Ferritic Stainless Steel Sheets Research Articles

A Machine Learning Approach for Modelling Cold-Rolling Curves for Various Stainless Steels.

Stainless steel is a cold-work-hardened material. The degree and mechanism of hardening depend on the grade and family of the steel. This characteristic has a direct effect on the mechanical behaviour of stainless steel when it is cold-formed. Since cold rolling is one of the most widespread processes for manufacturing flat stainless steel products, the prediction of their strain-hardening mechanical properties is of great importance to materials engineering. This work uses artificial neural networks (ANNs) to forecast the mechanical properties of the stainless steel as a function of the chemical composition and the applied cold thickness reduction. Multiple linear regression (MLR) is also used as a benchmark model. To achieve this, both traditional and new-generation austenitic, ferritic, and duplex stainless steel sheets are cold-rolled at a laboratory scale with different thickness reductions after the industrial intermediate annealing stage. Subsequently, the mechanical properties of the cold-rolled sheets are determined by tensile tests, and the experimental cold-rolling curves are drawn based on those results. A database is created from these curves to generate a model applying machine learning techniques to predict the values of the tensile strength (Rm), yield strength (Rp), hardness (H), and elongation (A) based on the chemical composition and the applied cold thickness reduction. These models can be used as supporting tools for designing and developing new stainless steel grades and/or adjusting cold-forming processes.

In-situ EBSD observation and simulation of free surface roughening and ductile failure in the ultra-thin ferritic stainless steel sheet

Surface roughening is one of the most fundamental issues which greatly affects the plastic deformation and ductile fracture behavior of ultra-thin sheet materials. The present study investigated the influence of free surface roughening on the micromechanical failure in the ultra-thin ferritic stainless steel (FSS) sheet for bipolar plate in proton exchange membrane (PEM) fuel cell under in-situ tensile deformation. The microstructural evolution during uniaxial tension was monitored using in-situ electron backscatter diffraction (EBSD) technique. Then the initial microstructure was directly mapped onto the finite element mesh of representative volume element (RVE) in the crystal plasticity finite element (CPFE) simulation. The CPFE model was incorporated with the microscopic ductile fracture criterion in terms of plastic energy dissipation in order to simulate the initiation of ductile fracture caused by the free surface roughening. The in-situ EBSD observations reveal that the microstructure of the ultra-thin FSS sheet exhibits heterogeneous plastic deformation and strain localization on the surface at the large deformation owing to the considerable free surface roughening. The CPFE simulations well reproduce the stress/strain heterogeneity and the local hotspots of the ductile fracture initiation. It reveals that the increase of surface roughness with the deformation results in the increased inhomogeneity of thickness which accelerates the strain localization, consequently the premature necking and failure in the ultra-thin steel sheet. Due to the free surface roughening which can facilitate strain localization, the initiation of ductile fracture is obviously observed in both the ridges and valleys in the free surface and its propagation along the pronounced localization band through the thickness, leading to material failure. The good correlation of free surface roughening to the onset of plastic instability and ductile fracture further advances the insight into the micromechanical failure during plastic deformation of the ultra-thin FSS sheet for bipolar plate in PEM fuel cell.

Influence of Different Contact Conditions on Friction Properties of AISI 430 Steel Sheet with Deep Drawing Quality

This article aims to investigate the influence of different contact conditions on the friction properties of an Nb-stabilized AISI 430 ferritic stainless-steel sheet with deep drawing quality. Three tribological tests were performed: pin-on-disk, bending under tension, and strip-tension test. Moreover, counter samples of a hard metal (WC-12%Co) with surface finishes of 0.27 and 0.54 μm were used in the friction tests under dry and lubricated conditions. The influence of the texture and relative elongation of the strip on formability also were investigated. A comparative analysis of the results revealed that the coefficients of friction, wear, lubricant efficiency, and hardness measured below the wear surface indicated a strong dependence on surface roughness and the friction test type. The coefficients of friction obtained from the pin-on-disk test were higher than those obtained from the formability tests. In addition, the coefficient of friction increased with increasing relative elongation during the formability tests; it was higher in the bending-under-tension test than in the strip-tension test, mainly owing to the increasing strip surface roughness. The contact pressure during the formability tests was non-uniform during strip sliding under the tool. According to our results, for each friction condition in a specific area of the forming die, there is a value of the coefficient of friction, depending on the kinematic conditions. Therefore, the results can be used as input data to define design guidelines, improve productivity, and improve product quality from this steel sheet.

