Hardness Gradient Research Articles

Tribological behavior and surface characteristics of severe surface plastic deformed as-cast and 3D-printed SS316L using LSP

The effect of laser shock peening (LSP) on as-cast and three-dimensional (3D)-printed SS316L alloys was studied. The LSP process was found more dominant than the 3D-printing technique over as-cast alloy steel for enhancing the properties. The process of laser shock peening results in microlevel changes in the structure of the material. This, in turn, leads to grain refinement and the increase of compressive residual stresses. After LSP treatment, the surface microhardness value increased by 33% in both as-cast and 3D-printed SS316L alloys with the hardness gradient from surface to subsurfaces. The wear rate of as-cast specimens and 3D-printed specimens were reduced to 1.28 10−6 and 1.14 10−6 g/m, respectively, and tensile value increased up to 22% after LSP treatment. The scanning electron microscopy images on the wear track confirm the vulnerability of the untreated SS316L alloys with adhesion wear and surface delamination. This difference was because the wear rate of the 3D-printed specimens was lesser than the as-cast SS316L alloys due to their layer-by-layer high-temperature finishing. The samples were characterized by optical microstructure to identify the surface refinement, and confirmed by the transmission electron microscope images along with specified area electron diffraction pattern for the accumulation of dislocation and grain refinement after LSP treatment. The accumulated residual stress was measured by X-ray diffraction technique and it found the highest compressive residual stress after LSP treatment was accrued by 3D-printed SS316L alloy with 310 MPa.

Design of explosively welded Fe–Al multilayer laminated composite pipes: A critical microscopy analysis of stand-off distance and post-weld heat treatment effects on interface properties

This study examined the microstructure and mechanical properties of explosively welded (EXW) Fe–Al multilayer laminated composite pipes, specifically SS321/AA1050/AA5083, with varying stand-off distances (SD) and post-weld heat treatments (PWHT). Findings revealed that higher collision forces at higher-SD produced a wavy interface with finer grains at the AA1050/AA5083 interface. In contrast, lower-SD yielded a smooth, flat interface. At the SS321/AA1050 interface, collision energy resulted in the formation of an uneven composite reaction layer comprising multiple intermetallics. Analysis of the SS321/AA1050 interface revealed that an increase in the PWHT temperature results in diffusion toward the reaction layer, transforming the unevenly Al-rich and Fe-rich phases into a uniform Al85(Fe,Cr,Ni)15 solid solution. The increase in the fraction of this phase made the reaction layer more brittle. At the AA1050/AA5083 interface, PWHT at higher temperatures caused a significant hardness decline extending further from the interface in samples with higher-SD. On the SS321 side of the laminated composite, higher SD resulted in higher kernel average misorientation (KAM) and increased hardness. Despite a decrease in hardness after PWHT at 350 °C, the hardness gradient from the matrix to the interface in laminates with higher SD remained relatively unaffected, whereas the lower SD counterparts experienced a noticeable stress relief.

Ultrasonic backscattering measurement of hardness gradient distribution in polycrystalline materials

It is crucial to obtain the internal hardness distribution in polycrystalline materials to evaluate the mechanical performance of components and monitor their service life. Current methods, however, fail to meet the non-destructive evaluation needs for materials with hardness gradient distributions. This paper, based on the principle of grain boundary scattering of ultrasound in polycrystalline materials, combined with the Transverse-to-Transverse Singly-Scattered Response (T-T SSR) theory, proposes an ultrasonic SSR model adapted to hardness gradient distributions. The model elucidates the influence of hardness gradient variations and grain dispersion on ultrasonic scattering. Using DREAM.3D, seven different-scale polycrystalline volumes were constructed to assess the relevance of volume-weighted average grain size and spatial correlation of hardness gradient materials. Finally, induction quenching was applied to 40Cr to induce a gradient hardness distribution internally, followed by ultrasonic backscatter experiments. The results indicate that the theoretical model and the spatial variance of measured signals align well over a relatively long time window. For the specimen with minor curvature, the theoretical hardness distribution obtained by the model is accurate, with an average error of 2.55 % compared to destructive testing data. However, the results for the larger curvature reveal limitations in the model.

