Austenitic-ferritic Stainless Steel Research Articles

Microstructure Evolution and Mechanical Properties of Ferrite–Austenite Stainless Steel Bimetals Fabricated via Wire Arc Additive Manufacturing

A bimetallic structure is implemented by wire arc additive manufacturing from 316L austenitic stainless steel to 430 ferritic stainless steel. Microstructure, interface characteristics, and tensile properties of 316L‐430 bimetals are investigated. In 316L/430 bimetals, defects like porosity and cracks are not present. The compositional distribution and microstructure evolution of 316L‐430 bimetallic deposition are significantly impacted by the deposition sequence and heat input. As heat input increases, microstructure evolution of 430 deposited layer is single F → F+ diffusely distributed (Cr, Fe)23C6 → F+ grain internal (Cr, Fe)23C6 → F + M, and 316L deposited layer presents typical austenite + skeletal δ ferrite. Microstructure of 430 deposited layers presents heterogeneous, grain coarsening, and martensite along grain boundaries, resulting in high‐temperature embrittlement. The highest hardness of 316L‐430 bimetals appears on 430 sides near the interface. The tensile strength of 430/316L bimetals is greater than that of 316L/430, and the tensile specimens exhibit brittle fracture.

An experimental investigation on predicting the fracture toughness distributions in ferritic-austenitic stainless steel dissimilar metal welds from spherical indentation tests

This study provides an experimental investigation on predicting the fracture toughness distributions in dissimilar metal welds (DMWs) connecting the nozzle of reactor pressure vessel (made of low-alloy steel) and the safety end of pipes (made of stainless steel). Indentation energy to fracture model with three criteria (namely the critical stress, critical strain, and critical damage variable, CDV) and the energy release rate (ERR) model were introduced into fracture toughness predictions from spherical indentation tests (SITs). It was found that consistency of the fracture toughness KIC calculated from the repeated tests using the critical strain criterion and ERR model is even higher than that from mini-CTs, while high dispersion predictions were observed in critical stress criterion. Compared with the CDV criterion, the ERR model focuses on the dissipation energy required to form unit area “equivalent-crack” (i.e., the rate) rather than the whole indentation energy till a critical depth, and thus it is less affected by the absolute value of damage variations in three repeated SITs. Source of errors were first investigated through the damage developments in uniaxial tensile specimens manufactured from different zones of the DMWs, which proves the CDV varies from 0.1 to 0.25 and thus shall not be regarded as a constant. Then, SEM observations were conducted on both the fracture surfaces from mini-CTs and the section surfaces from SITs, indicating both the changes in fracture mechanism (from ductile dominated to the joint effort of ductile void developments and cleavage micro-crack propagations) and the existence of small initial voids from welding lead to the errors in damage-mechanics-based predictions.

A comparative study of in-situ alloying in laser powder bed fusion for the stainless steel X2CrNiMoN20-10-3

This paper compares, for the first time, laser powder bed fusion (PBF-LB/M) processing of a powder mixture (PM), also known as in-situ alloying, with that of a pre-alloyed (PA) powder from gas atomization with the same chemical composition, using the example of X2CrNiMoN20-10-3 ferritic-austenitic stainless steel. The focus is on the differences in the microstructure formation mechanisms during PBF-LB/M between PM and PA using different energy inputs, in order to gain new insights into the process transferability of in-situ alloying to the processing of pre-alloyed powders. The microstructure investigations are carried out using electron backscattered diffraction (EBSD), energy dispersive (EDS) as well as wavelength dispersive X-ray spectrometry (WDS), X-ray diffraction (XRD) and magneto-inductive method (Feritscope®). The microstructures of samples produced from PM and PA differ significantly in terms of the resulting ferritic and austenitic phase fractions, so that a ferritic-austenitic microstructure forms for PM, while the PA is predominantly austenitic. The differences are mainly based on the increased chemical inhomogeneities for the PM in comparison to the PA state, which are discussed based on EDS map analysis through spatial statistics. With increasing energy input, the chemical homogeneity of the PM approaches that of the PA, but it cannot reach it even with maximum energy input. The formation of a ferritic-austenitic microstructure in the case of the PM leads to the formation of a finer microstructure compared to single-phase PA steel resulting in higher hardness of PBF-LB/M-built PM.

