Borided Steel Research Articles

The impact of post-treatments on the brittleness and wear resistance of borided 8 % Cr steel

The implementation of post-treatments for the enhancement of the mechanical behavior of boride layers has been conducted in recent years. For this reason, the borided 8 % Cr steel underwent austempering-interrupted treatment and austempering-interrupted + cryogenic treatments. Resulting microstructures have been characterized using light optical microscopy, scanning electron microscopy and energy dispersive X-ray spectrometry, and X-ray diffraction. The brittleness grade and dry sliding wear resistance of the borided steel with and without post-treatments were achieved through a Vickers microindentation approach (at various loads of 2 N, 3 N, and 5 N) and the ball-on-flat wear test (at a constant load of 20 N for 100 m). In addition, a non-borided material (hardening + tempering) was tested employing the ball-on-flat wear test to demonstrate the improvement of the boride layers.As expected, the wear resistance of the 8 % Cr steel subjected to conventional boriding was 8 times greater than that of the non-borided material due to the surface hardening achieved through the thermochemical treatment. On the other hand, the austempering-interrupted treatment conducted after conventional boriding resulted in a 1.2 times improvement in wear resistance. This enhancement was attributed to the transformation of the FeB–Fe2B dual-phase layer into a Fe2B single-phase layer, which reduced the brittleness grade of the boride layer from 4 to 3.1. Nonetheless, the Fe2B single-phase layer achieved the highest wear resistance, which was 1.3 times greater, after undergoing austempering-interrupted + cryogenic treatments (resulting in a slight decrease in the material's brittleness grade from 3.1 to 2.9) compared to the borided steel treated with austempering-interrupted alone.Grooving and debris were observed on the entire wear tracks for both non-borided material (ductile) and boride layers (brittle). Additionally, brittle layers exhibited the development of cracks and spalling, while ductile non-borided steel showed plastic deformation.

Improving the Mechanical and Electrochemical Performance of Additively Manufactured 8620 Low Alloy Steel via Boriding

In this study, mechanical and electrochemical performance of borided additively manufactured (AM) and wrought 8620 low alloy steel were investigated and compared to their bare counterparts. The microstructure of borided 8620 exhibited the presence of FeB and Fe2B phases with a saw tooth morphology. Both AM and wrought samples with boride layers showed a similar performance in hardness, wear, potentiodynamic polarization (PD), electrochemical impedance spectroscopy (EIS), and linear polarization resistance (LPR) experiments. However, borided steels exhibited about an 8-fold increase in Vickers hardness and about a 6-fold enhancement in wear resistance compared to bare ones. Electrochemical experiments of borided specimens (both AM and wrought) in 0.1 M Na2S2O3 + 1 M NH4Cl solution revealed a 3–6-fold lower corrosion current density, about a 6-fold higher charge transfer resistance, and about a 6-fold lower double-layer capacitance, demonstrating an improved corrosion resistance compared to their bare counterparts. Post-corrosion surface analysis revealed the presence of thick sulfide and oxide layers on the bare steels, whereas dispersed corrosion particles were observed on the borided samples. The enhanced wear and electrochemical performance of the borided steels were attributed to the hard FeB/Fe2B layers and the reduced amount of adsorbed sulfur on their surface.

Comparative Study of Conventional and Microwave-Assisted Boriding of AISI 1040 and AISI 4140 Steels

In this study, AISI 1040 and AISI 4140 steels were boriding using Ekabor-II commercial boriding powder with powder-pack boriding method using microwave and conventional heating methods. The samples were borided at 950 °C for 2 and 6 hours in an Ar atmosphere in a microwave oven of Enerzi-Mh2912-V8. Biphasic structure (FeB/Fe2B) was formed in all borided AISI 4140 samples and AISI 1040 samples borided for 6 hours. A single-phase structure was observed in AISI 1040 steel borided for 2 hours. Compared to the conventional method, a 1.5-1.6 times thicker boride layer was obtained in AISI 4140 and AISI 1040 steels with microwave-assisted powder-pack boriding. The highest hardness was measured as 1561.8 HV0.05 for boriding AISI 4140 steel and 1499.7 HV0.05 for boriding AISI 1040 steel. The Vickers indentation fracture toughness of borided steels with microwave energy varied between 2.31 and 3.46 MPa·m1/2. It was determined that in all samples borided by the microwave-assisted and conventional powder-pack boriding method, the adhesion strength between the boride layers and the substrate obtained was sufficient.