Hard antireflection coatings with enhanced mechanical properties based on gradient structure

Antireflection (AR) coatings with sapphire-like hardness are highly desired in various applications. Traditionally, hard AR coatings resist penetration, scratching, and abrasion by depositing a hard layer, such as silicon nitride (Si3N4), with a 1 to 2 µm thickness. However, thick Si3N4 thin films, fabricated by high-energy inductively coupled plasma assisted (ICP-assisted) sputtering, often introduce high residual stress, leading to severe buckling problems when the surface is scratched. To address these challenges, we employ a "Step up-step down" design method, integrating a hardness gradient structure with a series of SiOxNy films. This approach achieves high optical transmittance (Tave > 98.7 % at 420–720 nm), low residual stress (∼680 MPa), improved scratch resistance, and strong adhesion at the film-substrate interface (LC3 ∼180 mN). Our findings demonstrate significant potential for various mechanical and optical applications, including automotive and consumer electronics devices.

Influence of Ni content on electrochemical corrosion and tribological behavior of Cu-10Sn coatings by laser cladding

In this work, Cu-10Sn-xNi (x = 0, 3, 6, 9, and 12 wt%) alloy coatings were prepared using laser cladding technology. The influence of Ni content on the phase composition, microstructure, crystallographic characteristics, microhardness, electrochemical corrosion, dry sliding wear and corrosive wear behavior of the coatings were studied. The results indicated that the coatings formed good metallurgical bond with the substrate. The coatings were composed primarily of the α-Cu matrix and small γ precipitates. The Ni element effectively reduced component segregation, and with the increased of Ni content, the grain size in the coating was significantly refined, resulting in fine equiaxed grains. Under the effect of fine grain strengthening, the Cu-10Sn-12Ni (C12NS) coating achieved a higher microhardness of approximately 222.9 ± 5.3 HV0.2. In the electrochemical corrosion test, the increased in the number of grain boundaries significantly reduced the corrosion resistance of the Cu-10Sn-xNi coatings. The Cu-10Sn-6Ni (C6NS) coating exhibited excellent corrosion resistance, with an Icorr of only 1.37 μA/cm2. The results of the dry sliding wear test showed that under the influence of the hardness gradient between the hard precipitate phase and the soft matrix of the coating, the C6NS coating achieved satisfactory wear resistance with a specific wear rate of 3.30 × 104 μm3/Nm. Meanwhile, due to the good corrosive resistance, the C6NS coating showed the best tribological performance in 3.5 wt% NaCl environment, and the specific wear rate was 1.58 × 104 μm3/Nm.

Influence of Elements in Steel With Anode Plasma Electrolytic Boriding on Hardness Gradient and Tribological Properties

Abstract The influence of elements in steel on anode plasma electrolytic boriding has been studied. The modified layers and surfaces on steel samples were analyzed by a scanning electron microscope, an X-ray diffractometer, a surface profiler, a microhardness tester, and a ball-disc tribometer. With 1045 steel as the control group, the same treatment parameters (treatment with 5 min, 200 V of voltage, 5% boric acid, and 10% ammonium chloride) were implemented on various steel substrates. It was found that a low content of carbon would hinder the penetration of boron, and other metallic elements can improve the mircrohardness gradient and decrease the wear-rate. Chromium and manganese can increase the maximum microhardness than treated 1045 by about 15%, but have a detrimental effect on surface flatness. Nevertheless, manganese has the ability to rapidly create a layer of oxide that enhances the tribological characteristics, leading to a remarkably low average friction coefficient of 0.26 for 1046 steel. The presence of molybdenum in the element composition of 4140 steel results in a notable enhancement of surface properties, namely in terms of wear resistance, with a minimum wear-rate 2.1 × 10−6 g/Nm for 4140 steel. Nickel does not appear to have a notable impact on the surface characteristics of the modified samples.

