Pulsed FCAW of Martensitic Stainless Clads onto Mild Steel: Microstructure,Hardness, and Residual Stresses

The low carbon martensitic stainless AWS 410NiMo steel has in its chemical composition 13% chromium, 4% nickel, and 0.4% molybdenum (wt.%) and is used in turbine recovery, rotors, and high-pressure steam pump housings due to its resistance to impact at low temperatures, as well as to corrosion and cavitation. Those applications of the AWS 410NiMo steel frequently demand repair, which is performed by welding or cladding. Arc welding is a well-established technique for joining materials and presents several parameters that influence the mechanical performance of the weld bead. Although numerous welding processes exist, optimizing welding parameters for specific applications and materials is always challenging. The present work deals with a systematic study to verify the correlation between the pulsed fluxed core arc welding (FCAW) parameters, namely pulse current and frequency, welding speed, and contact tip work distance (CTWD), and the bead morphology, microstructure formation, residual stress, and hardness of the martensitic clad. The substrate used was the AISI 1020 steel, and the AWS 410NiMo steel was the filler metal for clad deposition. From the initial nine (9) samples, three (3) were selected for in-depth characterization. Lower heat input resulted in lower dilution, more elevated hardness, and lower compressive residual stresses. Therefore, the results highlight the need for selecting the proper heat input, even when using a pulsed FCAW procedure, to achieve the desired performance of the clad. In the present case, a higher heat input appears to be more advantageous owing to the lower convexity index, smooth hardness transition between fusion and heat-affected zones in addition to more elevated compressive stresses.     

Micromechanical Observation and Numerical Simulation for Local Deformation Evolution of Duplex Stainless Steel

The characteristics of local strain distribution and evolution of duplex stainless steel during the tensile process were studied using the digital image correlation (DIC) technique. In addition, the finite element inversion of nanoindentation experiments of austenitic and ferrite phases in duplex stainless steel was carried out to obtain the stress–strain response of the two phases. Further, based on the representative volume element (RVE) and the material parameters obtained from the finite element inversion method, the local stress and strain behavior of duplex stainless steel at microscale was simulated numerically. The results fit well with the experiments, showing that the austenite phase is softer than ferrite phase, with the larger strain zone concentrated in the austenite phase and the larger stress zone concentrated in the ferrite phase. The grain boundaries are prone to obvious stress and strain concentrations. The local stress and strain distributions are influenced by the shape and interaction of the grains, while the distribution features become more obvious as the load increases. The research results effectively reveal the two-phase interaction and local failure mechanism of duplex stainless steel, and may provide a reference for material preparation and safety design of related structures.     

A Study on the Ultrasonic Regulation of the Welding Performance and Residual Stress of 316L Stainless Steel Pipes

Due to its extreme service conditions, low-temperature pressure piping often needs post-welding stress measurement and control. Aiming at the phenomenon of local stress concentration in welded 316L pipes, this study used ultrasound to regulate the stress in the welded area at different times during and after the multi-layer welding of the pipeline butt joint for different time lengths. Mechanical properties such as tensile strength and hardness were tested for each comparison group, and the microcrystalline phases of the weld and its surrounding microstructure were analyzed. The transverse and longitudinal surface residual stresses of each comparison group were measured. The influence of high-energy ultrasound on the surface temperature field during and after welding was analyzed. The experimental results show that ultrasonic wave regulation can speed up heat exchange and radiation in the weld zone (WZ), refine the grains in the WZ, heat-affected zone (HAZ) and fusion zone (FZ) to some extent and reduce and homogenize residual stress to a certain degree. In the 120 mm area of the weld center, the residual stress measured after the mid-welding regulation was smaller than that of any other comparison group. This regulation result was the best, followed by that of hot regulation and finally that of offline regulation. The tensile strengths obtained by the mid-welding regulation and post-welding hot regulation of this group were the best, increasing by 17.2% and 24.3%, respectively, compared with the untreated groups.     

