Effects of shot peening on bending fatigue strength of spring steel SS 2090

Abstract

The influence of prior surface condition and of a shot peening treatment on the bending fatigue strength of a standard Si–Cr spring steel (SS 2090) has been investigated. This steel was initially hardened and tempered to a hardness of 52–54 HRC. After shot peening, compressive residual stresses had been introduced into a surface layer of depth ~0·3 mm, with the maximum value of ~1000 MN m^?2 being found close to the surface. The effect of this treatment was to increase the fatigue limit by ~40% to 890 MN m^?2. Coincident with this increase was a change in the site of fatigue initiation from a surface to a subsurface location beneath the compressive residual stress layer. The initiating inclusions, which were 20–40 μm in size, were analysed and found to be Al₂O₃. At stress amplitudes greater than the fatigue limit, initiation was invariably found to occur at the surface and was not always due to inclusions. Inclusion initiated failure has been modelled using the size and spatial distribution of inclusions in the test bars in addition to the variation of applied and residual stresses through the section. A crack propagation criterion based on linear elastic fracture mechanics is used, assuming that propagation is controlled by stress intensity threshold value. It is assumed that small cracks exist at oxide inclusions from the beginning of the fatigue life and that failure is associated with the propagation of one of these cracks.

MST/1392     

Tool life model for Al2O3 particle reinforced MMCs using genetic expression programming

Abstract

In the present work, the wear of cutting tools in the machining of 7075Al alloy composites reinforced with 10 and 25 wt-% and 16 μm average size Al₂O₃ particles was investigated. Machining tests were performed on both as cast and heat treated composites by turning process using a physical vapour deposition coated carbide tool CP500 at different cutting conditions. A new model was developed to predict the tool life by genetic expression programming. The training and validation data sets were obtained from the well established machining test results. The weight fraction of particle P_w, cutting speed V, feedrate f and heat treatment Ht were used as independent input variables, while tool life T was used as a dependent output variable. Different models for tool life were predicted on the basis of the training data set using genetic programming, and the accuracy of the best model was proved with validation data set. The test results showed that the genetic expression programming model has produced correlation coefficient R values of ～0·958 for the training data and 0·952 for the validation data. The predicted tool life results were compared with experimental results and found to be in good agreement with the experimentally observed ones.     

Creep of Pb–2·5Sb–0·2Sn alloy at low stresses

Abstract

The creep of a Pb–2·5Sb–0·2Sn alloy has been studied at stresses up to 6·5 MN m^?2 in the temperature range 318–348 K (0·53–0·58T_m) using helical specimens. At 333 K, a transition in the stress exponent from ~1 to 3 occurred at ~3 MN m^?2. The observed good agreements below the transition stress, both for experimental dE/do and predictions for Coble diffusional creep of lead, and for measured activation energy for creep and the activation energy for grain boundary self-diffusion in lead, suggest that grain boundary diffusional creep is the dominant mechanism. at low stresses. The presence of antimony does not seem to affect the magnitude of dE/do appreciably, and the results suggest that the grain boundary self-diffusivity of lead is not influenced by the presence of segregated antimony on the grain boundaries. The diffusional creep occurred above a threshold stress of magnitude ~0·5 MN m^?2, and its temperature dependence was characterised by an activation energy of ~20 kJ mol^?1, similar to the value of 23 ± 7 kJ mol^?1 typical of pure metals in the temperature range investigated. The stress exponent of ~3 observed for the power law regime suggests control by viscous glide of dislocations constrained by dragging of solute atmospheres. Preliminary tests on sagging beam specimens of as-worked material at an applied stress of 2·5 MN m^?2 and a test temperature of 333 K has provided the first direct evidence that anisotropic grain shape affects Coble creep. The specimen with the longest grain dimension along the stress axis underwent slower creep than the specimen with the longest grain dimension perpendicular to the stress axis. This observation is in qualitative agreement with theoretical predictions.

