Lean Duplex Steel Research Articles

Fatigue enhancement of welded thin-walled tubular joints made of lean duplex steel

In the present work, the fatigue strength and enhancement techniques of welded thin-walled (wall thickness of t = 2 mm) square hollow section (SHS) joints made of lean duplex stainless steel (grade EN 1.4162) are experimentally and numerically studied. Experimental fatigue tests were carried out on gas metal arc-welded tubular X-joints subjected to fully reversed (applied stress ratio of R = −1) constant amplitude in-plane bending moment. Geometrical improvement techniques were investigated by applying fully penetrating fillet welds and using structural reinforcements. The applications of high-frequency mechanical impact (HFMI) treatment were examined in the context of further enhancing the fatigue strength of the geometrically improved joints. In addition to the experimental work carried out in this study, the fatigue test data of welded thin-walled SHS and rectangular hollow section (RHS) joints were extracted from literature. Structural stress concentration factors were numerically and analytically obtained to validate the fatigue assessment model for welded thin-walled tubular joints using the structural hot-spot stress method. In addition, the effective notch stress (ENS) concept with the reference radius of rref = 0.05 mm was implemented. Compared to the joints in the as-welded state, fatigue strength improvements of 1.53–2.7 were found in the geometrically improved joints. On the other hand, the HFMI treatment did not lead to an additional increase in the fatigue strength capacity of the studied AW joints. The CIDECT guidelines for the structural hot-spot stress method were found conservative for thin-walled joints (t < 3 mm) and could be recommended for fatigue design.

Effects of preheating on laser beam–welded NSSC 2120 lean duplex steel

AbstractDuplex stainless steels show sustainable alternative for the conventional austenitic grades, with higher strength, higher resistance against stress corrosion cracking, and lower purchase cost. Thus, duplex stainless steel gains more attention in construction, oil and gas, and chemical industries. Among duplex stainless steels, low nickel and low molybdenum alloyed lean duplex stainless steel are a cost-effective substitution of austenitic grades. However, keeping the balanced ferrite/austenite phase ratio in the weld metal can be challenging, mostly for autogenous and low heat input welding processes. In our research, a newly developed NSSC 2120 lean duplex stainless steel grade was welded autogenously by fiber laser welding process. Different welding parameters and preheating temperatures were used during the experiments. The welds were evaluated by microscopic and metallographic techniques, and also by electrochemical corrosion measurements. The welding parameters and the preheating temperature greatly influenced the weld shape and the austenite content in the weld metal. It was found that the focus point distance from the sheet surface had significant effect on the weld geometry. Changing the focus point distance to + 2 mm, the penetration depth increased from 4.96 to 5.53 mm, and increased the austenite content by 2.6%. Due to the preheating the welds became wider and shallower, e.g., from 4.96 to 4.08 mm penetration depth, while the cross-section are increased from 5.10 to 6.12 mm2 at the same sample. The preheating resulted in more intergranular austenite formation, which meant maximum 4% increase in the weld metal. The increasing austenite content resulted in increasing pitting corrosion resistance in 3.5% NaCl electrolyte, the highest increase was 90 mV.

Advantages of lean duplex stainless steels in the pulp and paper industry

The performance of lean duplex stainless steels has been utilized by the pulp and paper industry since their introduction to the market almost 20 years ago. Experience has shown that this group of stainless steels has exceptional performance in, for example, alkaline environments towards typical deterioration mechanisms, i.e., uniform corrosion and stress corrosion cracking. The chemistry of the “lean” duplex steels is designed so that the content of volatile and expensive elements like nickel and molybdenum can be reduced to an absolute minimum without sacrificing the technical performance. This reduces the raw material cost and most importantly provides predictability of the steel price, which is often challenging with conventional austenitic and duplex stainless steels.Thanks to a dual phase microstructure and high nitrogen content, lean duplex steels have at least two times higher strength compared to standard austenitic stainless steels. This is often a preferred feature in pulp and paper construction, as it enables lighter structures and less material to be utilized. Today, lean duplex steels are widely available in various dimensions, from thin cold rolled sheets up to thick hot rolled plates. Lean duplex steels are also fully recyclable after the decommissioning stage of the equipment, thereby contributing to the circular economy.