Deformation Prediction and Experimental Study of 316L Stainless Steel Thin-Walled Parts Processed by Additive-Subtractive Hybrid Manufacturing

The hybrid process integrates two or more different processes, such as additive and subtractive manufacturing, which have gained appreciable consideration in recent years. The deformation of hybrid manufacturing is an essential factor affecting machining quality. The purpose of this paper is to study the effect of milling on stress release and surface deformation of additive manufacturing (AM) specimens in the process of additive and subtractive hybrid manufacturing (ASHM) of 316L stainless steel thin-walled parts, so as to effectively improve the forming quality of thin-walled parts manufactured by the combined processing of ASHM. To this end, a series of experiments were carried out to study the relationship between stress distribution and thermal stress deformation of 316L stainless steel thin-walled parts prepared by LMD, and the changes of stress and deformation of these thin-walled parts after subsequent milling. An infrared camera and laser distance sensor were used to record the temperature field data and deformation data to analyze the influence factors of temperature and stress on the machining results. Then, the finite element software was used to simulate the stress and deformation of the thin-walled parts in the additive manufacturing process and the subsequent milling process. Meanwhile, the model was verified through the experiments. In addition, the relationship between the milling force and the milling parameters of the AM parts was studied by orthogonal test and regression analysis.     

Effect of Internal Support on the Tensile Properties and Fracture Mode of 304 Stainless Steel Thin-Walled Tubes

Steel–tube composite structures contain multiple tubular components under tension. The enhancement of the mechanical properties of tubes under ultimate operating conditions is crucial for improving structural safety. In this study, 110 pieces of 304 stainless steel thin-walled tubes (SSTWTs) under five internal support conditions are investigated. The ultimate tensile strength, ultimate extension, and fracture energy of different groups of specimens are measured to understand the variation mechanism of fracture modes. The elastic modulus of tube filler is treated as a variable to establish a uniaxial tensile fracture matrix of 304 SSTWTs with different tube fillers and loading rates. The results demonstrate that flexible tube fillers can effectively limit the lateral necking of 304 SSTWTs. Under the middle fracture mode, the maximum increments in the ultimate strength, extension, and fracture energy of tubes are 10.81%, 24.56%, and 35.94%, respectively. Furthermore, as the support rigidity increases, the ultimate strength exhibits an overall increasing trend, while the extension and fracture energy initially increase and then decrease. Overall, this study provides a novel route for enhancing the performance of steel–tube composite structures under ultimate loading conditions, which is of great significance for improving the safety of the structural design and reducing the engineering construction cost.     

Microstructure and Mechanical Properties of Modified 316L Stainless Steel Alloy for Biomedical Applications Using Powder Metallurgy

AISI 316L stainless steel (SS) is one of the extensively used biomaterials to produce implants and medical devices. It provides a low-cost solution with ample mechanical properties, corrosion resistance, and biocompatibility compared to its counterpart materials. However, the implants made of this material are subjected to a short life span in human physiological conditions leading to the leaching of metal ions, thus limiting its use as a biomaterial. In this research, the addition of boron, titanium, and niobium with varying concentrations in the SS matrix has been explored. This paper explores the impact of material composition on modified SS alloy’s physical and mechanical properties. The study’s outcomes specify that the microhardness increases for all the alloy compositions, with a maximum increase of 64.68% for the 2 wt.% niobium added SS alloy. On the other hand, the tensile strength decreased to 297.40 MPa for the alloy containing 0.25 wt.% boron and 2 wt.% titanium additions compared to a tensile strength of 572.50 MPa for pure SS. The compression strength increased from 776 MPa for pure SS to 1408 MPa for the alloy containing niobium and titanium additions in equal concentrations.     

