Tribological behaviour of borided bearing steels at elevated temperatures

Borided steels are known to exhibit excellent wear resistance at room temperature. However, the sliding wear behaviour of borided steels at high temperatures is not known. In the present study, AISI 440C and 52100 bearing steels which are extensively used in industry, were borided by pack method at 950 °C for 2 h. X-ray diffraction analysis of boride layers on the surface of steels revealed various peaks of FeB, Fe₂B and CrB. The thickness and hardness of boride layers on the 52100 and 440C steels were 56 ± 6 and 47 ± 4 μm and 1970 and 2160 HK, respectively. Dry sliding wear tests of these borided steels were performed against Si₃N₄ bearing ball at a constant sliding speed and load at elevated temperatures. The temperature changed between room temperature and 600 °C. These tests indicated that the wear rates of unborided and borided steels increase with temperature and borided 52100 and 440C steels exhibit considerably lower wear rate at all temperatures, compared with unborided steels. At temperature of 600 °C, borided 52100 and 440C steels have a wear resistance of about 3 and 2.5 times higher than that of unborided steels, respectively. Examination of the worn surface of borided steels showed that, worn surfaces were covered with a discontinuous compact layer especially above temperature of 300 °C.     

Investigation of Boronizing Kinetics of AISI 51100 Steel

In this study, some mechanical properties of borided AISI 51100 steel with high C concentration were investigated. Boronizing heat treatment was carried out in solid medium consisting of Ekabor-II at 850, 900, and 950 °C for 2, 4, 6, and 8 h. Morphology and mechanical properties of boride layer, and the effect of chemical composition on properties and kinetics of borides were investigated. The results of this study indicated that the morphology of the boride layer has a saw-tooth nature, and its hardness is over 1500 HV. Depending on process time and temperature, the depth of boride layer ranged from 30 to 106 μm. Optical and SEM studies and XRD analysis revealed that borides formed on the surface of steel substrates have dominantly single Fe₂B boride phase in addition to small amount of Cr₂B.     

Oxidation resistance of boronized CoCrMo alloy

Boronizing of CoCrMo alloy has been performed by means of a powder-pack method using commercial LSB powders at 850, 900 and 950 °C for 8 h, respectively. In this study, the boronized CoCrMo alloy before and after oxidation tests were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The distribution of alloy elements of boronized samples from surface to interior was determined using energy-dispersive X-ray spectroscopy (EDS). XRD study showed the boride layer formed at 950 °C/8 h consisted of the phases Co₂B and CrB. Depending on boronizing temperature, the thickness of boride layer ranged from 2 to 11 μm. Cyclic oxidation behavior of the boride layer has been investigated at an oxidation temperature of 950 °C with a total exposure time up to 50 h in air. The test results indicated that the boronized CoCrMo alloy had superior oxidation resistance compared to unboronized sample.     

Preparation of Fe2B boride coating on low-carbon steel surfaces and its evaluation of hardness and corrosion resistance

Fe₂B coating was prepared on low-carbon steel by surface alloying. A series of experiments were carried out to examine some surface properties of boride coating. The surface heat treatment of coated low-carbon steel was performed at 700 °C, 800 °C and 900 °C for 2 h, 4 h, 6 h and 8 h under hydrogen atmosphere. The boride coating was revealed by XRD analysis and the microstructure of the boride coating was analyzed by scanning electron microscopy (SEM). Depending on the temperature and time of the process, the hardness of the borided low-carbon steel ranged from 99 to 1100 HV. The hardness showed a maximum (about 1100 HV) at 900 °C for 8 h. The corrosion resistance of the borided samples was evaluated by the Tafel polarization and electrochemical impedance spectroscopy (EIS). Shift in the corrosion potential (E_corr) towards the noble direction was observed, together with decrease in the corrosion current density (I_corr), increase in the charge transfer resistance (R_ct) and decrease in the capacitance (C_c), which indicated an improvement in corrosion resistance with increasing temperature and time of the treatment.     

