Boriding kinetics of H13 steel

It is known that boriding has been employed to increase the service life of parts such as orifices; ingot molds, and dies for hot forming made of AISI H13 steel. In this study, case properties and kinetics of borided AISI H13 steel have been investigated by conducting a series of experiments in Ekabor-I powders at the process temperature of 1073, 1173 and 1273 K for periods of 1-5 h. The presence of borides FeB and Fe₂B of steel substrate was confirmed by optical microscopy and scanning electron microscopy (SEM). The results of this study indicated that the morphology of the boride layer has a smooth and compact morphology, and its hardness was found to be in the range of 1650-2000 HV. Transition zone observed between the hard boride coating and the matrix was relatively softer than the substrate. The kinetics of boriding shows a parabolic relationship between layer thickness and process time, and the calculated activation energy for the process is 186.2 kJ/mol. Moreover, boriding parameter BOP, which is only a function of boride layer thickness and activation energy, has been suggested for the prediction of layer thickness in boriding of AISI H13. There is a reasonable correlation between the progress of boride layer thickness and proposed time-temperature-compensated parameter. Similar findings have been found when it is applied to another steels including tool and low alloy steels, as well as Armco iron.     

Effects of rare earth additions during surface boriding on microstructure and properties of titanium alloy TC21

Abstract

In the present paper, the effects of rare earth (RE) additions to the solid state boriding of titanium alloy TC21 have been studied. The microstructural evolution and phase transformations of the borided layers were examined using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction. Moreover, the microhardness for the borided layer was also determined by Vickers hardness test. The results showed that the addition of a small amount of RE elements in the boriding process can lead to an increased boron concentration in the surface layer coupled with the improved surface hardness and coating layer thickness. Furthermore, the presence of trace quantities of RE oxide (Ce₂O₃) in boride layers indicated that the RE elements as catalysts could not only influence but also accelerate boriding process.     

Electrochemical boriding and characterization of AISI D2 tool steel

D2 is an air-hardening tool steel and due to its high chromium content provides very good protection against wear and oxidation, especially at elevated temperatures. Boriding of D2 steel can further enhance its surface mechanical and tribological properties. Unfortunately, it has been very difficult to achieve a very dense and uniformly thick boride layers on D2 steel using traditional boriding processes. In an attempt to overcome such a deficiency, we explored the suitability and potential usefulness of electrochemical boriding for achieving thick and hard boride layers on this tool steel in a molten borax electrolyte at 850, 900, 950 and 1000 °C for durations ranging from 15 min to 1 h. The microstructural characterization and phase analysis of the resultant boride layers were performed using optical, scanning electron microscopy and X-ray diffraction methods. Our studies have confirmed that a single phase Fe₂B layer or a composite layer consisting of FeB + Fe₂B is feasible on the surface of D2 steel depending on the length of boriding time. The boride layers formed after shorter durations (i.e., 15 min) mainly consisted of Fe₂B phase and was about 30 μm thick. The thickness of the layer formed in 60 min was about 60 μm and composed mainly of FeB and Fe₂B. The cross sectional micro-hardness values of the boride layers varied between 14 and 22 GPa, depending on the phase composition.     

Surface Nanocrystallization of Commercial Pure Titanium Induced by SMAT and Its Properties

In this paper the preparation technique of surface nanocrystallization in commercial pure titanium was carried out by surface mechanical attrition treatment (SMAT). The mean grain size was calculated by using X-ray diffraction (XRD) and transmission electron microscope (TEM), and the results showed that the mean grain size of the surface was refined to nm Ievel after SMAT treatment. Nanocrystallized surface layers were formed after treated for 5, 15, 30 and 60 min. Microhardness experimental results implied the microhardness obviously increased on the surface layer and it also showed the variation of microhardness at the cross section. Corrosion test results showed the corrosion resistance of the surfaces in the original commercial pure titanium treated by SMAT was not improved in HCI solution. The corrosion micrographs were observed by scanning electron microscope (SEM).     

Surface nanocrystallization of Ti-6Al-4V alloy: microstructural and mechanical characterization

In this study, microstructural and mechanical properties of Ti-6Al-4V alloy, before and after the SMA treatment (SMAT) as well as the duplex SMAT/Nitriding process at different treatment conditions, were investigated in order to deepen the knowledge of these properties for biomedical devices. For that purpose, tribological (wear resistance, coefficient of friction) and mechanical (Vickers microhardness) tests were performed. To carry out the microstructural and surface topographical characterization of the samples, the scanning electron microscopy (SEM) and the 3D-SEM reconstruction from stereoscopic images have been used. By means of profiles deduced from the 3D images, the surface roughness has been calculated. The obtained results allowed to find an interesting SMAT condition which, followed by nitriding at low temperature, can greatly improve tribological and mechanical properties of Ti-6Al-4V alloy. It was also shown from SEM characterization and the original method of 3D-SEM reconstruction, that SMAT can reduce the machined grooves and consequently the roughness of the samples decreases. Moreover, we demonstrated, for the first time, that instead of usual etching method, the ionic polishing allowed to reveal the grains, the grain boundaries and the twins as well as the surface nanocrystalline layer generated by SMAT. Thus, the thickness of the SMATed layer decreases with the nitriding temperature, whereas the surface grain size increases.     