Possibilities to Use Physical Simulations When Studying the Distribution of Residual Stresses in the HAZ of Duplex Steels Welds

Dual phase steels combine very good corrosion resistance with relatively high values of mechanical properties. In addition, they can maintain good plastic properties and toughness at both room temperature and lower temperatures as well. Despite all the advantages mentioned above, their utility properties can be reduced by technological processing, especially by the application of the temperature cycles. As a result, in the material remain residual stresses with local stress peaks, which are quite problematic especially during cyclic loading. Moreover, determining the level and especially the distribution of such residual stresses is very difficult for duplex steels both due to the structure duality and in light of the very small width of the heat-affected zone (HAZ). This is why the paper presents the possibilities of using physical simulations to study the effect of temperature cycles in residual stresses’ magnitude and distribution, where it is possible to study the HAZ in more detail as well as on a much larger sample width due to the utilization of special samples. In the thermal–mechanical simulator Gleeble 3500, temperature-stress cycles were applied to testing samples, generating stress fields with local peaks in the testing samples. In addition, the supplied steel X2CrMnNiN21-5-1 had different phase rations in the individual directions. Therefore, as the residual stresses were measured in several directions and at the same time, it was possible to safely confirm the suitability of the used measurement method. Moreover, the effect of the stress and strain on the change of partial phases’ ratios was observed. It has been experimentally confirmed that annealing temperatures of at least 700 °C are required to eliminate local stress peaks after welding. However, an annealing temperature of 550 °C seems to be optimal to maintain sufficient mechanical properties.     

Stress Corrosion Cracking of an Austenitic Stainless Steel in Nitrite-Containing Chloride Solutions

This article describes the susceptibility of 316L stainless steel to stress corrosion cracking (SCC) in a nitrite-containing chloride solution. Slow strain rate testing (SSRT) in 30 wt. % MgCl₂ solution established SCC susceptibility, as evidenced by post-SSRT fractography. Addition of nitrite to the chloride solution, which is reported to have inhibitive influence on corrosion of stainless steels, was found to increase SCC susceptibility. The susceptibility was also found to increase with nitrite concentration. This behaviour is explained on the basis of the passivation and pitting characteristics of 316L steel in chloride solution.     

High Temperature Tensile Fracture Behavior of Copper-Containing Austenitic Antibacterial Stainless Steel

The mechanical properties and deformation microstructure of cast 304 Cu-containing austenitic stainless steel at 10⁻³/s strain rate in the range of 700~1200 °C were studied by Gleeble thermal simulator, metallographic microscope and scanning electron microscope. The results showed that the thermoplasticity of 304 Cu-containing austenitic stainless steel was higher than 60% when the temperature was higher than 1000 °C, and the tensile strength as a whole decreased with the increase in temperature. During the tensile process, the morphology and content of ferrite in the test steel were the main factors affecting the high-temperature thermoplastic of the billet. The inclusions near the fracture and the existence of ferrite at the grain boundary greatly affected the formation of microcracks and holes and the fracture.     

Neutron and X-ray Diffraction Analysis of Macro and Phase-Specific Micro Residual Stresses in Deep Rolled Duplex Stainless Steels

Experimental analyses of depth distributions of phase-specific residual stresses after deep rolling were carried out by means of laboratory X-ray diffraction and neutron diffraction for the two duplex steels X2CrNiMoN22-5-3 and X3CrNiMoN27-5-2, which differ significantly in their ferrite to austenite ratios. The aim of the investigation was to elucidate to which extent comparable results can be achieved with the destructive and the non-destructive approach and how the process induced phase-specific micro residual stresses influence the determination of the phase- and {hkl}-specific reference value d₀, required for evaluation of neutron strain scanning experiments. A further focus of the work was the applicability of correction approaches that were developed originally for single-phase materials for accounting for spurious strains during through surface neutron scanning experiments on coarse two-phase materials. The depth distributions of macro residual stresses were separated from the phase-specific micro residual stresses. In this regard, complementary residual stress analysis was carried out by means of incremental hole drilling. The results indicate that meaningful macro residual stress depth distributions can be determined non-destructively by means of neutron diffraction for depths starting at about 150–200 µm. Furthermore, it was shown that the correction of the instrumental surface effects, which are intrinsic for surface neutron strain scanning, through neutron ray-tracing simulation is applicable to multiphase materials and yields reliable results. However, phase-specific micro residual stresses determined by means of neutron diffraction show significant deviations to data determined by means of lab X-ray stress analysis according to the well-known sin²ψ-method.     

