Stainless Steel Test Specimens Research Articles

Metal Laser-Based Powder Bed Fusion Process Development Using Optical Tomography.

In this study, a set of 316 L stainless steel test specimens was additively manufactured by laser-based Powder Bed Fusion. The process parameters were varied for each specimen in terms of laser scan speed and laser power. The objective was to use a narrow band of parameters well inside the process window, demonstrating detailed parameter engineering for specialized additive manufacturing cases. The process variation was monitored using Optical Tomography to capture light emissions from the layer surfaces. Process emission values were stored in a statistical form. Micrographs were prepared and analyzed for defects using optical microscopy and image manipulation. The results of two data sources were compared to find correlations between lack of fusion, porosity, and layer-based energy emissions. A data comparison of Optical Tomography data and micrograph analyses shows that Optical Tomography can partially be used independently to develop new process parameters. The data show that the number of critical defects increases when the average Optical Tomography grey value passes a certain threshold. This finding can contribute to accelerating manufacturing parameter development and help meet the industrial need for agile component-specific parameter development.

Estimation of elastic properties of an additively manufactured lattice using resonant ultrasound spectroscopy.

Development of improved approaches in the characterization of additively manufactured structures continues to be a topic of interest for the advanced manufacturing community. This article will investigate an approach using resonant ultrasound spectroscopy (RUS) to determine the effective elastic constants of an orthotropic lattice structure. The evaluation is performed on a cube shaped 316 L stainless steel test specimen, constructed using selective laser melting techniques. The approach uses RUS techniques in conjunction with the assumption that in the frequency regime of interest, the wavelength of the diagnostic ultrasound is greater than the discrete structural features of the unit cell of the lattice; thus, the AM structure can be treated as an anisotropic continuum with effective material properties and symmetry inherited from the unit cell. The RUS analysis estimates the nine elastic coefficients associated with orthotropic sample symmetry, which, in turn, are used to determine the elastic moduli and Poisson ratios. Current results show good agreement between experiments and modeled data. Comparisons to published results are also in good agreement, indicating the potential applicability of this characterization technique for estimating the linear elastic properties of innovative additive manufactured metal lattice structures.

Analytical Modeling of Embedded Load Sensing Using Liquid-Filled Capillaries Integrated by Metal Additive Manufacturing

Additive manufacturing (AM) offers new manufacturing solutions for the integration of smart functionalities in engineering structures. In this paper, an analytical model is presented for an embedded load sensing element based on a liquid-filled capillary. During the additive manufacturing process, the capillary is integrated in the region where the strain is to be determined. The embedded capillary deforms as the structure deforms under an applied load, as such altering the pressure inside the capillary. The monitoring of the capillary pressure allows monitoring the loads and thus usage of the component. This paper presents a model describing the behavior of the sensing element under uniform tensile stress. The sensitivity of the load sensing element per unit longitudinal strain depends on the bulk modulus of the liquid inside the capillary and the Poisson coefficient of the surrounding material. The current work further compares the analytical model against static tension-compression tests of powder bed fused stainless steel (AISI 316L) test specimen with an integrated capillary filled with a liquid (water). Similarly, the validation of the model is then checked against a dynamic four-point bending test on a Ti-6Al-4V specimen produced by powder bed fusion.

Influence of stainless steel specimen topography on micro-abrasion and micro-abrasion-corrosion

This work carried out a systematic analysis to help understanding how surface roughness and relative orientation between surface topography and direction of abrasive entrainment can affect particle dynamics during both pure microabrasion and microabrasion-corrosion. For that, tests were carried out in AISI 304 austenitic stainless steel test specimens using a recently developed microabrasion-corrosion tester. A 3D load cell ensures controlled application and measurement of forces during the tests. For the microabrasion tests, a slurry composed of small ( ϕ=2.5 µm) silica particles dispersed in distilled water at a concentration of 20wt% was employed, whereas for the microabrasion corrosion tests the distilled water in the slurry was substituted by a corrosive solutions in distilled water (1 N H2SO4). Potentiodynamic curves varying from cathodic potentials into the transpassive region were obtained within the course of duration of each microabrasion-corrosion test. The specimens were ground using two different SiC papers (#80 and #4000) to vary their surface topography and were tested with the grooves oriented perpendicular and parallel to the direction of rotation of the ball. Although the effect of surface topography was slight under pure microabrasion conditions, it was more significant under microabrasion-corrosion conditions. It was proposed that under microabrasion conditions, the formation of a low friction passive layer on the specimens made particle entrainment more difficult and therefore the relative orientation between surface topography and ball rotation had a stronger effect on the amount of abrasives that entered the contact, affecting wear rates. Grooves parallel to the rotation of the sphere increased wear rates under microabrasion-corrosion conditions when compared with grooves oriented perpendicular to the contact. Parallel grooves also resulted in higher passivation currents than perpendicular grooves, probably due to more severe damage to the passive layer caused by more abrasives into the contact