An investigation of the effect of sensitization on the metallurgical characteristics of dissimilarly welded austenitic-ferritic stainless steel

Purpose The purpose of this study is to examine the effects of sensitization on the metallurgical characteristics of weld joints made up of austenitic stainless steel (AISI 316L) and ferritic stainless steel (AISI 430), using the gas tungsten arc welding (GTAW) process with ER316L filler wires. Design/methodology/approach A non-consumable tungsten electrode with a diameter of 1.6 mm was used during the GTAW procedure. The filler wire, ER316L, was selected based on the recommendation provided in literature. To explore the interconnections among the structure and properties of these weldments, the techniques including scanning electron microscopy and optical analysis have been used. In addition, the sensitization behaviour of the weldments was investigated using the double loop electrochemical potentio-kinetic reactivation (DLEPR) test. Findings Microstructural analyses revealed the occurrences of coarsened grains with equiaxed columnar grains and migrating grain boundaries in the weld zone. The results of the DLEPR test demonstrated that heat affected zone (HAZ) of AISI 430 was more susceptible to sensitization than HAZ of AISI 316L. Microstructure analysis also revealed the precipitation of large amounts of chromium carbide at the grain boundaries region of AISI 430 welded steel, causing more sensitization and, as a result, more failure or breaking at the side of AISI 430 weld in the dissimilar weldment of AISI 316L–AISI 430. Originality/value The present work has been carried out to determine the appropriate welding conditions for joining AISI 316L and AISI 430, as well as the metallurgical properties of the dissimilar weldment formed between AISI 316L and AISI 430. Owing to the difficulties in measuring the performance of these types of dissimilar joints given their unique mechanical and microstructural characteristics, research on the subject is limited.

Hydrogen embrittlement of high-strength steels in thiosulphate solutions

ABSTRACT The susceptibility to hydrogen embrittlement of supermartensitic stainless steel (S13Cr), martensitic-ferritic stainless steel (17Cr), carbon steel (P110) and austenitic-ferritic stainless steels (2205 and 2507) was evaluated by slow strain rate tests using a modified NACE TM-0177 solution, with 10–3M NaS2O3 substituting the saturated H2S standard solution. Tests were conducted at 25°C, pH 2.7 and under galvanostatic polarisation with a cathodic charging of 10 mA/cm2. S13Cr, 17Cr, P110 and 2507 steels were susceptible to hydrogen embrittlement inthiosulphate, with S13Cr and 17Cr being the most sensitive to this mechanism, while 2205 showed no sign of ductility loss. This harmful effect was attributed to the hydrogen sulphide generated from thiosulphate.

Constitutive Flow Equation for Cast Austenite–Ferrite Stainless Steels

Constitutive Flow Equation for Cast Austenite–Ferrite Stainless Steels

Detection and Separation of Close Flaws in Coarse-Grained Materials Using Ultrasonic Image Deconvolution

Ultrasonic inspection of coarse-grained steels is a common challenge in various industrial fields. This task is often difficult because of acoustic scattering that creates structural noise in the ultrasonic signals and images. Therefore, inspections usually use low-frequency probes, which achieve poor resolution with standard delay-and-sum (DAS) imaging techniques, such as the well-known total focusing method (TFM). The purpose of this paper is to evaluate the performance of a super-resolution ultrasonic imaging technique presented by Laroche et al. (IEEE Trans Comput Imaging 7:935–947, 2021) for the inspection of industrial coarse-grained materials. An image deconvolution problem (with spatially varying blur) is formulated, relying on a forward model that links the TFM image to the acoustic reflectivity map. The experiments consider an austenitic-ferritic stainless steel sample insonified using array probes at 3 MHz and 5 MHz placed in contact. The goal is to resolve two close reflectors corresponding to side-drilled holes (SDH) with diameter 0.4 mm spaced by 0.4 mm edge-to-edge and positioned at different depths (10, 20, 30, 40 mm). This configuration corresponds to a critical case where the distance between the two reflectors is much lower than the Rayleigh distance, that is the resolution limit of a DAS imaging system. These are typical cases where DAS images obtained from low-frequency inspections critically lack resolution, but where higher frequency probes cannot be used in practice, because a too week signal-to-noise ratio would affect the detection capability. As predicted by the Rayleigh criterion, TFM is not able to separate the reflectors. The proposed image reconstruction method successfully resolves the majority of the reflectors with a rather accurate distance estimation. In the context of coarse-grained structure inspection, subwavelength reflectors distant from each other by two times less than the resolution limit given by the Rayleigh criterion have been successfully detected and separated. This approach hence enables the use of low-frequency probes, in order to improve the signal-to-noise ratio, while keeping high resolution capability which can be particularly interesting for industrial applications. In particular, the proposed approach shows promising results for the sizing of a real crack in an industrial sample.