Residual Notch Stress Intensity Factors in Welded Joints Evaluated by 3D Numerical Simulations of Arc Welding Processes

Approaches based on calculating Residual Notch Stress Intensity Factors (R-NSIFs) assume the weld toe to be a sharp V-notch that gives rise to a residual singular stress distribution close to the weld toe. Once R-NSIFs are determined, they might be included in local fatigue criteria for the structural strength assessment of welded joints based on NSIFs due to external cyclic loading. However, the numerical calculation of R-NSIFs through finite element (FE) simulations of the welding process requires extremely refined meshes to properly capture the residual stress singularity. In this context, the Peak Stress Method (PSM) has recently been adopted to estimate R-NSIFs due to residual stresses by means of coarse meshes of 2D 4-node plane or 3D 8-node brick elements. The aim of this work is to investigate the applicability of the PSM to estimate R-NSIFs in a butt-welded joint using coarse meshes of 3D 10-node tetra elements. The R-NSIF distribution at the weld toe line is estimated by applying the PSM to coarse meshes of 3D 10-node tetra elements, and the results are in agreement with those obtained using 3D 8-node brick elements. Accordingly, the PSM based on tetra elements further enhances the rapid estimation of R-NSIFs using coarse meshes and could be effective in analyzing complex 3D joint geometries.     

Shell-to-Beam Numerical Homogenization of 3D Thin-Walled Perforated Beams

Determining the geometric characteristics of even complex cross-sections of steel beams is not a major challenge nowadays. The problem arises when openings of various shapes and sizes appear at more or less regular intervals along the length of the beam. Such alternations cause the beam to have different stiffnesses along its length. It has different bending and shear stiffnesses at the opening point and in the full section. In this paper, we present a very convenient and easy-to-implement method of determining the equivalent stiffness of a beam with any cross-section (open or closed) and with any system of holes along its length. The presented method uses the principles of the finite element method (FEM), but does not require any formal analysis, i.e., solving the system of equations. All that is needed is a global stiffness matrix of the representative volumetric element (RVE) of the 3D representation of a beam modeled with shell finite elements. The proposed shell-to-beam homogenization procedure is based on the strain energy equivalence, and allows for precise and quick determination of all equivalent stiffnesses of a beam (flexural and shear). The results of the numerical homogenization procedure were compared with the existing analytical solution and experimental results of various sections. It has been shown that the results obtained are comparable with the reference results.     

Numerical Study of the Effects of Residual Stress on Fretting Fatigue Using XFEM

Residual compressive stress can improve fretting fatigue strength. In this paper, the effects of residual stress on fretting fatigue of Al 2024-T351 alloy specimens are studied using a numerical approach. The extended finite element method combined with the cyclic cohesive zone model is adopted to model fretting fatigue crack growth behavior. It is shown that residual stress changes the fretting fatigue crack growth path and enhances fretting fatigue life. Crack initiation angle, depth of knee point, crack initiation life, crack propagation life and total life are greater for specimens with residual stress compared to specimens without residual stress. The effects of residual stress are more remarkable for specimens with a high intensity of residual stress. However, the effects of residual stress reduce at a high bulk load level.     

Experimental Study of the Post-Fire Mechanical and Material Response of Cold-Worked Austenitic Stainless Steel Reinforcing Bar

This paper is concerned with the behaviour of stainless steel reinforcing bar following exposure to elevated temperatures from a fire, followed by subsequent cooling. Stainless steel-reinforced concrete is an increasingly popular solution for structural applications which require corrosion resistance, excellent mechanical properties, and long life cycles with little maintenance. In addition, although stainless steel reinforcement has a higher initial cost compared with traditional carbon steel bars, the overall life cycle costs are likely to be quite similar, owing to the lack of maintenance required for stainless steel materials. There is no information available in the literature on the post-fire properties of austenitic stainless steel reinforcement, although these data are essential for any engineer who wishes to study the structural integrity of a reinforced concrete component or system following a fire. Accordingly, this paper presents a detailed discussion and analysis from the results of a series of laboratory experiments on three grades of austenitic stainless steel reinforcement following various levels of temperature exposure and also different cooling rates. Both the mechanical and metallurgical properties are examined, and the behaviour is compared to that of B500B carbon steel reinforcement. It is shown that the stainless steel bars retained their mechanical properties under the majority of the scenarios examined and to a greater degree than traditional materials. This is important for the rehabilitation and salvage of existing reinforced concrete structures following a fire and also to avoid unnecessary demolition and replacement.     

