Subsurface Hardness Research Articles

Laser melting of groove defect repair on high thermal conductivity steel (HTCS-150)

This paper presents laser melting repair of groove defect on HTCS-150 surface using Nd:YAG laser system. Laser melting process was conducted using JK300HPS Nd:YAG twin lamp laser source with 1064 nm wavelength and pulsed mode. The parameters are pulse repetition frequency (PRF) that is set from 70 to 100 Hz, average power (PA) of 50–70 W, and laser spot size of 0.7 mm. HTCS-150 samples were prepared with groove dimension of 0.3 mm width and depths of 0.5 mm using EDM wire cut. Groove defect repaired using laser melting process on groove surface area with various parameters’ process. The melted surface within the groove was characterized for subsurface hardness profile, roughness, phase identification, chemical composition, and metallographic study. The roughness analysis indicates high PRF at large spot size caused high surface roughness and low surface hardness. Grain refinement of repaired layer was analyzed within the groove as a result of rapid heating and cooling. The hardness properties of modified HTCS inside the groove and the bulk surface increased two times from as received HTCS due to grain refinement which is in agreement with Hall–Petch equation. These findings are significant to parameter design of die repair for optimum surface integrity and potential for repairing crack depth and width of less than 0.5 and 0.3 mm, respectively.

Material interactions in laser polishing powder bed additive manufactured Ti6Al4V components

Laser polishing (LP) is an emerging technique with the potential to be used for post-build, or in-situ, precision smoothing of rough, fatigue-initiation prone, surfaces of additive manufactured (AM) components. LP uses a laser to re-melt a thin surface layer and smooths the surface by exploiting surface tension effects in the melt pool. However, rapid re-solidification of the melted surface layer and the associated substrate thermal exposure can significantly modify the subsurface material. This study has used an electron beam melted (EBM) Ti6Al4V component, representing the worst case scenario in terms of roughness for a powder bed process, as an example to investigate these issues and evaluate the capability of the LP technique for improving the surface quality of AM parts. Experiments have shown that the surface roughness can be reduced to below Sa = 0.51 μm, which is comparable to a CNC machined surface, and high stress concentrating defects inherited from the AM process were removed by LP. However, the re-melted layer underwent a change in texture, grain structure, and a martensitic transformation, which was subsequently tempered in-situ by repeated beam rastering and resulted in a small increase in sub-surface hardness. In addition, a high level of near-surface tensile residual stresses was generated by the process, although they could be relaxed to near zero by a standard stress relief heat treatment.

In situ effect of the combination of fluoridated toothpaste and fluoridated gel containing sodium trimetaphosphate on enamel demineralization

ObjectiveThis in situ study evaluated the effect of the association of low-F (4500μg F/g) gel containing TMP and FT (1100μg F/g) on enamel demineralization. MethodsThis crossover and double-blind study consisted of five phases of seven days each. Volunteers (n=12) wore palatal appliances containing four enamel blocks. The cariogenic challenge was performed with 30% sucrose solution (six times/day). Treatments were: placebo toothpaste (PT, no fluoride/TMP); 1100μg F/g toothpaste (FT); FT+4500μg F/g+5%TMP gel (FT+TMP gel); FT+9000μg F/g gel (FT+9000 gel) and FT+12,300μg F/g (FT+Acid gel). After topical application of treatments for one min, two blocks were removed for analysis of loosely bound fluoride (CaF2), calcium (Ca), phosphorus (P) and firmly bound fluoride (FA) formed in enamel. After the seven-day experimental periods, the percentage of surface hardness loss (%SH), integrated subsurface hardness loss (ΔKHN), CaF2, Ca, P and FA retained were determined. Moreover, the biofilms formed on the blocks were analyzed for F, Ca, P and insoluble extracellular polysaccharide (EPS) concentrations. ResultsFT+TMP gel promoted the lowest%SH and ΔKHN (p<0.001). The highest concentration of CaF2 formed was observed for the FT+Acid gel (p<0.001), followed by FT+9000 gel > FT+TMP gel > FT > PT. CaF2 retained on the blocks was reduced across all groups (p<0.001). Similar values were observed for the Ca/P/F and EPS in enamel and biofilm for all fluoride groups. ConclusionThe association of FT+TMP gel significantly reduced enamel demineralization in situ. Clinical Significance: The association of treatments may be an alternative for patients with high caries risk.

