Effect Of Sigma Phase Research Articles

Effect of Sigma Phase on Corrosion Behavior of Duplex Stainless Steel

Effect of Sigma Phase on Corrosion Behavior of Duplex Stainless Steel

Effect of Sigma Phase in Wire Arc Additive Manufacturing of Superduplex Stainless Steel

In the present study, the thermal program in wire and arc additive manufacturing has been varied in terms of heat input and interpass temperature. Three walls were completed with subsequent Charpy V impact toughness and crack-tip opening displacement fracture toughness, together with a detailed microstructure characterization using light microscopy and scanning and transmission electron microscopy. The results clearly demonstrate that the formation of sigma phase may deteriorate the toughness of superduplex components. Such formation may take place under prolonged cooling time, which may occur when subsequent passes are deposited with too high interpass temperatures. This transformation behavior may limit the productivity in additive manufacturing of such steels and care must be taken in selection of proper combination of arc energy and interpass temperature.

Application of electromechanical impedance technique in the monitoring of sigma phase embrittlement in duplex stainless steel

Duplex stainless steels (DSS) have a structure composed of austenite and ferrite in approximately equal volume fractions which provides an excellent integration between mechanical properties and corrosion resistance. However, the formation of intermetallic phases, such as sigma (σ) phase could become a major problem, once a small percentage of this phase originates a drastic deterioration of toughness and ductility, phenomenon known as σ phase embrittlement. In the present work, the effect of the isothermal aging treatment at 800 °C (temperature in which σ phase can be formed) on the degradation of mechanical properties of UNS S31803 DSS was investigated. The results showed that σ phase precipitation occurred regularly and by eutectoid decomposition. The effect of sigma phase on the mechanical properties was analyzed by the microhardness and impact tests at room temperature. The assessment of DSS also includes an σ phase embrittlement monitoring alternative method based on the electromechanical impedance (EMI), using piezoelectric sensors. The basic idea of the σ phase embrittlement monitoring method is that the increase in the hardness and the decrease in the impact toughness will leads to the variation in the EMI signatures by modifying structural stiffness. The proposed methodology is evaluated using the impedance module (|Z|), resistance (R), reactance (X) and by the statistical indexes: Relative Variation of Natural Frequencies (RVNF) and Root Mean Square Deviation (RMSD). This method is considered a nondestructive test, presents accurate results with high sensitivity and reliability providing capacity of determining sigma phase embrittlement and has promising application potential in this and other materials.

Influence of the phase balance and cooling conditions on the microstructure and corrosion behavior of solution-treated 1.4462 duplex stainless steel

In this study, effect of cooling rate on the microstructure and corrosion behavior of the solution-treated 1.4462 alloy was investigated. 1.4462 alloy was heat-treated at 1000 °C, 1120 °C and 1200 °C for different duration in order to obtain various phase balance. Two different cooling regimes (rapid cooling and steady cooling) were applied following the solution treatment processes. Also, effect of sigma phase was investigated together with ferrite and austenite phases for steady cooled samples. After heat treatment processes, samples were characterized by an optical microscope (OM), image analysis and scanning electron microscope (SEM). Moreover, hardness tests and electrochemical corrosion tests were applied. According to results, Ferrite ratio and ferrite grain size were increasing with increasing solution treatment temperature and time. Also, ferrite phase was more dominant compared to sigma phase for hardness of the solution-treated samples. Moreover, existing of sigma in higher amount promoted the observation of Epit value.

Effect of secondary phases on 475 °C embrittlement of 1.4462 and 1.4501 duplex stainless steels at short-term heat treatment conditions

Duplex stainless steels (DSSs) are classified according to their chemical composition. Also, the formation of the secondary phases can be defined as a main problem of the DSSs. In this study, decomposition of chromium and iron was studied at short-term heat treatment conditions, and also the effect of the intermetallic phase on the decomposition of chromium and iron was investigated. 1.4462 duplex stainless steel (DSS) and 1.4501 super duplex stainless steel were used in experimental studies. Both alloys were aged at 800 °C for 30 min for obtaining the intermetallic based sigma phase. Then, sigma-free and sigma-containing samples for both 1.4462 and 1.4501 alloys were treated at 475 °C for 24 h and 75 h, respectively. Samples were characterized by an optical microscope (OM) and scanning electron microscope (SEM). Effect of sigma phase on the formation of 475 °C embrittlement was conducted by electron diffraction analysis (EDS) and Vickers hardness measurements. It was determined that decomposition of the chromium and iron in a ferrite grain is accelerated by the presence of sigma phase. Sigma phase morphology and the ratio of the alloys did not change for both alloys during the heat treatment processes at 475 °C. Also increasing chromium content promoted the decomposition of chromium and iron in the ferrite phase. It was clearly shown that the sigma is more detrimental to the corrosion properties compared to the decomposition of chromium/iron at 475 °C.