The influence of a pulsed-DC field on the growth of the Fe2B layer and the electrical behavior of the boriding media

Novel results about the growth kinetics of Fe2B layers produced by pulsed-DC powder-pack boriding (PDCPB) on AISI 1018 and 4140 steels were obtained. In the process, a DC-field is applied to ionize the B atoms and implements a programmable electronic control device (PECD) to maintain a constant B flux on both surfaces of the material. During PDCPB, the electrical resistivity (ER) was reduced due to the percolation of KBF4 in the boriding media. Nevertheless, the increase of ER was caused by the depletion of KBF4 after 1000s of the current induction. The PDCPB enhanced the growth kinetics of the Fe2B layer around of 9% for both borided steels compared to those estimated in the conventional powder-pack process.

Growth Kinetics, Microstructure Evolution, and Some Mechanical Properties of Boride Layers Produced on X165CrV12 Tool Steel

The powder-pack boriding technique with an open retort was used to form borided layers on X165CrV12 tool steel. The process was carried out at 1123, 1173, and 1223 K for 3, 6, and 9 h. As a result of boriding the high-chromium substrate, the produced layers consisted of three zones: an outer FeB layer, an inner Fe2B layer, and a transition zone, below which the substrate material was present. Depending on the applied parameters of boriding, the total thickness of the borided layers ranged from 12.45 to 78.76 µm. The increased temperature, as well as longer duration, was accompanied by an increase in the thickness of the FeB zone and the total layer thickness. The integral diffusion model was utilized to kinetically describe the time evolution of the thickness of the FeB and (FeB + Fe2B) layers grown on the surface of powder-pack borided X165CrV12 steel. The activation energy of boron for the FeB phase was lower than that for the Fe2B phase. This suggested that the FeB phase could be formed before the Fe2B phase appeared in the microstructure. The high chromium concentration in X165CrV12 steel led to the formation of chromium borides in the borided layer, which increased the hardness (21.88 ± 1.35 GPa for FeB zone, 17.45 ± 1.20 GPa for Fe2B zone) and Young's modulus (386.27 ± 27.04 GPa for FeB zone, 339.75 ± 17.44 GPa for Fe2B zone). The presence of the transition zone resulted from the accumulation of chromium and carbon atoms at the interface between the tips of Fe2B needles and the substrate material. The presence of hard iron and chromium borides provided significant improvement in the wear resistance of X165CrV12 steel. The powder-pack borided steel was characterized by a four times lower mass wear intensity factor and nine times lower ratio of mass loss to the length or wear path compared to the non-borided material.

Characterization of Boride Coatings on AISI 8620 Steels without and with Hydrogen Permeation

The present work studied the characterization of boriding coatings without and with exposed hydrogen permeation on AISI 8620 through microstructure and mechanical behavior. A dehydrated paste pack process was carried out for the formation of boride coatings on AISI 8620 at 1173, 1223, and 1273 K for 6 h of exposure time for each temperature. After boronizing, hydrogen was introduced into the borided steels by in situ cathodic charging using an H2SO4 acid electrolyte. Specimens borided without and with hydrogen were characterized by scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction. The experimental techniques of the Vickers microhardness and three-point bending were used. As a result, the hydrogen diffusion on steel AISI 8620 borided shows a significant increase in microhardness. Furthermore, three-point bend tests showed an increased strength on 8620 steel borided. However, the ductility and strength value on 8620 steel borided surfaces presented a drastic reduction due to hydrogen permeation. The results revealed that the borided coatings provided strength and reduced the hydrogen embrittlement index on AISI 8620 borided.