The effect of thermomechanical treatment and tempering on the subsurface microstructure and bendability of direct-quenched low-carbon strip steel

Recent results in the literature have shown that subsurface properties play a key role during the bending of steel plates. Now, for the first time, surface microstructure, surface texture, subsurface hardness and dislocation density have been studied to reveal the effect of tempering and thermomechanical treatment on the bendability of a direct-quenched strip steel. In the experiments, different thermomechanical treatments as well as non-isothermal tempering treatments were performed with slow heating to 570°C and slow cooling to simulate the tempering of large steel coils in a batch annealing furnace. The results show that in addition to the improved production efficiency obtained through direct quenching and a single tempering process, tempering improves bendability by reducing subsurface dislocation density and hardness without a significant loss of strip yield strength. The subsurface microstructure and texture of the strip are the result of thermo-mechanical processing and transformation behaviour. Upper bainite containing elongated Martensite-Austenite (MA) islands in addition to an intense shear texture component {112}〈111〉α leads to shear band formation, and therefore poorer bendability when the bend axis is perpendicular to the rolling direction. This texture is not affected by tempering. Therefore, tempering does not improve the bendability of steels with an unfavourable texture. When the subsurface layers comprise a softer ferritic microstructure, good bendability is obtained in the untempered direct-quenched condition with a modest improvement caused by tempering.

Fluoride concentration and amount of dentifrice influence enamel demineralization in situ

ObjectivesThis study evaluated the effect of conventional (CD, 1100ppm F) and low-fluoride (LFD, 550ppm F) dentifrices, applied in different quantities, on enamel demineralization, and on fluoride (F) concentrations in the dental biofilm formed in situ. MethodsFive combinations of dentifrices and quantities were tested: placebo (P–F-free) applied on all brush bristles; LFD applied by the transversal technique (0.3g–T1) or on all bristles (0.6g–T2); and CD applied in a pea-sized amount (0.15g–T3) or by the transversal technique (0.3g–T4), in order to produce comparable intensities (F concentration in the dentifrice×amount applied to the brush). Volunteers (n=13, 20–36 years old) wore palatal devices containing 4 bovine enamel blocks, and performed cariogenic challenges (30% sucrose solution) 6×/day, and brushing 3×/day, following a double-blind, cross-over and randomized protocol. On the 8th day, biofilm was collected 5 and 60min after brushing. The percentage of surface hardness loss (%SH), integrated loss of subsurface hardness (ΔKHN) and biofilm F concentrations (solid and fluid phases) were determined. Data were analyzed by repeated-measures ANOVA, Student-Newman-Keuls test, and Pearson’s correlation coefficient (p<0.05). ResultsSignificantly lower ΔKHN was observed for treatments with higher intensity (T2 and T4) in comparison with the lower intensity (T1 and T3). A strong correlation was observed between ΔKHN and F concentrations in total biofilm (r=−0.71) and biofilm fluid (r=−0.72) 5min after brushing. ConclusionsThe treatment intensity has a significant influence on the development of caries lesions in situ. Clinical significance: The intensity of treatment (amount of dentifrice×concentration) during brushing seems to be a more relevant parameter of clinical efficacy than simply observing the F concentration of the product. The use of a small amount of CD significantly reduced the protective effects against enamel demineralization.