Austenite reversion in lean duplex steel: Microstructural, dilatometric and magnetic characterization

Austenite reversion in lean duplex steel: Microstructural, dilatometric and magnetic characterization

Surface Quality and Mechanical Properties in Drawn Stainless Steel Profile Wires Caused by Process Parameters

Thin profile wires made of stainless steels are widely used for production of industrial screens used for filtration and separation processes. Industrial screens made of resistance spot welded thin profile wires are used e.g. in baskets for centrifuges or filtration screens and find a wide range of applications in various branches of the industry (mining, food and sugar industry, paper industry, wastewater treatment, oil and gas and many more). Regardless of the application, increased durability and surface quality of the screens is of paramount importance. Improvement of the longevity of manufactured items, process efficiency and environmental performance can be achieved only when interactions between initial wire rod quality and its susceptibility to metal forming operations are fully understood. Due to the complexity of the manufacturing process of industrial screens (precise and small gaps, narrow ranges of dimensional tolerances), forming of thin, so-called precision profiled wire is difficult. The influence of the charge, i.e. wire rod, parameters of the wire drawing / rolling process play a key role in obtaining semi-finished products characterized by high strength, low surface roughness and a low level of residual stresses. The paper discusses the influence of the initial state of wire rod and deformation parameters on the quality of thin austenitic and lean duplex steel profile wires as well as the possibility of replacing austenitic wires by lean duplex wires and their possible benefits.

Quantitative Analysis of Hydrogen‐Assisted Microcracking in Duplex Stainless Steel through X‐Ray Refraction 3D Imaging

While the problem of the identification of mechanisms of hydrogen‐assisted damage has and is being thoroughly studied, the quantitative analysis of such damage still lacks suitable tools. In fact, while, for instance, electron microscopy yields excellent characterization, the quantitative analysis of damage requires at the same time large field‐of‐views and high spatial resolution. Synchrotron X‐ray refraction techniques do possess both features. Herein, it is shown how synchrotron X‐ray refraction computed tomography (SXRCT) can quantify damage induced by hydrogen embrittlement in a lean duplex steel, yielding results that overperform even those achievable by synchrotron X‐ray absorption computed tomography. As already reported in the literature, but this time using a nondestructive technique, it is shown that the hydrogen charge does not penetrate to the center of tensile specimens. By the comparison between virgin and hydrogen‐charged specimens, it is deduced that cracks in the specimen bulk are due to the rolling process rather than hydrogen‐assisted. It is shown that (micro)cracks propagate from the surface of tensile specimens to the interior with increasing applied strain, and it is deduced that a significant crack propagation can only be observed short before rupture.

Metal forming driven surface engineering of thin profile wires for high precision industrial filtration screens

Screens made of drawn and spot-welded stainless steel precise profile wires are widely used to separate particles from fluids during various industrial applications. Increased durability and surface quality of these products are critical in maximising their wear resistance and extending the screen service life. Therefore, the paper discusses the development of metal forming technology considering the surface quality and residual stress level of austenitic and lean-duplex steel profile wires for the production of high-quality industrial screens. It is shown that surface engineering driven by metal forming is a key factor in extending the life and performance of these products.

Microstructural and magnetic characterization of a lean duplex steel: Strain-induced martensite formation and austenite reversion

Strain-induced α′-martensite and austenite reversion in a cold rolled UNS S32304 lean duplex steel were tracked by means of magnetic measurements, with emphasis on both Ms (saturation magnetization) and Hc (coercive field) parameters. Grain-averaged quality metrics derived from EBSD (electron backscatter diffraction) analysis were also used to distinguish the phases during austenite reversion. The material was cold rolled to a true strain (ε) of 1.61 and subjected to isothermal and continuous annealing, the latter conducted in the presence of an external magnetic field. The evolution of the α′-martensite fraction upon straining and after isothermal annealing was monitored by coupling the Ms values and thermodynamic simulations, as well as from EBSD analysis. For the isothermally annealed material (ε = 1.61), the overall behavior of Ms and hardness displayed similar trends with a strong decrease for temperatures higher than 500 °C, suggesting austenite reversion. Results confirmed the occurrence of austenite reversion for the temperature interval investigated here. At 800 °C, austenite reversion is complete, and the steel is fully recrystallized. Besides, from the EBSD analysis, evidence of ferrite transformation into austenite was rather noticeable, in accordance with thermodynamic simulations and magnetic probing. Complementary electron channeling contrast imaging (ECCI) revealed that precipitation reactions mainly occur in the recrystallized austenite at 700 and 800 °C. The Hc behavior of both, the strained and annealed conditions was inferred to be mostly driven by microstructural changes in ferrite.