Influence of Heat Treatment on the Structure and Properties of Austenitic-Ferritic Stainless Steel 03Kh24N8AD2M3

Influence of Heat Treatment on the Structure and Properties of Austenitic-Ferritic Stainless Steel 03Kh24N8AD2M3

Nondestructive Microstructural Characterization of Austenitic-Ferritic Stainless Steel Welded Joints by Double Loop Electrochemical Polarization Reactivation Portable Test

Austenitic-ferritic stainless steels are alloys with controlled additions of Cr, Ni, Mo and N which, after a suitable thermomechanical treatment, results in a balanced microstructure with similar proportions of ferrite (δ) and austenite (γ). Thus, it is possible to obtain good combinations of high mechanical properties and corrosion resistance through microstructural refinement, which also leads to a relatively good Pitting Resistance Equivalent value (PRE) at both phases. However, inadequate heat treatment and/or welding processes might result in the precipitation of deleterious phases, leading to poor mechanical properties and/or corrosion resistance. In this sense, the use of non-destructive microstructural characterization techniques becomes a valuable resource to access such alterations. Therefore, this work evaluates the precipitation of deleterious phase’s in welded thermal aged joints by portable Double Loop Electrochemical Polarization Reactivation (DL-EPR) taking into account a preliminary metallographic replica characterization. The results show that the proposed portable method can detect even a small percentage of deleterious phases, in addition to having a strong potential to be a non-destructive microstructural characterization technique.

Laves phase precipitation and sigma phase transformation in a duplex stainless steel microalloyed with niobium.

Duplex stainless steels are used to replace austenitic steels in industrial applications, especially in the chemical industry where corrosion resistance in chloride-containing media is an important requirement. According to literature, niobium has a great influence on the transformation of the phases of these steels (ferrite, austenite and sigma). Given the interest in evaluating this property in terms of wear resistance and corrosion of the material, the aim of this study was to evaluate the effect of niobium on the formation in austenitic-ferritic stainless steel SEW 410 Nr 1.4517 after heat treatment at 850ºC for 15, 30 and 60 minutes. The niobium contents studied were 0.2; 0.5 and 1.5 wt%. Measurements of microhardness, electrochemical tests in saline solution containing 3.5% sodium chloride and wear resistance were carried out. The results show that an increase in niobium content and sigma phase leads to an increase in hardness and wear resistance, associated with a decrease in corrosion potential. Therefore, the use of niobium in austeno-ferritic stainless steel is recommended when wear resistance is an important factor to be considered.

Tribological Evaluation of Ferritic-Austenitic Stainless Steel Coatings with Different Percentages of Silver

Stainless steels have been evaluated in the form of coatings or thin films deposited using magnetron sputtering in various investigations. In most of these investigations, mainly the microstructural evaluation of this type of coating has been reported, while the evaluation of wear resistance has not been widely studied. In the present investigation, ferritic-austenitic stainless steel coatings with different percentages of silver were deposited by means of unbalanced DC magnetron sputtering. A stainless steel target doped with different numbers of silver inserts (0, 1, 2, 3, and 4 inserts) was used, obtaining coatings with silver percentages of up to 7.7 at. %, in order to evaluate their tribological properties. During the synthesis, the first layer of the bilayer was deposited with argon and the other in an atmosphere of argon and nitrogen. The structure of the coatings was evaluated by use of X-ray diffraction (XRD), and the morphology and the chemical composition were studied by means of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively. The hardness was tested through nanoindentation, and the wear test was carried out using the pin-on-disc technique. The wear tracks were evaluated via profilometry, SEM, and EDS. In general, the deposited films exhibited BCC and FCC phases and a compact morphology, as well as good adherence to the substrate. The results of the evaluation of the mechanical properties revealed an increase in the hardness of the coatings when the silver content was augmented, obtaining values of 9.8 GPa and 12.1 GPa for coatings with 0 and 4 silver inserts, respectively. Likewise, it was found that the wear resistance of coatings was higher, up to two orders of magnitude compared to the 316L stainless steel substrate. Finally, the wear rate of the multilayers decreased slightly with an increase in the percentage of silver in the films produced, up to an order of magnitude for the coating with 4 silver inserts relative to the coating with 0 silver inserts.

