High Compressive Residual Stress Research Articles

Effects of fine particle peening on fatigue strength of a TRIP-aided martensitic steel

The effects of fine particle peening on the fatigue properties of a transformation-induced plasticity-aided martensitic steel with a chemical composition of 0.2% C, 1.5% Si, 1.5% Mn, 1.0% Cr, and 0.05% Nb (mass%) were investigated for applications in precision gears. When this steel was subjected to fine particle peening after heat treatment, higher fatigue limits and lower notch sensitivity were achieved compared with those of SNCM420 martensitic steel. The increased fatigue limits are principally associated with higher hardness and compressive residual stress on and just below the surface, as well as low surface roughness. Fine particle peening also increased the threshold stress intensity factor range in stage I, and decreased the crack propagation rate in stage II in this steel. The crack propagation behavior was mainly suppressed by (1) high hardness and high compressive residual stress, and (2) the strain-induced martensite transformation of retained austenite and high internal stress resulting from a difference in flow stress between the soft martensite matrix and the finely dispersed hard MA-like phase that developed during fatigue deformation.

A dynamic cellular vertex model of growing epithelial tissues

Intercellular interactions play a significant role in a wide range of biological functions and processes at both the cellular and tissue scales, for example, embryogenesis, organogenesis, and cancer invasion. In this paper, a dynamic cellular vertex model is presented to study the morphomechanics of a growing epithelial monolayer. The regulating role of stresses in soft tissue growth is revealed. It is found that the cells originating from the same parent cell in the monolayer can orchestrate into clustering patterns as the tissue grows. Collective cell migration exhibits a feature of spatial correlation across multiple cells. Dynamic intercellular interactions can engender a variety of distinct tissue behaviors in a social context. Uniform cell proliferation may render high and heterogeneous residual compressive stresses, while stress-regulated proliferation can effectively release the stresses, reducing the stress heterogeneity in the tissue. The results highlight the critical role of mechanical factors in the growth and morphogenesis of epithelial tissues and help understand the development and invasion of epithelial tumors.

A Novel Method for Achieving Gradient Microstructure in a Cu–Al Alloy: Surface Spinning Strengthening (3S)

A new designed surface strengthening method, surface spinning strengthening (3S), was applied to achieve gradient microstructure in the surface layer of a Cu–11 at.%Al alloy. According to the level of grain refinement, the gradient microstructure can be divided into four zones, including nanoscale grain zone, ultra-fine grain zone, fine grain zone and coarse grain zone from the surface to the matrix. Meanwhile, a plenty of grain boundaries and twin boundaries were introduced to inhibit the dislocation motion in the surface layer during the plastic deformation process. Consequently, the hardened layer with a microhardness gradient and high residual compressive stress was produced on the samples, and the yield strength of the Cu–11 at.%Al alloy was effectively improved after 3S processing due to the strengthening effect caused by the gradient microstructure.

Effects of laser peening on fretting wear behaviour of alloy 718 fretted against two different counterbody materials

This paper deals with the effects of laser peening on fretting wear behaviour of a nickel-based superalloy, alloy 718, fretted against two different counterbody materials (alumina and SAE 52100 steel). Laser peening was carried out on alloy 718. Microstructural characterization of laser peened surface was done by electron back-scattered diffraction and transmission electron microscopy. Surface roughness, nanoindentation hardness, and residual stress of both laser peened and unpeened samples were determined. Fretting wear tests were conducted on unpeened and laser peened samples using two different counterbody materials (alumina and SAE 52100 steel balls). The results show that nanocrystallites formed in the surface and near-surface regions and compressive residual stress were induced after laser peening. Hardness increased due to grain refinement at the surface and near-surface regions. There was no significant change in the surface roughness. The laser peened sample exhibited lower tangential force coefficient values compared to unpeened samples at all loads, which may be attributed to higher hardness. Samples fretted against alumina counterbody exhibited higher tangential force coefficient compared to samples fretted against steel counterbody. Owing to increased surface hardness and higher compressive residual stress, laser peened samples exhibited lower fretting wear damage compared to unpeened samples. Due to tribochemical reactions, the wear volume of unpeened and laser peened samples fretted against alumina counterbody was higher than that of the samples fretted against steel counterbody.