The Quantification of Corrosion Damage for Pre-stressed Conditions: A Model Using Stainless Steel

Stainless steel is used extensively within orthopaedics as part of varying surgical procedures such as trochanteric osteotomy, sternum closure, patella tendon repair and reconstruction, patellofemoral ligament reconstruction and internal fracture fixation. This study examines the changes in corrosion behaviour after applying plastic and elastic stresses. Furthermore it analyses how a tensile and a compressive load can affect the corrosion susceptibility of a material. Stainless steel test specimens were divided into three groups of seven to determine how corrosion susceptibility will change when a material is pre-stressed. Potentiodynamic testing was performed to investigate the pitting behaviour of ss304 after being exposed to pre-stressed conditions. All images graded exhibited both area and pit corrosion scores, which significantly differed between the 3 groups (p < 0.001). The samples that were destructively stressed under compression had a mean of 35.5 % corrosion damage (SD 14.5) which is significantly greater (p = 0.001) compared to a mean of 11 % (SD 4.5) corrosion damage present with a material destructively stressed under tension. This study has shown that a material’s corrosion susceptibility changes significantly after being pre-stressed regardless, whether this is mechanically loading it within its yield strength or destructively loading.

Transient eddy current oscillations method for the inspection of thickness change in stainless steel

Detection and evaluation of corrosion and defects in industrial structures are extremely important for accident reduction and product quality assurance. We propose a new nondestructive testing (NDT) method called transient eddy current oscillations (TECO) to detect a change in thickness of stainless steel. This method uses a charged capacitor to discharge through an induction coil (excitation coil) and the resultant magnetic field detected by using a Hall-effect sensor, which is placed inside the cylindrical excitation coil at the bottom side axial center. Homogeneous stainless steel test specimen of different thicknesses has been prepared to test the feasibility of the TECO method. Three different features were identified in the detected magnetic field signal: envelope (peak amplitudes), zero crossing difference and the energy. Experimental results show a good correlation between detected features and the thickness of the test material. To verify the experimental results, simulations were performed using a commercial finite element modeling software. Results suggest that the proposed method can be a potential NDT tool for the detection of corrosion and wall thickness in industrial structures.

Possibilities of CT Scanning as Analysis Method in Laser Additive Manufacturing

Laser additive manufacturing is an established and constantly developing technique. Structural assessment should be a key component to ensure directed evolution towards higher level of manufacturing. The macroscopic properties of metallic structures are determined by their internal microscopic features, which are difficult to assess using conventional surface measuring methodologies. X-ray microtomography (CT) is a promising technique for three-dimensional non-destructive probing of internal composition and build of various materials.Aim of this study is to define the possibilities of using CT scanning as quality control method in LAM fabricated parts. Since the parts fabricated with LAM are very often used in high quality and accuracy demanding applications in various industries such as medical and aerospace, it is important to be able to define the accuracy of the build parts. The tubular stainless steel test specimens were 3D modelled, manufactured with a modified research AM equipment and imaged after manufacturing with a high-power, high-resolution CT scanner. 3D properties, such as surface texture and the amount and distribution of internal pores, were also evaluated in this study.Surface roughness was higher on the interior wall of the tube, and deviation from the model was systematically directed towards the central axis. Pore distribution showed clear organization and divided into two populations; one following the polygon model seams along both rims, and the other being associated with the concentric and equidistant movement path of the laser. Assessment of samples can enhance the fabrication by guiding the improvement of both modelling and manufacturing process.