BORIDING OF STEELS: IMPROVEMENT OF MECHANICAL PROPERTIES - A REVIEW

Boriding on steels is an outstanding surface hardening technique widely used to improve the hardness, resistance to wear, corrosion, and oxidation. However, borided steels exhibit low ductility, strength, and toughness due to brittleness issues. The conventional pack boriding process induces brittleness in the borided case and core. Due to the brittleness, wide-scale industrial applications of the borided steels are limited. Many researchers have attempted to increase the ductility, strength, and toughness of the borided steels through reduction of the brittleness. This review examines the various aspects of boride surface modification of the steels, mainly the reasons for the brittleness and the efforts to reduce it.

A numerical-experimental study of AISI 316L borided steels under cyclic contact loading

AISI 316L borided steels under cyclic contact loading were evaluated. Boriding was carried out by two powder-pack processes: the continuous and interrupted processes. Boriding processes were developed at 900 °C for 1 h (continuous process) and 4 h (interrupted process). A monolayer mainly formed by Fe 2 B was obtained by the interrupted boriding with a thickness of 6.09 ± 0.31 μm and a surface hardness of 22.3 ± 1.4 GPa. In contrast, a FeB-Fe 2 B bilayer was formed by the continuous boriding with a total thickness of 13.08 ± 0.39 μm and a surface hardness of 26.3 ± 2.1 GPa. Adhesion tests were developed, in which the monolayer showed better adhesion than that the bilayer. Cyclic contact tests were carried out by loading a ball on the borided steel surface. Circular cracks because of the applied critical load (monotonic load) were observed on the borided steel surface. Subcritical loads were applied to the borided steels, where cohesive damage was caused in the bilayer, whereas no damage was observed in the monolayer at low subcritical loads. In contrast, circular cracks were observed in both treatment conditions at high subcritical loads. In addition, the high percentage of the FeB phase caused a more severe damage upon the bilayer system. Additionally, the thinner thickness of the boride layer, as well as the presence of compressive residual stresses (−0.75 GPa) in the monolayer system, caused a decrease on the maximum principal stress (2.4 GPa at the highest applied load) caused by contact loading compared to bilayer system (5.2 GPa at the highest applied load). Therefore, the interrupted boriding (monolayer system) showed a better mechanical resistance under cyclic contact loads than the continuous boriding (bilayer system). • FeB-Fe 2 B and Fe 2 B layers were obtained by continuous and interrupted boriding. • The bilayer system showed more severe damage under cyclic contact loads. • The plastic deformation of the substrate decreased the damage in the monolayer system.

Investigation of mechanical, kinetic and corrosion properties of borided AISI 304, AISI 420 and AISI 430

ABSTRACT In this study, three different types of stainless steels, AISI 304, AISI 420, and AISI 430 were pack borided under 850 °C–925 °C–1000 °C for 2, 4, and 6 h, respectively. Morphologies of coating were investigated by optical and scanning electron microscope (SEM). The maximum layer hardness was determined to be 1736 HV0.05, 1659 HV0.05, and 1572 HV0.05 for AISI 304, AISI 420, and AISI 430, respectively. Phases formed on the layer were determined by X-ray analysis (XRD) to confirm the presence of FeB, Fe2B, CrB, and MnB phases. Boride layer morphology was found to be a planar and compact structure. The activation energies of the coating were investigated. The corrosion resistance values of stainless steels were determined from the static immersion tests performed in 10% HCl solution. There was a remarkable increase in the corrosion resistance of borided steel compared to that of untreated samples.

Investigation of thermal residual stress of borided steel: finite-element simulation

Although a technologically interesting characteristic of boriding is the production of a hard, wear-resistant layer on the treated steel parts, thermal residual stresses may lead to crack formation in both the iron boride (FeB) layer and the vicinity of the iron boride–diiron boride (Fe2B) interface. In this study, the thermal residual stress distribution in borided tool steel without temperature gradients was investigated systematically by using finite-element analysis. A transient analysis was performed to obtain the stress change with cooling time. The models were cooled down from 823 to 293 K under steady-state conditions. The maximum compressive residual stress is about 690 MPa and tends to decrease with increasing layer thickness. For the case of the double-layer model with 60 μm thick diiron boride, the tensile residual stress occurs on the surface of the iron boride layer and its value changes within the range of 1870–2025 MPa with the iron boride layer thickness. However, the magnitude of the shear stress at the steel–diiron boride interface is not crucial in the model with a single boride layer. The shear stress at the iron boride–diiron boride interface in the double-layer model increases dramatically with the iron boride layer and exceeds 1050 MPa for a high percentage of iron boride (52%) in the total layer thickness.