Improvement of surface integrity of Nimonic C 263 super alloy produced by WEDM through various post-processing techniques

Wire electrical discharge machining (WEDM) process provides a cost effective solution to generate complicated shapes in various exotic materials, irrespective of their hardness. However, WEDM process removes the material by thermal erosion mechanism which in term affects the topography and mechanical characteristics of the material. In view of this, an effort is needed to eliminate the adverse effect of WEDM process on the final product surface. Therefore, the present work evaluates surface morphology, surface roughness, elemental composition, micro-hardness and residual stress of the machined surface produced by various WEDM strategies such as, rough cut and finish cut. Two post-processing techniques namely, grinding and etching-grinding are proposed to remove recast layer and also to improve the surface integrity of the machined surface. The proposed strategy found to be an efficient post-processing technique for complete removal of recast layer and generation of surface with an average surface roughness as low as 0.024 μm, high compressive residual stress as high as 500 MPa and basic elemental composition. The basic mechanism of these process are also explained in details.

A METHOD FOR OPTIMIZING CHEMICAL COMPOSITION OF STEELS TO REDUCE RADICALLY THEIR ALLOY ELEMENTS AND INCREASE SERVICE LIFE OF MACHINE COMPONENTS

A method for optimizing chemical composition of steel is proposed and a correlation is established to reduce cardinally alloy elements in existing steel grades that results in high compressive residual stresses at the surface of intensively quenched steel parts and increasing strength and ductility of material due to super- strengthening phenomenon. The algorithm of optimization consists in reducing alloy elements in existing alloy steel in 1.5 – 2 times and then lowering step-by-step content of steel, beginning from the most costly alloy element and ending the most cheaper one, until established correlation is satisfied. The range of reduction is minimal and during computer calculations can be chosen as 0,001wt%. The proposed approach can save alloy elements, energy, increase service life of machine components and improve environmental condition. The method is a basis for development of the new low hardenability (LH) and optimal hardenability (OH) steels.

Surface Integrity Analysis in Machining of Hardened AISI 4140 Steel

This study aimed to analyze the residual stresses and roughness in finishing milling of AISI 4140 steel, quenched and tempered up to hardness of 58 HRC. Machining operations were performed with the use of CBN inserts and by varying three basic cutting parameters (cutting speed, feed per tooth and cutting depth). Hardened materials are typically machined by abrasive processes, which in turn are more expensive and complex to be studied due to the undefined cutting geometry of the grinding wheel. A series of machining tests with milling process and CBN tools was implemented in order to study the resultant condition of the specimen´s surface. An experimental design was used and the results were statistically treated, enabling the generation of a model that aims to obtain roughness values due to the optimization of three adopted cutting parameters. The roughness values found in the range of Ra 0,16 to 0,4 µm indicate that it is possible to use the milling process with CBN tools for finishing, reducing machining time and the cost of the machined part. The generated residual stresses were compressive and the feed per tooth parameter showed greater influence in this result. The research was limited to test only one type of CBN insert, which was constantly replaced, preventing the influence of tool wear on responses. The geometry of the tool as well as the use of cutting fluid were not considered. Milling process with CBN inserts is confirmed as a possibility for replacing grinding process for finishing machining leading to significant gains in machining time. An optimized model was derived to predict the value of the roughness and three optimizations were made to specify the best cutting parameters to desirable answers such as better roughness, higher compressive residual stresses and low cutting forces, for example.

Increasing of the carbide cutting tool life by developing the multilayer coatings

The paper presents the results of studying the structural parameters and mechanical properties of multi-element coatings based on-TiZrN, TiZrAlN, TiZrCrN, and TiNbAlN. The phase composition, the structural parameters, and the mechanical properties of such coatings are shown to depend on the design of cathode evaporators and the machine layout at the moment of application. Compared to TiN coating, multi-element coatings have higher residual compressive stress, smaller CSR, higher microdeformations of the crystal lattice, better mechanical properties, yet weaker adhesion to the tool base. Based on studying their structural parameters and mechanical properties, we have designed double-layer coatings. Use of such multi-layer coatings prolongs the tool life of carbide plates compared to TiN coating.