Experimental investigation of cold-formed lean duplex stainless steel beam-columns

This paper describes a test program on cold-formed lean duplex stainless steel members in combined compression and minor axis bending. The test specimens were cold-rolled from flat strips of lean duplex stainless steel grade EN 1.4162. In this study, square and rectangular hollow sections were compressed at different eccentricities, in order to obtain a beam-column interaction curve for each series of tests. Initial overall geometric imperfections of the members were measured prior to testing. The ultimate loads and the failure modes of each specimen were obtained. The observed failure modes include local buckling, flexural buckling and interaction of local and flexural buckling. The test strengths obtained from this study and other available data were compared with the design strengths predicted by the American Specification, Australian/New Zealand Standard and European Code for stainless steel structures. It should be noted that these specifications do not cover the material of lean duplex stainless steel. Therefore, the suitability of the beam-column design rules in these specifications for lean duplex stainless steel is assessed in this study. Generally, these specifications are capable of predicting the beam-column strengths of the lean duplex stainless steel test specimens, and the design rules in the specifications are considered to be reliable. It is observed that the European Code generally provides quite conservative predictions for the beam-column specimens compared to the American Specification and Australian/New Zealand Standard predictions.

Strengthening of AISI 2205 duplex stainless steel by strain ageing

In this study, static strain ageing behavior of commercially available and solution heat treated duplex stainless steel was investigated and the effect of static strain ageing on the mechanical properties was also determined in detail. Some of as-received duplex stainless steel test specimens were pre-strained in tension by 5% and then aged at 100°C, 200°C, 300°C, 400°C, 500°C and 600°C for 30min in furnace. Some of duplex stainless steel test specimens were solution heat treated at 1050°C for 30min, water quenched and then pre-strained for 5% in tension shortly after the solution heat treatment.In order to identify the effect of static strain ageing on the mechanical properties, the tensile strength, the change in the strength due to ageing (ΔY), elongation fracture and hardness were determined. The test results showed that the mechanical properties were affected by static strain ageing mechanism which was applied at different temperatures for same time interval.

Effect of Nanocomposite Coating with Different Concentrations on the Fatigue Life of Stainless Steel316 with Different Surface Roughness

Abstract Many experimental researches succeeded to improve fatigue properties of materials by treating their surfaces. In this study, the benefits of nanocomposite material are used to investigate the fatigue life of stainless steel specimens with different surface roughness. Nanocomposite coating was prepared with different concentrations (0.3 %, 0.5 %, and 0.7 %) of multi wall carbon nano tubes MWCNT. This coating was applied on stainless steel test specimens with four different values of the surface roughness (0.3, 0.8, 1.6, and 2.5). It turned out that fatigue life increased more than four times compared to the original material when using 0.5 % and 0.7 % MWCNT concentration in coating composition with the 0.3 surface roughness, while no significant difference was detected at another roughness.

Experimental investigation of circular free-surface jet impingement quenching: Transient hydrodynamics and heat transfer

An experimental study of heat transfer during quenching of a cylindrical stainless steel test specimen has been performed. A subcooled water jet is directed onto the upward facing flat face of the cylinder. The test specimen is heated to an initial temperature slightly above 900 °C and then quenched. The resulting boiling curves and heat transfer distributions are presented for impingement velocities of 2.85 and 6.4 m/s (Re = 7900 and 18,900). High-speed imaging shows that three distinct regions on the quenched surface can be identified: an expanding circular wetted region surrounding the impinging point, annular transition zone just outside the wetting front, and a unwetted region outside this zone. The free-surface of the liquid in the wetted region is smooth in the nucleate and transition boiling regimes. The annular transition zone or the wetting front region outside the wetted region is characterized by a highly disturbed liquid–gas interface, which can be attributed to intense vapor generation. At the outer edge of the transition zone, the liquid is deflected away from the surface. The velocity of the wetting front significantly increases with the jet impact velocity, which indicates that the wetting front position is governed by the ability of the flowing liquid to transport the bubbles radially outwards from the wetted region.

Influence of boundary conditions on bi-phased polycrystal microstructure calculation

In this paper, polycrystalline bi-phased microstructure calculations were performed using finite element (FE) method and compared to mechanical fields measured experimentally using a digital image correlation (DIC) technique. From scanning electron microscopic (SEM) observations and electron back-scattered diffraction (EBSD) crystal orientation measurements performed on an austenitic-ferritic stainless steel test specimen, a quasi-2D numerical polycrystalline microstructure was constructed. Mechanical behaviors of austenitic (FCC) and ferritic (BCC) grains were modeled by two crystal plasticity laws, based on crystallographic slips and dislocation densities. FE mechanical fields were calculated using two types of boundary conditions: the displacements measured by DIC, and the average of the displacements measured by DIC. A better description of intraphase and intragranular strain heterogeneities is obtained using as boundary conditions the displacements measured instead of the average of the displacements measured.

