Evaluation of residual stress relaxation and its effect on fatigue strength of AISI 316L stainless steel ground surfaces: Experimental and numerical approaches

This paper is aimed at evaluating the residual stress relaxation and its effect on the fatigue strength of AISI 316L steel ground surfaces in comparison to electro-polished surfaces. An experimental evaluation was performed using 3-point and 4-point bending fatigue tests at Rr = 0.1 on two sets of notched spec-imens finished by electro-polishing and grinding. The residual stress fields were measured at the notch root of specimens, before and after fatigue tests, by means of the X-ray diffraction technique. It was found a degradation of about 35% for the 4-point bending fatigue limit at 2 106 cycles of the ground spec-imens in comparison to the electro-polished ones. This degradation is associated with a slight relaxation of the grinding residual stresses which remain significant tensile stresses at the stabilized state. While under the 3-point bending test, these residual stresses relax completely and provoke a noticeable increase of the fatigue limit estimated at about 50% in comparison to the 4-point bending fatigue test. The numerical evaluation of residual stress relaxation was carried out by FE analyses of the cyclic hard-ening behaviour of the ground layer. The isotropic and nonlinear kinematic model proposed by Chaboche was used and calibrated for the base material and the ground layer. The results show that residual stres-ses relax to a stabilized state characterized by elastic-shakedown response. This stabilization is occurred after the first cycle of the 4-point bending test corresponding to the higher stress concentration (Kt- 4p = 1.66), while it requires many cycles under the 3-point bending test corresponding to the lower stress concentration (Kt-3p = 1.54). The incorporation of stabilized residual stress values into the Dang Van’s cri-terion has permitted to predict with an acceptable accuracy the fatigue limits under both bending modes

Effect of interlamellar spacing on the elastoplastic behavior of C70 pearlitic steel: Experimental results and self-consistent modeling

The effect of pearlite microstructure characteristics on strength and deformation of C70 pearlitic steel was investigated. Tensile tests under X-ray diffraction coupled with self-consistent model have been used to identify the role of interlamellar spacing on the ferrite plasticity parameters and residual stress induced by plasticity. Tests have been carried out on two pearlitic microstructures with interlamellar spacing Sp = 170 and 230 nm respectively. Ferrite critical shear stresses ðs0c ðaÞÞ are equal to 75–86 MPa for interlamellar spacing Sp = 230 nm and 105–120 MPa for interlamellar spacing Sp = 170 nm. Moreover, the compressive residual stress measured in ferrite phase is higher in elasto-plastically deformed sample (total strain of E11 = 1.2%) having larger interlamellar spacing (rR Fea ¼ 161 MPa for Sp = 230 nm and rR Fea ¼ 77 MPa for Sp = 170 nm)

Low cycle fatigue life improvement of AISI 304 by initial and intermittent wire brush hammering

The effects of hammering by wire brush as a method of improving low cycle fatigue life of highly ductile austenitic stainless steel AISI 304 have been investigated through an experimental study combining imposed strain fatigue tests and assessment of surface characteristic changes under cyclic loading by SEM examinations and XRD analysis. It has been shown that the fatigue life of wire brush hammered surface was increased by 307% at an imposed strain rate of 0.2% and only 17% at an imposed strain rate of 0.5%, comparatively to the turned surface. This increase in fatigue life is explained in terms of fatigue damage that is related to crack networks characteristics and stability which are generated during fatigue on both turned and wire brush hammered surfaces. The improvement of brushed surface is attributed to the role of the surface topography, the near surface stabilized compressive residual stresses and superfi-cial cold work hardening on the fatigue crack network nucleation and growth. It is found that wire brush hammering produces a surface texture that favors, under cyclic loading, nucleation of randomly dispersed short cracks of the order of 40 lm in length stabilized by the compressive residual stress field that reached a value of r0 = 749 MPa. In contrast, turned surface showed much longer unstable cracks of the order of 200 lm in length nucleated in the machining groves with high tendency to propagate under the effect of tensile residual stress field that reached value of r0 = 476 MPa. This improvement is limited to strain rates lower than 0.5%. At higher strain rates, a cyclic plastic deformation induced martensitic phase alters furthermore the fatigue behavior by producing high cyclic strengthening of the bulk mate-rial. This phenomenon lead to a reduction in strain imposed fatigue life. It has also been established that wire brush hammering can be used as an onsite surface treatment to improve the residual fatigue life of components subjected to cyclic loading. The efficiency of this treatment is demonstrated if it is performed at a fraction of service lifetime Ni/Nr lower than 0.5