Surface Integrity and Corrosion Performance of Biomedical Magnesium-calcium Alloy Processed by Hybrid Dry Cutting-finish Burnishing

Biodegradable magnesium-calcium (MgCa) alloy is a very attractive orthopedic biomaterial compared to permanent metallic alloys. However, the critical issue is that MgCa alloy corrodes too fast in the human organism. Compared to dry cutting, the synergistic dry cutting-finish burnishing can significantly improve corrosion performance of MgCa0.8 (wt %) alloy by producing a superior surface integrity including good surface finish, high compressive hook-shaped residual stress profile, extended strain hardening in subsurface, and little change of grain size. The measured polarization curves, surface micrographs, and element distributions of the corroded surfaces by burnishing show an increasing and uniform corrosion resistance to simulated body fluid.

Friction stir welding: An alternative to fusion welding for better stress corrosion cracking resistance of maraging steel

Friction stir welding (FSW), a solid state welding process, is generally used for welding soft materials like aluminium and magnesium alloys. In this work, maraging steel, an ultra high strength steel, is welded using FSW process. Major problems associated with fusion welded maraging steel, i.e segregation of alloying elements and formation of reverted austenite pool, were suppressed by FSW. Slow strain rate stress corrosion characteristic of 18%Ni maraging steel (grade-250) and its weldments, by gas tungsten arc welding (GTAW) and FSW, has been investigated in air and 3.5% NaCl solution. It was observed that FSW joints demonstrated higher resistance to stress corrosion cracking (SCC) compared to both, base metal and gas tungsten arc weldments. Improved stress corrosion resistance of friction stir weldments was correlated to microstructural differences and residual stress. Fine grain structure, absence of segregation of alloying elements and high compressive residual stress in friction stir weld resulted in better SCC resistance compared to that of fusion weldment. It is brought out from the present study that FSW process can be used as an alternative to fusion welding process for better SCC resistance.

Investigation of residual stresses in a repair‐welded rail head considering solid‐state phase transformation

AbstractRepair welding for recovery from local damage of a rail head surface is known to cause high residual stress and can accelerate fatigue in the rail. This study examines repair‐welded rails by applying experimental and numerical approaches. In the former approach, two newly manufactured rail specimens and four repair‐welded rail specimens with two different weld depths were prepared, and their residual stresses were measured with a sectioning method. In the latter approach, a finite element repair welding simulation model was developed that adopted a prescribed temperature method with a moving block as an input heat source, and the thermal strain caused by the volume change due to solid‐state phase transformation was considered. Overall, the residual stresses correlated well between the experimental and numerical approaches. The measured high compressive residual stress of −290 MPa seems to be beneficial to prevent a crack initiation in the rail surface.

Microstructure and Residual Stress in Rotary Friction Welded Dissimilar Metals of AA7020 Aluminium Alloy with 316L Steel

Rotary friction welding (RFW) was used in the current study to join the dissimilar metals AA7020-T6 aluminium alloy and 316L steel. Neutron diffraction was performed to investigate the texture gradient around the weld line and to map the residual stress over the whole specimen. The texture analysis showed a weak shear component near the bond line of AA7020-T6-T6 side which indicated a plastic deformation of AA7020-T6 during welding. The shear bands were also observed in optical microstructures. Relatively high tensile residual stresses were observed near the bond line on the AA7020-T6 side, which were in-homogeneously distributed from the perimeter to the rod centre, while high compressive residual stresses were found in the sample centre at the bond line in the 316L steel.

