Gas Boriding Research Articles

РАЗРАБОТКА ТЕХНОЛОГИИ ГАЗОВОГО БОРИРОВАНИЯ В ЗАМКНУТЫХ ОБЪЕМАХ

Boriding is a promising process of chemical-thermal treatment, however, the duration of the process, the high cost of the components of the mixture for boriding, as well as the high costs of the processes of further cleaning of parts hardened by boriding from the remnants of the saturating medium reduce the degree of demand and distribution of boriding technology. This paper describes the results of the scientific school of prof. A.M. Guryev on the development of highly active saturating mixtures for gas boriding in sealed volumes and with the possibility of generating being saturated active gases from a powder mixture directly in the volume. The developed and proposed technologies do not require hardened surface cleaning operations, since there is no direct contact of the hardened part with the saturating powder. In addition, due to the closed volume in which active boron-containing gas is generated, there are no harmful emissions into the environment, which makes it possible to attribute the proposed saturation method to environmentally friendly, resource- and energy-efficient processes

The role of the carbon diffusion zone presence for the fracture toughness and wear resistance of a single-phase Fe2B layer produced on pure iron by gas boriding in N2–H2–B(CH3O)3 atmosphere

The aim of this work was to indicate the importance of the presence of the carbon diffusion zone on the fracture toughness of Fe2B layer. Single-phase Fe2B layer was produced as a result of gas boriding processes and simultaneous gas borocarburizing processes. The continuous gaseous processes were carried out at 1223 K for 2 h in N2–H2–B(CH3O)3 atmosphere. The temperature of trimethyl borate influenced its concentration in the gas atmosphere, therefore it was possible to carry out the two types of process: boriding and borocarburizing. As a result, in the case of the borided sample, the area directly of the Fe2B layer contained ferrite. Whereas, in the case of the borocarburized sample, pearlite grains appeared beneath the iron borides layer. Fracture toughness measurements were performed on the top surface of the specimens using a Vickers indenter under a load range from 0.981 N to 4.905 N. Both borided and borocarburized layers represent ceramic materials with a radial-median cracking mode. In the case of application of an indentation load lower than 3.924 N, an increase in indentation load from 0.981 N to 3.924 N resulted in increased fracture toughness. Whereas, a further increase in the load to 4.905 N was the reason for KC diminishing. In the case of high indentation load, the presence of a carbon diffusion zone in the gas borocarburized sample caused an increase in fracture toughness by about 50% compared to the gas-borided sample. The iron boride layers produced by gas boriding and simultaneous gas borocarburizing provided higher wear resistance in comparison to the pure iron without thermo-chemical surface treatment.

The importance of phase composition for corrosion resistance and thermal oxidation behavior of gas-borided layer produced on nickel alloy

Abstract Gas boriding was used to produce the borided layer containing a mixture of chromium and nickel borides on the Inconel®600-alloy. The borided sample was characterized by a higher corrosion potential (−0.953 V) than the non-borided sample (−1.005 V). The corrosion current density was significantly lower for the borided sample. The oxidation at 1000 °C for 24 h caused the formation of different oxides on the surface of the borided sample. Simultaneously, the presence of nickel and chromium borides was confirmed by XRD analysis after the oxidation test. It was concluded, that the gas boriding could be an effective barrier against corrosion and oxidation of Inconel®600-alloy.

Influence of nickel silicides presence on hardness, elastic modulus and fracture toughness of gas-borided layer produced on Nisil-alloy

