Stainless Steel UNS Research Articles

A New methodology of Nondestructive Testing for Quantitative Evaluation of Sigma Phase in Duplex Stainless Steels

Duplex Stainless Steels (DSSs) are widely used in the oil and gas industry. When the steel is subjected to a thermal gradient, as in welding may occur precipitation of intermetallic phases. Among these phases, there is the sigma phase, which even in small quantities reduces drastically the mechanical and corrosion resistance properties. According to some studies in the literature, the limit amount of sigma phase present in steel in welded joints must be in a maximum volume fraction of 2,5%. In search to improve the detection sensitivity of the sigma phase, this paper presents results obtained by Linear Sweep Voltammetry (LSV) in Duplex Stainless Steel UNS S31803 as a Nondestructive Testing (NDT) model. The innovation in the application of this test is to use a microcell-based system that allows a reduced area of ​​analysis obtaining density of currents in the order of microamperes and in the use of selective electrolytes to the elements of the oxides present in the intermetallic phases. With the use of these systems and through the optimization of control parameters it was possible to detect very low amounts of deleterious phases such as the sigma and chi phases.

Microstructure, Texture, Mechanical and Corrosion Performance of the Stainless Duplex Steel UNS S32205 Submitted to Warm Rolling

The UNS S32205 duplex stainless steel was warm rolled at 600°C with 60 and 80% of thickness reduction. The microstructure was characterized by optical, scanning and transmission electron microscopy, X-ray diffractometry and EBSD. The corrosion resistance was evaluated by electrochemical behavior in the chlorine ion environment using potentiodynamic polarization measurements. The tensile strength reached 1185 MPa and 1328 MPa, after warm rolling with 60 and 80%, respectively. In steel as-supplied, hot rolled and annealed, the tensile strength was 774 MPa. Ferrite microtexture presented the α-fiber and the rotated cube component, while the austenite enhanced the brass, copper, and cube components to a lesser extent. The substructure was characterized by intense formation of tangles and forests of dislocations and discrete subgrains in the ferritic phase and by planar gliding of dislocations and formation of dense dislocations walls in the austenite. Despite the existence of a certain similarity among the values of pitting potentials obtained for all samples, the number of pits observed was higher in the as-received sample, followed by the samples with 60 and 80% reduction. These results draw attention to innovative routes in the industrial production of duplex stainless steel of this class, even considering ductility lost. Keywords: Warm rolling; Mechanical strength; Texture; Substructure; Corrosion resistance

Produção e caracterização de compósito metálico laminado produzido por laminação à frio

O crescente interesse pelas indústrias por compósitos metálicos laminados resultou da necessidade de se desenvolver tecnologias de fabricação cada vez mais eficientes. Visto que é um assunto de pouco debate no Brasil e resultou no atraso do conhecimento científico do assunto, o presente trabalho objetivou a produção de um composto metálico laminado por meio do processo de laminação a frio na tentativa de melhorar as condições de competitividade do material nacional com o mercado exterior. O primeiro composto proposto era constituído por alumínio comercial AA1050 e aço inoxidável duplex UNS S32304. Uma vez as tentativas de sua produção foram ineficazes, o segundo foi substituído pelo aço inoxidável ferrítico AISI 430 com o objetivo de entender os mecanismos de ligação. O uso desse metal permitiu a produção de compósitos metálicos laminados e posterior estudo com ensaios de aderência (peeling) e análises microestruturais do mecanismo de aderência dos materiais.

Microstructural and Corrosion Properties of Cold Rolled Laser Welded UNS S32750 Duplex Stainless Steel