Autogenous Fiber Laser Welding of 316L Austenitic and 2304 Lean Duplex Stainless Steels.

This study presents results of experimental tests on quality of dissimilar welded joints between 316L austenitic and 2304 lean duplex stainless steels, welded without ceramic backing. Fiber laser welded butt joints at a thickness of 8 mm were subjected to non-destructive testing (visual and penetrant), destructive testing (static tensile test, bending test, and microhardness measurements) and structure observations (macro- and microscopic examinations, SEM, element distribution characteristics, and ferrite content measurements). Non-destructive tests and metallographic examinations showed that the welded joints meet the acceptance criteria for B level in accordance with EN ISO 13919–1 standard. Also the results of the destructive tests confirmed the high quality of the joints: specimens were fractured in base material with lower strength—316L austenitic stainless steel and a 180° bending angle was obtained confirming the high plasticity of the joints. Microscopic examination, SEM and EDS analysis showed the distribution of alloying elements in joints. The microhardness of the autogenous weld metal was higher by about 20 HV0.2 than that of the lean duplex steel. Ferrite content in the root was about 37% higher than in the face of the weld. The Schaeffler phase diagram was used to predict the phase composition of the welded joints and sufficient compliance with the magnetic method was found. The presented procedure can be used for welding of 316L–2304 stainless steels dissimilar welded joints of 8 mm thickness without ceramic backing.

Phase-specific residual stresses induced by deep drawing of lean duplex steel: measurement vs. simulation

The final geometry and fatigue behavior of deep drawn components in service is strongly influenced by deformation-induced residual stresses. For multi-phase materials, besides macro residual stresses (first kind), phase-specific residual stresses (second kind) occur on the microscale of the material. In order to influence the component characteristics positively it is important to predict the distribution of the residual stresses on both scales. A two-scale simulation and measurement approach is presented which allows for an efficient determination and validation of the phase-specific residual stresses. Finite-element simulations are performed to predict the deformation-induced macro residual stresses. A numerically efficient mean-field homogenization is used to estimate the total strain, the plastic strain and the eigenstrain on the grain level based on macroscopic stress, strain and stiffness data. The simulated residual stresses are compared to experimental data. Macro residual stresses are determined by means of incremental hole drilling method, whereas phase-specific residual stresses are analyzed with use of X-ray diffraction according to the $$\sin ^2\psi$$ method. The simulation and measurement approaches are applied to a representative deep-drawing process for the lean duplex stainless steel $$\mathrm {X2CrNiN23{-4}}$$ , which consists of a ferritic and an austenitic phase both with the same volume fraction. The results indicate that the proposed two-scale simulation approach is well suited for the prediction of phase-specific residual stresses after a deep drawing process of lean duplex steel.

Evaluation of Heat Input and Cold Wire Addition on Dimensional, Chemical and Metallurgical Characteristics of The Weld Metal Obtained by Submerged Arc Welding in UNS S32304 Lean Duplex Steel

Resumo A energia de soldagem (HI) máxima indicada para soldagem dos aços inoxidáveis duplex de 2,5kJ.mm-1 representa um fator limitante para o aumento de produtividade na soldagem dos mesmos, sendo a adição de arame frio uma alternativa interessante à essa questão. Avaliou-se a influência da energia de soldagem e da adição do arame frio nas características dimensionais, químicas e metalúrgicas do metal de solda (MS) obtido por arco submerso (SAW) em aço “lean” duplex UNS S32304, utilizando-se o consumível ER2209. Empregou-se o sistema ICETM que adiciona um arame frio entre dois arames energizados “twin” para executar cordões sobre chapas, empregando-se quatro diferentes HI e variando-se a porcentagem de arame frio em relação aos arames quentes em 0, 50 e 100%. Foram realizadas análise química, macrografia, análise dimensional, microscopia ótica e MEV/EDS do MS. A microestrutura do MS foi constituída de uma matriz ferrítica com a presença de austenita de contorno de grão, de Widmanstatten e intragranular e inclusões não metálicas, não sendo observado a ocorrência de precipitados deletérios como σ, χ e nitreto de cromo. A adição de arame frio aumentou a taxa de deposição e provocou a redução da diluição, produzindo soldas com maiores números equivalentes de resistência à corrosão por pite, principalmente para maiores HI, e maiores frações de austenita. Isso indica que o aumento da velocidade de alimentação de arame frio propicia o aumento da produtividade e da resistência à corrosão por pite do MS.

