Cold-rolled Austenitic Stainless Steel Research Articles

Evaluation of Permanent Strength of Cold-Rolled Austenitic Stainless Steel Sheet for Springs

Evaluation of Permanent Strength of Cold-Rolled Austenitic Stainless Steel Sheet for Springs

Effect of high current pulsed electron beam on surface microstructure and properties of cold-rolled austenitic stainless steel

High current pulsed electron beam (HCPEB) was used to modify the surface microstructure and performance of AISI 304 stainless steel with different cold-rolling deformation. The phase, microstructure, microhardness, and corrosion performance were characterized and analyzed after cold-rolling and HCPEB treatment. The results show that after cold-rolling, deformation-induced martensites (DMs) are generated in the deformation twin lamellae or at the intersections of multiple twin lamellae, resulting in a grid-like morphology, smaller size, and a certain orientation relationship with the parent austenite. The content of DMs increases with increasing cold-rolling deformation. After HCPEB treatment, many craters with different sizes and similar densities are produced on the sample surface, and the DMs are transformed into austenite again in the surface layer. The difference is that the grain size of the regenerated austenite after HCPEB is much smaller than that of the initial austenite (before cold-rolling). Such high-density equiaxed ultrafine grains (UFGs) of austenite formed after HCPEB present a twin feature, which is the inheritance of the orientation relationship between DMs and austenite formed during rolling. HCPEB treatment can effectively reshape the phase and microstructure of the surface layer of the cold-rolled deformed austenitic steel but reduce the surface hardness and increase pitting corrosion tendency.

Origin of deformation twins and their influence on hydrogen embrittlement in cold-rolled austenitic stainless steel

The effect of cold rolling on hydrogen embrittlement in stable 18Cr–1Mn–11Ni-0.15 N austenitic stainless steels was investigated. Alloy plates were cold-rolled to 15% or 30% reduction, then pre-charged with hydrogen and subjected to tensile testing with slow strain rate. Hydrogen-induced degradation of tensile elongation became increasingly severe with the increase in the degree of cold rolling. During cold rolling, deformation twins with various orientations were actively generated, and twins with specific orientations were vulnerable to hydrogen-induced cracking. Cold rolling also increased the density of defects, and thereby facilitated penetration of hydrogen into the steels. The combination of cracks generated at the twin boundaries, and the promoted hydrogen diffusion caused severe hydrogen embrittlement in the cold-rolled steels.

Modeling plastic anisotropy evolution of AISI 304 steel sheets by a polynomial yield function

In this study, a numerical model for the evolution of plastic anisotropy is investigated for the purpose of stamping method design by Finite Element (FE) analysis and proved experimentally via process simulations of a cold-rolled austenitic stainless steel (AISI 304) sheet. The plastic anisotropy of the sheets is described with a fourth-order homogenous polynomial yield function and this modelling approach is enhanced by plastic strain dependent material coefficients. Tensile tests of coupon specimens taken along the different directions from rolling direction, and flow strength and deformation anisotropies are described with the planar variations of yield stress and plastic strain ratio computed at four plastic strain levels (0.002, 0.02, 0.05 and 0.18). A new numerical approach is, then, applied to identify polynomial coefficients ensuring an orthotropic positive-definite, convex yield surface with a well-defined stress gradient at every loading point on plane stress subspace. The developed computational model is implemented into general purpose explicit FE analysis software Ls-Dyna by a user-defined material model subroutine (UMAT) and applied in the stamping simulation of AISI 304 steel rectangular cups for the house-hold applications. The computed thickness distributions and the flange geometries were compared with measurements and it was observed that the best predictions were done with material parameters at %5 plastic strain level.