Microstructure, mechanical, and tribological properties of transition metal (Nb, V, W) nitride coating on AISI-1045 steel by cathodic cage plasma deposition

AISI-1045 steel is a medium-carbon, medium-strength steel that usually requires surface engineering to be usable in industrial applications. Using the cathodic cage plasma deposition technique, transition metal (Nb, V, W) nitride coating is deposited on this steel using cathodic cage lids of these metals. The hardness of untreated steel (1.8 GPa) is upgraded to 11.2, 12.2, and 9.7 GPa for niobium nitride, vanadium nitride, and tungsten nitride coating, respectively. The elastic modulus, the ratio of hardness-elastic modulus (H/E, H2/E, and H3/E2), and the plasticity factor depict the improvement in mechanical and elastic properties. The sample treated with a niobium cage lid exhibits the Nb4N5 phase, the vanadium cage lid shows the VN phase (along with the Fe4N phase), and the tungsten cage lid consists of W2N3, WFeN2, and Fe4N phases. Among these coatings, the thickness of niobium nitride coating is maximum (1.87 μm), and a low deposition rate is obtained for tungsten nitride coating (0.83 μm). In addition to this coating, a nitrogen diffusion zone (∼60 μm) is also formed beneath the coating, which creates a hardness gradient between the coating and the substrate. The ball-on-disc wear tester shows that niobium nitride coating deposition reduces the wear rate from 19.5 × 10−3 to 8.8 × 10−3 mm3/N m and exhibits excellent wear performance.

Surface hardness imaging of a low-alloy steel using laser-induced breakdown spectroscopy

Conventional tactile hardness testing methods such as Brinell, Rockwell or Vickers rely on direct mechanical contact, which results in significant surface damage and prohibits their application to complex specimen geometries. Furthermore, the sample must have a certain thickness for tactile testing methods to be applied. In this work, we investigated the use of laser-induced breakdown spectroscopy as a fast non-quantitative alternative for visualizing surface hardness gradients. To eliminate the influence of different chemical compositions, we manually heat-treated low-alloy steel pieces cut from the same raw material and batch. By partially quenching a sample piece, we were able to obtain a hardness gradient along the longitudinal axis. We found a positive correlation between the ratio of ionic to atomic line intensities of iron (I Fe II 263.1 nm / I Fe I 358.1 nm) and the mechanical hardness of the sample surface. By scanning the surface and measuring the line intensity ratios, we were able to obtain a spatially resolved map directly correlating with the surface hardness distribution. Additionally, it was observed that the irradiation of laser pulses resulted in significant surface alterations, thereby invalidating subsequent measurements and scans at identical positions.

Gear contact fatigue prediction under mixed elastohydrodynamic lubrication with ellipsoidal asperity rough surface: Experimental and numerical investigation

A three-dimensional(3D) line-contact mixed lubrication model and contact fatigue life prediction model for gears are established, considering the elastoplastic contact of ellipsoidal asperities and their interactions. The localized lubrication states across the actual 3D rough surfaces are visually depicted. The direct asperity contact pressures calculation exhibits greater accuracy and universality. The proposed mixed lubrication model can be applied accurately and efficiently to heavy-load conditions. The synergistic mechanisms of multi-scale surface integrity parameters, such as surface morphology-lubrication coupling, hardness gradient, and residual stress, on gear contact fatigue has been revealed. The results reveal that surface roughness significantly affects the competitive failure mechanism between surface-initiated micropitting and subsurface-initiated macropitting. The increase in surface compressive residual stress is more sensitive to the enhancement of near-surface and subsurface fatigue lives, compared to the increase of peak residual stress and the depth of peak residual stress. The proposed physics-based gear contact fatigue life prediction model has been validated through gear contact fatigue experiments under various operating conditions. The maximum deviation between the predicted and experimental fatigue lives is within 10%. This paper presents theoretical methodologies and data-driven support for optimizing the design and manufacturing processes of gears, specifically focusing on enhancing anti-fatigue properties.