Influence of process duration on structure and chemistry of borided low carbon steel

In this study, we employed an ultra-fast boriding technique to grow hard boride layers on low carbon steel substrates using an induction furnace at 900 °C. The technique utilizes an electrochemical cell in which it is possible to achieve very thick (i.e., about 90 μm thick) boride layers in about 30 min. The effects of process duration on boride layer thickness, composition, and structural morphology were investigated using microscopic and X-ray diffraction (XRD) methods. We also developed an empirical equation for the growth rate of boride layers. XRD results revealed two principal boride phases: FeB and Fe₂B thickness of which was very dependent on the process duration. For example, Fe₂B phase was more dominant during shorter boriding times (i.e., up to 15 min.) but FeB became much more pronounced at much longer durations. The growth rate of total boride layer was nearly linear up to 30 min of treatment. However during much longer process duration, the growth rate assumed a somewhat parabolic character that could be expressed as d = 1.4904 (t)^0.5 + 11.712), where d (in μm) is the growth rate, t (in s) is duration. The mechanical characterization of the borided surfaces in plane and in cross-sections has confirmed hardness values as high 19 GPa at or near the borided surface (where FeB phase is present). However, the hardness gradually decreased to 14 to 16 GPa levels in the region where Fe₂B phase was found.     

Hard boride coating on iron aluminide (FeAl)

The mechanical properties of borided iron aluminide (FeAl) were investigated. Boronizing was carried out in a solid medium consisting of Ekabor powders at 940 °C for 2, 4 and 8 h. The formation of FeB on the surface of FeAl was confirmed by XRD analysis. Metallographic studies revealed an almost saw-tooth-like and compact boride layer on the FeAl. The thickness of boride layer ranged from 15 to 32 µm with standard deviation. The hardness of borided specimens decreased with the distance from the surface to the interior of the FeAl. The hardness of the boride on the FeAl ranged from 1362 to 1572 1572 HV while the hardness of the FeAl ranged from 374 to 436 436 HV. Radio Frequency-Glow Discharge Optical Emission Spectroscopy (RF-GDOES) allows the simultaneous measurement of the bulk composition and the depth profiles of all of the elements of interest.     

Plasma nitriding behavior of Ti6Al4V orthopedic alloy

The influence of plasma nitriding on mechanical, corrosion and tribological properties of Ti6Al4V has been investigated using X-ray diffraction, microhardness tester, scanning electron microscopy, pin-on-disc tribotester, electrochemical polarization and impedance spectroscopy. Plasma nitriding treatment of Ti6Al4V has been performed in 25%Ar-75%N₂ gas mixture, for treatment times of 1-4 h at the temperatures of 650-750 °C. The corrosion tests were carried out in Ringer solution at 37 °C, and the wear tests were performed in dry sliding conditions. XRD analyses confirm the formation of δ-TiN and tetragonal ?-Ti₂N phases in the modified layer. It was observed that the surface hardness and wear resistance increase as the treatment time and temperature increase. The electrochemical impedance measurements indicate a decrease in double layer capacitance value and increase in charge transfer resistance for the nitrided specimens compared to the untreated ones.     

Characterization of borided pure molybdenum under controlled atmosphere

In this study, the structural characterization and boriding kinetics of the molybdenum borides formed on the surface of borided pure molybdenum (Mo) have been investigated. Boronizing was carried out in solid medium with boron component forming Ekabor ® 2 (90% SiC, 5% KBF₄, 5%B₄C) powders at 1273 K, 1373 K for 2, 4, 6, 8 hours under a controlled atmosphere containing argon gas flow. The boride layer was characterized by the scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), Energy dispersive spectroscopy (EDS) and Vickers microhardness tester. X-ray diffraction analysis showed that the boride layers on molybdenum consisted of MoB and Mo₂B phases. However, the MoB phase was observed at certain boriding temperature and boriding times. The thickness of boronized layers almost ranged from 12 to 42.5 μm with boriding time. A parabolic relationship was observed between boride layer thickness and boriding time. The growth rate constant and activation energy for the boride layer were calculated. The hardness of borides compounds formed on the surface of molybdenum ranged from 925 to 1150 HV_0.05, whereas the hardness of the untreated molybdenum sample was 258 HV_0.05.     

