Kinetics of electrochemical boriding of low carbon steel

In this study, the growth kinetics of the boride layers forming on low carbon steel substrates was investigated during electrochemical boriding which was performed at a constant current density of 200 mA/cm² in a borax based electrolyte at temperatures ranging from 1123 K to 1273 K for periods of 5-120 min. After boriding, the presence of both FeB and Fe₂B phases were confirmed by the X-ray diffraction method. Cross-sectional microscopy revealed a very dense and thick morphology for both boride phases. Micro hardness testing of the borided steel samples showed a significant increase in the hardness of the borided surfaces (i.e., up to (1700 ± 200) HV), while the hardness of un-borided steel samples was approximately (200 ± 20) HV. Systematic studies over a wide range of boriding time and temperature confirmed that the rate of the boride layer formation is strongly dependent on boriding duration and has a parabolic character. The activation energy of boride layer growth for electrochemical boriding was determined as (172.75 ± 8.6) kJ/mol.     

Characterization of AISI 4140 borided steels

The present study characterizes the surface of AISI 4140 steels exposed to the paste-boriding process. The formation of Fe₂B hard coatings was obtained in the temperature range 1123-1273 K with different exposure times, using a 4 mm thick layer of boron carbide paste over the material surface. First, the growth kinetics of boride layers at the surface of AISI 4140 steels was evaluated. Second, the presence and distribution of alloying elements on the Fe₂B phase was measured using the Glow Discharge Optical Emission Spectrometry (GDOES) technique. Further, thermal residual stresses produced on the borided phase were evaluated by X-ray diffraction (XRD) analysis. The fracture toughness of the iron boride layer of the AISI 4140 borided steels was estimated using a Vickers microindentation induced-fracture testing at a constant distance of 25 μm from the surface. The force criterion of fracture toughness was determined from the extent of brittle cracks, both parallel and perpendicular to the surface, originating at the tips of an indenter impression. The fracture toughness values obtained by the Palmqvist crack model are expressed in the form K_C(π/2) > K_C > K_C(0) for the different applied loads and experimental parameters of the boriding process.     

Vanadium carbide coatings on die steel deposited by the thermo-reactive diffusion technique

Thermal reactive diffusion coating of vanadium carbide on DIN 1.2367 die steel substrate was performed in a powder mixture consisting of ferro-vanadium, ammonium chloride, alumina and naphthalene at 950, 1050 and 1150 °C for 1-5 h. The carbide layers were characterized by means of microstructure, microhardness, X-ray diffraction and chemical analysis. Depending on the coating process time and temperature, the thickness of the vanadium carbide layer formed on the substrate ranged from 2.3 to 23.2 μm. The hardness of vanadium carbide layers was about 2487 HV. Dry wear tests for uncoated and coated DIN 1.2367 die steel were carried out on pin-on-disk configuration and at a sliding speed of 0.13 m/s. The results showed superior wear properties of the coated samples. The kinetics of vanadium carbide coating by the pack method was also studied and the activation energy for the thermo-reactive diffusion process was estimated to be 173.2 kJ/mol.     

Growth kinetics of borided layers: Artificial neural network and least square approaches

The present study evaluates the growth kinetics of the boride layer Fe₂B in AISI 1045 steel, by means of neural networks and the least square techniques. The Fe₂B phase was formed at the material surface using the paste boriding process. The surface boron potential was modified considering different boron paste thicknesses, with exposure times of 2, 4 and 6 h, and treatment temperatures of 1193, 1223 and 1273 K. The neural network and the least square models were set by the layer thickness of Fe₂B phase, and assuming that the growth of the boride layer follows a parabolic law. The reliability of the techniques used is compared with a set of experiments at a temperature of 1223 K with 5 h of treatment time and boron potentials of 2, 3, 4 and 5 mm. The results of the Fe₂B layer thicknesses show a mean error of 5.31% for the neural network and 3.42% for the least square method.     