Microstructural and Statistical Investigation on Dissimilar Friction Stir Spot Welding of Austenitic and Ferritic Stainless-Steels

The increasing use of austenitic and ferritic stainless steel sheets, especially in the power plant industry, highlights the importance of research into the mechanical and metallurgical properties of these joints. Friction stir spot welding of these sheets was investigated using full-factorial experimental design. Considering the three main parameters of rotational speed, plunge depth and dwell time, each of which had three levels, led to 27 tests. The highest strength (6400 N) was related to sample number 8 with parameters 900 rpm, 2.7 mm and 20 s and this sample was selected for microstructural studies. The metallurgical properties of this sample were investigated and the microstructures of heat affected zone, thermos-mechanically affected zone and stir zone were extracted. Thermomechanical changes during the process and especially dynamic recrystallization in the stir zone due to heat application and severe plastic deformation by the process reduce the grain size, which in turn increases the bond strength. Using analysis of variance, it was found the direct effect of rotational speed and plunge depth on the lap shear strength is significant. The interaction effect of rotational speed and plunge depth and also, rotational speed and dwell time on the response were significant.

Microstructure Evolution and Mechanical Properties of AISI 430 Ferritic Stainless Steel Strengthened Through Laser Carburization

Abstract Carburization assisted by laser processing is a promising method to strengthen metallic materials. Direct laser beam carburization is implemented for the first time on thin AISI 430 ferritic stainless steel (FSS) sheets with graphite coating under different conditions. Microstructural morphology, phase constitution, carbon content, microhardness, and tensile behavior are investigated to evaluate the laser carburization effect. The carburized zone presents different morphologies according to the linear energy density of the laser beam. The least carbon content is around 0.4 wt% in the carburized zone where austenite becomes the leading phase. Delta ferrite is found in a cellular carburized area, which resembles a duplex microstructure. The hardness of carburized zone has been at least increased by 130%, the yield strength and ultimate tensile strength of a fully carburized sample can be increased by respectively 90% and 85%. This hardening effect is driven by the precipitation of carbides formed during solidification offering pinning points for dislocations and grain boundaries. These improvements could be useful to modify locally ferritic stainless steel to meet industrial needs such as wear-resistant surfaces.

Crystal plasticity finite element analysis of size effect on the formability of ultra-thin ferritic stainless steel sheet for fuel cell bipolar plate

The present study investigated the size effect on the formability of ultra-thin metallic bipolar plate for proton exchange membrane (PEM) fuel cell by crystal plasticity finite element method (CPFEM) in conjunction with the Marciniak-Kuczynski (MK) model. Mechanical behavior and crystallographic texture of a 0.08 mm-thick ferritic stainless steel (FSS) sheet were characterized by uniaxial tensile test and electron backscatter diffraction (EBSD) measurement. Nakazima formability test was carried out to measure the forming limit diagram (FLD) of ultra-thin FSS sheet. A hybrid cellular automata–Monte Carlo (CA–MC) model was developed to generate a ‘realistic’ representative volume element (RVE) that accurately reconstructed the measured texture and grain boundary misorientation distribution (GBMD). The predicted FLD by the CPFEM–MK model with the realistic RVE shows a good agreement with the experimental results. In order to explore the size effect on the formability, the forming limit analyses were performed using RVEs with various thickness-to-grain size ratios (t/d = 2∼10). The results reveal a significant degradation of the formability of the ultra-thin FSS sheet as t/d decreases. With decreasing number of grains through the thickness, the stress and strain heterogeneities in the surface grains are noticeably increased due to the less constraint by the subsurface grains, which plays an important role in the size effect. Furthermore, it is found that the surface strain hot spots in the material with low t/d can act as the geometrical imperfection to accelerate the failure, together with the increased stress triaxiality in the surface grains as the results of the localized strains and premature necking during the deformation. This attributes the decrease of forming limit strains to the early plastic flow instability in the ultra-thin sheet material with low thickness-to-grain size ratio.