Subsurface Deformation Generated by Orthogonal Cutting: Analytical Modeling and Experimental Verification

Subsurface deformation of the cutting process has attracted a great deal of attention due to its tight relationship with subsurface hardening, microstructure alteration, grain refinement, and white layer formation. To predict the subsurface deformation of the machined components, an analytical model is proposed in this paper. The mechanical and thermal loads exerted on the workpiece by the primary and tertiary shear zones are predicted by a combination of Oxley's predictive model and Fang's slip line field. The stress field and temperature field are calculated based on contact mechanics and the moving heat source theory, respectively. However, the elastic–plastic regime induced by the material yielding hinders the direct derivation of subsurface plastic deformation from the stress field and the work material constitutive model. To tackle this problem, a blending function of the increment of elastic strain is developed to derive the plastic strain. In addition, a sophisticated material constitutive model considering strain hardening, strain rate sensitivity, and thermal softening effects of work material is incorporated into this analytical model. To validate this model, finite element simulations of the subsurface deformation during orthogonal cutting of AISI 52100 steel are conducted. Experimental verification of the subsurface deformation is carried out through a novel subsurface deformation measurement technique based on digital image correlation (DIC) technique. To demonstrate applications of the subsurface deformation prediction, the subsurface microhardness of the machined component is experimentally tested and compared against the predicted values based on the proposed method.

Regularities of Thermal Diffusion Saturation with Nitrogen Combined with Standard Heat Treatment of VT22 Titanium Alloy

We establish the regularities of the thermal diffusion saturation of VT22 two-phase titanium alloy with nitrogen combined with standard heat treatment. The kinetic specific features of the nitriding of VT22 (α + β)-alloy caused by the temperature and time parameters of treatment are analyzed. The influence of these parameters on the phase-structural state of the material and its relationship with the level of subsurface hardening and the depth of nitrided layer are determined

Surface integrity of quenched steel 1045 machined by CBN grinding wheel and SiC grinding wheel

PurposeThe purpose of this paper is to evaluate surface integrity of quenched steel 1045 ground drily by the brazed cubic boron nitride (CBN) grinding wheel and the black SiC wheel, respectively. Surface integrity, including surface roughness, sub-surface hardness, residual stresses and surface morphology, was investigated in detail, and the surface quality of samples ground by two grinding wheels was compared.Design/methodology/approachIn the present work, surface integrity of quenched steel 1045 machined by the CBN grinding wheel and the SiC wheel was investigated systematically. All the specimens were machined with a single pass in the down-cutting mode of dry condition. Surface morphology of the ground specimen was observed by using OLYMPUS BX51M optical microscopy. Surface roughness of seven points was measured by using a surface roughness tester at a cut-off length of 1.8 mm and the measurement traces were perpendicular to the grinding direction. Sub-surface micro-hardness was measured by using HVS-1000 digital micro-hardness tester after the cross-section surface was polished. The residual stress was tested by using X-350A X-ray stress analyzer.FindingsWhen the cut depth is increased from 0.01 to 0.07 mm, the steel surface machined by the CBN wheel remains clear grinding mark, lower roughness, higher micro-hardness and higher magnitude of compressive stress and fine microstructure, while the surface machined by the SiC grinding wheel becomes worse with increasing of cut depth. The value of micro-hardness decreases, and the surface roughness increases, and the surface compressive stress turns into tensile stress. Some micro-cracks and voids occur when the sample is processed by the SiC grinding wheel with cut depth 0.07 mm.Originality/valueIn this paper, the specimens of quenched steel 1045 were machined by the CBN grinding wheel and the SiC wheel with various cutting depths. The processing quality resulted from the CBN grinding wheel is better than that resulted from the SiC grinding wheel.

Effect of the addition of nano-sized sodium hexametaphosphate to fluoride toothpastes on tooth demineralization: an in vitro study.

