigue properties of 9Cr steels for the steam generator of an FBR: Asada, Y., Dozaki, K., Ueta, M., Ichimiya, M., Mori, K., Taguchi, K., Kitagawa, M., Nishida, T., Sakon, T. and Sukekawa, M. Nucl Eng Des (M mar. 1993) 139 (3), 269–275

Various thermochemical treatments, including duplex treatments, were carried out to improve the three-body abrasive wear resistance of plain carbon steels. The treatments under investigation are gas carburizing, gas carbonitriding, gas nitrocarburizing, ion nitrocarburizing, ion nitriding and stainless steel wire flame sprayed coating. The results after dry-sand-rubber-wheel (DSRW) abrasive wear tests show that all the thermochemical treatments exhibit better wear performance than the stainless steel sprayed coatings. Gas carburized and ion nitrocarburized specimens show the best wear resistance. Duplex treatments under investigation do not result in improved wear performance compared with single gas carburizing treatment.

The improvement of three-body abrasive wear of plain carbon steel by thermochemical treatments

Various thermochemical treatments, including duplex treatments, were carried out to improve the three-body abrasive wear resistance of plain carbon steels. The treatments under investigation are gas carburizing, gas carbonitriding, gas nitrocarburizing, ion nitrocarburizing, ion nitriding and stainless steel wire flame sprayed coating. The results after dry-sand-rubber-wheel (DSRW) abrasive wear tests show that all the thermochemical treatments exhibit better wear performance than the stainless steel sprayed coatings. Gas carburized and ion nitrocarburized specimens show the best wear resistance. Duplex treatments under investigation do not result in improved wear performance compared with single gas carburizing treatment.

Compound Layer Characteristics Resulting from Plasma Nitrocarburising in Atmospheres Containing Carbon Dioxide Gas Additions

Plasma ferritic nitrocarburising of iron-carbon alloys and low alloy steels has been performed at 570°C in atmospheres containing carbon dioxide gas additions. Variations in compound layer structure, thickness, porosity, and composition have been investigated. It has been found that the formation of a predominantly & phase compound layer structure is favoured by the use of high nitrogen atmospheres, and cementite formation isfound to be delayed by the presence of carbon dioxide in the system. Initiation of cementite is found to be strongly dependent on substrate composition, and the formation of this cementite phase is not necessarily via pore formation or a denitriding lnechanism, as is commonly found with gaseous nitrocarburising. The compound layer thickness was found to increase with increasing carbon dioxide content, except where the substrate contained carbides, when a continuous reduction in compound layer thickness was observed. Some degree of porosity in the & subregion may develop during plasma nitrocarburising with high nitrogen additions. The pores are of an intergranular type, and the pore walls can dissolve oxygen when in direct contact with an open atmosphere. Proper process control and the addition of carbon dioxide to the atmosphere results in a reasonably thick compound layer composed predominantly of & phase.