Friction and wear behaviors of a gradient nano-grained AISI 316L stainless steel under dry and oil-lubricated conditions

A gradient nano-grained (GNG) surface layer was fabricated on an AISI 316L stainless steel (SS) by using the surface mechanical rolling treatment (SMRT). Reciprocating dry and oil-lubricated sliding tests of the GNG 316L SS in air at room temperature were conducted in comparison with the coarse-grained (CG) counterpart. Worn surface morphologies and subsurface microstructures were investigated for both 316L SS samples. 316L SS with a GNG surface layer shows a significantly improved wear resistance, especially under oil-lubricated condition. The notably wear resistance enhancement of the GNG 316L SS is attributed to the GNG surface layer with high strain accommodation ability and high hardness, which can reduce the wear volume in the running-in stage effectively.     

Effects of cryogenic cooling on surface layer alterations in machining of AISI 52100 steels

Abstract

Microstructural phase transformations, commonly known as white layer formation in hard turned steel components, have in recent times become an interesting research topic in machining as they are related to the surface integrity and functional performance of components. Three main theories have been proposed to justify the mechanisms of white layer formation: (1) rapid heating and quenching; (2) severe plastic deformation; and (3) surface reaction with the environment. Coolant application also affects the surface microstructural alterations resulting from machining operations, which have a significant influence on product performance and life. The present work aims at understanding the effects of cryogenic coolant application on the machined surface alterations during machining of hardened AISI 52100 bearing steel. Experiments were performed under dry and cryogenic cooling conditions using cubic boron nitride tool inserts with varying initial work material hardness, tool shape, cutting speed and feedrate. Optical and scanning electron microscopes (SEM) were used to analyse the affected layer in the machined subsurface, while X-ray diffraction technique was utilised to investigate the microstructural phase composition. The experimental results prove that the microstructural phase changes are heavily influenced by the cutting process parameters and the use of cryogenic cooling, in some cases leading to the total removal of martensite.     

Atomic-scale dissecting the formation mechanism of gradient nanostructured layer on Mg alloy processed by a novel high-speed machining technique

Severe plastic deformation(SPD)-induced gradient nanostructured(GNS)metallic materials exhibit supe-rior mechanical performance,especially the high strength and good ductility.In this study,a novel high-speed machining SPD technique,namely single point diamond turning(SPDT),was developed to produce effectively the GNS layer on the hexagonal close-packed(HCP)structural Mg alloy.The high-resolution transmission electron microscopy observations and atomistic molecular dynamics sim-ulations were mainly performed to atomic-scale dissect the grain refinement process and corresponding plastic deformation mechanisms of the GNS layer.It was found that the grain refinement process for the formation of the GNS Mg alloy layer consists of elongated coarse grains,lamellar fine grains with deformation-induced-tension twins and contraction twins,ultrafine grains,and nanograins with the grain size of～70 nm along the direction from the inner matrix to surface.Specifically,experiment results and atomistic simulations reveal that these deformation twins are formed by gliding twinning partial dis-locations that are dissociated from the lattice dislocations piled up at grain boundaries.The corresponding deformation mechanisms were evidenced to transit from the deformation twinning to dislocation slip when the grain size was below 2.45 μm.Moreover,the Hall-Petch relationship plot and the surface equivalent stress along the gradient direction estimated by finite element analysis for the SPDT process were incorporated to quantitatively elucidate the transition of deformation mechanisms during the grain refinement process.Our findings have implications for the development of the facile SPD technique to construct high strength-ductility heterogeneous GNS metals,especially for the HCP metals.     

温度对40Cr13钢等离子表面渗铌层组织的影响

为提高马氏体不锈钢的耐蚀和耐磨性能,选择40Cr13不锈钢为基材、纯铌板为靶材,采用双辉等离子表面冶金技术在不锈钢表面制备合金化层.用SEM、GDOES、XRD等方法分析渗铌温度对铌合金层组织、成分、相组成、表面形貌及硬度的影响,并对渗层形成机制及表面硬化机理进行了研究.结果表明:在900~1 000℃形成的铌合金层组织均匀致密,合金层主要由Nb2C、Nb C、Fe2Nb、Cr2Nb及铌组成;合金层表面粗糙度随渗铌温度的提高而增加;合金层厚度随渗铌温度改变发生不同变化规律,950℃渗铌形成的渗层约13μm,900和1 000℃渗铌后合金层厚度均为7μm左右;不同温度渗铌后试样的表面硬度与基体相比均有较大幅度的提高,1 000℃渗铌后试样表面硬度高达约985 HV0.025,900℃渗铌后约758 HV0.025,而950℃渗铌后表面硬度最低,约698 HV0.025.     