This study evaluated the effect of toothpastes containing 1100ppm F associated with nano-sized sodium hexametaphosphate (HMPnano) on enamel demineralization in vitro using a pH-cycling model. Bovine enamel blocks (4mm×4mm, n=72) selected by initial surface hardness (SHi) were randomly allocated into six groups (n=12), according to the test toothpastes: without fluoride or HMPnano (Placebo), 550ppm F (550F), 1100ppm F (1100F), 1100F plus HMPnano at concentrations of 0.25% (1100F/0.25%HMPnano), 0.5% (1100F/0.5%HMPnano), and 1.0% (1100F/1.0%HMPnano). Blocks were treated 2×/day with slurries of toothpastes and submitted to five pH cycles (demineralizing/remineralizing solutions) at 37°C. Next, final surface hardness (SHf), integrated loss subsurface hardness (ΔKHN), integrated mineral loss (gHAp×cm-3), and enamel fluoride (F) concentrations were determined. Data were analyzed by ANOVA and Student-Newman-Keuls test (p<0.001). Toothpaste with 1100F/0.5%HMPnano led to the lowest mineral loss and the highest mineral concentration among all groups, which were 26% (SHf) and 21% (ΔKHN) lower and ~58% higher (gHAp×cm-3) when compared to 1100F (p<0.001). Similar values of enamel F were observed for all fluoridated toothpastes (p>0.001). The addition of 0.5%HMPnano to a 1100 F toothpaste significantly enhances its effects against enamel demineralization when compared to its counterpart without HMPnano in vitro. Toothpaste containing 1100ppm F associated with HMPnano has a higher potential to reduce the demineralization compared to 1100ppm F. This toothpaste could be a viable alternative to patients at high risk of caries.

Enhanced Cu-to-Cu direct bonding by controlling surface physical properties

Cu-to-Cu direct bonding is one of the key technologies for three-dimensional (3D) chip stacking. This research proposes a new concept to enhance Cu-to-Cu direct bonding through the control of surface physical properties. A linear relationship between bonding strength and the value of the bonding face (H: subsurface hardness, R: surface roughness) was found. Low vacuum air plasma and thermal annealing were adopted to adjust the surface physical conditions. Instead of surface activation, an acceleration in copper atom diffusion due to plasma-induced compressive stress accounts for the improvement in bonding strength.

Laser polishing of 3D printed mesoscale components

Laser polishing of various engineered materials such as glass, silica, steel, nickel and titanium alloys, has attracted considerable interest in the last 20 years due to its superior flexibility, operating speed and capability for localised surface treatment compared to conventional mechanical based methods. The paper initially reports results from process optimisation experiments aimed at investigating the influence of laser fluence and pulse overlap parameters on resulting workpiece surface roughness following laser polishing of planar 3D printed stainless steel (SS316L) specimens. A maximum reduction in roughness of over 94% (from ∼3.8 to ∼0.2μm Sa) was achieved at the optimised settings (fluence of 9J/cm2 and overlap factors of 95% and 88–91% along beam scanning and step-over directions respectively). Subsequent analysis using both X-ray photoelectron spectroscopy (XPS) and glow discharge optical emission spectroscopy (GDOES) confirmed the presence of surface oxide layers (predominantly consisting of Fe and Cr phases) up to a depth of ∼0.5μm when laser polishing was performed under normal atmospheric conditions. Conversely, formation of oxide layers was negligible when operating in an inert argon gas environment. The microhardness of the polished specimens was primarily influenced by the input thermal energy, with greater sub-surface hardness (up to ∼60%) recorded in the samples processed with higher energy density. Additionally, all of the polished surfaces were free of the scratch marks, pits, holes, lumps and irregularities that were prevalent on the as-received stainless steel samples. The optimised laser polishing technology was consequently implemented for serial finishing of structured 3D printed mesoscale SS316L components. This led to substantial reductions in areal Sa and St parameters by 75% (0.489–0.126μm) and 90% (17.71–1.21μm) respectively, without compromising the geometrical accuracy of the native 3D printed samples.

Toothpaste with Nanosized Trimetaphosphate Reduces Enamel Demineralization.