Effect of homogenization temperature on microstructure and mechanical properties of low-carbon high-boron cast steel

The effects of quenching treatment on the microstructure, hardness, impact toughness, and wear resistance of low-carbon high-boron cast steel (LCHBS) containing 0.15–0.3 %C, 1.4–1.8 %B, 0.3–0.8 %Si, 0.8–1.2 %Mn, 0.5–0.8%Cr, 0.3–0.6%Ni, and 0.3–0.6%Mo have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and via an electron probe microanalyzer (EPMA), X-ray diffraction (XRD) analysis, impact tester, hardness tester, and wear tester. The as-cast matrix of LCHBS consists of pearlite and ferrite. There is 8–10 vol.% Fe₂(B, C) type borocarbides in the matrix. The micro-hardness of Fe₂(B, C) is 1430–1480 Hv. Fe₂(B, C) shows no obvious change and the matrix completely transforms into lath martensite upon quenching at 900 °C to 1100 °C. The microhardness of the matrix and the macrohardness of the LCHBS sample show a slight increase with an increase of homogenization temperature. When the homogenization temperature exceeds 1050 °C, no distinct change in the hardness is observed. The change of homogenization temperature has no apparent effect on the impact toughness of LCHBS. The mass losses of LCHBS increase distinctly when the wear load increases. The homogenization temperature is less than 1000 °C and the wear rate of LCHBS decreases with an increase of temperature. The wear rate does not display any obvious change after exceeding a homogenization temperature of 1000 °C.     

La2O3包埋渗硼TB2钛合金的腐蚀与磨损性能

为了改善TB2合金的表面性能,采用4％La2O3(质量分数)包埋渗硼法对TB2合金进行1100℃,20 h渗硼处理,研究TB2钛合金的渗硼层组成与厚度以及腐蚀与磨损性能.结果表明,La2O3在渗硼过程中促进硼化物层的生长,提高其连续性和致密性,TiB晶须长度从16.80增至21.84μm.这是因为La2O3能与B反应生...     

纯镍渗硼硅处理工艺及组织性能研究

采用粒状渗剂分别在渗硼硅温度为850、900、950℃,保温时间为2、8 h的工艺参数下对纯镍表面进行固体渗硼硅处理。用光学显微镜(OM)对渗层横断面进行了显微组织观察,用显微硬度计测试渗层的硬度分布,用M200型磨损试验机研究未渗硼硅和渗硼硅纯镍的耐磨性,采用循环氧化试验研究未渗硼硅和渗硼硅纯镍的抗高温氧化性。结果表明,纯镍渗硼硅后,渗层为硅化物层和硼化物层,且硅化物和硼化物的显微硬度都大于基体硬度,渗层厚度随着渗硼硅时间和温度的增加而增加,其范围约为36~237?m,用X射线衍射仪(XRD)分析出渗层为硼化物层(Ni2B)和硅化物层(Ni3Si、Ni5Si2和Ni2Si)。磨损试验结果表明渗硼硅后试样的耐磨性得到提高。抗高温氧化试验结果显示未渗硼硅纯镍试样抗高温氧化性优于渗硼硅后纯镍试样。     

Characteristics and wear performance of borided Ti6Al4V alloy

In this study, a 10 µm thick uniform boride layer, composed of TiB₂ and TiB phases, was formed on the surface of a Ti6Al4V alloy using a pack boriding technique. The hardness of the boride layer was over 2000 HV. Beneath the boride layer, a boron diffusion zone (BDZ) appeared with a thickness of about 50 µm. The microstructure of the BDZ was composed of randomly oriented TiB whiskers mixed with the structure of the base metal. In the BDZ, hardness decreased gradually towards the base metal owing to the reduction of the TiB volume fraction. The borided alloy exhibited excellent wear resistance along with a lower coefficient of friction against sapphire ball under both dry and smear lubricated sliding conditions when compared to the as-received state.     