Numerical modelling of stainless steel preloaded bolted connections

AbstractThe use of stainless steel in construction has become more popular in recent years. It is used for a wide range of structural applications in aggressive environments where reliable performance over long periods with little maintenance is required. Although structural design standards are available for stainless steel, currently there are no rules covering the design of preloaded slip‐resistant bolted connections because of the lack of knowledge about their long‐term viscoplastic behaviour. Viscoplastic creep and stress relaxation in the preloaded bolt assemblies will lead to a certain loss of clamping force and may cause the failure of the connection if not accounted for. This paper presents the development of material models and finite element models for bolt assemblies based on an extensive experimental study of creep, relaxation and tension effects on austenitic, ferritic, duplex and lean duplex steel plates and bars for different loading rates. These models were verified against slip tests with stainless steel bolt assemblies according to EN 1090‐2 and then used in a parametric study to extend the scope of the connections investigated. Both experimental programmes were carried out in the European RFCS research project SIROCO (Execution and reliability of slip‐resistant connections for steel structures using Carbon Steel and Stainless Steel) as well as finite element calculations.

Hydrogen Induced Damage in Heavily Cold-Drawn Wires of Lean Duplex Stainless Steel

The paper addresses the sensitivity to hydrogen embrittlement of heavily cold-drawn wires made of the new generation of lower alloyed duplex stainless steels, often referred to as lean duplex grades. It includes comparisons with similar data corresponding to cold-drawn eutectoid and duplex stainless steels. For this purpose, fracture tests under constant load were carried out with wires in the as-received condition and fatigue-precracked, in air and exposed to ammonium thiocyanate solution. Microstructure and fractographic observations were essential means for the cracking analysis. The effect of hydrogen-assisted embrittlement on the damage tolerance of lean duplex steels was assessed regarding two macro-mechanical damage models that provide the upper bounds of damage tolerance and accurately approximate the failure behavior of the eutectoid and duplex stainless steels wires.

Rietveld and Impedance Analysis of Cold and Hot Rolled Duplex and Lean Duplex Steels for Application in Paper and Pulp Industry

In this study, X-Ray Diffraction (XRD) and Rietveld Refinement were performed to identify and quantify the ferrite and austenite phase of cold and hot rolled duplex stainless steels (UNS S31803) and lean duplex stainless steels (UNS S32304). Electrochemical impedance spectroscopy (EIS) was applied to evaluate the chemical behavior of duplex and lean duplex stainless steels in white, green, and black liquors of paper and pulp industry. Rietveld analysis results showed a higher austenite content than the standard limit for duplex steels in the hot rolled condition. The hot rolling condition plays a major role in improving corrosion resistance in white liquor mainly for the lean duplex steel.

Cover Picture: Materials and Corrosion. 5/2016

Cover:Pit morphologies of welded samples after polarization measurements (FCAW 2307 root region): (a) BM, (b) FZ, (c) HAZ and (d) optical microstructure of welded sample showing HAZ and FZ.More detailed information can be found in: D. C. Sicupira, G. S. Frankel, F. Lins, Pitting corrosion of welds in UNS S32304 lean duplex steel, Materials and Corrosion 2016, 67, page 440.