Study of the process parameters influence on crack formation in laser alloying of grey cast iron

This study aimed to investigate the influence of process parameters on crack formation in laser alloying or cladding of grey cast iron. For this purpose, the effects of laser power and feeding rate of Ni-based alloying powders were examined. The microstructure and hardness of the coating and the interface of the coating with cast iron (bonding zone) were studied. The results showed that the dilution ratio is crucial in crack formation, explaining the challenges in achieving a defect-free laser alloying coating on cast iron. The higher dilution ratio of laser alloying resulted in higher dissolved carbon and bigger (Nb, Ti)C carbides formation than in laser cladding coatings. In this study, cracks appeared in the coating due to the combination of the high amount of carbide in the layer and a sharp hardness gradient at the interface with the cast iron substrate. An empirical relation was proposed for dilution ratio as a function of specific energy density, which combined the most critical process parameters on crack formation.

A numerical model for total bending fatigue life estimation of carburized spur gears considering the hardness gradient and residual stress

A numerical model for total bending fatigue life estimation of carburized spur gears considering the hardness gradient and residual stress

Manufacturing of new dual phase steel with a combination of macro-laminated and micro-laminated morphology

ABSTRACT In this study, a new dual phase steel with a combination of macro-laminated and micro-laminated morphology was fabricated for the first time. Friction stir welding, heat treatment, and cold rolling were employed to produce the new laminated DP steel. The microstructure of produced DP steels consisted of elongated ferrite and martensite phases in the St52 layers. The microhardness profiles and stress–strain curves of produced laminated DP steels showed that the best mechanical properties corresponded to the traverse speed of 70 mm/min due to less hardness gradient. The failure morphology indicated good interfacial bonding that prevented delamination during the tensile deformation.

Corrosion studies of duplex surface-treated M420 stainless steel by plasma nitrocarburizing and WCrAlTiSiN multilayer coating

M420 stainless steel has good mechanical properties, but its corrosion resistance is poor, which limits its application in marine conditions. Single-technology processing has been unable to meet the development needs of modern industry. In this work, plasma nitrocarburizing (PNC) and CrN/CrTiAlSiN/WCrTiAlN multi-layer coating were carried out on M420 martensitic stainless steel by hollow cathode-assisted plasma treatment and multi-arc ion plating, respectively. The modified layers were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. The results show that the nitrocarburized layer effectively improves the hardness gradient between the coating and the substrate. The presence of the composite strengthening layer enables the sample to obtain a thick and dense corrosion-resistant surface, effectively blocking the invasion of the corrosive medium. The corrosion rate of the PNC + C sample decreased by an order of magnitude. Combining hollow cathode-assisted rapid nitrocarburizing technology and multi-arc ion plating technology can significantly improve the mechanical properties and corrosion resistance of M420 stainless steel in a shorter time. The duplex surface-treated M420 stainless steel by plasma nitrocarburizing and WCrAlTiSiN multilayer coating exhibited excellent corrosion resistance compared to the nitrided and coated M420 stainless steel.

A gradient strengthening method of wire-arc additive manufacturing coupling with surface rolling and its application

The gradient distribution of properties, which satisfies the service requirements of critical components operating in extreme environments, plays a significant role in enhancing the durability of these components. Here, a new method of wire-arc additive manufacturing (WAAM) coupling with surface rolling was proposed to construct the gradient distributed properties. The present study focuses on a typical application of this method, wherein the surface of an exhaust valve in a marine diesel engine is processed to achieve a gradient property. The gradient strengthening mechanisms were investigated through numerical simulation and physical experiments. Simulation results demonstrated that the residual tensile stress resulting from WAAM was significantly reduced and mostly converted to compressive stress after surface rolling. The initial maximum residual tensile stress of 208.6 MPa, located at the surface of the deposited valve, was reduced to −167.8 MPa after surface rolling. The depth of residual compressive stress was extended 6–7 mm from the rolled surface to the deposited zone. Analysis of microstructures revealed that three distinct zones with different microstructural characteristics were generated through the proposed method, thereby forming the gradient microstructures. Specifically, the surface rolled zone was characterized as nanocrystallines (<1 μm), refined grains (1–10 μm), and deformed grains (>10 μm), while the deposited zone and substrate corresponded to columnar grains and equiaxed grains, respectively. A wide gradient strengthening layer with an approximate thickness of 8 mm was achieved on the processed valve surface, exhibiting a hardness gradient from 314 MPa at the substrate to 566 MPa on the rolled surface. The gradient strengthening mechanisms were attributed to intrinsic material strengthening, grain boundary strengthening and dislocation strengthening.