MST/1139     

Optimizing Single Point Diamond Turning for Mono-Crystalline Germanium Using Grey Relational Analysis

In this paper the results of an experimental study conducted for precision machining of mono-crystalline germanium with single point diamond turning (SPDT) have been reported. The input parameters include the top rake angle, tool overhang, depth of cut, tool feed rate, and rotational speed of the workpiece. The flat profile is generated on a disk of mono-crystalline germanium possessing three performance characteristics: surface roughness (R_a), profile error (P_t), and waviness error (W_a). The process parameters are optimized to obtain the best surface finish with minimum profile and waviness errors by using the Taguchi method. The grey relational analysis is employed for carrying out multiresponse optimization of performance parameters. The best value of surface finish obtained after multiresponse optimization is 10.7 nm having a profile error and a waviness error of 0.202 µm and 0.046 µm, respectively.     

Optimization of photochemical machining process parameters for manufacturing microfluidic channel

In the present study, the investigation on photochemical machining (PCM) of stainless steel (SS-304) by ferric chloride as etchant is reported. SS-304 is machined by PCM process to obtain accurate dimensions and better geometrical features. Weighted grey relational analysis (WGRA) technique is used in optimization of PCM process parameters. DoE (L₂₇) orthogonal array is applied to evaluate machining parameters, such as concentration of etchant, etching time, and temperature of etchant. The multiobjective optimization technique is used to optimize material removal rate (MRR), surface roughness (R_a), undercut (U_c) and etch factor (EF). Weighted grey relational grade is calculated to minimize U_c and surface roughness and to maximize MRR and EF. The quality characteristics MRR, EF, U_c, and R_a are reporting the improvement after the confirmatory test. The optimum machining parameters are processed to manufacture the microfluidic channel used in biomedical applications. The microfluidic channels and its assembly with Y-type for mixing of fluid with a size of 100 µm, 200 µm, and 300 µm are developed and investigated.     

Integrated ANN–GA for estimating the minimum value for machining performance

In this study, we proposed a new approach in estimating a minimum value of machining performance. In this approach, artificial neural network (ANN) and genetic algorithm (GA) techniques were integrated in order to search for a set of optimal cutting condition points that leads to the minimum value of machining performance. Three machining cutting conditions for end milling operation that were considered in this study are speed (v), feed (f) and radial rake angle (γ). The considered machining performance is surface roughness (R _a). The minimum R _a value at the optimal v, f and γ points was expected from this approach. Using the proposed approach, named integrated ANN–GA, this study has proven that R _a can be estimated to be 0.139?µm, at the optimal cutting conditions of f?=?167.029?m/min, v?=?0.025?mm/tooth and γ?=?14.769°. Consequently, the ANN–GA integration system has reduced the R _a value at about 26.8%, 25.7%, 26.1% and 49.8%, compared to the experimental, regression, ANN and response surface method results, respectively. Compared to the conventional GA result, it was also found that integrated ANN–GA reduced the mean R _a value and the number of iterations in searching for the optimal result at about 0.61% and 23.9%, respectively.     

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 R_0.1 and R_0.5. It is found that the bending fatigue limit at R_0.1 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.     

TRANSIENT RETARDATIONS IN FATIGUE CRACK GROWTH FOLLOWING A SINGLE PEAK OVERLOAD

Retardation in the fatigue crack growth rate following the application of a single peak overload in a fatigue loading sequence has been studied for a low carbon structural steel. Tests have been performed at load ratios of R= 0.2 and R= 0.6 at a baseline stress intensity range, ΔK_b, corresponding to fatigue crack growth rates in the Paris regime. Single peak overloads were applied at a crack-length to specimen-width ratio of a/W= 0.5. At the load ratio of R= 0.6 monotonic or “static” fracture modes were observed upon application of the overload, and these produced an immediate increase in growth rate. A subsequent retardation is attributed to the presence of a residual compressive stress field ahead of the crack tip. A similar retardation was observed at a load ratio of 0.2. The importance of residual stress was established by performing stress relieving experiments. In addition, removal of the surface deformation after an overload by machining “T” sidegrooves resulted in an extended transient, which could not be explained by residual machining stresses.     

An Investigation of Machining-Induced Residual Stresses and Microstructure of Induction-Hardened AISI 4340 Steel

Excessive induction hardening treatment may result in deep-hardened layers, combined with tensile or low compressive residual stresses. This can be detrimental to the performance of mechanical parts. However, a judicious selection of the finishing process that possibly follows the surface treatment may overcome this inconvenience. In this paper, hard machining tests were performed to investigate the residual stresses and microstructure alteration induced by the machining of induction heat-treated AISI 4340 steel (58–60 HRC). The authors demonstrate the capacity of the machining process to enhance the surface integrity of induction heat-treated parts. It is shown how cutting conditions can affect the residual stress distribution and surface microstructure. On the one hand, when the cutting speed increases, the residual stresses tend to become tensile at the surface; and on the other hand, more compressive stresses are induced when the feed rate is increased. A microstructural analysis shows the formation of a thin white layer less than 2 µm and severe plastic deformations beneath the machined surface.     