The two-stage gas boriding in N2−H2−BCl3 atmosphere was applied to producing a two-zoned borided layer on Nisil-alloy. The process was carried out at 910 °C for 2 h. The microstructure consisted of two zones differing in their phase composition. The outer layer contained only a mixture of nickel borides (Ni2B, Ni3B) only. The inner zone contained additionally nickel silicides (Ni2Si, Ni3Si) occurring together with nickel borides. The aim of this study was to determine the presence of nickel silicides on the mechanical properties of the borided layer produced on Ni-based alloy. The hardness and elastic modulus were measured using the nanoindenter with a Berkovich diamond tip under a load of 50 mN. The average values of indentation hardness (HI) and indentation elastic modulus (EI) obtained in the outer zone were respectively (16.32±1.03) GPa and (232±16.15) GPa. The presence of nickel silicides in the inner zone reduced the indentation hardness (6.8−12.54 GPa) and elastic modulus (111.79−153.99 GPa). The fracture toughness of the boride layers was investigated using a Vickers microindentation under a load of 0.981 N. It was confirmed that the presence of nickel silicides caused an increase in brittleness (by about 40%) of the gas-borided layer.

Importance of trimethyl borate temperature used during gas boriding for microstructure, nanomechanical properties and residual stresses distribution on the cross-section of the produced layer

The aim of this work was to indicate the possibility of applying organic compounds as a boron source for gas boriding. In the present work the trimethyl borate was used as an organic boron source for gas boriding process. The process was carried out at 950 °C for 2 h in gaseous atmosphere composed of N2–H2–B(CH3O)3. The temperature of trimethyl borate influenced on its concentration in gas atmosphere. As a result, depending on B(CH3O)3 temperature of 20 °C or 50 °C, it was possible to arranging the two types of process: boriding and borocarburizing, respectively. In the case of gas boriding the single-phase Fe2B layer was produced. The high temperature of B(CH3O)3 caused release of free atoms of carbon, therefore there existed favorable conditions for carburizing. The produced borocarburized layer consisted of two zones: an outer Fe2B borided layer and an inner carburized zone. The thickness of boride layer was higher after boriding process than simultaneous borocarburizing process, 10.8 μm and 7.8 μm, respectively. Whereas, the depth of zone of carbon diffusion was equal ca. 400 μm. For nanomechanical properties, as well as, the residual stress distribution the nanoindentation tester Anton Paar NHT3 equipped with the Berkovich diamond tip under a maximum load of 10 mN was used. In both layers, the highest hardness HIT (7.8–17.9 GPa) and highest Young's modulus (222–368 GPa) were measured in Fe2B layer. However, the presence of thick zone of carbon diffusion was the reason for gradually decrease in hardness in the cross section of borocarburized layer. Moreover, the presence of carburized zone advantageous influenced on residual stresses distribution across the layer. The gradually changes of residual stresses from compressive to tensile were observed in the case of simultaneous gas borocarburized layer. Such a situation was more advantage than those obtained for gas borided layer.

Gas Technique of Simultaneous Borocarburizing of Armco Iron Using Trimethyl Borate

The gas boriding process is an appropriate technique used for increasing the hardness and wear resistance of iron and steels. However, the boron halides (e.g., BCl3, BF3) are rarely used as a boron source during gas boriding in industry due to the toxic character of these reagents. The possibility of the use of organic compounds as a boron source in plasma assisted processes was the instigation to determine the possibility of applying these agents for gas boriding. In the present work trimethyl borate was used as an organic boron source. The use of a N2–H2–B(CH3O)3 atmosphere ensured the appropriate conditions for the simultaneous gas borocarburizing of Armco iron. The process was carried out at 1223 K (950 °C) for 2 h. The produced layer consisted of two zones: an outer zone containing a diffusion of boron atoms and an inner zone containing a diffusion of carbon atoms, under the outer zone. Due to the reduction of trimethyl borate with hydrogen, free atoms of carbon were released for the gas atmosphere. Therefore, there existed favorable conditions for carburizing. Unfortunately, the formation of a carburized layer was the reason for the difficult diffusion of boron atoms. As a consequence, the boron diffusion front was hindered, and the outer boride layer was relatively thin (ca. 7.8 µm). The boride layer contained only Fe2B phase, which was characterized by high hardness in the range from 1103 HV0.01 to 1546 HV0.01. The presence of iron borides in the outer layer was also the reason for increased wear resistance in comparison with untreated Armco iron.