Friction and wear of borided AISI O1 steel with carbon nanomaterial deposit

Carbon nanomaterials (CNs) were deposited on borided AISI O1 steel by chemical vapor deposition. The deposit, characterized by scanning electron microscopy and X-ray diffraction, presents CNs such as carbon nanotubes, as well as cementite and amorphous carbon. The tribological effects of friction and wear under dry conditions were analyzed with a ball-on-disk with a tungsten carbide (CW) ball and roughness meter. The results show that the coefficient of friction considerably decreases and stabilizes to ~ 0.24 for the sample with CNs. It also is concluded that CNs act as a protective coating that prevents the surface of the borided steel from wear.

Wear Performance Under Dry and Lubricated Conditions of Post Boriding Heat Treatment in 4140 Steel

AbstractThe tribological performance of borided, post borided heat-treated and uncoated 4140 steel, under dry and lubricated conditions, was studied. The boriding process was conducted by powder-pack method at 900 °C with 1 h of exposure time. After boriding, a heat treatment (quenching and tempering) was conducted to obtain a hardened core microstructure of the 4140 borided steel. The tribological tests were performed with an Al2O3 ball of 6.35 mm of diameter, setting sliding distances of 25, 50, and 100 m. The loads used were 20 and 40 N in dry and lubricated conditions, respectively. Overall samples, the coefficient of friction was reduced around five times when lubricant was used, in comparison to dry tests. After 100 m of sliding distance, it was observed that the wear resistance increased around ten times with boriding. The dry results showed that borided (B) exhibited better wear-rate kB = 7.06 × 10−7 mm3 N−1 m−1 than borided and heat-treated (BHT) kBHT = 9.93 × 10−7 mm3 N−1 m−1, attributed to its higher hardness. Nevertheless, the lubricated results showed that the surface characteristics of BHT improved its wear resistance kBHT = 5.25 × 10−8 mm3 N−1 m−1 over kB = 1.68 × 10−7 mm3 N−1 m−1 of borided only, because it permitted a better adsorption of the lubricant.

Tribological Behavior of a Self-Lubricant Surface Film of H<sub>3</sub>BO<sub>3 </sub>Formed on a Borided Biomedical Steel by a Post-Treatment

This paper evaluates the tribological behavior of borided AISI 316L steel after being exposed to a secondary process to form a thin film of a solid lubricant. The process known as Short Annealing Process (SAP), allows creating a thin film of boric acid (H3BO3) on the surface of metallic materials previously exposed to boriding. The H3BO3 film acts like solid lubricant due to its lamellar crystalline structure. First samples of AISI 316L were exposed to boriding to temperatures of 875, 925 and 975 °C during 2, 4 and 6 h each temperature. Then, samples were heated to 750 °C during 5 min and cooled to room temperature at 60 % of Relative Humidity. The tribological behavior of the treated samples was evaluated by pin-on-disk test equipped with friction coefficient measurement system. Samples were characterized by Scanning Electron Microscopy, X-Ray Diffraction and Vickers microhardness test. The results showed an evident influence of the experimental parameters on the thickness of the boride layers and their mechanical properties. The layer ́s thickness was ranged from 10.51±0.71 to 51.57±5.12 μm. The hardness of the coatings was increased from 264 to 1685 HV. Finally, the Coefficient of Friction was diminished from values of 0.7 for the as-received material to 0.29 for the borided samples and to 0.06 for those after SAP, which indicates that the post-treatment SAP enhances the tribological properties of the biomedical steel.