Electrochemical Evaluation of Corrosion on Borided and Non-borided Steels Immersed in 1 M HCl Solution

In this study the corrosion resistances of AISI 1018 and AISI 304 borided and non-borided steels were estimated using polarization resistance and electrochemical impedance spectroscopy (EIS) techniques. Boriding of the steel samples was conducted using the powder-pack method at 1223 K with 6 h of exposure. Structural examinations of the surfaces of the borided steels showed the presence of a Fe₂B layer with isolated FeB teeth on the AISI 1018 steel, whereas a compact layer of FeB/Fe₂B was formed on the AISI 304 steel. Polarization resistance and EIS of the borided and non-borided steels surfaces were performed in a corrosive solution of 1 M HCl. The EIS data were analyzed during 43 days of exposure to the acid solution. Impedance curves obtained during this period for the borided and non-borided steels were modeled using equivalent electrical circuits. The results of both electrochemical techniques indicated that boride layers formed at the steel surfaces effectively protect the samples from the corrosive effects of HCl. The main corrosion processes observed on the boride layers were pitting and crevice corrosion.     

Tribology and cyclic oxidation behavior of plasma nitrided valve steel

In this study, the tribology and cyclic oxidation behavior of plasma nitrided DIN 1.4871 austenitic valve steel were investigated. For this purpose plasma nitriding treatments were carried out in nitrogen and hydrogen with ratio N₂/H₂: 1/3 at 10 Torr pressure. Nitriding cycles of 400, 450, 500 and 550 °C for 7 h were selected. To remove oxide layer and to enhance diffusion, an effective sputter cleaning procedure was applied in argon and hydrogen gases. The pin-on-disc sliding wear experiments were performed at a load of 6 N and sliding velocity of 0.1 m/s in normal atmosphere under dry condition. Cyclic oxidation tests used to evaluate the oxidation characteristics of the samples consisted of 50 cycles each 30 min at 750 °C. The structure and properties of the samples were examined by optical and scanning electron microscopy (SEM), microhardness measurements and X-ray diffraction. The results indicated plasma nitriding at all temperatures increased the wear resistance of valve steel when sliding against bearing steel. The 550 °C nitrided layer, with CrN, Fe₄N and Fe_2-3N on the surface, was most effective in improving wear resistance. In the case of cyclic oxidation, the results showed that oxidation resistance depends strongly on nitriding temperature. Nitriding at 450 °C produced a layer of predominantly “S” phase which was more effective in improving the oxidation resistance of valve steel.     

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

New results about the fracture toughness (K_c) 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.     

Effect of DC plasma nitriding temperature on microstructure and dry-sliding wear properties of 316L stainless steel

A wear resistant nitrided layer was formed on 316L austenitic stainless steel substrate by DC plasma nitriding (DCPN). The structural phases, micro-hardness and dry-sliding wear behavior of the nitrided layer were investigated by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS), micro-hardness tester and ring-on-block wear tester. The results show that a single expanded austenite layer (S-phase) and a single CrN nitride layer were formed at 400 °C and 480 °C, respectively. In addition, the S-phase layers formed on the samples exhibited the best dry-sliding wear resistance under ring-on-block contact configuration test. Wear of the untreated 316L was sever and characterized by strong adhesion, abrasion and oxidation mechanism, whilst wear of the DCPN-treated 316L was mild and dominated by plastic deformation, slight abrasion and frictional polishing.     

Mechanical behavior of borides formed on borided cold work tool steel

In this study, some mechanical properties of borided cold work low-alloy tool steels were investigated. Boronizing was performed in a solid medium consisting of Ekabor-I powders at 1000°C for 2, 4 and 6 h. The substrate used in this study was high-carbon, low-alloy tool steel essentially containing 1.18 wt.% C, 0.70 wt.% Cr, 0.30 wt.% Mn, 0.10 wt.% V and 0.25 wt.% Si. The presence of borides (FeB+Fe₂B) formed on the surface of steel substrate was confirmed by optical microscope and X-ray diffraction (XRD) analysis. The hardness of the boride layer formed on the surface of the steel substrate and unborided steel substrate were 1854 and 290 kg/mm², respectively. Experimental results revealed that longer boronizing time resulted in thicker boride layers. Optical microscope cross-sectional observation of the borided layers revealed denticular morphology. The fracture toughness of the boride layers measured by means of a Vickers indenter with a load of 3 N was in the range of 2.52–3.07 MPa m^1/2.     