Modelling and measurement of residual stresses in autogenously welded stainless steel plates: Part 1 – fabrication and modelling

Fabrication details and finite element derived predictions of residual stress are presented for two types of 316 L stainless steel test specimens of simple geometry, both containing an autogenous (no filler material) weld bead. The specimens were conceived in order to validate analytical weld residual stress predictions, which are only available for components of simple geometry and which do not account for phase transformations. The numerical modelling presented was undertaken by four different participants as part of a small ‘round robin’. The simplicity of the specimens permitted conclusions to be drawn concerning the relative importance of various modelling parameters. It is demonstrated that the assumptions made regarding the material hardening behaviour have the most significant effect on the predicted residual stress fields. Maximum differences in residual stress magnitudes of approximately 250 MPa were seen, but in general the variations were around 100 MPa. This underlines the need for caution when considering numerical residual stress predictions and underlines the need to validate predictions with measurements.

Oxidative dissolution of ruthenium deposits onto stainless steel by permanganate ions in alkaline medium

Abstract During spent nuclear fuel reprocessing ruthenium is liable to form black ruthenium deposits on the stainless steel walls of process equipments. These deposits promote corrosion and can eventually obstruct the off-gas lines. The results of decontamination of 304L stainless steel test specimens covered with RuO(OH)2· x H2O deposits by permanganate ions in alkaline medium are described. The ruthenium deposits were dissolved by oxidation of RuO(OH)2 to RuO4 2− ions, while the permanganate ions were reduced to MnO4 2− ions and then to manganese dioxide MnO2. The parameters affecting the kinetics of oxidative dissolution of these deposits were examined. The results indicate that the oxidative dissolution kinetics depends on the instantaneous surface area of the deposit, and that the dissolution rate increases with the concentrations of MnO4 − and OH− ions.

Joining stainless steel parts by means of weld bonding

Recent published works in the field of weld bonding present new results relevant to project and design of structural metal joints. However, it is still difficult to find technological data in the open literature to support and control manufacturing conditions. Gaps of knowledge are easily found in the set up of process parameters, in the selection of adhesives and in the understanding of the long term performance of weld bonded joints in hostile environments. The aim of this paper is to contribute to an extension of the actual knowledge on the weld bonding technology and to provide useful insights into its application to the joining of stainless steel. The research is accomplished by the weld bonding of stainless steel test specimens using three different commercial adhesives with varying working times and different exposures to water. The relative performance and quality of the weld bonded joints is evaluated against alternative solutions based on adhesives and conventional spot-welding. The experimental investigation is supported by finite element computer simulations with the purpose of establishing the welding parameters that allow spot-welding through the adhesives.

Static and Fatigue Performance of Weld-Bonded Stainless Steel Joints

Recent published works in the field of weld-bonding present new results relevant to the manufacturing of structural metal joints. The set-up of process parameters, the selection of adhesives and the basic understanding of the long-term performance of these joints in hostile environments are now sufficiently well established for industrial implementation. However, it is still difficult to find reliable data in the open literature that may assist project and design engineers in choosing to replace conventional spot-welded and/or adhesive-bonded by weld-bonded joints. Important gaps of knowledge can still be found on the evaluation and understanding of the static and fatigue performance of these joints. This work addresses the aforementioned difficulties by evaluating the influence of the adhesives and of the applied load characteristics on the mechanical performance of weld-bonded structural metal joints. The presentation is supported by laboratory controlled experimental procedures using stainless steel test specimens and two different types of commercial adhesives.

Application of quenching to create highly triaxial residual stresses in type 316H stainless steels

It has been proposed that highly triaxial residual stress fields may be sufficient to promote creep damage in thermally aged components, even in the absence of in-service loads. To test this proposal, it is necessary to create test specimens containing highly triaxial residual stress fields over a significant volume of the specimen. This paper presents results from an experimental and numerical study on the generation of triaxial residual stresses in stainless steel test specimens. Spray water quenching was used to generate residual stress fields in solid cylinders and spheres made from type 316H stainless steel. A series of finite element simulations and measurements were carried out to determine how process conditions and specimen dimensions influenced the resulting residual stress distributions. The results showed that highly compressive residual stresses occurred around the surfaces of the cylinders and spheres and tensile residual stresses occurred near the centre. Surface residual stresses were measured using the incremental centre hole-drilling technique, while internal residual stresses were measured using neutron diffraction. Overall there was good agreement between the predicted and measured residual stresses. The level of triaxiality was found to be very sensitive to the heat transfer coefficient, and could be controlled by adjusting the cooling conditions and changing the dimensions of the steel samples. This differed from other processes, such as welding and shot-peening, where the magnitudes and distributions of residual stresses are ill-defined and the volume of material subjected to a triaxial residual stress state is relatively small.