This double-blind crossover study assessed the effects of a low-fluoride (low-F) dentifrice containing nanosized sodium trimetaphosphate (TMP) on enamel demineralization in situ. Nineteen subjects wore palatal appliances containing 4 blocks of bovine enamel and were randomly assigned to brush their teeth with placebo (without F/TMP), 250-ppm F (250F), 250F plus 0.05% nanosized TMP (250F-TMPnano), and 1,100-ppm F (1,100F) dentifrices during 7 d, under cariogenic challenge. Enamel surface hardness and cross-sectional hardness (ΔKHN [Knoop hardness number]), as well as F, calcium (Ca), and phosphorus (P) concentrations, were determined. Also, biofilm that formed on the blocks was analyzed for F, Ca, P, and insoluble extracellular polysaccharide concentrations. Data were submitted to analysis-of-variance models and Student-Newman-Keuls test ( P < 0.05). The 250F-TMPnano dentifrice promoted the lowest ΔKHN among all groups ( P < 0.001), while the percentage of surface hardness loss was similar to 1,100F. Also, similar F, Ca, and P concentrations in enamel were observed for 1,100F and 250F-TMPnano. In the biofilm, the highest F content was observed for 1,100F; Ca content was similar between 1,100F and 250F-TMPnano; and P content was similar among all groups. Similar extracellular polysaccharide values were observed for 250F-TMPnano and 1,100F ( P < 0.001), ionic activity of CaHPO40, CaF+, and HF0 ( P < 0.05) and degree of saturation of hydroxyapatite and CaF2 ( P < 0.05). It was concluded that the protective effect of 250F-TMPnano dentifrice was similar to a conventional dentifrice for most of the variables studied, having a more pronounced effect on the subsurface lesion when compared with the conventional toothpaste (1,100F). Knowledge Transfer Statement: Although toothpastes containing ≥1,000-ppm fluoride are more effective than low-fluoride formulations against dental caries, their early use can lead to side effects. This has prompted intensive research on alternatives to increase the anticaries effect of low-fluoride toothpastes. The present in situ study demonstrated that the addition of sodium trimetaphosphate nanoparticles to toothpastes containing 250-ppm fluoride significantly enhances the protective effect of this formulation against enamel demineralization to levels comparable to a 1,100-ppm fluoride toothpaste in terms of most of the variables studied. Most important, this formulation promoted the lowest loss of subsurface hardness among all groups, suggesting that caries lesions would take longer to develop under clinical conditions when compared with a conventional (1,100-fluoride) toothpaste.

Effect of Red Scale on the Bendability of Ultrahigh-Strength Strip Steel

The effect of red scale on the bendability of a thermomechanically rolled and direct quenched pilot-scale strip steel has been studied by comparing the bending behaviour of adjacent areas with and without red scale. The yield strength of the studied 8 mm thick strip was 960 MPa. The local microstructure and texture below the different scale surfaces were characterized using FESEM and FESEM-EBSD, chemical compositions were determined using GDOES, microhardness profiles were measured and bendability was determined using three-point brake press bending. Red scale was found to significantly affect bendability especially when the bend axis is transverse to the rolling direction. The minimum usable punch radius for defect-free bends in the absence of red scale was 12 mm (1.5 x thickness) while under red scale it was 30 mm (3.75 x thickness). Beneath the red scale the microstructure 50 to 400 μm below the surface was clearly different to that in the absence of red scale. Without the red scale the microstructure was mainly granular bainite with small fraction of upper bainite and polygonal ferrite. Below the red scale the microstructure was a mixture of upper bainite and granular bainite. As a result of the microstructural differences, the subsurface hardness changed substantially from 360 HV in the absence of red scale to 410 HV with red scale. The chemical composition did not change as a result of the presence or absence of red scale, which rules this factor out as possible cause of differences in bendability or final microstructure. Possible explanations for the observed effects of red scale on subsurface microstructure, and microstructure on bendability, are discussed in the paper.

Structural life enhancement on friction stir welded AA6061 with optimized process and HFMI/PIT parameters

This novel study presents an unconventional approach to find the best governing process parameters of high frequency mechanical impact technique based on multi-objective optimization method. In this investigation, the post-weld mechanical treatment is aimed to enhance fatigue resistance of structural friction-stirred weld subjected to fluctuating loads by obtaining nominal sub-surface hardness. The experimental study was conducted on aluminium alloy AA 6061 with thickness of 6 mm under varied parameters centred on indenter diameter, air pressure and impact frequency. The investigation began with obtaining optimum parameters for single response by using conventional Taguchi method with L9 orthogonal array. Next, advanced optimization approach by means of multi-objective Taguchi method attempts to consider the multiple responses simultaneously which are sub-surface hardness and structural life. As the final results, the optimum value was acquired by calculating the total normalized quality loss and multiple signal-to-noise ratios based on unequal desirability. The significant level of the parameters was evaluated by using analysis of variance. Furthermore, the second-order model for predicting the objectives was derived by applying response surface methodology. It can be summarized that, first, the affecting parameters to obtain superior structural life can be ordered at significant level of ca. 65, 25 and 10% for air pressure, impact frequency and indenter diameter, respectively. Secondly, using subsequent post-weld mechanical treatment, the life cycle number can be extended up to 12 times on friction-stirred weld. Finally, based on the experimental confirmation test, the proposed method can effectively estimate the structural life and surface hardness within the acceptable range of relative error.