Identification and quantification of martensite in ferritic-austenitic stainless steels and welds

This paper aims at the phase identification and quantification in transformation induced plasticity duplex stainless steel (TDSS) base and weld metal containing ferrite, austenite, and martensite. Light optical microscopy (LOM) and electron backscatter diffraction (EBSD) analysis were employed to analyze phases. Samples were either mechanically or electrolytically polished to study the effect of the preparation technique. Mechanical polishing produced up to 26% strain-induced martensite. Electrolytic polishing with 150 g citric acid, 300 g distilled water, 600 mL H3PO4, and 450 mL H2SO4 resulted in martensite free surfaces, providing high-quality samples for EBSD analysis. Martensite identification was challenging both with LOM, due to the similar etching response of ferrite and martensite, and with EBSD, due to the similar lattice structures of ferrite and martensite. An optimized Beraha color etching procedure was developed that etched martensite distinctively. A novel step-by-step EBSD methodology was also introduced considering grain size and orientation, which successfully identified and quantified martensite as well as ferrite and austenite in the studied TDSS. Although here applied to a TDSS, the presented EBSD methodology is general and can, in combination with knowledge of the metallurgy of the specific material and with suitable adaption, be applied to a multitude of multiphase materials. It is also general in the sense that it can be used for base material and weld metals as well as additive manufactured materials.

Processing of a newly developed nitrogen-alloyed ferritic-austenitic stainless steel by laser powder bed fusion – Microstructure and properties

In this work, a novel alloy design of a stainless steel with a ferritic-austenitic microstructure is derived for PBF-LB/M (powder bed fusion-laser beam/metal). The alloy was developed based on X2CrNiMo17-12-2 steel, for which an austenite volume content of approx. 54 vol% in the PBF-LB/M state was achieved using a reduced Ni equivalent. Partial substitution of Ni by Mn increases the N solubility of the alloy. By melting and further gas-atomizing this melt in an N2 atmosphere, an N content of 0.27 mass% was set in the produced steel powder. This leads to both high strength and high corrosion resistance of the PBF-LB/M-processed steel. However, microstructural investigations in the PBF-LB/M state confirm a microstructure consisting of ferrite, austenite, and Mo- and Cr-rich nitrides of M2N type. The nitrides were not completely eliminated by a subsequent heat treatment of the PBF-LB/M samples. As a result of the solution annealing, the microstructure approaches the thermodynamic equilibrium so that the austenite volume content increases from 54.2 vol% to 92.7 vol%. The higher Cr and N contents result in a higher corrosion resistance of the investigated steel compared to PBF-LB/M-processed X2CrNiMo17-12-2, regarded as the reference material. In addition, the measured strengths are significantly higher due to the larger amounts of austenite/ferrite interfaces and the N-induced solid-solution strengthening effect compared to X2CrNiMo17-12-2.

Microstructure and Formability of Laser Welded Dissimilar Butt Joints of Austenitic-Ferritic Stainless Steels

Dissimilar laser welding of ferritic, type EN 1.4509, and austenitic, type EN 1.4307, stainless steel sheets was conducted at different energy inputs 30 and 80 J/mm and under different shield gases Ar and N and without shielding gas to evaluate the microstructure and hardness of the welded zone. The formability tests, using Erichsen principle, were carried out to determine the deformation behaviour of the dissimilar welded joints under biaxial straining. The fusion zone microstructure analysis revealed that the predominant phase structure is columnar coarse ferritic grains with slightly small content of austenite in the ferrite grain boundaries. The formability of the welded joints under Ar and N shielding gases is significantly improved, i.e., higher plasticity, compared with welded joints without shielding gas at both energy inputs.

Effect of material position on microstructure and mechanical properties of friction stir welded dissimilar austenite–ferrite stainless steels joints

The friction stir welding of 304 austenitic stainless steel and 430 ferritic stainless steel was performed by placing 304 plate on advancing side and retreating side of the tool, respectively. The results indicated that when the 304 plate was placed on advancing side, the weld metal showed superior flow characteristics and better material mixing compared to the weld when the 304 plate was placed on retreating side. The stir zone showed different characteristics including banded structure, subgrains and fine composite structure of austenite and ferrite because of dynamic recrystallization. The strain rate in austenitic stainless steel had a major effect on grain refinement and plastic deformation. The mechanical properties tests indicated that grain refinement combined with substructure and boundary martensite increased hardness and tensile strength when placing 304 plate on advancing side. The voids in stir zone resulted in a significant decrease in strength and elongation when placing 304 plate on retreating side. Overall, placing austenitic stainless steel on advancing side is more suitable for friction stir welded dissimilar austenite–ferrite stainless steels.