Suppressing precipitation during the reverse transformation from martensite to austenite in a cold-rolled austenite stainless steel

The precipitation behavior and its influence on the tensile properties of cold-rolled austenitic stainless steel, isothermally aged at 750 °C with and without a pulsed electric field, was investigated. During reverse transformation annealing, a large amount of coarse precipitate forms on the grain boundary and grain interior. However, when a pulsed electric field is applied in reverse transformation annealing, only a small amount of precipitate forms in the stainless steel; moreover, the suppressed precipitation effect did not significantly change with an increased annealing time. Simultaneously, the heat treatment comparison test at the almost same temperature clarifies that the pulsed electric field plays a critical role in inhibiting precipitate formation, rather than temperature. The corresponding mechanism reveals that the pulsed electric field disrupts the diffusion of solute Cr and Mo to the precipitate interface, which restricts precipitation growth. Furthermore, the growing precipitate is dissolved to reduce the difference in electrical resistance between the matrix and precipitate. Therefore, the precipitation behavior is strangled in the early stages with the pulsed electric field treatment, and the ductility of the cold-rolled stainless steel is further improved. The pulsed electric field was explored as a novel pathway to suppress precipitation behavior, and the results confirmed that the pulsed electric field was not only effective in impeding the precipitate growth of austenitic stainless steel, but could also be a useful technique to inhibit deleterious precipitation in other metallic materials.

Effect of Cryorolling–Annealing and Bake-Hardening on the Microstructure and Mechanical Properties of AISI 301LN Grade Austenitic Stainless Steel

Steel is a valuable material for automotive and structural applications. Due to global warming, vehicle manufacturers are trying to lower the weight of the automobiles without hampering the safety. Austenitic stainless steel has recently gained attention for its superior strength and ductility along with lower maintenance costs. AISI 301LN grade austenitic stainless steel produces a high percentage of strain-induced martensite on cold rolling than other grades of austenitic stainless steel, like AISI 304, AISI 316, and AISI 301, etc. After annealing at elevated temperatures, cold-rolled austenitic stainless steel produces ultra-fine grain structures (~ 0.2–0.5 µm) which improve the yield strength due to the Hall–Petch effect. In the present work, cold rolling was performed at liquid nitrogen (~ 196 °C) temperature, followed by annealing at 700 °C for 20 min ,and finally bake-hardened at 170 °C for 2 h, exhibiting nearly 1000 MPa yield strength, ~ 1200 MPa ultimate tensile strength, and ~ 26% ductility. These demonstrate better mechanical properties (~ 20% improvement in tensile strength) than previously obtained results. The micro-structures and texture of the newly developed advanced steel were examined by TEM, EBSD, and XRD characterization techniques.

Predictions of material properties in cold-rolled austenitic stainless steel tubular sections

Material properties in cold-rolled stainless steel members are significantly different from those in annealed sheets (e.g. considerable enhancement in 0.2% proof stress), which provides a potential to reduce the initial cost of stainless steel structures in the design stage. This study is aimed at proposing reasonable predictive formulas for the enhanced material properties in cold-rolled stainless steel tubular sections. A total of 130 tensile coupons were extracted from three types of material statuses (i.e. the coil, the circular hollow section and the square/rectangular hollow section) occurred in four cold-rolling production routes. Tensile coupon tests were then conducted to illustrate the change of material properties from coils to square/rectangular hollow sections (SHS/RHSs) as well as the detailed distributions of material properties in cross-sections. Based on test data, predictive formulas were developed for all material parameters in cold-rolled stainless steel hollow sections according to the “through thickness averaged plastic strain” method originally proposed by Gardner's group in Imperial College London. The predictions of the proposed formulas were evaluated using the test results in this study and in existing literature. Comparisons indicate that proposed models provide not only accurate predictions for the key material properties (e.g. the 0.2% proof stress, the 1% proof stress and the ultimate tensile strength), but also reasonable predictions for full-range stress-strain curves in cold-rolled stainless steel hollow sections. The predictions of the proposed models can be used in finite element analysis and advanced design methods for cold-rolled stainless steel hollow sections.

The significant role of heating rate on reverse transformation and coordinated straining behavior in a cold-rolled austenitic stainless steel

The effect of heating rate on the mechanism of reverse transformation from martensite to austenite during continuous heating process was studied in a cold-rolled 304 stainless steel, and the corresponding mechanical properties were analyzed. It indicated that the reverse transformation occurred diffusionally at slow heating rate range (< 10 °C/s). However, rapid heating rates (> 40 °C/s) resulted the reverse transformation to occur via martensitic shear-type. At medium heating rates (10–40 °C/s), the reversion mechanism gradually changed from diffusional to shear-type. The diffusional reversed austenite was characterized by equiaxed, defect-free, nano/ultrafine grains with grain size of 100–500 nm, while the martensitic shear-type reversed austenite exhibited lath-type/banded grains with high defects density. At 100 °C/s, an excellent combination of high-strength and high-ductility with significant work hardening ability was obtained. Moreover, the product of strength and ductility was enhanced over three times from 11.14 GPa (at 2 °C/s) to 35.78 GPa (at 100 °C/s). The strain gradient due to the heterostructure, the glide of preexisting mobile dislocations and the controlled release from TRIP effect contributed to the improved mechanical properties. The study also indicated that reversion mechanism had no effect on austenite texture.