Research on the Influence of Cold Drawing and Aging Heat Treatment on the Structure and Mechanical Properties of GH3625 Alloy.

With the development of the petroleum industry, the demand for materials for oilfield equipment is becoming increasingly stringent. The strength increase brought about by time strengthening is limited in meeting the needs of equipment development. The GH3625 alloy with different strength levels can be obtained through cold deformation and heat treatment processes. A study should be carried out to further develop the potential mechanical properties of GH3625. In this study, the GH3625 alloy was cold drawn with different reductions in area (0-30%) and heat treated, and its mechanical properties were tested. The microstructure of the alloy during deformation and heat treatment was characterized by methods such as optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) based on the principles of physical metallurgy. The strength increase caused by dislocation strengthening was calculated from the dislocation density, tested by X-ray diffraction (XRD). The calculated value was compared to the measured value, elucidating the strengthening effect of cold deformation and heat treatment. The results showed that the yield strength and yield ratio of the cold-drawn alloy significantly reduced after aging at 650 °C and 760 °C. Heat treatment can make a cold-deformed material recover, ablate dislocations, and greatly reduce the dislocation density in the microstructure of the GH3625 alloy, which was the main factor in the decrease in yield strength. The work-hardening gradient of the cold-drawn material varied greatly with different reductions in area. When the reduction in area was small (10%), the hardness gradient was obvious. When it increased to 30%, the alloy was uniformly strengthened as the deformation was transmitted to the axis. This study can provide more mechanical performance options for GH3625 alloy structural components in the petrochemical industry.

The effect of fine particle peening on aviation gear performance, part II: Rolling contact fatigue

AbstractFine particle peening (FPP) technology offers an effective method to achieve outstanding surface integrity and fatigue performance in aviation gears. However, the lack of sufficient research on the correlation between process parameters, surface integrity parameters, and fatigue life undermines the full potential of FPP in strengthening aviation gears. Therefore, we have developed an elastic–plastic contact fatigue life model to predict the fatigue life of AISI 9310 gear steel after different strengthening processes. This model can consider surface integrity parameters such as surface micro‐topography, residual stress gradient, and hardness gradient. To validate the model, we conducted rolling contact fatigue tests and compared the results with the predictions from our model. The influence of process parameters on the rolling contact fatigue of AISI 9310 gear steel was further investigated. A process window for FPP was established, and an optimization method was proposed in order to provide guidance for selecting process parameters for FPP on high‐performance aviation gears.

Effect of Scanning Electron Beam Pretreatment on Gas Carburization of 22CrMoH Gear Steel

22CrMoH was selected for the gear steel material in this work, and the temperature field change in the scanning electron beam was analyzed to determine the optimal scanning parameters and explored the effect of scanning electron beam pretreatment (Abbreviated as: SEBP) on gas-carburizing (GC) efficiency and organizational properties of gear steel. The results showed that the scanning electron beam caused the material to form a thermally deformed layer 110 μm thick, and it promoted the adsorption of carbon atoms on the surface and their inward diffusion. Under the same gas-carburizing conditions, the carburizing efficiency was improved, and the thickness of the carburized layer increased from 0.78 to 1.09 mm. Furthermore, the hardness of the GC specimens with the SEBP increased from 615 to 638 HV0.05 at 0.1 mm of the sample surface, whereas the hardness of the cross-sectional region decreased gradually, indicating that the scanning electron beam enhanced the adhesion between the carburized layer and matrix zone. A comparative analysis of the microstructures of the GC specimens with and without the SEBP showed that the carbide particles in the surface layer of the samples become smaller and that of volume fraction of residual austenite reduced in size. In terms of the mechanical properties, the surface friction coefficient decreased from 0.87 to 0.46 μ and the GC specimen with the SEBP had a higher cross-sectional hardness gradient. Its friction coefficient was reduced from approximately 0.8 to almost 0.45 μ, and the wear amount of the specimens with SEBP was 47.7% of that of the matrix specimens.