The main goal of this work was to study the effect of plastic deformation on weldability of duplex stainless steel (DSS). It is well known that plastic deformation prior to thermal cycles can enhance secondary phase precipitation in DSS which can lead to significant change of the ferrite-austenite phase ratio. From this point of view one of the most important phase transformation in DSS is the eutectoid decomposition of ferrite. Duplex stainless steels (DSSs) are a category of stainless steels which are employed in all kinds of applications where high strength and excellent corrosion resistance are both required. This favorable combination of properties is provided by their biphasic microstructure, consisting of ferrite and austenite in approximately equal volume fractions. Nevertheless, these materials may suffer from several microstructural transformations if they undergo heat treatments, welding processes or thermal cycles. These transformations modify the balanced phase ratio, compromising the corrosion and mechanical properties of the material. In this paper, the microstructural stability as a consequence of heat history due to welding processes has been investigated for a super duplex stainless steel (SDSS) UNS S32750. During this work, the effects of laser beam welding on cold rolled UNS S32750 SDSS have been investigated. Samples have been cold rolled at different thickness reduction (ε = 9.6%, 21.1%, 29.6%, 39.4%, 49.5%, and 60.3%) and then welded using Nd:YAG laser. Optical and electronical microscopy, eddy’s current tests, microhardness tests, and critical pitting temperature tests have been performed on the welded samples to analyze the microstructure, ferrite content, hardness, and corrosion resistance. Results show that laser welded joints had a strongly unbalanced microstructure, mostly consisting of ferritic phase (~60%). Ferrite content decreases with increasing distance from the middle of the joint. The heat-affected zone (HAZ) was almost undetectable and no defects or secondary phases have been observed. Both hardness and corrosion susceptibility of the joints increase. Plastic deformation had no effects on microstructure, hardness or corrosion resistance of the joints, but resulted in higher hardness of the base material. Cold rolling process instead, influences the corrosion resistance of the base material.

Precipitation condition and effect of volume fraction on corrosion properties of secondary phase on casted super-duplex stainless steel UNS S32750

PurposeThe secondary phase decreased the corrosion resistance because of the segregation of Cr and Mo. Therefore, this paper aims to study the precipitation condition and the effect of secondary phase with volume fraction on corrosion behavior.Design/methodology/approachSecondary phase precipitated approximately from 375°C to 975°C because of saturated Cr and Mo at grain boundary by growth of austenite. Therefore, heat treatment from 800°C to 1,300°C was applied to start the precipitation of the secondary phase.FindingsThe secondary phase is precipitated at 1,020°C because of segregation by heterogeneous austenite. The growth of austenite at 1,000°C needs the time to saturate the Cr and Mo at grain boundary. When the volume fraction of austenite is 56 per cent (14 min at 1,000°C), the secondary phase is precipitated with grain boundary of austenite. The secondary phase increased the current density (corrosion rate) and decreased the passivation. That is checked to the critical pitting temperature (CPT) curves. The 1 per cent volume fraction of secondary phase decreased CPT to 60°C from 71°C.Research limitations/implicationsThe precipitation of secondary phase not wants anyone. Casted super-duplex stainless steel (SDSS) of big size precipitates the secondary phase. This study worked the precipitation condition and the suppression conditions of secondary phase.Social implicationsManufacturers need precipitation condition to make high-performance SDSS.Originality/valueThe corrosion resistance of SDSS is hard the optimization because SDSS is dual-phase stainless steel. The precipitation of the secondary phase must be controlled to optimize of the corrosion resistance of SDSS. Anyone not studied the precipitation condition of secondary phase and the effect of secondary phase with volume fraction on corrosion behavior of SDSS.

The effect of low pH in erosion-corrosion resistance of high chromium cast irons and stainless steels

Corrosive wear is a form of material degradation that is known to occur on equipment and components that handle suspended solids in aqueous solutions. This surface deterioration process is generally enhanced when the fluids contain chlorides that are acidic in nature. Such environments are encountered in many applications, one such example being the minerals processing industry.High chromium white cast irons are widely used in many engineering applications as they extend the service life beyond that of stainless steels. In this work, the corrosive wear of austenitic stainless steel UNS S31600 and martensitic stainless steel UNS S42000 are compared with high chromium white cast irons (27WCI and 37WCI) by using a submerged jet technique at perpendicular incidence. The testing medium consisted of 3.5% NaCl liquid solution (pH 7 and pH 3) with suspended silica sand particles. The post-test methodology characterised the material loss within the directly impinged zone and turbulent zone where abrasive damage is involved. In neutral conditions, the presence of chromium carbides in white cast irons provided benefits, whilst in the acidic conditions, the austenitic structured materials (for stainless steels and white cast irons) suffered lower overall material loss compared with the martensitic structured materials.