Effect of electro discharge machining (EDM) on the AISI316L SS white layer microstructure and corrosion resistance

The localised corrosion resistance of austenitic stainless steels is strongly influenced by the quality of finished surface. EDM machining induces substantial changes by the high thermal gradients generated by electric sparks. Experimental techniques such as roughness measurement, scanning electron microscopy (SEM), energy dispersive microanalysis (EDX) and X-ray diffraction technique, reveal microgeometrical, microstructural, chemical and mechanical changes. These changes lead to white and heat-affected layers with a depth less than 100 μm. The white layer is a melted material characterised by dendritic structure and constituted by austenite, chromium carbide and ε-carbide. The heat-affected layer is characterised by very large grain size comparatively to the bulk material. Electrochemical test coupled with metallographic examinations using SEM reveals a weakening of the resistance to pitting and intergranular corrosion comparatively to diamond polished surface. This weakening is correlated to differences in structure and chemical composition of white layer. Susceptibility to stress corrosion cracking has been attributed to the field of tensile residual stresses resulting from thermal effects. The removal of the white layer material by polishing or wire brushing restores the corrosion resistance of the AISI316L SS

Potential fatigue strength improvement of AA 5083-H111 notched parts by wire brush hammering: Experimental analysis and numerical simulation

The effects of milling as machining process and a post-machining treatment by wire-brush hammering, on the near surface layer characteristics of AA 5083-H111 were investigated. Surface texture, work-hardening and residual stress profiles were determined by roughness measurement, scanning electron microscope (SEM) examinations, microhardness and X-ray diffraction (XRD) measurements. The effects of surface preparation on the fatigue strength were assessed by bending fatigue tests performed on notched samples for two loading stress ratios R0.1 and R0.5. It is found that the bending fatigue limit at R0.1 and 107 cycles is 20% increased, with respect to the machined surface, by wire-brush hammering. This improvement was discussed on the basis of the role of surface topography, stabilized residual stress and work-hardening on the fatigue-crack network nucleation and growth. The effects biaxial residual stress field and surface work-hardening were taken into account in the finite element model. A multi-axial fatigue criterion was proposed to predict the fatigue strength of aluminum alloy notched parts for both machined and treated states

Evaluation by the double loop electrochemical potentiokinetic reactivation test of aged ferritic stainless steel intergranular corrosion susceptibility

An experimental design method was used to determine the effect of factors that significantly affect the response of the double loop-electrochemical potentiokinetic reactivation (DL-EPR) test in controlling the susceptibility to intergranular corrosion (IGC) of UNS S43000 (AISI 430) ferritic stainless steel. The test response is expressed in terms of the reactivation/activation current ratio (I r /I a pct). Test results analysed by the analysis of variance (ANOVA) method show that the molarity of the H 2SO4 electrolyte and the potential scanning rate have a more significant effect on the DL-EPR test response than the temperature and the depassivator agent concentration. On the basis of these results, a study was conducted in order to determine the optimal operating conditions of the test as a nondestructive technique for evaluating IGC resistance of ferritic stainless steel components. Three different heat treatments are considered in this study: solution annealing (nonsensitized), aging during 3 hours at 773 K (500 °C) (slightly sensitized), and aging during 2 hours at 873 K (600 °C) (highly sensitized). The aim is to find the operating conditions that simultaneously ensure the selectivity of the attack (intergranular and chromium depleted zone) and are able to detect the effect of low dechromization. It is found that a potential scanning rate of 2.5 mV/s in an electrolyte composed of H 2SO4 3 M solution without depassivator, at a temperature around 293 K (20 °C), is the optimal operating condition for the DL-EPR test. Using this condition, it is possible to assess the degree of sensitization (DOS) to the IGC of products manufactured in ferritic stainless steels rapidly, reliably, and quantitatively. A time-temperature-start of sensitization (TTS) diagram for the UNS S43000 (France Inox, Villepinte, France) stainless steel was obtained with acceptable accuracy by this method when the IGC sensitization criterion was set to I r /I a > 1 pct. This diagram is in good agreement with the time-temperature-start of precipitation (TTP) diagram that delineates the domain of low dechromization consecutive to chromium carbide precipitation