强化处理对GCr15钢球残余应力和微观组织的影响

实验研究了强化处理后钢球残余应力和微观组织的改变.本文研究了经强化处理后,钢球沿径向表面残余应力大小、残余奥氏体含量及硬度分布;用光学显微镜、扫描电子显微镜对经表面形变强化后的轴承钢球进行显微组织及裂纹观察.研究表明:轴承钢球次表层残余应力为压应力,随电解抛光深度的增加,逐渐会出现一个最大压应力的峰值,而后压应力值逐渐减小.强化处理过程中会发生应力诱发残余奥氏体转变为马氏体,但对压应力分布影响不大.将静态Herzt接触理论应用于钢球的动态碰撞强化中,分析探讨钢球裂纹的形成,与实验结果相一致,具有重要指导意义.     

Experimental observation and numerical modeling of formation of local plastic zones in hardened surface layers due to contact overloading

The problem of formation of plastic zones in case-hardened metallic bodies due to contact overloading is studied both experimentally and numerically. Metallic materials exposed to surface hardening demonstrate spatial variation of the material hardness and yield strength with a decreasing profile with depth and belong to the class of so-called plastically graded materials. The presented experimental program employs micro-Vickers hardness tests to map the variation in material hardness and corresponding yield strength for both virgin and loaded case-hardened specimens made of a chromium tool steel. It is shown that, depending on the profile of the yield strength in the near-surface zones and contact parameters, a plastic deformation can originate underneath the hardened layer. The distribution of the effective plastic strain extracted from the micro-hardness increment measurements are found in good agreement with the results of finite element simulations of a plastically graded material subjected to similar loading conditions. Numerical analysis reveals significant perturbations in the stress field distribution within the hardened layer due to formation of a closed-shaped plastic zone in the gradient layers, including development of a tensile stress on the boundary between the elastic and plastic zones as well as an overall increase in the effective stress intensity. It is shown that the hardened layer behaves similar to an elastic beam on a compliant foundation. These stress field perturbations in the hardened layers with low deformation capacity can greatly affect the durability and serviceability of surface treated mechanical parts.     

Effect of ultrasonic shot peening on microstructure and properties of 301SS

Ultrasonic shot peening (USP) is an efficient way to improve the mechanical behavior of 301 stainless steel by inducing severe plastic deformation on its surface. However, this surface treatment induces complex microstructural evolutions, such as grain refinement and phase transformation. Therefore, a better understanding of those evolution mechanisms is critical to optimize the USP treatment. In this work, we rely on various electron microscope observations to compare a specimen before and after a 5-min shot peening treatment. We found an affected layer of ~450?µm with a significant increase in hardness on the top surface by a factor of 2.7 times. Inside this layer, we noticed a nanoscale grain layer of ~130?µm, the most strengthened layer, containing nanoscale grain of α′, with few γ and a large amount of low angle grain boundaries on the topmost. Afterward, we observed coarse grains with deformation twins, which seem to be a preferential site for martensite nucleation, especially at their intersection, and a high density of dislocation. We also conducted experiments to determine a possible scenario for the microstructural evolution, based on those observations.     

A Study on Formation and Growth Mechanism of Nitride Layers During Plasma Nitriding Process of Plastic Injection Mold Steel