Formation of Secondary Phases and Their Effect on the Mechanical Properties of Joints Formed by TLP Bonding Using Fe–B–Si Insert Metal in Duplex Stainless Steel

We studied the effects of secondary phases on the mechanical properties of joints formed by transient liquid-phase bonding in nitrogen-containing duplex stainless steel UNS S32750. Transient liquid-phase bonding was carried out using Fe–B–Si amorphous insert metal over the temperature range of 1423–1473 K for 0–1000 s under a vacuum of 5 × 10−5 Torr. Area fraction of austenite (γ) in the joint region decreased with increasing bonding temperature and holding time. In particular, when the holding time was sufficiently prolonged, γ phase-depleted region were observed in the base metal adjacent to the joint. Secondary phases formed at the joint after short holding times were predominantly chromium borides. For specimens bonded with longer holding times (up to 1000 s), boron nitrides were formed at the interface with the joint. Formation of secondary phases in a joint area during the bonding process was expressed by diffusion equation. Tensile strength was improved in accordance with increase of bonding temperature and holding time. Especially, the bonding efficiency was over 98% for the specimen held for 1000 s at 1473 K. Tensile strength of the joint after short holding times was affected by brittle residual eutectic constituents and borides, whereas tensile strength after longer holding time was influenced by boron nitride formed at the joint interface. The results showed that the mechanical properties were strongly related to the secondary phases (CrB, BN) formed during the bonding process.

Effect of cooling rate after heat treatment on pitting corrosion of super duplex stainless steel UNS S 32750

PurposeIn this study, super duplex stainless steel (SDSS) was heat-treated. The purpose of this study is to assess the effect of the cooling rate after heat treatment on the pitting corrosion of SDSS.Design/methodology/approachThe heat treatment from 1,000°C to 1,300°C was applied to SDSS to check the effect of the cooling rate.FindingsThe heat treatment temperature produced a different SDSS microstructure, and the cooling rate led to the growth of austenite. The casted SDSS indicated the presence of heterogeneous austenite, and the precipitation secondary phase under 1.6 per cent precipitated to bare metal. By applying heat treatment and cooling SDSS, its corrosion resistance changes because of the change in the chemical composition. The cooling rate at 5,600 J/s has the highest critical pitting temperature (CPT) at 1,100°C, and the cooling rate at 1.6 J/s has the highest CPT at 1,200°C. Low cooling rate (0.4 J/s) made the secondary phase at all temperature range.Research limitations/implicationsThe effect of secondary phase not consider because that is well known to decreasing corrosion resistance.Practical implicationsSolution annealing is taken into account to optimize the corrosion resistance. But that is not consider the cooling rate at each temperature. This study assessed the effect of the cooling rate at each temperature point.Social implicationsManufacturers need to know the effect of the cooling rate to optimize the corrosion resistance, and this study can be applied in the industrial scene.Originality/valueSDSS is hard the optimization because SDSS is a dual-phase stainless steel. Corrosion resistance can be optimized by controlling heat treatment temperature and the cooling rate. Anyone not studied the effect of the cooling rate at each temperature. The effect of the cooling rate should be considered to optimize the corrosion resistance.

A New Method to Recycle Stainless–Steel Duplex UNS S31803 Chips

Due to the increased consumption of raw materials, energy, and the waste it generates, recycling has become very important and fundamental for the environment and the industrial sector. The production of duplex stainless–steel powders with the addition of vanadium carbide in the high energy mechanical milling process is a new method for recycling materials for the manufacture of components in the industrial sector. This study aims to reuse the chips from the duplex stainless–steel UNS S31803 by powder metallurgy with the addition of Vanadium carbide (VC). The mechanical milling was performed using a planetary ball mill for 50 h at a milling speed of 350 rpm and a ball-to-powder weight ratio of 20:1, and the addition of 3 wt % of VC. The material submitted to milling with an addition of carbide has a particle size of less than 140 μm. After milling, the sample went through a stress relief treatment performed at 1050 °C for 1 h and the isostatic compaction process loaded with 300 MPa. The sintered powders and material was characterized by scanning electron microscopy, X-ray diffraction, and micro-hardness tests. The milling process with an addition of 3% VC produced a particle size smaller than the initial chip size. The measurement of micrometric sizes obtained was between 26 and 132 μm. The sintered material had a measurement of porosity evaluated at 15%. The obtained density of the material was 84% compared to the initial density of the material as stainless–steel duplex UNS S31803. The value of the microhardness measurement was 232 HV. The material submitted for grinding presented the formation of a martensitic structure and after the thermal treatment, the presence of ferrite and austenite phases was observed. Thus, in conclusion, this study demonstrates the efficacy in the production of a metal-ceramic composite using a new method to recycle stainless–steel duplex UNS S31803 chips.