Experimental study on residual stresses in cold rolled austenitic stainless steel hollow sections

This paper focuses on the residual stresses in cold rolled austenitic stainless steel hollow sections. A series of residual stresses tests were carried out. Eight cross sections were tested, including four rectangular hollow sections and four circular hollow sections. The fabrication route of the test specimens was measured and introduced. Sectioning method modified with T rosettes strain gauges was used to reveal the residual stresses, including the longitudinal and transverse bending and membrane residual stresses. Totally, residual stresses of 160 points were recorded. Plane stress condition assumption was used in processing the residual strains. Test results indicate that the dominated residual stress was the longitudinal bending residual stress for RHS, and longitudinal and transverse bending residual stress for CHS. The magnitudes of all the other types of residual stress were less than 50 MPa. Residual stress distribution models were proposed based on the test data, and compared with test data available in literatures. Comparisons show the residual stresses in cold formed section highly depend on manufacturing processes and material types.

Inhomogeneity of plastic deformation in austenitic stainless steel after surface mechanical attrition treatment

Inhomogeneity of microstructure evolution in cold-rolled austenitic stainless steel after surface mechanical attrition treatment (SMAT) was investigated. A characteristic deformation pattern was obtained for all studied specimens. Selected areas were examined through X-ray diffraction (XRD) and scanning electron microscopy (SEM). Estimation of the α′-martensite volume fraction below the treated surfaces showed, that some of the studied areas are characterised by significant different amount of this phase (from 10 to 22% - BB, from 19 to 31% - GIXD). The performed finite element (FE) numerical analysis showed, that the reason for this may be the presence of an air gap between the impacted material and fixation and also relatively short high stress time duration due to surface inclination during the surface treatment. Annealing at 550 and 650°C greatly increased the volume fraction of α′-martensite (up to 47% - BB) and formed Fe2O3 as well as Fe3O4, whereas annealing at 700°C resulted in both disappearance of α′-martensite and in reduction of oxides.

Study on Fatigue Crack Propagation Behavior in Corrosion Environment of a Cold-Rolled Austenitic Stainless Steel

Fatigue crack growth testing and measurement of fracture toughness with single edge notch tension (SENT) specimens of American Iron and Steel Institute (AISI) 301L-MT stainless steel were performed in the air, distilled water, and 3.5% and 9% NaCl solutions. Type 301L-MT steel exhibited the same resistance to corrosion for various environments in near-threshold region. The crack growth in the Paris region could be accelerated in a [Cl−] containing environment by decreasing the threshold of stress corrosion KISCC, but independent of [Cl−] concentration. When specimens were fatigue precracked in different conditions, it was found that the fracture toughness of SENT specimens in the aqueous environment decreased 23% to 30% compared with that in the air. The normalized formula was established to describe the behavior of fatigue crack propagation of type 301L-MT steel. Three parameters, ΔKthcf, β, and JIQ, were used to clarify the effect of H-embrittlement mechanism on fatigue crack propagation.

単純強圧延オーステナイト系ステンレス鋼中のヘテロナノ組織が集合組織と延性に及ぼす影響

単純強圧延オーステナイト系ステンレス鋼中のヘテロナノ組織が集合組織と延性に及ぼす影響

Microstructure and Property Evolution for Hot-Rolled and Cold-Rolled Austenitic Stainless Steel 316L

Grain boundary character distribution plays an important role in determining the functional and mechanical properties of polycrystalline materials. The aim of this work was to achieve improved coincident site lattice (CSL) fraction without increasing low angle grain boundary (LAGB) proportion. We utilized single-step thermo-mechanical processing route involving rolling followed by short heat treatment and compared the effect of rolling temperature. Our results indicated that rolling at elevated temperature led to significant increase in the fraction of special boundaries while keeping the fraction of LAGB very low, as desired. We conducted thermal stability of our sample-conditions at elevated temperatures for various lengths of time and found the microstructure of the samples to be stable up to 1000 °C. This study showed that even commercially suitable process (single step processing with short heat treatment duration) could lead to microstructure with considerable increase in CSL boundaries fraction, improved hardness values and good thermal stability.