Effect of Multi-Objective Optimization for Double Pulse Resistance Spot Welded Joints of Ferritic Stainless Steel

In this research work, an attempt is made to optimize the parameters of double pulse resistance spot welding process of AISI 409M ferritic stainless steel sheets by multi objective Taguchi approach. The tensile shear strength of the weld joint and indentation at the weld surface are taken as the response quality characteristics, which are required to be maximized and minimized respectively. Three control parameters, such as second pulse current, second pulse weld time and cooling time were taken for the analysis. Furthermore, a linear response surface model has been developed to correlate tensile shear strength and indentation values with process parameters. The results showed that, the second pulse current value was found to be the most influential parameter affecting tensile shear strength and indentation in double pulse resistance spot welding. Results of the optimization process were also validated by confirmation test.

Effects of UIT on Microstructure and Properties of 441 Ferritic Stainless Steel Laser Welded Joint

Effects of UIT (Ultrasonic impact treatment) on the microstructure and properties of laser welded 1.5mm 441 ferritic stainless steel sheets were studied. Results indicate that the surface of the butt joint generated an ultrasonically induced strengthening layer after UIT. The average microhardness of the layer was increased by 18.1%. The tensile strength of the specimens was increased. Moreover, UIT can decrease the residual stress of welded specimens, the maximum longitudinal tensile stress in the central area was decreased by 78.4%, and the transverse residual stress was converted into compressive stress. The cupping value was increased from 34.5 mm to 37.0 mm following UIT. In addition, the fracture position was changed from the central weld to the edge weld of the cupping hemisphere. The mechanical properties were increased due to the generation of strengthening layers and the decrease of residual stress by UIT.

Effective stress and fatigue life prediction with mean stress correction models on a ferritic stainless steel sheet

The deformation behavior and fatigue strengths under plane bending loads were investigated using standard sheet specimens, and the effective stress model was proposed. When the experimental data were fitted to the proposed and Walker model, the relationship between the fully-reversed equivalent stress amplitude and failure cycles was according to the regression line of the linear-logarithmic formula, and the coefficient of determination was 0.9707. The failure cycles by the observation were in good agreement with those by the prediction using the proposed model. The Goodman and other conventional relations had insufficient goodness of fit.

Cold-formed stainless steel RHS members undergoing combined bending and web crippling: Testing, modelling and design

The behaviour and resistances of cold-formed stainless steel rectangular hollow section (RHS) members undergoing combined bending and web crippling were studied based upon experimental and numerical investigations. A test program consists of 4 pure bending tests, 7 pure web crippling tests and 23 web crippling-bending interaction tests was conducted. The RHS specimens were cold-rolled from lean-duplex and ferritic stainless steel sheets. Numerical models were built and validated against the test results. Upon validation, parametric studies comprised of 312 finite element analyses were undertaken. The obtained test and numerical results were compared with nominal resistances predicted from the American, Australian/New Zealand and European design standards for stainless steel structures. Moreover, the provisions in the North American Specification for cold-formed steel members were also evaluated. The comparison results indicate that the codified design provisions are generally safe to use for design of cold-formed stainless steel RHS members undergoing combined bending and web crippling, among which the European provision yields overly-conservative predictions. The codified web crippling-bending interaction curves can be applied for designing the stainless steel RHS members undergoing combined bending and web crippling, whilst improved predictions could be achieved by employing recently proposed bending and web crippling design rules.

Laser treatment of 430 ferritic stainless steel for enhanced mechanical properties

Thin AISI 430 ferritic stainless steel sheets containing 18% Cr were hardened by continuous-wave laser beams under different conditions. A symmetric double laser treatment platform was developed to operate two lasers simultaneously in order to achieve a homogenous microstructure through the specimen and avoid the overheating associated with single laser treatments on one surface. The influence of linear energy density and shielding gas on microstructure and hardness is investigated for both single and double laser treatments. Tensile behavior and oxidation effect during treatment are also studied. With a peak temperature around Tm, the single-track laser yields local microhardness that is 90% higher than the base material, while samples completely treated by the multi-track laser present a 60% increase in yield stress and 45.8% in ultimate tensile stress. These improvements in mechanical behaviors are driven by the formation of martensite and chromium carbides upon rapid cooling despite poor carbon content and dramatic grain coarsening. These results could be useful in order to locally modify the behavior of medium chrome ferritic stainless steel to meet industrial requirements.