Evaluating the mechanical behavior of electrochemically borided low-carbon steel

In this study, mechanical properties of borided low carbon steels were investigated with different testing methods namely tensile, Erichsen cupping and three-point bending tests along with fracture toughness and hardness measurements. For boriding, a new electrochemical based boriding called as CRTD-Bor (Cathodic Reduction and Thermal Diffusion based boriding) was chosen. The main advantages of CRTD-Bor are its high boride growth rate and its environmentally friendly nature. Cross-sectional scanning electron microscope inspections along with thin film X-ray diffraction analyses confirmed the fast boride layer formations (i.e. about 40 μm thick Fe2B layer in 5 min). On the other hand, some mechanical degradations were observed on the borided substrates such as decrease in yield and ultimate strength values. However, the improvement of flexural stress were detected which reached to 2.5 times higher after 30 min of boriding. The fracture toughness values were found as 0.44 MPa·m1/2, 1.34 MPa·m1/2 and 3.7 MPa·m1/2 for sole FeB, the interphase of FeB/Fe2B and Fe2B regions, respectively. Furthermore, the surface hardness of borided steels was measured around 1400 HV which exhibited excellent adhesion to the substrate as long as the boride layer composed of single Fe2B.

Corrosion and tribocorrosion behaviours of AISI 321, AISI 431 and FDMA borided and non-borided steels

Corrosion and tribocorrosion behaviours of non-borided and borided steel materials were evaluated under sliding contact in a 3.5 wt% NaCl solution, using an alternative tribometer integrated with a potentiostat for electrochemical control. Alloy-steel samples were borided by a powder-pack boriding process during 1 h at 950 °C. Microstructural examinations and phase analysis were performed employing Scanning Electron Microscopy (SEM) and x-ray diffraction. The surface profiles of wear tracks were obtained using a high resolution surface profilometer. Under corrosion conditions, it was found that a negative effect of boriding treatment was observed in the corrosion resistance of the investigated steels due to the presence of porosity and microcracks within FeB and Fe2B coating layers. On the other hand, the tribocorrosion tests indicated the positive effect of boride coating which induced a shift of the polarization curves of the borided steels to higher corrosion potentials contrary to the Tafel polarization curves of the non-borided steel materials which shifted to lower corrosion potentials. Moreover, even though the borided steels presented lower corrosion performances, the synergistic effect between corrosion and wear acting together resulted in a decrease of their coefficient of friction (COF) values and material losses with respect to the non-borided steel materials. In addition, the line profile of their surfaces exhibited less pronounced wear behaviour showing shallowed wear depth leading subsequently to a lower wear volume and loss compared to the non-borided steel materials. Microscopic observations revealed that wear-induced corrosion was more dominant in the corrosion-wear synergism.

Sliding Wear Behavior of Friction Couples Primarily Selected for Corrosion Resistance: Iron Boride/Iron Boride and Iron Boride/Yttria-Stabilized Zirconia

Wear mitigation in a sliding couple is challenging if wear has to be minimized on both surfaces. In this paper, ball-on-disk testing is performed on sliding couples where both surfaces (ball and disk) are treated for wear resistance. Studied materials are pack borided H13 tool steel (ASTM A681), pack borided AISI 420 stainless steel (ASTM A276) and plasma sprayed yttria-stabilized zirconia (YSZ). Borided H13 steel exhibits a single phase Fe2B layer, while AISI 420 has a double phase layer, with FeB on the outer surface. Both FeB/Fe2B and FeB/YSZ couples generate three-body abrasion. In the latter case, mass transfer occurs from the ball to the disk as well. Friction coefficient is ~0.6 for the AISI 420/Fe2B and FeB/Fe2B sliding pairs, with less vibration on the latter and wear rates close to 10−3 mm³·(N·m)−1 for both the ball and the disk. In comparison, the FeB/YSZ pair has a friction coefficient of ~0.65, a similar total mass loss, but a much higher wear rate for YSZ than for FeB.

EFFECT OF INTERRUPTED BORIDING ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF 16MnCr5 STEELS

The microstructure, microhardness and tensile properties of continuously borided and interrupted borided 16MnCr5 steel were compared. In the continuous process, boriding was carried out continuously at 1273[Formula: see text]K for 16[Formula: see text]h. In the interrupted process, after 5[Formula: see text]h of boriding at 1273[Formula: see text]K, the crucible containing the specimens and the boriding powders was removed from the furnace and cooled to 873[Formula: see text]K in still air for 30[Formula: see text]min and this procedure was repeated three times. Both boriding treatments were achieved in a solid medium using the powder pack method with commercial Ekabor-I powders as the boron source. X-ray diffraction studies indicated the presence of FeB and Fe2B phases in the boride layers for both boriding treatments. The transition zone was more pronounced in the continuously borided steel. The average hardness values of the boride layers were 1759 and 1648[Formula: see text]HV0.1for the continuously borided and interrupted borided steels, respectively. The hardness of the boride layers were considerably higher than the 200[Formula: see text]HV0.1hardness of the untreated steel. No significant difference in tensile properties could be observed between the two different boriding procedures.