Kinetics and mechanical study of plasma electrolytic carburizing for pure iron

In this work, plasma electrolytic surface carburizing of pure iron in aqueous solution consisting of water, glycerin and NH₄Cl was investigated. Surface carburizing was carried out in 20% glycerin solution treated at 750 °C, 800 °C, 900 °C and 950 °C temperatures for 5, 10 and 30 min. The formation of hard carbon-rich layer on the surface of pure iron was confirmed by XRD analysis. Metallographic and SEM studies revealed a rough and dense carburized layer on the surface of the pure iron. Experimental results showed that the thickness of the carburized layers changes with the time and temperature. The average thickness of the carburized layer ranged from 20 to 160 μm. The hardness of the carburized samples decreased with the distance from the surface to the interior of the test material. The average hardness values of the carburized layers on the substrate ranged 550-850 HV, while the hardness of the substrate ranged from 110 HV to 170 HV. The dominant phases formed on the pure iron were found to be a mixture of cementite (Fe₃C), martensite (Fe + C) and austenite (FCC iron) confirmed by XRD. Wear resistance in all plasma electrolytic carburized samples is considerably improved in relation to the untreated specimen. After carburizing, surface roughness of the samples was increased. Friction coefficients were also increased because of high surface roughness.     

The effect of nanostructured surface layer on the fatigue behaviors of a carbon steel

A nanostructured surface layer was formed on a carbon steel by means of surface mechanical attrition treatment (SMAT). The microstructure of the surface layer of the SMATed sample was characterized by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microhardness and residual stress distribution along the depth from the SMATed surface layer were measured at the same time. Fatigue behaviors of the carbon steel subjected to the SMAT process were investigated. A nanostructured layer with average grains size of ∼12.7 nm was formed, of which microhardness is more than twice as high as that in matrix and residual compressive stress can reach about −400 MPa with maximum depth of ∼600 μm. The fatigue strength of as-received sample is 267 MPa and that of SMATed sample is 302 MPa based on fatigue life 5 × 10⁶ cycles. The SMAT process has improved the fatigue strength by as much as 13.1% for the carbon steel. It is shown that the SMAT is an effective method to render the material with the features, such as a nanostructured and work-hardened surface layer as well as compressive residual stresses, which can pronouncedly improve the fatigue strength of the carbon steel.     

Surface modification of Al-20Si alloy by high current pulsed electron beam

Hypereutectic Al-20Si (Si 20 wt.%, Al balance)alloy surface was treated with high current pulsed electron beam (HCPEB) under different pulse numbers. The results indicate that HCPEB irradiation induces the formation of metastable structures on the treated surface. The coarse primary Si particle melts, producing a “halo” microstructure with primary Si as the center on the melted surface. A supersaturated solid solution of Al is formed in the melted layer caused by Si atoms dissolving into the Al matrix. Cross-section structure analysis shows that a 4 μm remelted layer is formed underneath the top surface of the HCEPB-treated sample. Compared with the matrix, the Al and Si elements in the remelted layer are distributed uniformly. In addition, the grains of the Al-20Si alloy surface are refined after HCPEB treatment, as shown by TEM observation. Nano-silicon particles are dispersed on the surface of remelted layer. Polygonal subgrains, approximately 50-100 nm in size, are formed in the Al matrix. The hardness test results show that the microhardness of the α(Al) and eutectic structure is increased with increasing pulse number. The hardness of the “halo” microstructure presents a gradient change after 15 pulse treatment due to the diffusion of Si atoms. Furthermore, hardness tests of the cross-section at different depths show that the microhardness of the remelted layer is higher than that of the matrix. Therefore, HCPEB technology is a good surface modification method for enhancing the surface hardness of hypereutectic Al-20Si alloy.     

A diffusion model for describing the bilayer growth (FeB/Fe2B) during the iron powder-pack boriding

In this paper, a diffusion model is proposed for studying the bilayer growth kinetics (FeB/Fe₂B) on pure iron substrate during the powder-pack boriding in the temperature range of 1023-1273 K.This model based on Fick's laws was solved, under certain assumptions, considering a parabolic growth of iron borides.For this purpose, a computer simulation program was created for predicting the boride layer thickness as a function of process parameters (temperature, time and surface boron content). A fairly good agreement was observed between the simulation calculations and experimental data derived from the literature.     