Mechanism of hot-rolling crack formation in lean duplex stainless steel 2101

The thermoplasticity of duplex stainless steel 2205 (DSS2205) is better than that of lean duplex steel 2101 (LDX2101), which undergoes severe cracking during hot rolling. The microstructure, microhardness, phase ratio, and recrystallization dependence of the deformation compatibility of LDX2101 and DSS2205 were investigated using optical microscopy (OM), electron backscatter diffraction (EBSD), Thermo-Calc software, and transmission electron microscopy (TEM). The results showed that the phase-ratio transformations of LDX2101 and DSS2205 were almost equal under the condition of increasing solution temperature. Thus, the phase transformation was not the main cause for the hot plasticity difference of these two steels. The grain size of LDX2101 was substantially greater than that of DSS2205, and the microhardness difference of LDX2101 was larger than that of DSS2205. This difference hinders the transfer of strain from ferrite to austenite. In the rolling process, the ferrite grains of LDX2101 underwent continuous softening and were substantially refined. However, although little recrystallization occurred at the boundaries of austenite, serious deformation accumulated in the interior of austenite, leading to a substantial increase in hardness. The main cause of crack formation is the microhardness difference between ferrite and austenite.

On the stress corrosion cracking of lean duplex steel in chloride environment

Duplex stainless steel having attractive combination of austenitic and ferritic properties is being used in industry such as petrochemical, pulp and paper mills. In this study, the corrosion and stress corrosion behavior of duplex stainless steel in 3.5% sodium chloride environment was investigated by weight loss measurements, electrochemical DC testing and slow strain rate test (SSRT). Weight loss data showed no significant corrosion after 1700 hours. Electrochemical polarization test in 3.5% NaCl solution exhibited a uniform corrosion rate of 0.008 mpy (calculated using Tafel analysis) showing passivity in the range of 735-950 mV. A comparison of the slow strain rate test in 3.5% NaCl solution with air shows almost a similar stress strain curve for duplex stainless steel. In comparison, the stress strain curves for 0.15% carbon steel show a loss of about 25% tensile elongation for the same comparison. The excellent corrosion and especially resistance to localized corrosion (pitting) is responsible for no loss of ductility in duplex stainless steel.

Avaliação da soldagem multipasse de chapas espessas de aços inoxidáveis lean duplex UNS S32304 soldadas pelos processos SMAW, GMAW e FCAW -: resistência à corrosão

Os aços inoxidáveis duplex (AID) vêm se apresentando como uma excelente alternativa para aplicações em que alta resistência à corrosão e alta resistência mecânica são requeridas. Contudo, os AID, incluindo os aços inoxidáveis lean duplex, apresentam soldabilidade inferior em relação aos aços inoxidáveis austeníticos. Esse trabalho tem como objetivo a avaliação da resistência à corrosão de juntas soldadas multipasse de chapas de 22 mm de espessura da liga inoxidável lean duplex UNS S32304, utilizando-se os processos SMAW, GMAW e FCAW e consumíveis com dois tipos de composição química, 22%Cr9%Ni3%Mo e 23%Cr7%Ni, totalizando seis experimentos. As juntas soldadas foram analisadas usando-se a técnica de polarização anódica potenciodinâmica para determinação do potencial de pite do material. O eletrólito estudado foi uma solução salina de cloreto de sódio 3,5% m/v. A junta de aço lean duplex soldada pelo processo GMAW com o consumível 22%Cr9%Ni3%Mo apresentou o melhor comportamento frente à corrosão no meio salino. A região da raiz da solda apresentou o maior potencial de pite em relação ao topo das juntas soldadas. As juntas de aço lean duplex soldadas usando-se o processo FCAW apresentaram um comportamento característico de corrosão generalizada no meio de solução salina, ao contrário das demais amostras.

Technical and economical stainless steel alternatives for civil engineering applications

Systematic laboratory and field exposure tests allow to compare nine different stainless steels (three ferritic, three austenitic, and three duplex grades) for civil engineering applications. The influence of surface finish was also taken into account by choosing five different industry‐specific features. The three duplex stainless steels revealed excellent corrosion resistance under most of the test conditions. Especially for applications in civil engineering the lean duplex steels offer distinct advantages. The manganese‐alloyed 1.4376 and the ferritic steel 1.4509 cannot be recommended as alternative materials as they did not perform satisfactorily.

Ferrite measurement in lean duplex steel S32101 and its welded joints

The results of testing the ferrite content of S32101 lean duplex steel and its welded joints by means of a ferritescope, as well as the influence of such factors as the sample thickness, measurement area shape and state, etc., are presented. The application of correction coefficients is discussed. The problems arising during measurement of ferrite content in welded joints and the ways of preventing them are specified. The influence of MAG welding parameters on the ferrite content in S32101 lean duplex steel butt welded joints is also presented.