Effects of grinding process on residual stresses in nanostructured ceramic coatings

This paper investigates the depth profiles of residual stresses using the sin² method combined with grazing incident X-ray diffraction (GIXD) technique. It specifically focuses on the effects of grinding process on the residual stresses in the thermally sprayed nanostructured WC/12Co and Al₂O₃/13TiO₂ (n-WC/12Co and n-Al₂O₃/13TiO₂) coatings on low carbon steel substrates. The influence of grinding parameters, such as depth of cut (DOC), table feedrate, abrasive grit size and wheel bond type, on residual stresses is studied. The conditions and limitations of X-ray diffractometry for residual stress measurements are discussed. Discussed also is the difference between the average and actual depth profiles of residual stresses. The paper introduces a method for retrieving the actual depth profiles from the measured average depth profiles. Finally, the influence of peak broadening of grain size, anisotropy from different diffraction planes and surface finish of the samples on the measurement results is explored.     

WEDM of Mg/CRT/BN composites: Effect of materials and machining parameters

The current work presents a detailed exploration on real-time wire electric discharge machining (WEDM) experiments and grey relational analysis (GRA)–based multi-criteria optimization of material and machining characteristics for lowered surface roughness (R_a) and improvised material removal rate (MRR) of the newly developed magnesium/boron nitride/cathode ray tube (Mg/BN/CRT) hybrid metal matrix composites (MMCs). The composites were fabricated through powder metallurgy (PM) route by reinforcing silica-rich E-waste CRT panel glass powder crushed for different particle sizes (10, 30, and 50?µm) at various weight percentages (5%, 10%, and 15%) and with 2% boron nitride (BN). Taguchi-based orthogonal array procedure was utilized to formulate the experimental plan for WEDM considering reinforcement level and size, pulse on time (P_on), pulse off time (P_off), and wire feed (W_f) as the input process parameters. ANOVA results reveal that P_on and wt% of reinforcement has more effect on R_a and MRR than any other considered parameters. The developed mathematical model for R_a and MRR predicted values similar to that of experimental results. Multi-criteria optimization was done through GRA technique and the so recommended optimum parameter set furnishes higher MRR (22.34?mm³/min) and reduced R_a (2.87?µm).     

Effect of strain on formation of ultrafine ferrite in surface of hot rolled microalloyed steel

Abstract

The phenomenon of ultra grain refinement of ferrite in surface layers of hot rolled strip has been studied in a low carbon, niobium microalloyed steel. Wedge specimens were used, to vary the nominal equivalent strain applied during rolling from zero to approximately unity, and the cooling rate after rolling was varied from ~ 20 to 1 K s ^-1. In contrast with previous work, which contended that a very coarse austenite grain size and a low rolling temperature near the Ar ₃ were essential to obtain ultrafine ferrite in surface layers, such ultrafine layers were observed after rolling coarse austenite at up to 150 K above the Ar ₃ and after rolling fine grained austenite near the Ar ₃. In the case of coarse grained austenite, a critical nominal rolling strain needed to be exceeded to trigger the surface layer phenomenon, upon which cooling rate had little effect on the surface layer's grain size. Refining the prior austenite grain size had the further beneficial effect of refining the grain size at the centre of the rolled product, for example to 2·6 μm, while the surface layer was refined to 0·7 μm.     