Ion nitriding modifies composition of surface layer in steel used in plastic mold application and this consequently improves their lifecycle. In this study, pulsed plasma nitriding technique was used to produce a protecting hard layer on AISI P20 steel at three process temperatures of 450°C, 500°C, and 550°C for durations of 2.5, 5, 7.5, and 10 h at a constant gas mixture of 75% N₂–25% H₂. Surface morphology was studied by optical and scanning electron microscope and the phases formed on the surface layer were determined by X-ray diffraction (XRD). Elemental depth profile was measured by techniques including energy dispersive spectroscopy, wavelength dispersive spectrometer, and glow discharge spectroscopy and for identifying hardness profile, microhardness variations from surface to core of samples were recorded. Results showed that, thickness of compound layer of plastic mold steel AISI P20 was negligible. Moreover in ion nitriding of AISI P20, nitride were formed and grown in some preferred directions and upward diffusion of carbon and downward diffusion of nitrogen occurred during ion nitriding of AISI P20. XRD results showed that, ?-nitride is the dominant phase after plasma nitriding in all strategies. Furthermore, ion nitriding improved hardness of AISI P20 up to three times and as time and temperature increased, hardness and hardness depth of diffusion zone increased considerably.     

Plasma nitriding of AISI 52100 ball bearing steel and effect of heat treatment on nitrided layer

In this paper an effort has been made to plasma nitride the ball bearing steel AISI 52100. The difficulty with this specific steel is that its tempering temperature (~170–200°C) is much lower than the standard processing temperature (~460–580°C) needed for the plasma nitriding treatment. To understand the mechanism, effect of heat treatment on the nitrided layer steel is investigated. Experiments are performed on three different types of ball bearing races i.e. annealed, quenched and quench-tempered samples. Different gas compositions and process temperatures are maintained while nitriding these samples. In the quenched and quench-tempered samples, the surface hardness has decreased after plasma nitriding process. Plasma nitriding of annealed sample with argon and nitrogen gas mixture gives higher hardness in comparison to the hydrogen–nitrogen gas mixture. It is reported that the later heat treatment of the plasma nitrided annealed sample has shown improvement in the hardness of this steel. X-ray diffraction analysis shows that the dominant phases in the plasma nitrided annealed sample are ε (Fe_2 − 3N) and γ (Fe₄N), whereas in the plasma nitrided annealed sample with later heat treatment only α-Fe peak occurs.     

高能电脉冲-超声滚压耦合技术对淬火态GCr15钢表面强化研究

采用高能电脉冲辅助超声滚压技术对高频淬火态GCrl5轴承钢进行了表面强化处理,并对表层硬度梯度、表面粗糙度以及摩擦磨损性能进行了表征.与普通超声滚压技术相比,声电耦合处理后样品在提高表面硬度的同时强化层深度提高约100μm,表面粗糙度Ra由1.4μm降低至0.23μm,并且在电脉冲作用下位错运动与越过能垒的能力都得到增强,从而促进表面微裂纹得到愈合,表面质量显著提高,摩擦磨损性能提高约50％.对高频淬火态GCr15轴承钢而言,脉冲电流的电致塑性效应能够促进位错运动,提高材料表面塑性变形能力,从而使超声滚压产生的塑性变形向次表层发展,显著提高强化效果.     

Effect of Surface Mechanical Attrition Treatment on Tribological Behavior of the AZ31 Alloy

By surface mechanical attrition treatment(SMAT),a gradient nano structure(GNS) from the surface to center was generated in the AZ31 alloy sheet.The tribological behavior of AZ31 alloy with GNS was systematically investigated by using dry sliding tests,a 3D surface profile-meter and a scanning electron microscope equipped with an energy-dispersive spectrometer.The experimental results indicate that the Mg alloy with GNS exhibits better wear resistance comparing to the as-received sample,which is associated to the alteration of wear mechanism at different sliding speeds.The Mg alloy with GNS presents the wear mechanism of the abrasive wear at 0.05 m/s and the oxidative wear at 0.5 m/s,respectively.Moreover,the GNS can effectively promote the reaction between the oxygen and worn surface,which leads to a compact oxidation layer at 0.5 m/s.The effect of oxidation layer on the wear resistance of the Mg alloy was also discussed.     