Composite effects of residual stress and hardness gradient on contact fatigue performance of rough tooth surfaces and optimization design of detailed characteristic parameters

Residual stress and hardness gradient significantly affect the contact fatigue failure process of rough tooth surfaces, and the meticulous design of their characteristic parameters can help improve the fatigue resistance of gears. In this paper, an efficient computational method for rough tooth surface contact is established based on the reconstruction modeling of asperities and mixed lubrication analysis. By comprehensively considering the effects of residual stress and hardness gradient, the Dangvan multiaxial fatigue criterion is adopted to predict and analyze the contact fatigue life of the gear surface. The correlation between the surface residual stress, maximum residual stress depth, surface hardness, and effective hardening layer depth, among other characteristic parameters, and the contact fatigue performance of tooth surfaces is established. The results show that: (1) Increasing the magnitude of surface residual compressive stress and surface hardness can significantly reduce the risk of fatigue failure in the near-surface layer of the gear. To achieve the same fatigue performance, there is a linear negative correlation between the design values of the two parameters. (2) Affected by the double stress peaks in the rough tooth surface stress field, the maximum residual stress depth significantly affects the depth at which the highest failure risk occurs. With changes in roughness, the optimal design value of the maximum residual compressive stress depth changes from about 0.5 times the Hertzian contact half-width depth in the subsurface layer (Sa < 0.2 μm) to about 15 μm in the near-surface layer (Sa > 0.4 μm). (3) Considering both process cost and gear surface fatigue resistance, the design threshold for the effective hardening layer depth should be greater than the Hertzian contact half-width. This paper establishes a composite correlation between the meticulous characteristic parameters of residual stress, hardness gradient, and the contact fatigue performance of tooth surfaces, providing theoretical support for optimal design of tooth surface anti-fatigue performance.

Ultrahigh surface hardness and excellent hardness gradient induced by ultrafine dual-carbides in a novel stainless bearing steel

A novel alloy composition design (Fe–14Cr–13Co–5Mo–2Ni–1W–1V-0.16C-0.12Nb-0.074Si-0.027Mn, wt. %) was proposed for modification on that of the conventional CSS-42L stainless steel. The present homogeneous structure without carburizing displays an excellent synergy of yield strength and fracture toughness, which should improve the performance of the corresponding case-carburized steels under the contact/bending fatigue conditions. An ultrahigh surface hardness (71 HRC) and an excellent hardness gradient along the depth are achieved in the present case-carburized gradient structure. Higher surface hardness at a temperature range of 200–400 °C is also observed in the present case-carburized gradient structure, as compared to the other case-carburized advanced bearing/gear steels, such as the M50 steel, the M50NiL steel and the conventional CSS-42L steel. Besides the previously reported M6C carbides in the case-carburized conventional CSS-42L steel, M7C3 carbides are newly discovered in the present case-carburized gradient structure due to the novel chemical design. The ultrahigh surface hardness at both room and high temperatures and the excellent hardness gradient distribution in the present case-carburized gradient structure should be attributed to the increased amount of dual-carbides, as well as the ultrafine dual-carbides embedded in the ultrafine martensite matrix. The superior mechanical properties of the present case-carburized gradient structure should be desirable for real bearing/gear applications.

Influence of carbon layer hardness characteristics on RCF performance of carburized bearing steel and optimization of heat treatment process

Being a pivotal element within high-speed train transmission systems, the mechanical attributes of bearings are of great significance to ensure the quality and safety of train operation. In this study, the rolling contact fatigue performance of bearing materials was tested to investigate the effects of different carbon layer characteristics on the rolling contact fatigue behavior. Combined with the thickness of carburizing layer, the elastoplastic coupling model of rolling contact fatigue damage of G20CrNi2MoA carburizing bearing steel was established. The model revealed the relationship between the number of load cycles and the degree of subsequent material damage. The model was used to calculate and predict the loading frequency of G20CrNi2MoA carburized bearing steel with excellent fatigue characteristics, which was 2.02 × 108 times before spalling. In addition, the carburizing heat treatment process for bearings was optimized based on the hardness gradient curves obtained, which provided valuable insights for improving the actual production process.