Effect of heat treatment on the microstructure and microhardness of cold-sprayed tin bronze coating

Tin bronze (TB) powder was deposited on a stainless steel substrate by cold spraying. Post-deposition heat treatment was conducted in an electrical resistance furnace under nitrogen atmosphere at a temperature of 850 °C for 3 h. The effect of heat treatment on the microstructure and microhardness of cold-sprayed TB coating was investigated. It was found that the as-sprayed TB coating presented a dense microstructure. Heat treatment significantly influenced the microstructure and microhardness of cold-sprayed TB coating. A distinguishable diffusion layer of about 150 μm was formed in the coating near the coating/substrate interface. A compound was precipitated in the diffusion layer. The microhardness in the coating was changed gradually along the coating from the interface to the coating surface after heat treatment. The microhardness in the diffusion layer was high owing to the precipitation of hard phase, while it was much low in other area due to the obvious grain growth during annealing.     

Microstructure and properties of borocarburized and laser-modified 17CrNi6-6 steel

Two-step process: carburizing followed by boriding was applied to the formation of borocarburized layers. The boride layer formed on the substrate of changeable chemical and phase composition (e.g. borocarburized layer) was called “gradient boride layer”, in contrast to “typical boride layer”, formed on the substrate of constant chemical and phase composition. Until now, the typical heat treatment of borocarburized layer consisted of treatment through hardening: quenching in oil and low-temperature tempering. In this paper, instead of treatment through hardening, laser-heat treatment was employed. The properties of such layer were compared to the properties of typical carburized layer. Three zones characterized the microstructure of laser-modified borocarburized layer: iron borides (FeB+Fe₂B) of modified morphology, hardened carburized zone (heat affected zone) and carburized layer without heat treatment. X-ray microanalysis indicated the increased boron concentration close to the surface due to the occurrence of a mixture of FeB and Fe₂B borides. Near to the hardened carburized zone, Fe₂B phase occurred in the laser-modified boride zone. Laser-heat treated borocarburized layer was characterized by higher microhardness at the surface than that obtained in case of carburized layer. It was caused by the iron borides (FeB+Fe₂B) occurrence at the surface, as a consequence of boriding process. However, the carburized layer was characterized by considerably larger hardened zone. Higher abrasive wear resistance, but lower low-cycle fatigue strength in comparison with the carburized layer, characterized the gradient boride layer formed by borocarburizing and laser surface modification. The indentation craters obtained on the surface of laser-heat treated borocarburized layer revealed sufficient cohesion (HF3 standard). The use of laser-modified borocarburized layers may be advantageous under conditions of high abrasive wear of mating parts. In case of parts, which require high resistance to fatigue, the carburized layer is irreplaceable.     

Gradient formation of boride layers by borocarburizing

In this study borocarburizing was used for the formation of gradient boride layers. The microstructure, microhardness profiles and the low-cycle fatigue strength during radial compression of carburized, borided and borocarburized layer have been compared. The gradient borocarburized layers, formed by boriding of previously carburized substrate, are characterized by two zones in diffusion layer: iron borides zone and carburized zone. After borocarburizing the iron borides show a tendency towards a loss of the needle-like nature. The hardness gradient between iron borides and low-carbon substrate is reduced. The microhardness beneath the iron borides decreases to 900 HV in carburized zone and next gradually decreases to 400–450 HV in the core of steel. The highest resistance to low-cycle fatigue during radial compression has been observed in case of carburized and through hardened layer. The fatigue strength of gradient boride layer (borocarburized and through hardened) is a little lower. The typical borided and through hardened layer is characterized by the lowest resistance to low-cycle fatigue during radial compression. The profiles of stresses after boriding and borocarburizing have been compared. The obtained profile of stresses and the lower values of tensile stresses at the surface can be the reason for higher frictional wear resistance of borocarburized layers and for higher fatigue strength of these layers, too.     