Large-strain Bauschinger effect in austenitic stainless steel sheet

Large-strain Bauschinger effect in cold-rolled austenitic stainless steel sheet is investigated after large amounts of prestrain. The material is prestrained in uniaxial tension, and the tensile properties of the prestrained material are measured in different angles with respect to the prestraining direction. By comparing the differences in the yield stresses in different orientations, the effect of prestraining on material anisotropy is studied. The method is applied to AISI 304-type stainless steel sheet. The test results are analyzed using a combined isotropic–kinematic hardening model. The results indicate that this kind of material shows a considerable Bauschinger effect. Transient and permanent softening is observed in the experiments. The experimental Young's modulus also seems to decrease with prestrain.

Investigation of experimental conditions in magnetic adaptive testing

Influence of the magnetizing and pick-up conditions on the optimum magnetic descriptors of magnetic adaptive testing (MAT) was studied by measuring a series of cold-rolled austenitic stainless steel samples. It was found that the way of magnetization and of picking-up the signal had no qualitative influence on relative values of the descriptors.

Fine-grained Structures Developed along Grain Boundaries in a Cold-rolled Austenitic Stainless Steel.

Fine-grained structures developed by cold-rolling in a 310S austenitic stainless steel have been investigated by using transmission electron microscopy (TEM). Particular emphasis is laid on the role of initial grain boundaries in the transmutation process of twin-matrix (T-M) lamellae into fine-grained structures. Microstructural evolution in a cold-rolled 310S steel is characterized by the development of dense T-M lamellae and their following collapse caused by shear band formation: shear bands destroy the T-M lamellae, and a fine-grained structure develops with increasing the area of shear bands. In the process of this grain refinement, such fine-grained structures have been found not only in shear bands but also in the area along initial grain boundaries. The fine-grained structure preferentially appeared around the intersections of grain boundaries with shear bands, and it expanded along the grain boundaries which were almost parallel to the rolling direction. In selected area diffraction patterns obtained from these fine-grained areas, large scatterings of crystal orientations were observed. On the basis of these results, the role of initial grain boundaries on the grain refinement is discussed.

Micro Shear Bands in Cold-rolled Austenitic Stainless Steel

Shear bands (SBs) in cold-rolled austenitic stainless steel 310S have been investigated by using transmission electron microscopy (TEM). Shear Bands appear in the region of twin-matrix lamellae which have already developed at moderate strain. Inside the shear band, fine grains are observed, and the shape of each grain is somewhat elongated along the shear direction. These features found in 310S are similar to those observed in materials with very low stacking fault energy such as α-brass. Selected area diffraction patterns obtained from a SB reveals that local orientations inside the SB are distributed between those in the matrix and the twinning region. Using dark field images of the SB, it is shown that some of the misorientations of neighboring grains in the SB reach nearly 20 degrees. Based on these observations, the formation mechanism of SBs is discussed.

X-Ray Stress Measurement of Materials Having Nonlinear sin2 .PSI. Diagram.

An X-ray diffraction technique for measurement of stress of materials having a nonlinear sin2 ψ diagram was proposed. A specimen was prepared from a cold-rolled austenitic stainless steel JIS type SUS304. Various stresses σa were applied to the specimen and peak positions of diffraction lines were measured at seven ψ angles. Peak positions oscillated with sin2 ψ. A straight line was fitted to oscillatory peak positions in the sin2 ψ diagram for each applied stress σa. The straight lines for various applied stresses intersect at a point in the sin2 ψ diagram. The slope M and the intercept N of the straight lines vary in proportion to an applied stress σa. The stress constants K and L can be determined experimentally as the reciprocals of the slope of the strtaight lines of M and N versus σa, respectively. The stress and the stress constants can also be determined from a straight line connecting two end points in the sin2 ψ diagram.