Effect of Combined Grinding–Burnishing Process on Surface Integrity,Tribological, and Corrosion Performance of Laser-Clad Stellite 21 Alloys

Herein, the influence of the grinding–burnishing on surface integrity, mechanical properties, and corrosion performance of Stellite 21 alloys coating deposited by laser cladding is investigated. The as-clad specimens are first ground followed by further modification by ball burnishing at forces of 424 N and 509 N. Results show that the grinding–burnishing enhances surface finish by lowering R_a from 2.6 to 0.73 μm and R_z from 13 to 4.9 μm, respectively. Surface porosity is found to decrease from 3.8% to 0.9%. Hardness is increased from 609 HV to 702 HV, with a surface alteration as deep as 250 μm, while wear resistance increases by reducing worn volume from 4.15 to 2.95 mm³. Because of high hardness, the grinding–burnishing increases impact resistance by lowering indent depth by 20%. Grains flatten and surface undulations are remarkably reduced due to burnishing. Finally, grinding–burnishing at 509 N improves the corrosion resistance by increasing positive corrosion potential from −0.41 to −0.14 V and lowering corrosion current density from 6.34 × 10⁻⁴ A cm⁻² to 2.19 × 10⁻⁵ A cm⁻², as compared to grinding. This synergistic grinding–burnishing can be a plausible post-treatment route for the laser-clad alloys.     

COMPARISON OF PLASTIC WORK OF FATIGUE CRACK PROPAGATION IN LOW CARBON STEEL MEASURED BY STRAIN-GAGES AND ELECTRON CHANNELING

Abstract— The plastic work to propagate a fatigue crack by a unit area, U, measured by the foil strain gage technique requires an extrapolation to estimate the contribution closer than 100 μm to the crack tip. This is due to the size of the strain-gages used, 200 × 210 μm. Conversely, the electron channeling technique for determining U is useful mainly close to the crack tip where subgrains form. In the present work U was measured by both techniques in the same low carbon steel at ΔK= 8 MN/m^3/2. The contribution to U from closer than 100 μm of the crack tip was determined to be 1·7 × 10⁶ J/m² using electron channeling and 2·0 × 10⁶ J/m² by extrapolation. The measured contribution to U from further than 100 μm from the crack tip was 3·6 × 10⁶ J/m² giving 5·3 × 10⁶ J/m² for U. Thus, a large amount of energy is absorbed outside the region where sub-grains form. The non-hysteretic plastic work was found to be about four orders of magnitude smaller than the hysteretic plastic work, and may be neglected. A map of the plastic zone results from the strain-gage measurement. Rice's theory predicts the measured plastic zone sizeif the proper material's strength is employed in the formula.     

Monotonic subcritical crack growth in T42 high speed steel

R-curve behaviour in the microstructurally short crack regime has been reported mainly in ceramics, composites and polymers; this paper describes it for a metallic material: brittle cast and wrought T42 high speed steel. Continuum mechanics has demonstrated the general admissibility of sub-critical growth of cracks with a cohesive zone. Results now reported show that, in a metallic material, growth of microstructurally short cracks under monotonic loading, as in fatigue, is governed by microstructure (texture). Natural cracks, i.e. produced by the hot forging operation, or induced by the application of stress in the range 0.5 to 1.1 GPa in four-point bending experiments, of depths extending to 25μm were always associated with MC carbides. At comparable stress levels cracks were nucleated in compression -- surprisingly some transverse to the compressive axis. Observations of crack nucleation and subsequent studies of subcritical growth of these microcracks were made by surface replica microscopy. Crack extension was easy within the carbide stringers (a characteristic feature of hot-worked high speed steels), but, at higher stresses, took place between these bands to reach up to ∼ 100 μm (surface) length. Dormant cracks were shallow, no more than 6 μm deep; whereas those responsible for failure, at stresses ranging from 0.6 to 1.9 GPa, had a semicircular geomerty -- identified by scanning electron fractography. Step-wise monotonic subcritical crack growth is modelled asR -curves and it is shown that the maximum estimated (microscopic) applied stress intensity factor K _a can vary from 0.5 to 1.0 K_1C, the macroscopic fracture toughness independently determined using sharp artificial long cracks. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of cutting speed and tool rake angle on residual stress distribution in machining 2024-T351 aluminium alloy — unlubricated conditions

The residual stress distribution in the machining of 2024-T351 aluminium alloy was measured using an electrolytic etching technique. Ring-shape specimens were machined under unlubricated orthogonal conditions with high-speed steel tools having rake angles of 10, 15, 20 and 25° at cutting speeds ranging between 0.5 and 1.25 m sec^–1. The results of the investigation show that the residual stresses are compressive at the machined surface and decrease with depth beneath the machined surface. The maximum (near-surface) residual stress and the depth of the severely stressed region increase with an increase in the cutting speed. There seems to be little change in the residual stress distribution due to a change in the rake angle. The results are interpreted in terms of the variations in the amount of surface-region deformation produced by changes in cutting conditions.     