Surface roughness analysis in finishing end-milling of duplex stainless steel UNS S32205

Duplex stainless steel (DSS) UNS S32205 presents high resistance to oxidation, corrosion, and corrosion under stress associated with good mechanical properties, thus being widely used in oil exploration and chemical treatment systems. Due to these characteristics, studies on surface quality are justified in the machining of such materials. In this work, dry machining end-milling tests were performed with positive geometry cutting tools (coated carbide inserts with tip radius of 0.4 mm) with different cutting parameters. Each cutting parameter was evaluated in three levels, combined through Box-Behnken Design of experiments (BBD): cutting speed (30, 45, 60 m/min), feed per tooth (0.05, 0.1, 0.15 mm/tooth), and depth of cut (0.2, 0.4, 0.6 mm). Two surface roughness parameters (Ra and Rz) were measured in the machined surfaces under different cutting combinations. Roughness profiles were analyzed for the highest and lowest measured values of surface roughness, and Abbott-Firestone Curve (AFC) and amplitude-probability distribution (APD) represented its statistical profile. In this way, appropriate combination of cutting parameters for minimum Ra and Rz parameters obtained after finishing end-milling of UNS S32205 were determined, as well as AFC and APD of these values, thus allowing the characterization of the performance of tool-workpiece pair in DSS cutting, resulting in Ra and Rz values of 0.182 and 1.302 μm, respectively. Low surface roughness values were validated using BBD, representation of the statistical profile, and optimization of cutting parameters improved the little bibliographical material in DSS milling.

Welding and electrochemical behavior of ferritic AISI 430 and austeno-ferritic UNS 32205 dissimilar welds

Stainless steel UNS 32205 and AISI 430 welded by arc welding process with the help of Mo-based austenitic and duplex electrodes. Microstructure studies of the welds revealed the complex microstructure with different solidification mode. The duplex weld showed the ferritic mode of solidification with the presence of various forms of austenite. While Mo-based austenitic weld illustrated ferritic-austenitic mode having ferrite as a cores of the subgrains in the austenite phase. The analysis of X-ray diffraction revealed the existance of carbide phase in the welds due to the presence of more carbon. Electron backscatter diffraction showed strong texture of both welded zone. Higher ferrite content was measured in duplex electrode weld. Higher degree of sensitization was measured in AISI 430 base metal and duplex weld. Higher pitting corrosion and film resistance were measured in duplex base metal and duplex electrode weld. More intergranular corrosion was observed in the heat affected zone of AISI 430.

Mechanistic study on micromilling of the super duplex stainless steel UNS S32750

Super duplex UNS S32750 is an important steel used in the deep oil industry that could be applied in the fabrication of microsensors and microactuators. However, there are no previous studies that could estimate cutting forces, no information in the literature about specific cutting force values for super duplex nor for similar stainless steels in micromilling. The present article analyzes three stainless steels and their behavior during micromilling: super duplex, which is a dual phase material, an austenitic and a ferritic stainless steel, chosen with similar mechanical properties to each phase of the first one. A set of experiments explored different feeds per tooth, as it is an important factor to identify mechanical behavior in micro scale, and cutting speed in order to develop a mechanistic force approach. The results showed that force coefficients for the dual phase material are located between the values of the austenitic and ferritic steels, indicating that heterogeneous super duplex steel microstructure can be modeled as a combination of ferritic and austenitic stainless steel coefficients. Lower feed per tooth experiments presented higher fluctuation of the maximum peak forces, maintaining average forces inside the predicted range.

Influence of small amount and different morphology of secondary phases on impact toughness of UNS S32205 Duplex Stainless Steel

Duplex Stainless Steels (DSSs) are biphasic austenite-ferrite stainless steels, with higher mechanical properties and corrosion resistance than the other stainless steels grades. Impact toughness is a very important mechanical property and even though DSSs have a lower impact toughness as compared to the austenitic grades, the transition to ductile to brittle behavior is more gradual and it occurs at lower temperature than ferritic grades. On the other hand, DSSs suffer from embrittlement due to secondary phase precipitation, which affects all mechanical and corrosion properties, in particular impact toughness, even in low amount.In this research paper, the influence of a small amount (< 2%) and different morphologies of secondary phases (coarse and finely dispersed) on the ductile to brittle transition of standard Duplex Stainless Steel UNS S32205 was studied. Two isothermal heat treatments were conducted on the solubilized DSS in order to precipitate the same amount but different morphologies of secondary phases. Charpy tests were conducted on a temperature range between 20 °C and − 196 °C.The wrought material retained a very good impact toughness even at − 90 °C (100 J), but a small volume fraction of secondary phases affected the impact toughness even at room temperature. Coarser secondary phases affect the impact toughness largely as compared to small and finely dispersed particles.