Effect of added stabilizing elements on thermal activation process of plastic deformation in 18Cr ferritic stainless steel

The effects of a trace amount (~0.1 at.%) of added stabilizing elements (Nb, Ti, and Zr) on the tensile properties of 18Cr ferritic stainless steel sheets at strain rates ranging from 10−3–102 s−1 (from ordinary quasi-static deformation to dynamic deformation) were investigated. The additional Ti or Zr elements formed coarse carbide or nitride precipitates, which consumed interstitial solute elements (C and/or N) in the ferrite matrix. In the Nb-added steel, however, only fine NbN precipitates with a few hundred nanometers in size were observed locally, implying a small amount of solute Nb in the ferrite matrix. Ti or Zr additions had only a slight effect on the deformation resistance of 18Cr ferritic steel sheets and strain rate sensitivity (m) of the flow stress, however, the Nb addition increased the deformation resistance and significantly reduced the strain rate sensitivity. The measured activation volume (v*) values of the Ti- and Zr-added steels (62 b3 and 66 b3) were almost equivalent to one of the 18Cr base steel (64 b3). This result suggested that a general thermally activated process for screw dislocation motion is controlled by double-kink nucleation, which is slightly affected by Ti or Zr additions. The high v* value (106 b3) of the Nb-added steel indicated that the thermally activated process of plastic deformation would be associated with the substitutional solute Nb atoms (act as short-range obstacles) interacting with the moving dislocations. These results showed that the Nb element played a distinct role in plastic deformation from the other stabilizing elements of Ti and Zr.

Analysis of the Visual Appearance of AISI 430 Ferritic Stainless Steel Flat Sheets Manufactured by Cool Rolling and Bright Annealing

This article reports on the relation between the surface topography and the optical reflectance, both total and diffuse, of different samples of AISI 430 ferritic stainless steel. Gaussian filters with different cutoff wavelengths were applied to the height maps of the surface topography of the samples, to separate the different scales of surface roughness involved in optical scattering in the visible range of the spectrum. Significant anisotropy, related to the rolling process, was found in the topography. An effective roughness slope parameter was defined from the dependence of the ratio between the root mean square height and the autocorrelation length on the cutoff wavelength. This roughness slope demonstrated an exceptionally good linear relationship with CIE 1931 luminance, which was calculated from the diffuse reflection spectra. The color uniformity of the samples was analyzed based on their CIE L*a*b* coordinates under daylight and LED illumination. The results confirmed the strong influence of manufacturing process on the surface characteristics of AISI 430 ferritic stainless steel sheet products with a bright finish.

An Associated and Nonassociated Flow Rule Comparison for AISI 439-430TI Forming: Modeling and Experimental Analysis

Abstract The plastic anisotropy behavior of ferritic stainless steel (FSS) sheets was analyzed and modeled under associated and nonassociated flow rule approaches. Three orthotropic flow functions, known as quadratic Hill48 and nonquadratic (Yld2000-2d and BBC2005), were developed and employed under an associated and nonassociated flow rule hypothesis. For the NAFR based on the initial anisotropy, the mechanical behavior was described by the nonexponential model functions of Yld2000-2d and BBC2005 to predict the directional dependence of mechanical parameters. It provided a considerable advantage in terms of flexibility and good agreement with the experiment. According to the results, the polynomial fit functions of the transverse versus longitudinal true plastic strain curve were used to describe the designated properties corresponding to a selected level of strain. To describe the evolution of anisotropic hardening and potential plastic hardening, seven different loading conditions were considered. The proposed evolutionary non-AFR Yld2000-2d and BBC2005 criteria showed good accuracy in predicting the evolution of hardening yield and Lankford coefficients depending on the plastic deformation.

Quantification of the Severity of Ridging in Ferritic Stainless Steel Sheets Using a Profilometric Technique

A new method to quantify the ridging phenomenon in ferritic stainless steels has been developed based on the evaluation of surface profiles after the tensile elongation of 100 mm wide sheet specimens. The ridging components of the surface profiles are extracted by a tailored spline filtering procedure. A ridging index is proposed to quantify the severity of the surface defect based on surface profile height and spacing parameters. The procedure is independent of the type of profilometer used as long as unfiltered raw profiles can be recorded. The reproducibility of the measurement method and its correlation with the visual assessment of strained specimens is discussed.