Formation of nanocrystalline layers by surface severe plastic deformation and pulsed plasma electrolytic carburizing

Surfaces of various kinds of metallic materials spheres were treated by nanocrystalline surface severe plastic deformation and then pulsed nanocrystalline plasma electrolytic carburizing to study nanocrystalline substrate effect on formation and nano-hardness of hard nanocrystalline layer. The surface layers of the metallic materials developed by the nanocrystalline surface severe plastic deformation were characterized by means of high resolution scanning electron microscope. Nearly equiaxed nanocrystals with grain sizes ranging from 15 to 90 nm were observed in the near surface regions of all metallic materials, which are low carbon steel and commercially pure titanium. The effect of substrate nanocrystallization on growth kinetics and hardness of formed nanocrystalline carbide layer was studied with the means of figure analysis and nanohardness tests. Figure analysis show the length to diameter ratio and distribution curve of nanocrystals and it has been found that the achieved properties of hard layer (growth rate, nano-hardness, nanostructure...) are related to these factors. It was also clarified that these techniques and surface nanocrystallization can be easily achieved in most of metallic materials. Results indicate that the resultant hardened carburized layers exhibited excellent hardness profile. Investigation of the layer characteristics showed strong dependence followed from the treatment experimental parameters as well as the shape of nanocrystals.     

聚甲醛钢背铜塑三层结构复合材料 的加工工艺与性能研究

介绍了以钢为基座, 青铜粉为过渡层, 聚甲醛塑料为表面层的三层结构复合材料的复合 加工工艺及其工艺对性能的影响。试验的结果表明, 在一定的温度范围内, 制品的表面硬度随压制 温度的升高而增加, 随塑料层厚度的增大而减小; 在一定的范围内, 青铜粉的粒径越大, 塑料层与 钢背间的结合强度越大, 且表面硬度也越大, 摩擦系数也越小。      

Microstructure and Properties of Low Temperature Composite Chromized Layer on H13 Tool Steel

Low temperature composite chromizing is a process composed of a plain ion-carbonitriding or ion-nitriding at 550～580℃, followed by a low-temperature chromizing in a salt-bath of 590℃. The microstructure and properties of the low temperature composite chromized layer on H13 tool steel were investigated using metallography,X-ray diffraction, microanalysis, hardness and wear tests. It was found that this low temperature process was thermodynamically and kinetically possible, and the composite chromized layer on H13 steel, with a thickness of 3～6μm,consisted of three sub-layers (bands), viz. the outer Cr-rich one, the intermediate (black) one, and the inner, original white layer. After chromizing, the former diffusion layer was thickened. The results of X-ray diffraction showed that the composite chromized layer contained such nitrides and carbides of chromium as CrN, Cr2N, (Cr, Fe)23C6, and (Cr, Fe)7C3, as well as plain α-(Fe, Cr). A high surface microhardness of 1450～1550 HV0.025, which is much higher than that obtained by the conventional ion carbonitriding and ion nitriding, was obtained. In addition, an excellent wear resistance was gained on the composite chromized layer.     

Characterization and Properties of Nanostructured Surface Layer in a Low Carbon Steel Subjected to Surface Mechanical Attrition

A nanostructured surface layer was synthesized on a low carbon steel by using surface mechanical attrition (SMA)technique. The refined microstructure of the surface layer was characterized by means of different techniques,and the hardness variation along the depth was examined. Experimental results show that the microstructure is in homogeneous along the depth. In the region from top surface to about 40 /zm deep, the grain size increases fromabout 10 nm to 100 nm. In the adjacent region of about 40~80 /zm depth, the grain size increases from about 100nm to 1000 nm. The grain refinement can be associated with the activity of dislocations. After the SMA treatment,the hardness of the surface layer is enhanced significantly compared with that of the original sample, which canprimarily be attributed to the grain refinement.     

Combined effect of shot peening,subcritical austenitic nitriding,and cryo-treatment on surface modification of AISI 4140 steel

Shot peening is a simple but effective severe plastic deformation process to synthesize ultrafine grains in micro- to nanometer range on metallic surfaces. In this work, shot peening on AISI 4140 steel specimens was done in a novel centrifugal air blast shot peening reactor with shot velocity of 5.8?m/s for 3?h. Characterization of the shot peened surface (XRD, micro-hardness, SEM, and TEM) showed that surface undergoes significant plastic deformation with marked increase in microstrain of lattice, dislocation density, and surface hardness. XRD profiles and TEM analysis confirmed formation of ultrafine grain structure in the nanometer range. These specimens were then subjected to austenitic nitriding at 610°C for 4?h followed by cryo-treatment at???185°C for 32?h. Characterization of pre-shot peened nitrided and cryo-treated surfaces showed that there was marked improvement in surface hardness (from 695 to 797 HV_0.05) and effective case depth (from 19 to 54?µm) in comparison with un-shot peened nitrided and cryo-treated specimens. It was demonstrated that presence of ultrafine grain structure and austenitic phase during nitriding plays synergetic role to improve content and diffusion kinetics of nitrogen in AISI 4140 steel surface.     