Comparison of pulsed and continuous current gas tungsten arc welding in dissimilar welding between UNS S32750 and AISI 321 in optimized condition

In this research, mechanical and corrosion properties of dissimilar joint between duplex stainless steel UNS S32750 duplex stainless steel and AISI 321 austenite stainless steel welded by pulsed current gas tungsten arc welding (PCGTAW) and continuous current gas tungsten arc welding (CCGTAW) were compared. The welding parameters and the filler metal were the same in both methods. Microstructural characterization of joint was determined by optical and scanning electron microscope. The pitting potential was evaluated by potentiodynamic polarization test in 3.5 wt.% NaCl solution at room temperature. Mechanical properties of optimal joint were investigated by impact test and vickers microhardness. Microstructural evaluation shows that the equal amounts of ferrite and austenite were obtained in PCGTAW weldment, while in CCGTAW weldment fraction of phases changed to 40% ferrite and 60% austenite. TiN precipitations were formed in fusion line of CCGTAW weldment on the side of AISI 321 austenite stainless steel. The average values of hardness in PCGTAW and CCGTAW weld zone were 241 and 225 HV0.2, respectively. The higher amount of ferrite in PCGTAW weldment in comparison with CCGTAW weldment was the reason of higher hardness average. The toughness of PCGTAW and CCGTAW was 106 and 102 J, respectively. Based on cyclic polarization test, pitting potential of weldment welded with PCGTAW and CCGTAW was 1.03 and 0.8 V, respectively, which is a proof of better pitting corrosion resistance of the weld zone in PCGTAW method.

Evolution of interface character distribution in duplex stainless steel processed by cross-rolling and annealing

Duplex stainless steel UNS S31803 samples were cross-rolled with a true strain of ε=2 followed by annealing at 1323K for 2min and 240min, respectively. The distributions of intervariant boundary planes in the precipitated austenite (A) from ferrite (F) and phase boundary planes conforming to Kurdjumov-Sache (K-S) orientation relationship (OR) were characterized by electron backscatter diffraction (EBSD) and the five-parameter analysis (FPA) method, respectively. The intervariant boundary planes with misorientation angle of 60° around <111> and <011> occur frequently and tend to terminate on the {111} plane. At the grain size level of 4μm, the phase boundary appears to be connected with the K-S OR terminating on {110}F‖{111}A at the early stage of annealing. When the grain size reaches approximately 20μm, phase boundary was modified into {541}F‖{533}A due to twinning in austenite during annealing.

Experimental study of electrical discharge drilling of stainless steel UNS S30400

In this study, overcut and taper angle were investigated in machining of stainless steel UNS S30400 against three different electrical discharge machining parameters which are electric current (Ip), pulse on-time (Ton) and pulse off-time (Toff). The electrode used was of 1 mm diameter with aspect ratio of 10. Dimensional accuracy was measured by evaluating overcut and taper angle. Those two measurements were performed using optical microscope model (Olympus BX41M, Japan). The experimentation planning, evaluation, analysis and optimization have been carried out using DOE software version 10.0.3 RSM based method with total number of twenty experiments. The research reveals that, discharge current was found to have the most significant effect on overcut and taper angle followed by pulse on-time and pulse off-time. As the discharge current and pulse on-time increase, overcut and taper angle are increased. However, when pulse off-time increases, overcut and taper angle decrease. The outcome result of this study will be very useful in the manufacturing industry to select the appropriate parameters for the selected work material. The model has shown a great accuracy with percentage error of less than 5%.

INFLUÊNCIA DE DIFERENTES TRATAMENTOS TÉRMICOS DE RECOZIMENTO NAS FRAÇÕES DE FASES E NAS PROPRIEDADES MECÂNICAS DO AÇO INOXIDÁVEL DUPLEX UNS S31803

Neste trabalho a influência de diferentes tratamentos térmicos nas frações de fases (ferrita, austenita e sigma) e seus efeitos nas propriedades mecânicas de um aço inoxidável duplex UNS S31803 foram investigados. A quantificação dessas fases foi feita por técnicas de microscopia óptica e microscopia eletrônica de varredura e as propriedades mecânicas avaliadas através de ensaios de tração uniaxial e impacto Charpy-V. As análises das fases primárias indicaram que a fração de austenita foi superior à de ferrita em todas as condições estudadas. A análise de fase sigma indicou que para uma mesma temperatura de recozimento, quanto maior o tempo de encharque, maiores as frações de sigma encontradas e maiores os valores de limite de resistência. Além disso, as amostras recozidas em menor tempo de encharque foram as que alcançaram os maiores valores de energia de absorvida.