Evaluation and Prediction of the Bending Behavior of Circular Hollow Steel Tube Sections Using Finite Element Analysis

Circular hollow steel tube columns are widely used in high-rise buildings and bridges due to their ductility and lower weight compared to reinforced concrete. The use of this type of steel section has several advantages over using reinforced concrete members. The present study investigates the bending behavior of steel circular hollow sections when subjected to bending loads. The variations in material characteristics with regard to position along the cross-section of a steel tube member is first considered in this experimental study, providing for a more accurate definition of the material behavior in the model. A supported beam tested by two-point loads is the loading type that is used to study the bending performance of steel tubes. Ten circular hollow beam specimens were prepared and tested up to and post the failure stage with the following dimensions: thickness (2, 3, and 6 mm), diameter (76.2, 101.6, and 219 mm), and span (1000, 1500, and 2000 mm). A finite element analysis has been conducted for these ten specimens using the ANSYS program. The finite element model is compared to experimentally obtained data to verify that both local and global behaviors are correctly considered. The load-deflection results of this analysis showed a good agreement with the experimental results. A parametric study also was performed that considered two variables, which were the effect of the presence of circular rings and the change of opening location in the length direction on the specimens’ behavior. This study showed that the presence of the circular rings in the specimen led to an increase in its ultimate strength (of 53.24%) compared with the non-presence of these rings. In contrast, the presence of openings at 30, 40, and 50% from the specimen length reduced the strength capacity by 8.76, 14.23, and 17.88%, respectively.     

Semi-Empirical Prediction of Turned Surface Residual Stress for Inconel 718 Grounded in Experiments and Finite Element Simulations

The surface residual stress after machining, especially for finishing, has a vital influence on the shape stability and fatigue life of components. The current study focuses on proposing an original empirical equation to predict turned surface residual stress for Inconel 718 material, taking tool parameters into consideration. The tool cutting-edge angle, rake angle, and inclination angle are introduced for the first time in the equation based on the Inconel 718 material turning experiments and finite element simulations. In this study, the reliability of simulation parameters’ setting is firstly calibrated by comparing the residual stresses and chips of the experiments and simulations. The changing trends of turned forces, temperatures of lathe tool nose, and surface residual stress with turning parameters are analyzed. Then, the predictive equation of surface residual stress is proposed considering relationships between the back-rake angle, the side-rake angle, and the tool cutting-edge angle, rake angle, and inclination angle. Moreover, the genetic algorithm optimizes the objective function to obtain the undetermined coefficients in the prediction equation. Finally, the predicted accuracy of the surface residual stress is proven by comparing the experimental, simulation, and prediction values. The results indicate that the predictive equation of surface residual stress has a good accuracy in predicting turned surface residual stress for Inconel 718 materials. The correlation coefficient, R, and absolute average error between the predicted and the simulated value are 0.9624 and 13.40%, respectively. In the range of cutting parameters studied and experimental errors, this study provides an accurate predictive equation of turned surface residual stress for Inconel 718 materials.     