Nanocrystallization of zirconium subjected to surface mechanical attrition treatment

A nanostructured surface layer with thickness of about 20?μm was formed on commercially pure zirconium using surface mechanical attrition treatment (SMAT). The microstructural features of the surface layer were systematically investigated using optical microscopy (OM), x-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), respectively. Based on the results obtained, a grain refinement mechanism induced by plastic deformation during SMAT of Zr is proposed. At?the initial stage of SMAT, twinning dominates the plastic deformation of Zr and divides the coarse grains of Zr into finer twin plates. With increasing strain, intersection of twins occurs, and dislocation slips are activated, becoming the predominant deformation mode instead of twinning. As a result of the dislocation slips, high-density dislocation arrays are formed, which further subdivide the twin plates into subgrains of size about 200-400?nm. With a further increase of strain, the dislocations accumulate and rearrange to minimize the energy state of the high-strain-energy subgrains, the dense dislocation walls convert to grain boundaries, and the submicronic grains are subdivided, leading to the formation of nanosized grains at the top of the treated surface.     

Influence of Surface Nanocrystallization on Ti Ion Implantation of Pure Iron

In order to increase the depth or concentration of Ti ion implantation of pure iron, the surface mechanical attrition treatment(SMAT), which can fabricate a nanometer-grained surface layer without porosity and contamination in a pure iron plate, was used before ion implantation. Ti ion was implanted into the SMA treated sample and coarse-grained counterpart by using a metal vapor vacuum arc source implanter. The changing of depth and concentration of Ti was studied in a function of implantation time.By optical microscopy, transmission electron microscopy and X-ray diffraction, the grain size of the nano structured surface was studied. Micro-hardness, friction and wear behavior of nano surface layers were studied. By energy dispersive X-ray spectroscopy and Auger electron spectroscopy, the chemical composition and concentration of Ti ion in the surface implantation layer were studied. Experimental results showed that the concentration of Ti increased dramatically compared with untreated coarsegrained samples, which is attributed to the existence of higher density of defects(supersaturated vacancies, dislocations, non-equilibrium grain boundaries etc.) and compression stress field in the SMA treated nanocrystallined surface layer. The interaction between the defects and the implanted solute atoms leads to the increment of solid solubility. But the implantation depth showed inconspicuous change. It is shown that the ion range is just relevant to the energy and mass of the ion, dose of injection,the mass and density of target material.     

Surface mechanical attrition treatment-induced dissolution of Cu4Ti precipitates in Cu–4wt-%Ti alloy

ABSTRACT

The microstructural evolution of an overaged Cu-4wt-%Ti alloy associated with surface mechanical attrition treatment (SMAT) has been studied by X-ray diffraction, scanning electron microscope and transmission electron microscopy. The results show that β-Cu₄Ti precipitates in the topmost surface layer were dissolved after SMAT, and Cu solid solution phase with a fine grain size of approximately 25?nm was observed. Dislocation activities were the main deformation mechanism of lamellar structure in the overaged Cu–4wt-%Ti alloy. The twinning in lamellar Cu phase was inhibited by the β-Cu₄Ti precipitates.     

X80管线钢焊接接头表面自身纳米化

采用表面机械研磨(SMAT)技术对X80管线钢的焊接接头进行了表面自身纳米化处理,利用金相显微镜(OM)、透射电子显微镜(TEM)和X射线衍射技术(XRD)研究了表面自身纳米化处理后试样表面微观结构的变化。结果表明:经SMAT处理后,可以在X80管线钢的焊接接头表面形成一定厚度的等轴、取向随机的纳米晶粒;随距处理表面深度的增加,晶粒尺寸逐渐增大;SMAT处理时间的进一步延长对表层晶粒尺寸影响不大;SMAT处理可以实现X80管线钢的焊接接头组织的连续化和均匀化。     

Mo离子注入对纯铜表面纳米层稳定性的影响

采用表面机械研磨处理(SMAT)对纯铜进行表面改性,通过金属蒸汽真空弧离子注入技术在纳米表层注入Mo离子。利用光学显微镜(OM)、X射线衍射分析仪(XRD)和扫描电镜(SEM)观察SMAT处理效果,表面存在纳米层和变形层,通过原子力显微镜(AFM)表征纳米层的晶粒尺寸。结果表明:晶粒尺寸得到了显著的抑制,表面纳米层的晶粒在退火后长大到163nm,而注入了Mo离子的只长大到72nm。此外,SMAT并离子注入后材料表面的硬度仅达到SMAT试样的3.5倍,是纯Cu基体硬度的7倍左右。Mo离子的分散和由SMAT及离子注入引入晶体缺陷的反应促使了这些优化现象的产生。     