Investigation of plastic deformation heterogeneities in duplex steel by EBSD

An EBSD analysis of a duplex steel (austeno-ferritic) deformed in tension up to fracture is presented. The main purpose of the paper is to describe, qualitatively and quantitatively, the differences in the behavior of the two phases during plastic deformation. In order to do so, several topological maps are measured on the deformed state using the electron backscatter diffraction technique. Distributions of grain size, misorientation, image quality factor and texture are then analyzed in detail.International audienceAn EBSD analysis of a duplex steel (austeno-ferritic) deformed in tension up to fracture is presented. The main purpose of the paper is to describe, qualitatively and quantitatively, the differences in the behavior of the two phases during plastic deformation. In order to do so, several topological maps are measured on the deformed state using the electron backscatter diffraction technique. Distributions of grain size, misorientation, image quality factor and texture are then analyzed in detail

Numerical and experimental analysis of residual stress and plastic strain distributions in machined stainless steel

In this study, a numerical approach has been developed to predict the near surface residual stresses and plastic strain resulting from turning in orthogonal cutting configuration. This approach is based on the Arbitrary Lagrangian-Eulerian (ALE) formulation using the commercial finite element code Abaqus-Explicit. The coefficients of the used material behavior law and friction model required for the simulation are identified experimentally in this study. The simulated results are validated by experiments carried out on AISI 316L stainless steel. Using this method, the effect of the depth of cut (Doc) and the cutting speed (Vc) on the surface properties has been established. The simulated residual stress gradient resulting from machining has been experimentally validated by X-ray diffraction measurements. The simulated plastic strain gradient has been validated by an experimental microhardness-strain relationship established in this study

Application of multireflection grazing incidence method for stress measurements in polished Al–Mg alloy and CrN coating

Multi-reflection grazing incidence geometry, referred to as MGIXD, characterized by a small and constant incidence angle, was applied to measure low surface stresses in very thin layers of Al–Mg alloy and CrN coating. These two materials were selected in order to deal with the low and high levels of residual stress, respectively. The influence of different mechanical treatments on residual stresses was studied for Al–Mg samples. It was found that both rolling and mechanical polishing influence the distribution and amplitude of residual stress in surface layers. In the case of CrN coating, a very high compressive stress was generated during the deposition process. The stress distributions determined by the MGIXD method is in good agreement with the classic sin2 technique results for all studied samples. In performing stress measurements for a powder sample, it was found that the application of the Göbel mirror in the incident beam strongly reduces statistical and misalignment errors. Additionally, the root mean square values of the third order lattice strain within diffracting grains were determined

Influences of up-milling and down-milling on surface integrity and fatigue strength of X160CrMoV12 steel

This paper aims to compare the influences of the two peripheral milling modes, up-milling and down-milling, on surface integrity and fatigue strength of X160CrMoV12 high-alloy steel. The experimental investigations showed an important difference between integrity of both milled surfaces. The down-milled surface is lowly work-hardened and well finished (lower roughness), but subjected to tensile residual stresses and severely damaged by folds of metal and short micro-cracks. The up-milled surface is highly work-hardened and subjected to compressive residual stresses, but poorly finished (higher roughness) and damaged by a density of micro-cavities due to carbide extraction. The results of 3-point bending fatigue tests revealed that the fatigue limit at 2 × 106 cycles of the up-milled state is largely higher of about 26% in comparison with the down-milled state. The effects of surface integrity induced by each milling mode on fatigue strength were evaluated using a HCF behaviour predictive approach based on Dang Van’s multiaxial criterion. The predictive results estimated that the pre-existing micro-cracks play a dominant role in the fatigue strength degradation of the down-milled surface while the other surface effects seem to be lower. On the contrary, the fatigue strength of the up-milled surface is less affected by the pre-existing micro-cavities. The detrimental roughness effect (stress concentration effect) is significantly reduced by the beneficial effects of superficial hardening and compressive residual stresses. So, this study revealed that up-milling is the more appropriate mode for a better surface integrity towards fatigue strength of X160CrMoV12 steel than the down-milling mode