Comparison Between the Parameters Obtained by Electrochemical DL EPR Techniques and Electrochemical Impedance Spectroscopy in the Corrosion Resistance of Duplex Stainless Steel UNS 31803

Os acos inoxidaveis tem ampla utilizacao em diversos segmentos industriais, porem quando submetidos a temperaturas superiores a 300oC podem dar origem a novas fases e precipitados que modificam as caracteristicas originais do material, como por exemplo, a resistencia a corrosao. Este artigo tem como objetivo comparar os parâmetros eletroquimicos relacionados a corrosao localizada obtidos atraves das tecnicas eletroquimicas de DL-EPR e espectroscopia de impedância eletroquimica (EIS). Para isto, amostras do aco inoxidavel duplex UNS S31803 foram tratadas termicamente as temperaturas de 645 °C, 820 °C e 960 °C durante 12 horas. Esses tratamentos resultaram na precipitacao de fases intermetalicas, sobretudo a fase sigma. Os ensaios de corrosao foram conduzidos em uma solucao 2 M H2SO4+ 0,5 M NaCl + 0,01 M KSCN, sendo o ensaio de EIS conduzido no potencial de circuito aberto (OCP) e no potencial de passivacao. A presenca de intermetalicos e fase sigma provocou a sensitizacao das amostras tratadas em 645 e 820oC, o que nao foi observado na amostra tratada em 960oC, que apresentou o efeito de recuperacao. Os resultados de cada tecnica foram comparados sendo observada uma boa relacao entre EIS e DL-EPR para o ensaio conduzido no potencial de passivacao, enquanto que o ensaio conduzido no potencial de OCP nao apresentou correspondencia entre as tecnicas ao comparar os parâmetros de simulacao de forma isolada.

Guidelines for Selecting Plugs Used in Thin-Walled Tube Drawing Processes of Metallic Alloys

In this paper, some practical guidelines to select the plug or set of plugs more adequate to carry out drawing processes of thin-walled tubes carried out with fixed conical inner plug are presented. For this purpose, the most relevant input parameters have been considered in this study: the tube material, the most important geometrical parameters of the process (die semiangle, α , and cross-sectional area reduction, r ) and the friction conditions (Coulomb friction coefficients, μ 1 , between the die and the tube outer surface, and μ 2 , between the plug and the tube inner surface). Three work-hardening materials are analyzed: the annealed copper UNS C11000, the aluminum UNS A91100, and the stainless steel UNS S34000. The analysis is realized by means of the upper bound method (UBM), modelling the plastic deformation zone by triangular rigid zones (TRZ), under the validated assumption that the process occurs under plane strain conditions. The obtained results allow establishing, for each material, a group of geometrical parameters, friction conditions, a set of plugs that make possible to carry out the process under good conditions, and the optimum plug to carry out the process using the minimum amount of energy. The proposed model is validated by means of an own finite element analysis (FEA) carried out under different conditions and, in addition, by other finite element method (FEM) simulations and real experiments taken from other researchers found in the literature (called literature simulations and literature experimental results, respectively). As a main conclusion, it is possible to affirm that the plug that allows carrying out the process with minimum quantity of energy is cylindrical in most cases.

Effects of austenite regeneration heat treatment on the TLP bonding of duplex stainless steel UNS S32750 using Fe-B-Si insert metal

This study discusses microstructural evolution during transient liquid phase (TLP) bonding and the effect of austenite regeneration heat treatment with regard to nitrogen-containing duplex stainless steel UNS S32750. TLP bonding was performed between the temperature range of 1423–1473 K for 0–1000 s under a vacuum of 5 × 10−5 Torr using an amorphous Fe-B-Si insert metal. The results showed that the volume fraction of austenite (γ) decreased with an increased bonding temperature and holding time. In the case of a prolonged holding time, a depleted area of the γ phase was observed in the base metal adjacent to joints. In order to prevent this phenomenon, an austenite regeneration heat treatment process was performed. The austenite volume fraction of the base metal and joint area increased with an increase in austenite regeneration heat treatment time; deviation of the austenite volume fraction between the base metal and joint area decreased. In this study, homogenization behaviors of joint microstructures were treated through a diffusion model. Bonding and austenite formation processes at the joints were divided into 4 steps: base metal dissolution, boride disappearance and joint widening, austenite growth to the joint, and homogenization.