Ultra-fast boriding of nickel aluminide

In this study, we explored the possibility of ultra-fast electrochemical boriding of nickel aluminide (Ni₃Al) in a molten borax electrolyte. Electrochemical boriding was performed at 950 °C for 15 min and at current densities ranging from 0.1 to 0.5 A/cm². The boride layers formed on the test samples were 50 to 260 μm thick depending on the current density. The mechanical, structural, and chemical characterization of the boride layers was carried out using a Vickers micro-hardness test machine, optical and scanning electron microscopes, and a thin film X-ray diffractometer. The hardness of boride layer was in the range from 800 to 1200 ± 50 HV depending on the load and the region from which the hardness measurements were taken. X-ray diffraction studies confirmed that the boride layers were primarily composed of Ni₃B, Ni₄B₃ and Ni₂₀AlB₁₄ phases. Structurally, the boride layer was very homogenous and uniformly thick across the borided surface area.     

钛合金表面阳极微弧等离子体渗硼层的研究

采用阳极微弧等离子体技术研究了钛合金表面渗硼层的微观组织和性能。通过光学显微镜、扫描电镜(SEM)、X射线衍射仪(XRD)、能谱仪(EDS)表征分析了渗硼层的表面和截面的微观组织、形貌、相结构、渗层元素分布。借助摩擦磨损试验机测试了渗硼层的耐磨性,运用电化学工作站对渗硼后的TC4材料进行了耐腐蚀性测试。结果表明,钛合金表面阳极微弧等离子体渗硼技术制备的渗硼层连续致密。渗硼层主要由金属间化合物TiB2和TiB组成,其与氧化层共同作用,能显著提高钛合金表面的耐磨性。渗硼后的TC4钛合金耐腐蚀性较基体有所降低。表面阳极微弧等离子体技术是一种新型的钛合金表面改性方法。     

An investigation on boriding kinetics of AISI 316 stainless steel

Boronizing was performed by using a solid medium of Ekabor powders at 1073, 1148 and 1223 K for 2, 4 and 8 h. After boronizing, the major dominant phase was found to be Fe₂B and the minors were CrB and Ni₂B. Boride coating resulted in smooth and dense feature confirmed by optical and SEM. The thickness of boride layer varied from 7 to 87 μm depending on the process time and temperature. Boride layer has a hardness of over 1700 HVN, while the substrate's hardness was about 180 HVN. The growth kinetics of boride layer was found to obey a parabolic rate demonstrating a solid diffusion limited process. The kinetic rates for different process times were plotted by using Arrhenius equation. From this measurement, the activation energy of boride growth for this study was determined as 199 kJ/mol. In addition, the possibility of predicting the iso-thickness of boride layer variation was studied and an empirical relationship between process parameters and boride layer thickness was established. EDS studies showed that Cr concentrated in the coating layer and Ni and Fe concentrated in the substrate.     

Improved Fatigue Behavior of Pipeline Steel Welded Joint by Surface Mechanical Attrition Treatment (SMAT)

A pipeline steel X80 with welded joint was subjected to surface mechanical attrition treatment(SMAT).After SMAT,a nanostructure surface layer with an average grain size of about 10 nm was formed in the treated sample,and the fatigue limit of the welded joint was elevated by about 13% relative to the untreated joints.In the low and the high amplitude stress regimes,both fatigue strength and fatigue life were enhanced.Formation of the nanostructured surface layer played more important role in the enhanced fat...     

Mechanical and Wear Properties of Nanostructured Surface Layer in Iron Induced by Surface Mechanical Attrition Treatment

A porosity-free and contamination-free surface layer with grain sizes ranging from nanometer to micrometer in Fe samples was obtained by surface mechanical attrition treatment (SMAT) technique. Mechanical and wear properties of the surface layer in the SMATed and annealed Fe samples were measured by means of nanoindentation and nanoscratch tests, respectively. Experimental results showed that the hardness of the surface layer in the SMATed Fe sample increased evidently due to the grain refinement. The elastic moduli of the surface layers in the SMATed and annealed Fe samples were unchanged, independent of grain size in the present grain size regime. Compared with the original Fe sample, the wear resistance enhanced and the coefficient of friction decreased in the surface layer of the SMATed Fe sample.     