Effects of Build Direction on the Mechanical Properties of a Martensitic Stainless Steel Fabricated by Selective Laser Melting

Mechanical properties and microstructure are investigated for a martensitic stainless steel (AISI 420) fabricated by selective laser melting (SLM) in three build directions. The tensile specimens built by SLM are classified into three groups. Group A is horizontally built in the thickness direction, Group B is horizontally built in the width direction, and Group C is vertically built in the length direction. The loading direction in tensile test is parallel to the build direction of Group C, but perpendicular to that of Groups A and B. Experimental results indicate build direction has significant effects on the residual stress, hardness, and tensile properties of SLM builds. Microstructural analyses indicate the as-fabricated SLM AISI 420 builds exhibit elongated cells and acicular structures which are composed of martensite and retained austenite phases growing along the build direction. Such anisotropy in the microstructure leads to anisotropic mechanical properties as Group C specimens (length direction) exhibit greater yield stress, ultimate tensile stress, and elongation than the specimens of Groups A (thickness direction) and B (width direction). The residual compressive stress in the gauge section also contributes to the superior tensile properties of Group C (length direction), as compared to Groups A (thickness direction) and B (width direction), which exhibit residual tensile stress in the gauge section.     

Investigation on the Residual Stress State of Drawn Tubes by Numerical Simulation and Neutron Diffraction Analysis

Cold drawing is widely applied in the industrial production of seamless tubes, employed for various mechanical applications. During pre-processing, deviations in tools and their adjustment lead to inhomogeneities in the geometry of the tubes and cause a gradient in residuals. In this paper a three dimensional finite element (3D-FE)-model is presented which was developed to calculate the change in wall thickness, eccentricity, ovality and residual macro-stress state of the tubes, produced by cold drawing. The model simulates the drawing process of tubes, drawn with and without a plug. For finite element modelling, the commercial software package Abaqus was used. To validate the model, neutron strain imaging measurements were performed on the strain imaging instrument SALSA at the Institute Laue Langevin (ILL, Grenoble, France) on a series of SF-copper tubes, drawn under controlled laboratory conditions, varying the drawing angle and the plug geometry. It can be stated that there is sufficient agreement between the finite element method (FEM)-calculation and the neutron stress determination.     

Effect of Cold Swaging on the Bulk Gradient Structure Formation and Mechanical Properties of a 316-Type Austenitic Stainless Steel

The present study aimed to discover the effect of cold swaging reduction on the bulk gradient structure formation and mechanical properties of a 316-type austenitic stainless steel. The initial rod was subjected to radial swaging until 20–95% reduction of initial rod diameter, at room temperature. According to finite element simulation, higher plastic strain was accumulated in the surface layer compared to the center region during swaging. Microstructural investigations revealed three-stage gradient structure formation in the center and edge regions of the deformed rod. Meanwhile, cold swaging resulted in the development of strong 111ǁBA, 001ǁBA, and weak 111ǁBA texture components in the center and edge, respectively. Significant tensile strengthening was observed after cold swaging. For instance, the yield strength (YS) increased from 820 MPa to 930 MPa after 40–80% reduction respectively, without the loss of ductility (δ–14%). This unique aspect of the mechanical behavior was attributed to the gradient structure of the cold swaged material and explained in detail.     

Full-Field Temperature Measurement of Stainless Steel Specimens Subjected to Uniaxial Tensile Loading at Various Strain Rates

This article presents a study on the effect of strain rate, specimen orientation, and plastic strain on the value and distribution of the temperature of dog-bone 1 mm-thick specimens during their deformation in uniaxial tensile tests. Full-field image correlation and infrared thermography techniques were used. A titanium-stabilised austenitic 321 stainless steel was used as test materials. The dog-bone specimens used for uniaxial tensile tests were cut along the sheet metal rolling direction and three strain rates were considered: 4 × 10⁻³ s⁻¹, 8 × 10⁻³ s⁻¹ and 16 × 10⁻³ s⁻¹. It was found that increasing the strain rate resulted in the intensification of heat generation. High-quality regression models (Ra > 0.9) developed for the austenitic 321 steel revealed that sample orientation does not play a significant role in the heat generation when the sample is plastically deformed. It was found that at the moment of formation of a necking at the highest strain rate, the maximum sample temperature increased more than four times compared to the initial temperature. A synergistic effect of the strain hardening exponent and yield stress revealed that heat is generated more rapidly towards small values of strain hardening exponent and yield stress.