Crystal plasticity finite element–Marciniak-Kuczynski approach with surface roughening effect in predicting formability of ultra-thin ferritic stainless steel sheets

Forming Limit Diagrams (FLDs) of 0.1 and 1 mm thick ultra-thin ferritic stainless steel (FSS) sheets are predicted using a combination of the crystal plasticity finite element (CPFE) approach and the Marciniak-Kuczynski (M-K) model. In a scale of few microns, especially for thin materials, surface roughness significantly affects sheet formability; therefore, the M-K model is further modified to address the surface roughening effect. The CPFE combined M-K model with the surface roughening effect can reproduce experimental FLDs of two FSS sheets, unlike the method without considering the surface roughening effect. The results highlight the importance of surface roughening, particularly in thinner sheets, where the portion of surface roughness is an order higher than that in the thicker sheets. The evolution of the deformation texture under various loading paths is further investigated and discussed. The findings coincide with reports of previous studies.

On the Intergranular Corrosion Properties of Thin Ferritic Stainless Steel Sheets Welded by Fiber-Laser

In the present investigation, thin sheets of stabilized and unstabilized ferritic stainless steel were welded in butt joint configuration using irradiation of a 1070 nm fiber-laser. Using optical microscopy, the microstructural evolution upon alternating heat input was characterized. In addition to that, hardness and tensile tests were carried out on the specimens. Detailed focus was given to the intergranular corrosion properties, which were investigated on basis of the Strauss test with different times of exposure to the corrosive environment. Following these tests, the mechanical properties of the joints were characterized using tensile tests. A combination of the latter with an inspection by μ-CT analysis allows for the proposition of an intergranular corrosion rate with regard to the degradation of the joint strength.

Crystal Plasticity Analysis of the Relation between Micro‐Texture and Surface Ridging for a 21%Cr Ferritic Stainless Steel

The micro‐texture of ferritic stainless steel generated by rolling and annealing processes is supposed to behave plastic anisotropy and heterogeneous strain response during forming, especially for the high Cr content ferritic stainless steel with single ferrite phase. The relation between crystallographic orientations and their strain responses during loading are analyzed for a 21% Cr ferritic stainless steel sheet. The crystallographic orientation distributions in surface and center layers as well as in cross section are characterized by an electron backscatter diffraction (EBSD) method. It indicates that texture gradients along thickness with center layer having a higher density of <111>//ND texture are formed. Some grain clusters with similar oriented grains gathering together to be parallel to the rolling direction are observed. Then, the experimental orientation distributions are condensed and incorporated into a developed crystal plasticity finite‐element model (CPFEM), in which the multiple slip systems and the interaction between neighbor grains are considered. The simulation results confirm that the heterogeneous distributions of normal strain and shear strain preserved for the different crystal orientation grains result in the surface ridging. The texture gradients can help to decrease the tendency of surface ridging. The simulation results show a reasonable agreement with the tensile test results.

Effects of cold rolling and annealing on the ridging behaviour of ferritic stainless steel

A systematic study was carried out to evaluate the effects of cold rolling and annealing processes on the ridging severity of ferritic stainless steel (FSS). Both microstructural refinement and texture optimisation were achieved with an optimised processing schedule to improve the ridging resistance of FSS. Coarse grain bands comprising of primarily {112} oriented grains were found to form inside cold-rolled and annealed FSS sheets. The effects of annealing schedules and the resultant microstructure and texture on the ridging behaviour of FSS are discussed. The results indicate that the reduction in the fraction of {001} orientation by optimisation of the rolling processes contributes to reduce the ridging height of the FSS during the subsequent forming process. Through extending soaking time and under 880 °C during final annealing step, a maximum of 30% reduction in ridging height can be achieved with a refined microstructure and texture of FSS, improving the surface quality of FSS during subsequent forming processes.

Process parameter optimization and performance comparison of AISI 430 and AISI 1018 in resistance spot welding process

Resistance spot welding (RSW) process plays an important role in the sheet metal fabrication process, in which metal joining is obtained by heat developed through electric resistance against the current flow in the metals contact area. In this work, the impact of weld time, squeeze time, weld current and hold time on weld strength of 1.2 mm thickness ferritic stainless steel (AISI 430) and low carbon mild steel (AISI 1018) sheets by using RSW process were investigated and compared. The Design of experiment (DoE) was used to formulate the number of experiments with variations in input conditions. Welding of similar metals of AISI 430 and AISI 1018 was done according to the number of experiments obtained using DoE. The welded samples were subjected to tensile shear failure load and micro-hardness testing. The output responses were analyzed through Response surface methodology (RSM) to find the optimum control parameters to achieve higher weld strength.