The growth of single Fe2B phase on low carbon steel via phase homogenization in electrochemical boriding (PHEB)

In this study, we introduce a new electrochemical boriding method that results in the formation of a single-phase Fe₂B layer on low carbon steel substrates. Although FeB phase is much harder and more common than Fe₂B in all types of boriding operations, it has very poor fracture toughness; hence, it can fracture or delaminate easily from the surface under high normal or tangential loading. We call the new method “phase homogenization in electrochemical boriding” (PHEB), in which carbon steel samples undergo electrochemical boriding for about 15 min at 950 °C in a molten electrolyte consisting of 90% borax and 10% sodium carbonate, then after the electrical power to the electrodes is stopped, the samples are left in the bath for an additional 45 min without any polarization. The typical current density during the electrochemical boriding is about 200 mA/cm². The total original thickness of the resultant boride layer after 15 min boriding was about 60 μm (consisting of 20 μm FeB layer and 40 μm Fe₂B layer); however, during the additional phase homogenization period of 45 min, the thickness of the boride layer increased to 75 μm and consisted of only Fe₂B phase, as confirmed by glancing-angle x-ray diffraction and scanning electron microscopy in backscattering mode. The microscopic characterization of the boride layers revealed a dense, homogeneous, thick boride layer with microhardness of about 16 GPa. The fracture behavior and adhesion of the boride layer were evaluated by the Daimler-Benz Rockwell C test and found to be excellent, i.e., consistent with an HF1 rating.     

Improvement of corrosion and wear resistances of AISI 420 martensitic stainless steel using plasma nitriding at low temperature

The influence of low temperature plasma nitriding on the wear and corrosion resistance of AISI 420 martensitic stainless steel was investigated. Plasma nitriding experiments were carried out with DC-pulsed plasma in 25% N₂ + 75% H₂ atmosphere at 350 °C, 450 °C and 550 °C for 15 h. The composition, microstructure and hardness of the nitrided samples were examined. The wear resistances of plasma nitrided samples were determined with a ball-on-disc wear tester. The corrosion behaviors of plasma nitrided AISI420 stainless steel were evaluated using anodic polarization tests and salt fog spray tests in the simulated industrial environment.The results show that plasma nitriding produces a relatively thick nitrided layer consisting of a compound layer and an adjacent nitrogen diffusion layer on the AISI 420 stainless steel surface. Plasma nitriding not only increases the surface hardness but also improves the wear resistance of the martensitic stainless steel. Furthermore, the anti-wear property of the steel nitrided at 350 °C is much more excellent than that at 550 °C. In addition, the corrosion resistance of AISI420 martensitic stainless steel is considerably improved by 350 °C low temperature plasma nitriding. The improved corrosion resistance is considered to be related to the combined effect of the solid solution of Cr and the high chemical stable phases of ?-Fe₃N and α_N formed on the martensitic stainless steel surface during 350 °C low temperature plasma nitriding. However, plasma nitriding carried out at 450 °C or 550 °C reduces the corrosion resistance of samples, because of the formation of CrN and leading to the depletion of Cr in the solid solution phase of the nitrided layer.     

Studies on thermal oxidation of Mg-alloy (AZ91) for improving corrosion and wear resistance

The present investigation concerns development of an adherent stable oxide layer on the surface of a Mg alloy, AZ91 (8.3-9.7% Al, 0.13% Mn, 1.0% Zn, 0.50% Si, 0.10% Cu, 0.03% Ni, 0.30% other, balance Mg) by thermal oxidation to improve wear and corrosion resistance in simulated body fluid. Oxide layer was developed by thermal oxidation of freshly polished AZ91 samples in air at 200 °C, 300 °C and 400 °C for 8, 16 and 25 h. Following thermal oxidation, a detailed characterization of the oxide layer was undertaken in terms of microstructure and phases. The wear, corrosion resistance and biocompatibility were evaluated in details. The oxide layer mainly consists of oxides of magnesium (MgO₂ and MgO) phases. The corrosion resistance (in simulated body fluid) and wear resistance of the oxidized surface were improved. Thermally oxidized Mg-Al alloy (AZ91) treated at 200 °C for 25 h have shown best biocompatibility in terms of cell (L-929 mouse fibroblast cell line) proliferation.