Possibility as a magnetic recording media of austenitic stainless steel using stress-induced phase transformation

A new magnetic recording media which uses the phase transformation of austenitic stainless steel, is proposed. The transformation from austenite to martensite occurs due to tribological stresses, such as indentation and sliding, has the potential to induce phase transformation which in turn generates a locally ferro-magnetized surface. Surface observations were made using magnetic force sensing systems having a sensing mechanism similar to that of magnetic force microscope (MFM). Two-dimensional distribution of the magnetic force measured by the sensing systems showed many possible applications in magnetic recording, and magnetic field freezing, etc. Scratching experiments with a load of 0.4 mN verified the possibility of the material's application as a recording media with spatial resolution of sub-micrometer. Memorizing phenomena of magnetic field in a stainless steel plate was also observed. After a uniform tensile stress generated the phase transformation of a stainless steel thin plate in a magnetic field, the magnetic force distribution on the steel surface was measured. The distribution was similar to that of the magnetic field which was induced by an exciting coil. The results indicated that information about the external magnetic field distribution was frozen in the stainless steel test specimen. By using this phenomenon, an analogue-type recording system capable of memorizing a two-dimensional distribution of a magnetic field at any arbitrary point in time by an external force trigger is proposed.

Hydrogen embrittlement of 22-5 duplex stainless steel

The influence of hydrogen on 22Cr-5Ni duplex stainless steel with an austenitic-ferritic microstructure was investigated. Notched test specimens were cathodically charged in 1N H 2 SO 4 containing NaAsO 2 at a current density of 0.4 mA cm -2 while undergoing tensile strain over a wide range of crosshead speeds (833, 83, and 8.3 μm s -1 and 833, 83, and 9.8 nm s -1 ) at room temperature of 22 ± 2°C and at 70 ± 2°C. For comparison, the same number of specimens was tested in air. The degree of embrittlement was evaluated by calculating the elongation and stress to fracture in air and solution. Then the elongation to fracture ratio E sol /E air and stress to fracture ratio σ sol /σ oir were calculated at each applied crosshead speed. A marked reduction in the mechanical properties, particularly the E sol /E air ratio, was exhibited by the duplex stainless steel test specimens when the embrittlement test was carried out at room temperature using slow crosshead speeds of 83 and 9.8 nm s -1 . However, when the embrittlement test was carried out at 70°C at the same crosshead speeds, less reduction in the E sol /E air ratio was observed. A transgranular cleavage fracture mode was observed in both the ferritic and austenitic phases of duplex stainless steel specimens when tested at room temperature with slow crosshead speeds. Large faceted cleavage fracture was predominant in the ferritic phase, while cleavage associated with slight plastic deformation was exhibited by the austenitic phase. In keeping with the results concerning mechanical properties, less change in fracture morphology was exhibited by the duplex stainless steel specimens when the test was carried out at 70°C. In this case, cleavage fracture was predominant only in the ferritic phase, while the austenitic phase exhibited quasicleavage fracture associated with the extensive plastic deformation.

An Experimental Study of the Relative Effects of Transverse and Longitudinal Ribbing of the Heat Transfer Surface in Forced Convective Boiling

This work describes an experimental investigation into the effects of surface geometry variations on heat transfer performance in forced flow boiling. Ribs of rectangular cross section were machined on flat stainless steel test specimens of 1.60 mm thickness. The height of the ribs was kept constant at 0.50 mm and a number of test specimens with rib width varying from 0.50 mm to 2.0 mm and rib spacing ranging from 0.50 mm to 3.5 mm were produced. In order to maintain a constant microsurface roughness of 0.7 μm over the entire test specimen, all test surfaces were subjected to vequa-blasting after machining. Precautions were taken to ensure that aging of the heat transfer surface had been established so that the data recorded were reproducible. Experimental data are reported for water at atmospheric pressure with flow velocities of 0.20 m/s to 1.40 m/s. Inlet subcoolings were varied from 5°C to 30°C. The results for the longitudinal ribbed surfaces (LRS) were compared with those for the transverse rectangular ribbed surfaces (TRS) and both sets of data were compared with the heat transfer performance of an optimum flat roughened surface.