Remineralizing Potential of a Low Fluoride Toothpaste with Sodium Trimetaphosphate: An in situ Study

Objective: To evaluate the effect of a low-fluoride (F) toothpaste supplemented with sodium trimetaphosphate (TMP) on enamel remineralization in situ. Design: Bovine enamel blocks were selected on the basis of their surface hardness (SH) after caries-like lesions had been induced, and randomly divided into 4 treatment groups, according to the toothpastes used: without F or TMP (placebo); 500 ppm F; 500 ppm F plus 1% TMP; and 1,100 ppm F. The study design was blinded and crossover and performed in 4 phases of 3 days each. Eleven subjects used palatal appliances containing 4 bovine enamel blocks which were treated 3 times per day during 1 min each time, with natural slurries of saliva and toothpaste formed in the oral cavity during toothbrushing. After each phase, the percentages of surface (%SH_R) and subsurface hardness recovery (%ΔKHN_R) were calculated. F, calcium (Ca), and phosphorus (Pi) contents in enamel were also determined. Data were analyzed by 1-way, repeated-measures ANOVA, followed by the Student-Newman-Keuls test (p < 0.05). Results: Toothpaste with 500 ppm F + TMP and 1,100 ppm F showed similar %SH_R and %ΔKHN_R as well as enamel F, Ca, and Pi concentrations. Conclusion: The addition of TMP to a low-fluoride toothpaste promoted a similar remineralizing capacity to that of a standard (1,100 ppm F) toothpaste in situ.

Machinability studies on INCONEL 718

The main objective of proposed work is to determine the influence of controllable parameters on machining characteristics of Inconel-718 and to achieve the optimum parameters for sustainable and efficient turning. Understanding the consequences of advanced tool materials together with higher cutting speeds on the formation of residual stresses and therefore the underlying mechanisms of small structural alteration within the subterranean layer thereby becomes terribly crucial for predicting product quality and more optimizing the machining conditions. Controllable cutting parameters such as cutting velocity, feed rate and depth of cut were selected at different level for experimentations in accordance with the Taguchi L9 array method using Minimum Quantity Lubrication (MQL) cutting condition and three different tools namely PVD TiAlN carbide, Cubic boron nitride and ceramic. Extensive study is done on the resulting surface roughness, surface subsurface hardness, tool wear and chip morphology. The results obtained from each of the tool were thoroughly analyzed and finally the optimized parameters are obtained for efficient machining of Inconel 718.

The enhancement effect and kinetics of rare earth assisted salt bath nitriding

A rapid salt bath nitriding technology was primarily developed by adding rare earth lanthanum (RE La) in the salt bath media on the basis of normal salt bath nitriding technique (NT). Characterization of the modified surface layers was made by means of optical microscopy, X-ray diffraction, energy dispersive spectroscopy (EDS) and micro-hardness tester. The results showed that the nitriding efficiency could be significantly enhanced assisted by RE La, and the enhancement effect was RE La concentration dependent. The thickness of compound layer was increased from 13.3 μm up to 18.5 μm with 5% La addition at the temperature of 833 K for 2 h. Higher surface hardness, modestly higher sub-surface hardness and superior micro-hardness profile were obtained due to the solution of RE La atoms and consequent lattice expansion in the surface layer. Meanwhile, kinetics analysis showed that the diffusion coefficient was increased and activation energy was decreased with enhancement effect of RE La, the following kinetics formulas were obtained, dNT=0.1exp−21990/Tt and dRE=4.3×10−3exp−16319/Tt for NT and RE treatment, respectively. And the confidence level of the kinetics is examined to be perfectly agreed with the data. The main enhancement mechanism is that RE La can accelerate the decomposition rate of CNO− and thus increase the concentration of active nitrogen atoms in the salt bath medium, and especially increase the number of nitrogen atoms that meet the requirements for diffusing toward the substrate by reducing the energy barrier.