Microstructural evolution and mechanical properties of oil jet peened aluminium alloy,AA6063-T6

Grain size refinement by severe surface plastic deformation is one way of improving the surface properties. This paper describes the microstructural evolution due to severe surface plastic deformation by oil jet peening in aluminium alloy, AA6063-T6. Detail characterization of the treated surfaces using X-ray diffraction analysis and transmission electron microscopy revealed the formation of submicron size grains at and near the surface. The nozzle-traveling velocity decides the peening intensity and coverage and affects the surface properties. The specimen peened at low nozzle-traveling velocity exhibited an ultrafine grain size (∼210 nm) with high surface hardness (∼0.88 GPa), compressive residual stress (−102 ± 7 MPa) and dislocation density. The hardness is high at the surface and the depth of hardened layer is ∼400 μm. Formation of high-density dislocations and associated grain refinement resulted in increased surface hardness. Presence of surface modified layer will be beneficial in improving the fatigue and tribo behavior.     

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.     

Analysis of plastic deformation in diamond like carbon films-steel substrate system with tribological tests

The influence of plastic deformation of the substrate on the tribological properties of diamond like carbon (DLC) films was investigated in DLC films-steel substrate system. The tribological properties of DLC films deposited on different hardness steel were evaluated by a ball on disk rotating-type friction tester at room temperature under different environments. In dry nitrogen, DLC films on soft steel exhibited excellent tribological properties, especially obvious under high load (such as 20 N and 50 N). However, DLC films on hard steel were worn out quickly at load of 20 N. Plastic deformation was observed on soft steel after tribological tests. The width and depth of plastic deformation track increased with increase of the experimental load. Super low friction and no measurable wear were kept in good condition even large plastic deformation under high load conditions in DLC films-soft steel system. In open air, DLC films on soft steel exhibited high coefficient of friction and DLC films on ball were worn out quickly. Plastic deformation was not observed on soft steel because the contact area increased and the thick hardened layer on contact surface were formed by DLC films and debris particles together on the steel substrate. The wear track on steel became deep and wide with increase of loads and DLC films were worn out. The experimental results showed that super low friction and high wear resistance of DLC films on soft steel can be attributed to the good adhesion and plastic deformation. Plastic deformation played an active role in the tribological properties of DLC films on soft steel in the present work.     

Metallurgical investigation of a prematurely failed roller bearing used in the support and tilting system of a steel making converter used in an integrated steel plant

An extensive metallurgical investigation was carried out on samples of a failed roller bearing from the support and tilting system of a basic oxygen furnace (BOF) converter used in the steel melting shop of an integrated steel plant. The converter bearing was fabricated from low-carbon, carburizing grade steel and had failed in service within a year of fitting to a repaired shaft. Microscopic observations of both the broken roller and inner-race samples revealed subsurface cracking and preponderance of brittle oxide and other macroinclusions. Electron probe microanalysis (EPMA) studies confirmed that the brittle oxides that formed stringers were alumina, and the other macroinclusions were complex silicates. Both the alumina and silicate inclusions were deleterious to contact-fatigue properties. Microstructurally, the carburized regions of the broken roller and of inner-race samples contained high-carbon tempered martensite. Microhardness measurements revealed that although the core hardness of the roller and the inner-race samples were similar, the surface hardness of the roller was approximately 8.5 HRC units harder than that of the inner-race. Scanning electron microscope (SEM) observations of the roller fracture surface revealed striations indicative of fatigue, and energy-dispersive spectrometric (EDS) analyses corroborated a high incidence of silicate inclusions at crack sites. The study suggests that the failure of the bearing occurred because the hardness difference between the roller bearing and the inner-race surfaces resulted in wear of the inner-race. The wear led to shaft misalignment and play during service. The misalignment, coupled with the presence of inclusions, caused fatigue failure of the roller bearing.