Complex-shaped hardened parts fatigue limit prediction according to the witness sample study results

The aim of this study is to investigate the possibility of assessment of the effect of preparatory surface plastic deformation by hydraulic shot blasting on the fatigue strength of cylindrical parts of different diameters (10-40 mm) of D16T alloy with circular notches of semicircular section, based on measurements of residual stress (initial deformations) of a witness sample. The residual stresses of smooth parts were used to calculate the residual stresses of parts with stress raisers. These were used to predict the increment of these parts fatigue limit caused by hardening hydraulic shot blasting. It was found that the highest compressive residual stresses in the smooth parts obtained through calculations differ from the observed values not more than by 7%, and in notched parts by 8%. Using the criterion of mean integral residual stresses, we calculate the increments of the fatigue limit of parts due to superficial hardening. The discrepancy between the experimental and calculated increment values of the fatigue limit of hardened parts with raisers does not exceed 17%.

Effect of high energy shot peening on the microstructure and mechanical properties of Mg/Ti joints

The effects of high energy shot peening (HESP) on the microstructures and mechanical properties of tungsten inert gas (TIG) welded-brazed Mg/Ti joints were investigated, in terms of surface characteristics, residual stress variation, grain size, dislocation density and tensile strength of the resulting joints. The results show that HESP under an optimized Almen intensity of 0.16 mm N resulted in the surface defects elimination, high compressive residual stress, fine-grain strengthening and strain strengthening of Mg/Ti joint. All these factors contributed to the significant improvement in mechanical properties of the joints, leading to a notable enhancement (24.5%) in tensile strength of the Mg/Ti joint to 241 MPa over the joints without HESP treatment (193.5 MPa). However, further increment in Almen intensity to 0.24 mm N resulted in severe internal cracks within the joint, which deteriorated the mechanical properties of Mg/Ti joints.

Mechanical Behavior and Fatigue Performance of Carburized Steel Specimens

Carburization is popular in design and fabrication of mechanical parts, such as gears, to improve fatigue performance. However, it is still open how to characterize the mechanical property of carburized steel and to quantify effects of the carburization to fatigue life of carburized parts. In the present paper four types of specimens differently treated and carburized are experimentally investigated. The experiments confirm significant increments in hardness and yield stress due to carburization. The fracture strain of the carburized steel is significantly smaller than that of the base material. Although the fatigue performance of the carburized steel is slightly worse than the base material, the solid carburized specimen shows significantly longer fatigue life. The fatigue limit increases from ca. 300 MPa for the base material to 550 MPa for the tensile carburized specimens. Detailed measurements display that the carburized layer in a carburized specimen possesses high compressive residual stresses, which arises the fatigue performance of the carburized steel.

Influence of laser shock peening pulse density and spot size on the surface integrity of X2NiCoMo18-9-5 maraging steel

Abstract Maraging alloys are a special class of steels with exceptional mechanical properties, suitable for critical and added value applications in aeronautical, aerospace engineering, and the tooling industry. Researchers have been studying several surface engineering techniques to enhance surface characteristics of maraging and tool steels with the aim of extending service life. The goal of the research presented in this paper was to investigate the influence of laser shock peening (LSP) pulse density and spot size on the surface integrity of X2NiCoMo18-9-5 maraging steel. Different pulse densities and spot sizes were chosen for this purpose. Surface integrity was analysed through roughness, residual stress, and microhardness measurements. The influence of each processing parameter was statistically evaluated using the analysis of variance (ANOVA) confirming a significant influence. LSP without the use of an absorbent coating generated high compressive residual stresses in the surface layer with relatively low surface roughness, a combination of characteristics which may have a beneficial influence on mechanical properties of 18% Ni maraging steel. According to the measurements taken in this research, maximal compressive residual stresses were not achieved using the smallest laser beam spot while maintaining constant pulse energy.

Compound technology of manufacturing and multiple laser peening on microstructure and fatigue life of dual-phase spring steel

The present work proposes an advanced double quenching and tempering heat treatment based laser surface modification process of dual-phase spring steel. Multiple laser peening without coating process utilized the decarburized surface as the protective layer for the further cold working process. The electron backscattering diffraction analysis on crystallographic orientation of individual grains and phase map exhibits a perfect dual-phase steel. Also, the high resolution transmission electron microscopic study explains the high strain induced microstructural grain refinement features and plastic deformation behaviors. The laser peening technique taking an advantage that it induces a large and high magnitude compressive residual stress with good thermal stability. The micro and nano-hardness profile provides better surface and sub-surface mechanical properties. The controlled average surface roughness is achieved in this course of work. The stress-strain characteristics on tensile properties are analyzed through the pre-fatigued specimens. The fully reversed high cycle fatigue test indicates that the current laser peening has substantially improves the fatigue life of the specimens.