Evaluation of boron mobility on the phases FeB, Fe2B and diffusion zone in AISI 1045 and M2 steels

The present study evaluates the growth kinetics of boride layers at the material surface on AISI 1045 and M2 steels during the paste boriding process. This surface hardening technique produces on the material two characteristic phases FeB, Fe₂B and a transition zone, denominated diffusion zone, in the layer/substrate interface. The thermochemical treatment was done at three different temperatures: 1193, 1223 and 1273 K with two treatment times: 2 and 6 h for the 1045 steel, and 1223, 1253 and 1273 K with the same treatment times for M2 steel, modifying the boron potentials in equilibrium at the substrates surfaces. Using the mass balance equation, and assuming a linear concentration profile at the interfaces, the mobility of boron was determined on both types of steels. The influence of boron potential, treatment time and temperatures is clearly observed on the growth kinetics of boride layers.     

The effect of surface nanocrystallization on plasma nitriding behaviour of AISI 4140 steel

A plastic deformation surface layer with nanocrystalline grains was produced on AISI 4140 steel by means of surface mechanical attrition treatment (SMAT). Plasma nitriding of SMAT and un-SMAT AISI 4140 steel was carried out by a low-frequency pulse excited plasma unit. A series of nitriding experiments has been conducted at temperatures ranging from 380 to 500 °C for 8 h in an NH₃ gas. The samples were characterized using X-ray diffraction, scanning electron microscopy, optical microscopy and Vickers microhardness tester. The results showed that a much thicker compound layer with higher hardness was obtained for the SMAT samples when compared with un-SMAT samples after nitriding at the low temperature. In particular, plasma nitriding SMAT AISI 4140 steel at 380 °C for 8 h can produced a compound layer of 2.5 μm thickness with very high hardness on the surface, which is similar to un-SMAT samples were plasma nitrided at approximately 430 °C within the same time.     

A model for studying the kinetics of the formation of Fe2B boride layers at the surface of a gray cast iron

The present work estimates, using a kinetic model, the growth kinetics of Fe₂B boride layers generated at the surface of a gray cast iron via the powder-pack boriding considering three different temperatures (1173, 1223 and 1273 K) and four treatment times (2, 4, 6 and 8 h). By the use of the mass balance equation at the (Fe₂B/substrate) interface under certain assumptions and considering the effect of the boride incubation time, it was possible to estimate the corresponding parabolic growth constant in terms of two parameters and β(T) depending on the boron content in the Fe₂B phase and on the process temperature, respectively. The mass gain at the material surface and the instantaneous velocity of the (Fe₂B/substrate) interface were also estimated. A fairly good agreement was observed between the experimental parabolic growth constants taken from a reference work (Campos-Silva et al., Characterization of boride layers formed at the surface of gray cast irons, Kovove Mater. 47 (2009) 1-7.) and the simulated values of the parabolic growth constants. Furthermore, the boride layer thicknesses were predicted and experimentally verified for three process temperatures and four treatment times.     

Fatigue properties of a S45C steel subjected to ultrasonic nanocrystal surface modification

An ultrasonic nanocrystal surface modification (UNSM) technique, at 3 different vibration strike numbers (34,000 times/mm², 45,000 times/mm², 68,000 times/mm²) was used to modify the surface structure and properties of S45C. These three process conditions respectively produced 2 μm, 12 μm and 30 μm nanocrystal layers. UNSM technique improves the following mechanical properties: microhardness, surface roughness, and compressive residual stress. Also, fatigue life increased with the vibration strike number. UNSM C3 (with the vibration strike number of 68,000 times/mm²) has improved the fatigue strength by as much as 33% for S45C. Optical microscope pictures show that cracks usually initiate from intergranular microcracks on the surface and then extend along the tip traces of UNSM which are considered as process defects. A simple math model (tearing adhesive plaster model) has been made to analyze the initiation and growth of cracks. Though most of the cracks initiate at the surface of specimens, surface nanocrystal layers can help to retard crack initiation. In S45C, the efficiency of crack resistance is more than 48%.the vibration strike number     

Nanolayered multilayer coatings of CrN/CrAlN prepared by reactive DC magnetron sputtering