Nanomechanical properties and fracture toughness of hard ceramic layer produced by gas boriding of Inconel 600 alloy

Abstract Gas-boriding in N2−H2−BCl3 atmosphere resulted in the formation of a thick layer on Inconel 600 alloy. The microstructure of layer produced at 920 °C for 2 h consisted of a mixture of chromium borides and nickel borides. The objective of investigations was to determine the influence of the chemical and phase compositions of borided layer on its mechanical properties. The nanoindentation was carried out using Berkovich diamond tip under a load of 50 mN. The gas-borided layer was characterized by high indentation hardness HIT from 1542.6 HV to 2228.7 HV and high elastic modulus EIT from 226.9 to 296.4 GPa. It was found that the mixture with higher percentage of chromium borides was the reason for the increase in HIT and EIT values. The fracture toughness (KC) was measured using Vickers microindentation technique under a load of 0.98 N. The presence of high compressive stresses in normal direction to the top surface caused the strong anisotropy of the borided layer, in respect of fracture toughness. The high difference between the lowest (0.5763 MPa·m1/2) and the highest (4.5794 MPa·m1/2) fracture toughness was obtained. This situation was caused by the differences in chemical and phase compositions of tested areas, presence of porosity and residual stresses. Generally, the higher KC values were obtained in areas with lower chromium content.

Nanomechanical characterization and fracture toughness of FeB and Fe2B iron borides produced by gas boriding of Armco iron

FeB and Fe2B hard ceramic phases were produced in Armco iron using gas-boriding in N2–H2–BCl3 atmosphere. This process was carried out at 920°C (1193K) for 3h and caused acceleration in the diffusion of boron into the surface of a base material in comparison with other acceptable methods of diffusion boriding. FeB and Fe2B layers were characterized by a strong zonation, obtaining the average thickness of 32μm and 125μm, respectively. Young's moduli and hardness of iron borides were measured using the nanoindenter with a Berkovich diamond tip under load of 50mN. The higher average indentation hardness (HIT=20.95±0.93GPa) and Young's modulus (EIT=308.86±26.44GPa) were characteristic of FeB phase. Fe2B boride was characterized by HIT=17.42±0.80GPa, and EIT=252.96±15.57GPa. The fracture toughness of iron borides was measured by microindentation technique using a Vickers diamond indenter under load of 100gf (about 0.981N). The average fracture toughness (Kc), measured in FeB zone, was equal to 1.79±0.70MPa·m1/2. Fe2B phase was characterized by higher fracture toughness, obtaining Kc=2.42±0.66MPa·m1/2. However, at FeB/Fe2B interface the increase in brittleness was detected. Such a situation was caused by the differences in coefficients of thermal expansion of both iron borides and their mechanical properties. It could provide the preliminary cracks at this interface after cooling. During indentation, the value of shear stress probably exceeded the value of normal compressive stress. It could cause the failure at this interface, facilitating cracks' propagation.

Corrosion Behavior of Hard Boride Layer Produced on Nimonic 80A-Alloy by Gas Boriding

The gas boriding in N2–H2–BCl3 atmosphere was applied in order to produce a wear resistant surface layer on Nimonic 80A-alloy samples. The microstructure, microhardness and corrosion resistance of the boride layer were investigated. The produced layer consisted mainly of the compact boride zone (with average thickness 71 μm), including the mixture of nickel and chromium borides of high hardness (up to 1861 HV). In order to evaluate the corrosion behavior, the two methods of corrosion tests were used: potentiodynamic corrosion test in 5% NaCl solution and immersion corrosion test in a boiling solution of H2O, H2SO4 and Fe2(SO4)3. The results showed that gas boriding could provide the excellent corrosion resistance if the whole surface of a Nimonic 80A-alloy sample was covered by the continuous boride layer. Otherwise, as a consequence of selective boriding, the significant difference in electrochemical potentials caused an accelerated uniform corrosion of the base material.