A comparative study of borided pure niobium, tungsten and chromium

Pure niobium (Nb), tungsten (W) and chromium (Cr) were boronized at 940 °C for 2, 4 and 8 h. The borided samples were characterized by X-ray diffraction, Scanning electron microscope and microhardness tests. Tribological investigation was conducted. X-ray study showed the presence of NbB₂, WB, and CrB. The hardnesses of boride layers formed on the pure Nb, W and Cr were 2500, 2500 and 1700 HV, respectively, whereas the hardnesses of the pure Nb, pure W and pure Cr were 110, 445 and 115 HV, respectively. Nb boride layers ranged in thickness from 8 to 22 μm, whereas W boride layers ranged in thickness from 10 to 42 μm, and the thickness of Cr boride layer varied from 4 to 12 μm with boronizing time. The boriding of W resulted in thicker boride layer compared to the boriding of Nb and Cr at given time. The frictional behaviour and wear mechanicms differ in modes and scales.     

钛合金表面微粗糙化对成骨细胞黏附及增殖行为的影响

采用表面机械研磨(SMAT)法对Ti-25Nb-3Mo-3Zr-2Sn(TLM)钛合金处理15min,而后将处理前后的钛合金样品与成骨细胞共培养1h、24h。采用X射线衍射(XRD)、光学显微镜(OM)、透射电镜(TEM)、扫描电镜(SEM)、电子能谱(EDX)及激光共聚焦显微镜(CLSM)对处理前后样品表面的结构进行了分析,结果显示SMAT处理不会改变TLM钛合金的物相、晶粒尺度及化学组成,处理前后物相均由单一的β相组成,晶粒尺寸均在10~20μm且没有引入新的杂质元素,但SMAT处理会改变TLM钛合金的表面粗糙度,处理前的样品表面平整,平均粗糙度Ra为(0.2±0.03)μm,而处理后的样品表面坑洼不平,平均粗糙度Ra增至(2.6±0.4)μm。随后的细胞实验结果显示,对比SMAT处理前,成骨细胞在处理后的微粗糙表面铺展得更为充分,增殖得更加迅速。研究表明,SMAT处理可以改善TLM钛合金表面的成骨细胞生长环境,从而为开发出更符合临床应用的钛金属种植体提供新的思路。     

Influence of different electron beam surface pre-treatments on the results of subsequent boriding-part I

Boriding produce thick hard layers on cast iron components, which can improve their wear and corrosion behaviour. However, this potential cannot be fully exploited by a simple boriding due to the material specific presence of graphite. In that context, this paper presents results of two fundamentally different electron beam liquid surface treatments (remelting, cladding with nickel-based additive) and their possibilities and limitations regarding subsequent boriding. The boriding behaviour under conventional high temperatures (760 °C–860 °C), and experiments on low-temperature boriding (600 °C–700 °C) were investigated. Under identical treatment conditions, the compound layer thicknesses generated on the unalloyed surfaces (remelting) were approx. 50 %–75 % greater than those of the alloyed surfaces (cladding). A two-layered boride layer structure were generated, though with different phase compositions. Nevertheless, the hardness of all borided layers were comparable. Surface hardness measurements revealed that the supporting effect of substrates plays a decisive role up to a boride layer thickness of approx. 57 μm. In this layer-thickness range, the compound hardness of the alloyed substrates is higher than that of the unalloyed substrates. This knowledge should prove decisive for the selection of layer composites for corrosive and/or tribologically stressed components.     

Q235钢固体粉末渗硼及渗层生长动力学行为

各类材料渗硼工艺不同,硼的扩散也不同,其中有许多现象往往不能定量分析.采用固体粉末法对Q235钢进行了渗硼,得到的渗硼层为锯齿状,垂直于钢表面楔入基体.用sigma Plot 10.0软件对试验数据进行了分析和拟合,得出了渗硼层等厚度图,为制定渗硼工艺提供了依据:利用此图,既可以对设定的渗硼时间和温度预测渗硼层厚度,又可以用一定的固体渗硼厚度值确定渗硼时间和温度.通过动力学研究得到了渗层相组成为单一的Fe2B相硼,在不同温度下的扩散速率常数:K800℃=1.074×10-13m2/s,K850℃=1.622×10-13m2/s,K900℃=3.921×10-13m2/s,平均扩散激活能为134.473 kJ/mol.     

Pack boronizing of AISI H11 tool steel: Role of surface mechanical attrition treatment

The role of surface mechanical attrition treatment (SMAT) on pack boronizing of AISI H11 type tool steel is addressed. SMAT induced plastic deformation, enabled nanocrystallization at the surface, reduced the grain size and increased the volume fraction of non-equilibrium gain boundaries, increased the accumulation of defects and dislocations at the grain boundaries and within the grains. These features helped to promote the diffusion of boron during boronizing and increased the case depth and hardness of the borided layer. Duplex treatment on SMATed H11 steel samples helps to achieve a higher case depth when compared to the single stage treatment. The findings of the study suggest that SMAT can be used as a pre-treatment for boronizing of H11 tool steel.