Single-phase CrN and CrAlN coatings were deposited on silicon and mild steel substrates using a reactive DC magnetron sputtering system. The structural characterization of the coatings was done using X-ray diffraction (XRD). The XRD data showed that both the CrN and CrAlN coatings exhibited B1 NaCl structure with a prominent reflection along (2 0 0) plane. The bonding structure of the coatings was characterized by X-ray photoelectron spectroscopy and the surface morphology of the coatings was studied using atomic force microscopy. Subsequently, nanolayered CrN/CrAlN multilayer coatings with a total thickness of approximately 1 μm were deposited on silicon substrates at different modulation wavelengths (Λ). The XRD data showed that all the multilayer coatings were textured along {2 0 0}. The CrN/CrAlN multilayer coatings exhibited a maximum nanoindentation hardness of 3125 kg/mm² at a modulation wavelength of 72 Å, whereas single layer CrN and CrAlN deposited under similar conditions exhibited hardness values of 2375 and 2800 kg/mm², respectively. Structural changes as a result of heating of the multilayer coatings in air (400-800 °C) were characterized using XRD and micro-Raman spectroscopy. The XRD data showed that the multilayer coatings were stable up to a temperature of 650 °C and peaks pertaining to Cr₂O₃ started appearing at 700 °C. These results were confirmed by micro-Raman spectroscopy. Nanoindentation measurements performed on the heat-treated coatings revealed that the multilayer coatings retained hardness as high as 2250 kg/mm² after annealing up to a temperature of 600 °C.     

One way of surface alloying treatment on iron surface based on surface mechanical attrition treatment and heat treatment

A method of surface alloying treatment has been developed: Ni powders were welded into the surface of iron plates by Surface Mechanical Attrition Treatment (SMAT), followed by annealing at certain temperature for 30 min. A Ni-Fe alloy layer with thickness about 100 μm in the sample surface was fabricated on pure iron plate. Scanning electron microscope (SEM), glow discharge spectrum (GDS), and X-ray diffraction (XRD) methods were used to analyze the microstructure, the composition and the phases of the alloy layer. Studies on the interface microstructure indicated that there was significant atomic diffusion and formation of multilayer of intermetallic compound and solid solution in SMAT process. Subsequent annealing accelerates the alloying process. The corrosion test shows the sample by SMAT treated with Ni powders exhibit the best corrosion resistance.     

Laser controlled melting of pre-treated zirconia surface

Laser treatment of pre-prepared zirconia surface is carried out. The pre-prepared surface, prior to laser treatment, consists of 50 μm carbon film and 7% titanium carbide particles, which are imbedded in the carbon film. The microstructural and morphological changes in the laser treated surface layer are examined using optical and scanning electron microscopes, energy dispersive spectroscopy, and X-ray diffraction. The fracture toughness of the laser treated surface is measured and the residual stress formed at the surface vicinity is determined from the X-ray diffraction technique. It is found that the microhardness of the laser treated surface increased slightly due to the dense layer formed at the surface vicinity. However, the laser treatment process reduces the fracture toughness of the surface due to improved surface hardness and the residual stress formed in the surface vicinity.     

Laser surface alloying of Ni-plated steel with CO2 laser

Laser surface alloying of low carbon steel electroplated with thin (10 μm) Ni using an 850 W CW CO₂ laser is reported for the first time. Fe-Ni binary alloys of different concentrations are formed by varying laser traverse speed from 0.5 to 5 m/min. The phase transformation from α to α + γ is discussed as a function of Ni contents. Development of microstructure in the modified zone is analysed in terms of solidification rate and Ni concentration. A three-fold increase in the microhardness of the binary alloy is observed. Formation of homogenous, adherent and crack free surface alloys is reported.     

Surface modification of Al-12.6Si alloy by high current pulsed electron beam

The paper investigates the microstructure and property modifications of Al-12.6Si alloy induced by high current pulsed electron beam (HCPEB) treatment. The scanning electron microscope (SEM) results show a fine and equiaxed grain structure of several micrometers is obtained on the top surface of the melted layer. Underneath the top surface, a remelted layer with about 10 μm thickness is obtained and a supersaturated solid solution of Al is formed in the remelted layer. XRD analysis shows that the relative strength of diffraction peaks of Al (1 1 1) and Si (1 1 1) crystal planes is increased after HCPEB treatment. As a result, relative wear resistance of HCPEB-treated sample is significantly improved by a factor of 2.5 due to fine grain strengthening and solid solution strengthening. Therefore, the HCPEB treatment has a good application future in treating Al-Si alloys.