On the fatigue behavior of medical Ti6Al4V roughened by grit blasting and abrasiveless waterjet peening

Flat fatigue specimens of biomedical Ti6Al4V ELI alloy were surface-processed by high pressure waterjet peening (WJP) without abrasive particles using moderate to severe conditions that yield roughness values in the range of those obtained by commercial grit blasting (BL) with alumina particles. Fatigue behavior of WJP and BL specimens was characterized under cyclical uniaxial tension tests (R=0.1). The emphasis was put on a comparative analysis of the surface and subsurface induced effects and in their relevance on fatigue behavior. Within the experimental setup of this investigation it resulted that blasting with alumina particles was less harmful for fatigue resistance than abrasiveless WJP. BL specimens resulted in higher subsurface hardening and compressive residual stresses. Specimens treated with more severe WJP parameters presented much higher mass loss and lower compressive residual stresses. From the analysis performed in this work, it follows that, in addition to roughness, waviness emerges as another important topographic parameter to be taken into account to try to predict fatigue behavior. It is envisaged that optimization of WJP parameters with the aim of reducing waviness and mass loss should lead to an improvement of fatigue resistance.

Influence of Composition and Hot Rolling on the Subsurface Microstructure and Bendability of Ultrahigh-Strength Strip

The effect of subsurface microstructure on the bendability of three 8-mm-thick low-alloyed hot-rolled and direct-quenched ultrahigh-strength strip steels with yield strengths in the range 800 to 1100 MPa has been investigated. Rolling to lower finish rolling temperatures increased austenite pancaking, leading to the formation of ferritic/granular bainitic subsurface microstructures that are softer than the upper bainitic microstructures found with higher finish rolling temperature. In addition, increased austenite pancaking was found to increase the intensities of ~{112}〈111〉α and ~{110}〈112〉α to {110}〈111〉α texture components in the surface layers, especially in upper bainitic microstructures. It is shown that the bendability of ultrahigh-strength steels is governed by subsurface hardness and crystallographic texture. Bendability was found to be related to mean microhardness 0.1 to 0.4 mm below the surface, such that excellent bendability was achieved with a relatively soft subsurface layer down to a depth of 0.4 mm, i.e., 5 pct of the sheet thickness. Intense ~{112}〈111〉α texture combined with upper bainite containing MA islands in the subsurface region is shown to be detrimental to bendability when the bend axis is perpendicular to the rolling direction probably as a result of geometrical softening combined with high hardness.

Characterization of the microstructure evolution and subsurface hardness of graded stainless steel produced by different mechanical or thermochemical surface treatments

Abstract The work presented in this paper is dedicated to a multi-scale characterization of plastically graded materials obtained with mechanical (ultrasonic shot peening) and thermo-chemical (nitriding followed by over-carburizing, nitrogen ion implantation, and low-temperature carburizing) surface treatments. The benefits of several characterization techniques (scanning electron microscopy with electron backscattering diffraction, spectrometry (glow discharge optical emission spectrometry and energy-dispersive X-ray spectrometry), instrumented indentation, and X-ray diffraction), at different scales, and the possible existence of a signature of the studied surface treatments on the material are examined. It is found that nitriding followed by over-carburizing has the largest microstructure signature: there is a clear correlation between changes in the grain sizes, the volume fraction of precipitates, the carbon content and the hardness profile. In contrast, specimens subjected to low-temperature carburizing and ion implantation show no changes at the microstructure scale. However, for both treatments, hardness varies over the same depth as the chemical composition. Specimens subjected to ultrasonic shot peening present little or no microstructural changes. For the specimen treated with the highest intensities, the depth of the area showing high misorientations is different from the depth over which hardness varies. Thus, the sole observation of the microstructure changes leads to the identification of a depth impacted by the treatment smaller than the one identified using the misorientations.