Applicability of Solid Lubricant Coatings in Cold Rod Extrusion of Stainless Steels

Cold extrusion is an established technology for the production of dimensionally accurate components in large series. Due to the high material and energy efficiency, a resource-saving manufacturing of high-performance parts is possible. Forming at room temperature leads to an advantageous grain structure and work hardening of the material, resulting in components with favorable operating characteristics. Nevertheless, a challenge is the generation of residual stresses during forming, which are influencing the fatigue behavior. The modification of the tribological conditions is one method for influencing the parts’ residual stress state. However, the high strength and work hardening of the materials formed at room temperature leads to high tribological loads between billet and die. These challenges are intensified by the increasing use of stainless steels due to growing demands for corrosion resistant components. The aim followed within this paper is therefore to investigate the applicability of typical lubricant coatings in the forward rod extrusion of stainless steels. For this purpose, the ferritic stainless steel X6Cr17 (DIN 1.4016) and the ferritic-austenitic stainless steel X2CrNiMoN22-5-3 (DIN 1.4462) are extruded with an equivalent plastic strain of ε̅ ≈ 1. The research is performed with a molybdenum disulfide (MoS2), a soap and a polymer-based lubricant coating. For reproducing different contact conditions, the die geometry is varied with die opening angles of 60°, 90° and 120°. The suitability of the lubricants is evaluated using the integrity of the lubricant coating after forming. From the correlations between process forces, temperatures and surface integrity, recommendations for the application of the researched lubricants are derived.

Microstructure and Mechanical Properties of Friction Stir Welded Austenitic-Ferritic Stainless Steels Using Staggered Joint Configuration

Dissimilar friction stir welding of 304 austenitic stainless steel and 430 ferritic stainless steel was performed by using staggered butt joint configuration. Under proper heat input, welding experiments were carried out to study the effect of tooth dimension on material flow, microstructure evolution and mechanical properties of the joint. The results indicate that the material flow tends to improve with reduced the tooth width. However, when the tooth width is reduced to 1 mm, the mixing degree of base metals in stir zone becomes worse due to the lack of softened metal. It is noted that joint 2 with a tooth width of 2 mm is characterized by river pattern and shows various flow directions in different regions of the stir zone. This pattern is formed by the mixed structure of austenite and ferrite with various grain sizes. Joint 2 presents the maximum tensile strength in all specimens and failed in the ferritic base metal.

Comparative analysis of the corrosion resistance of UNS S31200 duplex stainless steel and its analogue

Austenitic-ferritic stainless steels, more commonly known as duplexstainless steels, have become increasingly popular among foreign producers, but in Russian Federation development of these steels has begun relatively recently. The purpose of this study was to perform a comparative analysis of the corrosion resistance of UNS S31200 austenitic-ferritic stainless steels and its Russian analogue. The corrosion resistance equivalents PREN and MARC were determined and compared for these two steels. Corrosion resistance of analogue-steel is lower than S31200′s due to the reduced content of chromium and nitrogen. Using metallographic techniques in combination with thermodynamic modeling a large amount of undesired σ-phase was found in analogue-steel. This is due to insufficient high cooling rates or incorrect selection of heat treatment modes.

Effect of Grain Size on the Anodic Dissolution of Lean Duplex UNS S32202 Austenitic-Ferritic Stainless Steel

The effect of grain size on the anodic dissolution of lean duplex UNS S32202 dual-phase austenitic-ferritic stainless steel was evaluated. Grain coarsening was achieved by heat treatment, and grain size and grain boundary densities determined by automatic image analysis after etching. Potentiodynamic electrochemical testing in acidic chloride medium allowed isolating the anodic dissolution behavior of the crystallographic phases of the material. A relationship between grain boundary density (for grain sizes in the micrometer range) and dissolution rate has been found, showing that reducing grain size enhances active corrosion rates in environments that promote active behavior. This leads to new possibilities of industrial adjustment of the corrosion behavior of duplex stainless steels via grain size control.

Investigation of microstructural behavior and mechanical properties of dissimilar weld joints of austenitic-ferritic stainless steel

Joining of AISI 316L (austenitic stainless steel) and AISI 409M (ferritic stainless steel) was carried out using MIG welding at different root gaps and notch angles. The mechanical properties of similar and dissimilar welded joint were examined using tensile test. The results show that the interfacial microstructures and mechanical properties are strongly affected by joining configuration. When compared in similar material welded joints, it was observed that the yield strength and tensile strength of 2 mm root gap with 60° notch angle was highest. Also, the welded butt joint with 2 mm root gap and 60° notch angle showed highest strength among all dissimilar welded joints.