Influence of gas boriding on corrosion resistance of Inconel 600-alloy

Purpose: The aim of this study was to analyse the corrosion behaviour of gas-borided layers produced on Inconel 600-alloy. Two types of the borided layers were produced: fully borided and partially borided layer. The results obtained for gas-borided specimens were compared to untreated Inconel 600-alloy. Design/methodology/approach: In this paper, gas boriding in N2-H2-BCl3 atmosphere was applied to produce the boride layers on Inconel 600-alloy. This process was carried out at 910°C (1193 K) for 2 h. Microstructure observations were carried out using a light microscope. The hardness measurements were performed using a Vickers method under a load of 0.981 N. In order to evaluate the corrosion resistance, the immersion corrosion test in a boiling solution of H2O, H2SO4 and Fe2(SO4)3 was used. Findings: The gas-borided layers consisted of a mixture of nickel borides (Ni3B, Ni2B, Ni4B3, NiB) and chromium borides (CrB, Cr2B). The high thickness of compact boride layer (76-79 μm) as well as high hardness (up to 2061 HV) were obtained. Based on corrosion resistance tests it was found that in case of untreated sample the strong intergrannular attack was observed. Whereas the corrosion behavior ofgas-borided Inconel 600-alloy was more complicated and resulted from the surface condition. Research limitations/implications: The obtained results indicated that gas-boriding in N2-H2-BCl3 atmosphere could be a suitable corrosion protection if the whole surface would be covered with boride layer. Practical implications: The parameters of gas boriding in N2-H2-BCl3 atmosphere used in this study (temperature of 910°C for 2 h) allowed to produced layers of a higher thickness in comparison with other acceptable method of boriding e.g. powder-pack boriding. Originality/value: Based on the results it was found that gas-boriding in N2-H2-BCl3 atmosphere is a suitable method to protect Inconel 600-alloy from corrosion.

Fracture toughness of hard ceramic phases produced on Nimonic 80A-alloy by gas boriding

A hard boride layer was produced on Nimonic 80A-alloy using gas boriding in N2–H2–BCl3 atmosphere. This process was carried out at 920°C (1193K) for 2h. Proposed gas boriding accelerated the diffusion of boron into the surface of workpiece in comparison with other acceptable methods of diffusion boriding. The comparable thickness of boride layer was obtained after considerably shorter duration and at lower temperature. The microstructure, microhardness and fracture toughness were in detail studied. The boride layer consisted of a mixture of hard ceramic phases, i.e. nickel and chromium borides. The fracture toughness of borided layer was measured by microindentation technique using a Vickers diamond indenture under loading 100gf (about 0.981N). The measurements were carried out through a cross-section of the specimen in order to determine the differences in brittleness distribution. The changeable surface percentage of hard ceramic phases (chromium and nickel borides׳ mixtures) strongly influenced the fracture toughness which obtained the values from 0.609 to 4.436MPam1/2.

Microstructural characterization and some mechanical properties of gas-borided Inconel 600-alloy

Abstract The excellent resistance of Ni-based alloys to corrosion and oxidation is well-known. Boriding can be applied to these alloys in order to obtain suitable wear protection. In this paper, two-stage gas boronizing in N2–H2–BCl3 atmosphere is proposed for the producing the boride layer on Inconel®600-alloy. This process consists in two stages alternately repeated: saturation by boron and diffusion annealing. Such a gas boriding is applied in order to accelerate the saturation by boron and its diffusion. It turns out to be more effective because of eliminating the excess of boron, diffusing into the substrate, during the second stage. Microstructure and some mechanical properties of the produced layer are presented. Microstructural characterization is studied with using an optical microscope, scanning electron microscope, energy-dispersive x-ray microanalysis and x-ray diffraction. The diffusion zone consists of the mixture of nickel and chromium borides, occurring in the compact boride zone and in the area located beneath, at grain boundaries. The improved hardness and wear resistance characterize the layer. The formed boride layer is significantly thicker than those-obtained by the pack-boronizing or paste process at comparable temperature and time. Simultaneously, the measured depth of layer is slightly smaller than that-reported for electrolytic boriding.