Microabrasive wear behavior of borided steel abraded by SiO2 particles

Boriding treatment was applied to AISI 1020 steel to improve its wear resistance. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy, Vickers microhardness testing, fracture toughness, and confocal microscopy. The microscale abrasive wear behavior was also investigated. SiO2 abrasive particles were used as abradant with slurry concentrations of 0.5 and 1.0g/cm3. Normal loads of 0.49 and 0.98N were used. Fe2B phase was identified in the boride layer via XRD analysis. The Fe2B layer was 169μm thick with a mean hardness of 1608±101 HV0.05 and fracture toughness of 5.35±1.43MPam1/2. A reduction in the hardness of the outermost surface of the boride layer was observed owing to the formation of a porous region. Boriding treatment improved the wear resistance of the steel substrate. Sliding abrasive wear was the main mechanism under all tested conditions. The presence of micro-rolling abrasion and fracture-based mechanisms was observed for untreated and borided samples, respectively.

Fracture Toughness of Thick Boride Layers Estimated by the Cross-Sectioned Scratch Test

New results about the fracture toughness (Kc) of thick boride layers estimated by the cross-sectioned scratch test are presented in this study. The FeB-Fe2B layers developed at the surface of borided AISI 1018 and AISI 1045 steels and the Fe2B layer formed on the borided AISI 1045 steel exposed to a diffusion annealing process (DAP) were used for this purpose. The cross-sectioned scratch tests were performed with a Vickers diamond stylus drawn across the thick boride layer under a constant load to produce a half-cone-shaped fracture near to the top surface of the borided steels. The height of the half-cone-shaped fracture as a function of the cross-sectioned scratch loads was used to determine the fracture toughness of the FeB and Fe2B layers. The results showed a fracture resistance of $$\sim2.8\,{\text{MPa}}\sqrt m$$ for the FeB layer formed at the surface of borided AISI 1045 steel. Likewise, the effect of the DAP on the surface of the borided AISI 1045 steel promoted the formation of an exclusively Fe2B layer, with an increase in the fracture toughness of the whole boride layer around $$5\,{\text{MPa}}\sqrt m$$ . Finally, the principle of the technique can be used to minimize the influence of the anisotropic properties on the fracture toughness along the depth of boride layers.

Surface modification of chromium-silicon martensitic steel by forming hard borides

In this study, an alternative surface hardening method called as “CRTD-Bor” (Cathodic Reduction and Thermal Diffusion based Boriding) was introduced for the heavy-duty applications of medium carbon, chromium-silicon martensitic steel, also known commercially Silchrome 1. The influences of process parameters (e.g. electrolyte temperatures and both electrolysis and phase homogenization durations) on the chemistry, thickness and hardness of boride structures were investigated to yield a modified surface within the industrially desired compositions, namely single Fe2B or the layer containing max. 10% of FeB in vol. Furthermore, adhesion as well as thermal oxidation behaviors of borided substrates were examined. Cross sectional SEM investigations revealed that it was possible to grow 35μm or 45μm thick boride layers within the preferred compositions after 40min and 55min of CRTD-bor, respectively. Thin film XRD analyses confirmed single Fe2B formation with minor Cr2B peaks. The hardness of boride layers varied in the range of 1400±200HV. The grown boride layers exhibited the ideal adhesions to the steel matrix with either perfect HF1 or acceptable HF3 qualities according to their constitutions. CRTD-Bor boriding process improved the oxidation resistance of the steel at 650°C remarkably by forming thin ~7μm thick protective layer composing of mixed iron-, chromium - oxides and borates as well as boron oxide (B2O3) which was firstly identified in the hexagonal crystalline structure after oxidizing borided steel samples at 450°C and 650°C.