Enhancing the durability of spinal implant fixture applications made of Ti-6Al-4V ELI by means of cavitation peening

The surface treatment technology known as ‘cavitation peening’ was employed in this study in order to enhance the durability of spinal implant fixture applications, which are subject to fretting fatigue. Cavitation peening can be realized by a technique in which a high-speed water jet is injected into water through a nozzle. It utilizes a phenomenon by which surface impacts due to collapsing cavitation bubbles induce work-hardening by introducing residual compressive stress near the surface. A fretting fatigue test was conducted on a spinal implant rod made of Ti-6Al-4V ELI in accordance with the ASTM F1717 standard, which is the established method for testing spinal implants after they are treated by cavitation peening. The residual stress was evaluated by using X-ray diffraction analysis. The hardness over the cross-sectional area was also measured using an indentation test. The obtained results show that cavitation peening drastically improves the fretting fatigue properties of spinal implant fixtures by as much as 2.2 times compared to untreated ones. This can be attributed to a significant increase in the hardness from 5.0 to 9.6GPa and a high compressive residual stress of over 600MPa induced by cavitation peening.

Characterization of AISI 316L Stainless Steel Pattern via DC Plasma Nitriding

Aiming to produce a pattern on the surface of austenitic stainless steel, AISI 316L, cylindrical samples were plasma nitrided with copper grids (the same are used in Transmission Electron Microscopy (TEM)) accommodated on the surface. Samples were positioned at the cathode and process was performed according to following constant parameters: temperature of 400 °C, pressure of 3 Torr and treatment time of 2h. In order to evaluate the difference in the pattern produced, three different gas mixtures were used: poor in nitrogen (5% N2/95% H2), rich in nitrogen (95% N2/5% H2) and a gas mixture with addition of argon (80% N2/10% H2/10% Ar). After nitriding, samples were characterized by optical interferometry (OI), scanning electron microscopy (SEM), optical microscopy (OM), micro hardness test and, X-ray diffraction (XRD). The results showed the presence of a mask (TEM grid) promotes the surface patterning of steel. This patterning is associated with high residual compressive stress induced by the introduction of a large amount of nitrogen.

Analysing the coupled effects of compressive and diffusion induced stresses on the nucleation and propagation of circular coating blisters in the presence of micro-cracks

Abstract This paper presents a study of delamination of coating with micro-cracks under compressive residual stress coupled with diffusion induced stress. Micro-cracks in coating provide a passage for corrosive species towards the coating-substrate interface which in turn produces diffusion induced stress in the coating. Micro-cracks contract gradually with increasing compressive residual stress in coating due to thermal expansion mismatch which blocks the species diffusion towards the interface. This behaviour reduces diffusion induced stress in the coating while compressive residual stress increases. With further increase in compressive residual stress, micro-cracks reach to the point, where they cannot be constricted any further and a high compressive residual stress causes the coating to buckle away from the substrate resulting in delamination and therefore initiating blistering. Blistering causes the contracted micro-cracks to wide open again which increases diffusion induced stress along with high compressive residual stress. The high resultant stress in coating causes the blister to propagate in an axis-symmetric circular pattern. A two-part theoretical approach has been utilised coupling the thermodynamic concepts with the mechanics concepts. Thermodynamic concepts involve corrosive species transportation through micro-cracks under increasing compression, eventually causing blistering, while fracture mechanics concepts are used to treat the blister growth as a circular defect propagation. The influences of moduli ratio, thickness ratio, thermal mismatch ratio, poisson's ratio and interface roughness on blister growth are discussed. Experiment is reported for blistering to allow visualisation of interface and to permit coupled (diffusion and residual) stresses in the coating over a full range of interest. The predictions from model show excellent, quantitative agreement with the experimental results.