Two-stage gas boriding of Nisil in N2–H2–BCl3 atmosphere

Boriding can be applied to a wide range of steels, Ni-based alloys, Co-based alloys, molybdenum or sintered carbides. If Ni-based alloys have to be applied under conditions of appreciable mechanical wear (adhesive or abrasive), these materials will require suitable wear protection. Boronizing offers a suitable alternative for protecting these materials from wear. In this paper, two-stage gas boronizing in N2–H2–BCl3 atmosphere was proposed for producing the boride layer on Ni-based alloy called Nisil. This process consisted in two stages: saturation by boron and diffusion annealing. The stages were alternately repeated. The process was applied in order to accelerate the saturation by boron and its diffusion. It turned out to be more effective because of the elimination of the excess boron from the atmosphere and because of the boron diffusion into the substrate during the second stage. The microstructure consisted of nickel borides (Ni2B, Ni3B) and nickel silicides (Ni2Si, Ni3Si) formed as the result of the movement of silicon towards the core by following the boron diffusion front. The formed boride layer was 1.5 times thicker than that obtained by pack-boronizing using powder without SiC at comparable temperature and time.

Simulation of the growth kinetics of boride layers formed on Fe during gas boriding in H2-BCl3 atmosphere

The modeling of the boriding kinetics is considered as a necessary tool to select the suitable process parameters for obtaining boride layer of an adequate thickness. Therefore, the simulation of the growth kinetics of boride layers has gained much attention for last years. The majority of the published works described the kinetics of the pack-boriding or paste-boriding. In this study, the model of growth kinetics of two-phase boride layer (FeB+Fe2B) on pure Fe was proposed for gas boriding. Displacements of the two interfaces (FeB/Fe2B and Fe2B/substrate) resulted from a difference of the arrival flux of interstitial boron atoms to one phase and the departure flux of the boron atoms from this phase to the second phase. The mass balance equations were formulated. The measurements of thickness of both zones (FeB and Fe2B), for different temperature of boriding, were used for calculations. Based on the experimental data, the parabolic growth constants AFeB and BFe2B versus the temperature of boriding were determined. The linear relationships were accepted. As a consequence, the activation energies (QFeB and QFe2B) were calculated. The calculated values were comparable to other data derived from gas boriding. The presented model can predict the thicknesses of the FeB and Fe2B zones (XFeB and YFe2B, respectively) formed on pure Fe during gas boriding. Additionally, the diffusion annealing after boriding was analyzed. This process was carried out in order to obtain a single-phase boride layer (Fe2B). The relationship between the reduction in FeB zone (dXFeB) and the growth in Fe2B phase (dYFe2B) was determined. The time tXFeB=0, needed for the total elimination of FeB phase in the boride layer was calculated and compared to the experimental data.

An alternative method of gas boriding applied to the formation of borocarburized layer

The borocarburized layers were produced by tandem diffusion processes: carburizing followed by boriding. An alternative method of gas boriding was proposed. Two-stage gas boronizing in N2–H2–BCl3 atmosphere was applied to the formation of iron borides on a carburized substrate. This process consisted in two stages, which were alternately repeated: saturation by boron and diffusion annealing. The microstructure and microhardness of produced layer were compared to those-obtained in case of continuous gas boriding in H2–BCl3 atmosphere, earlier used. The first objective of two-stage boronizing, consisting in acceleration of boron diffusion, has been efficiently implemented. Despite the lower temperature and shorter duration of boronizing, about 1.5 times larger iron borides' zone has been formed on carburized steel. Second objective, the absolute elimination of brittle FeB phase, has failed. However, the amount of FeB phase has been considerably limited. Longer diffusion annealing should provide the boride layer with single-phase microstructure, without FeB phase.