Effect of cryogenic CO2 and LN2 coolants in milling of aluminum alloy

The research work was carried out on the end milling of Al 6082-T6 alloy with cryogenic CO₂, LN₂ and wet conditions. The highest axial force (F_z), normal force (F_y) and feed force (F_x) values were recorded on cryogenic LN₂ machining. Use of cryogenic LN₂ helped to reduce the cutting temperatures up to 38.29% and 32.8% when compared with wet and cryogenic CO₂ conditions, respectively. The conventional fluid coolant offered a better surface roughness value (R_a) over cryogenic coolants at a feed rate of 0.015 and 0.02 mm/tooth and cutting speed of 100 m/min. The workpiece surface quality degraded in cryogenic machining conditions during the slot end milling operation of aluminum alloy under the given machining parameters.     

Machining Parameters Optimization of Titanium Alloy using Response Surface Methodology and Particle Swarm Optimization under Minimum-Quantity Lubrication Environment

The present paper depicts an application of response surface methodology (RSM) and particle swarm optimization (PSO) technique for optimizing the machining factors in turning of titanium (Grade-II) alloy using cubic boron nitride insert tool under minimum quantity lubricant (MQL) environment. The three machining factors, i.e., cutting speed (V_c), feed rate (f) and side cutting edge angle (approach angle π), are designed as three factors by using RSM design, which is withal subject to several constraints including tangential force (F_c), tool wear (V_Bmax), surface roughness (R_a) and tool-chip contact length (L). The multiple regression technique was used to establish the interaction between input parameters and given responses. Moreover, the results have been presented and optimized process parameters are acquired through multi-response optimization via desirability function as well as the PSO technique. The lower values of V_c (200 m/min), f (0.10 mm/rev) and higher values of ? (90°) are the optimum machining factors for minimizing the aforementioned responses. It was also observed that the selected responses predicated on PSO are much closer as that of the values acquired in view of the desirability function approach. Henceforth, PSO has the potential to cull appropriate machining factors while turning titanium (Grade-II) alloys under MQL conditions.     

Adhesion of tin droplets impinging on a stainless steel plate: effect of substrate temperature and roughness

We photographed impact of small tin droplets on stainless steel surfaces of varying temperature and roughness. To achieve high impact velocities the test surfaces were mounted on the rim of a rotating fly wheel. Substrate temperature (T_s) was varied from 120 to 220 °C and surface roughness (R_a) kept at either 0.05 or 2 µm. We kept constant the impact velocity (30 m/s) and droplet diameter (0.6 mm). To form a coating 60 droplets were deposited randomly on each stainless steel test coupon. Deposition efficiency was evaluated by dividing the mass adhering to the coupon by the mass of sixty droplets prior to impact. The maximum deposition efficiency was achieved at a substrate temperature of 160 °C. For T_s < 160 °C the deposition efficiency was higher on a rough surface (R_a = 2 µm) than on a smooth surface (R_a = 0.05 µm), since splats did not adhere well to the smooth surface. For T_s≥ 160 °C the deposition efficiency was higher on a smooth surface (R_a = 0.05 µm) than on a rough surface (R_a = 2 µm), since splats splashed less on the smooth surface.

© 2003 Elsevier Science Ltd. All rights reserved.     

The effect of matrix stresses on fibre pull-out forces

A pull-out test was developed to measure the bond strengths and frictional forces between steel wires, and polycarbonate and epoxy matrices when the matrix was under tensile stress. Some debonding occurred due to the matrix stress. Despite this, the nominal bond strength, in the polycarbonate case, increased with increasing matrix applied stress. When the pull-out force had caused complete debonding, sliding under approximately constant friction coefficient,, occurred. The value of for steel sliding in polycarbonate was 0.6, and for epoxy it was 0.19. The values were reduced to 0.12 and 0.10 respectively when the steel was coated with a fluorocarbon release agent. The normal stresses at the interface, in the absence of any applied stresses, were found to be about 7 MN m^–2 in the polycarbonate, and 3.0 MN m^–2 in the epoxy case. It was observed that the frictional forces due to these residual stresses could be less than one third of those generated by the applied stresses on the matrix. Thus residual stresses are not as important for fibre reinforcement as are matrix Poisson's shrinkage stresses.