Characterization of complex (B + C + N) diffusion layers formed on chromium and nickel-based low-carbon steel

Combined surface hardening with boron, carbon and nitrogen was used for low-carbon chromium and nickel-based steels. The microstructure, boron and nitrogen contents, carbon profiles and chosen properties of borided layers produced on the carbonitrided steels have been examined. These complex (B+C+N) layers are termed borocarbonitrided layers. Gas boriding applied to these steels that have been previously carbonitrided enables the production of wear-resistant diffusion layers. After combined surface hardening with boron, carbon and nitrogen in the microstructure two zones have been observed: iron borides (FeB+Fe 2B) and carbonitrided zone. The iron borides show the tendencies towards lose of the needle-like nature. Although borocarbonitriding causes reducing of the depth of iron borides zone, it also reduces the microhardness gradient across the case. Probably, the brittleness of borocarbonitrided layer is lower. In the consequence, the frictional wear resistance of borocarbonitrided layers is essentially higher than that obtained in case of only borided layers. The similar values of wear intensity factors are characteristic for borocarburized layers. The carbonitriding used before boriding make possible the application of lower temperature and shorter time in comparison with carburizing process.

The importance of carbon content beneath iron borides after boriding of chromium and nickel-based low-carbon steel

The complex (B+C) diffusion layers have been formed on chromium- and nickel-based low-carbon steels. Gas boriding applied to these steels that have been previously carburized enables the production of wear-resistant borocarburized layers. After combined surface hardening with boron and carbon in the microstructure two zones have been observed: iron borides (FeB+Fe2B) and carburized zones. The iron borides in borocarburized layer show the tendencies towards lose of the needle-like nature. The depth of iron borides zone depend on pre-carburizing parameters. Although borocarburizing causes reducing of the depth of iron borides zone, it also reduces the microhardness gradient across the case. An increase of distance from the surface is accompanied by a gradual decrease of carbon content and microhardness in carburized zone. Probably, the brittleness of borocarburized layer is lower. In the consequence, the frictional wear resistance of borocarburized layers is essentially higher than that obtained in case of only borided layers. There is every indication that the carbon content beneath iron borides has an important effect on wear resistance, too. The highest wear resistance of borocarburized layer has been observed in case of about 1.2% C below iron borides.

Gas Boriding Conditions for the Iron Borides Layers Formation

Gas Boriding Conditions for the Iron Borides Layers Formation

Study of screw mixing of tungsten-nickel-copper heavy alloy

Combined surface hardening with boron, carbon and nitrogen was used for low-carbon chromium and nickel-based steels. The microstructure, boron and nitrogen contents, carbon profiles and chosen properties of borided layers produced on the carbonitrided steels have been examined. These complex (B+C+N) layers are termed borocarbonitrided layers. Gas boriding applied to these steels that have been previously carbonitrided enables the production of wear-resistant diffusion layers. After combined surface hardening with boron, carbon and nitrogen in the microstructure two zones have been observed: iron borides (FeB+Fe2B) and carbonitrided zone. The iron borides show the tendencies towards lose of the needle-like nature. Although borocarbonitriding causes reducing of the depth of iron borides zone, it also reduces the microhardness gradient across the case. Probably, the brittleness of borocarbonitrided layer is lower. In the consequence, the frictional wear resistance of borocarbonitrided layers is essentially higher than that obtained in case of only borided layers. The similar values of wear intensity factors are characteristic for borocarburized layers. The carbonitriding used before boriding make possible the application of lower temperature and shorter time in comparison with carburizing process.

Boriding of steels in gaseous media

1. Gas boriding in boron chloride is not very promising for mass production due to technological difficulties. 2. Boriding is most promising in a hydrogen atmosphere with application of thin coatings (0.1–0.5 mm) of amorphous boron. The process is characterized by: a) technological simplicity of all operations; b) a high utilization coefficient of boron and low expenditure of hydrogen; c) the possibility of local boriding; d) the absence of waste products contaminating the environment; e) formation of single-phase (Fe2B) diffusion coatings, which is of particular importance for hardening parts subject to dynamic and impact loads in operation.