Effects of Sigma Phase on the Embrittlement Behavior of Super Duplex Stainless Steel F55 under SSRT with a Cathodic Charge

Effects of Sigma Phase on the Embrittlement Behavior of Super Duplex Stainless Steel F55 under SSRT with a Cathodic Charge

Effect of sigma phase on thermophysical properties of Fe55Cr45 alloy

This paper presents the results of an experimental and modelling study to determine the thermophysical properties of α and σ phases in Fe55Cr45 (wt%) alloy. A comprehensive analytical model was used to predict the temperature dependent variation of enthalpy and specific heat, which are compared with experimental results. σ phase was synthesized from homogenized α alloy through heat treatment at 973 K for 300 h. Analysis of high temperature phase stability showed a magnetic transformation for α phase at 831 K and σ→α structural transformation for sigma phase at 1120 K with an enthalpy of 2.6 ± 0.2 kJ mol−1. Room temperature linear thermal expansion coefficient (CTE) for α and σ phases were obtained as 7.39 × 10−6 and 15.03 × 10−6 K-1 respectively. While the α phase exhibited an increase of 118% in CTE up to 1000 K, only 1.7% increase was observed for σ phase. Room temperature thermal conductivity for σ was much lower (3.1 W m−1 K−1) than that of the α phase (16.22 W m−1 K−1). The systematic experimental investigations carried out in the present study, established that formation of σ has an adverse effect on the heat transport properties of structural steels and their welds.

Effect of Sigma Phase Precipitated at 850 °C on Corrosion Behaviour of UNS S82441 Duplex Stainless Steel

Duplex stainless steels (DSS) are alloys with binary microstructure consisting of ferrite (δ) and austenite (γ), combining high mechanical properties and corrosion resistance by pitting and stress corrosion, due to the two phases austenite/ferrite. However, the formation of secondary and intermetallic phases, during solidification processes, heat treatment or welding in duplex steels tend to cause the degradation of its main properties. The present study aim to investigate the corrosion behaviour of UNS S82441 aged at 850°C for 30, 300 and 3000 minutes, due to the formation of sigma phase. Corrosion resistance evaluation was performed by means of stress corrosion testing, potenciodinamic polarization and mass loss. The microstructural characterization and morphology confirmed the presence of sigma phase in the UNS S82441 duplex stainless steel, and that the amount of this phase increases with the time of heat treatment of aging. The aging time influences negatively the corrosion resistance of this steel, with a gradual drop of up to 50% on passivation regime for the condition of aging at 850°C for 3000 minutes. Keywords: Duplex Stainless Steel. Corrosion. Sigma Phase. Heat Treatment of Aging.

Dubleks Paslanmaz Çeliklerde Sigma Fazının Korozyon Dayanımına Etkileri

800x600 Normal 0 21 false false false TR X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Normal Tablo"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman",serif;} Duplex stainless steels consistsof approximately half austenitic and half δ-ferritic phases and are preferred due to applications demanding both satisfactory corrosion and strength properties versus solely ferritic and austenitic grades. In this work; deleterious sigma phase that may form in microstructures of S32205 (2205) duplex stainless steel plates was intended to be quantified after various thermal conditions such as heat treatments, welding and service life and also the effects of sigma phase on microstructural and corrosion properties were investigated. Sigma phase was formed in two distinct raw material by applying heat treatments, and microstructural and corrosion states were analyzed. For this purpose; 800°C was adjusted as heat treatment temperature and two seperate specimens were heated through 5,5 and 16 hours of duration time in consequenceof the most rapidly formation rate of sigma phase at this temperature.

Effect of sigma phase on the low cycle fatigue property of Z3CN20.09M cast duplex stainless steel in high temperature water

The effect of sigma phase on the low cycle fatigue (LCF) property of Z3CN20.09M cast duplex stainless steel (DSS) was investigated by corrosion fatigue cracking test system and scanning electron microscopy (SEM). The results showed that the fatigue lives of the aged Z3CN20.09M DSS specimens decreased with the increase of aging time. The Cr‐depleted zones around the sigma phase are sensitive to pitting and preferential to forming fatigue cracks. Moreover, the precipitation of sigma phase was also found to degrade markedly the fatigue property of the DSS as it leads to fracture of ferrite during fatigue tests.

Effects of sigma phase and carbide on the wear behavior of CoCrMo alloys in Hanks' solution

This study aims to elucidate the synergy effects of the σ phase and carbide on the wear behavior of low-carbon (LC) and high-carbon (HC) cobalt–chromium–molybdenum (CoCrMo) alloys, by using pin-on-disc tests under Hanks' lubricated conditions. Fractured or torn-off σ-phase precipitates were observed to be the main reason for abrasion for both LC and HC alloys. Carbides were torn off at the initial high contact pressure to form pitting; σ-phase precipitates around the pitting were uprooted and led to micro cracks, which is considered as surface fatigue of HC alloy. In contrast, strain-induced martensite observed on the worn surface was contributed to the increase of hardness and abrasion resistance of LC alloy.

Characterization of Sigma Phase in Duplex Stainless Steels Using Non-destructive Ultrasonic Techniques

Abstract Duplex stainless steels have wide applications in industry due to their high mechanical strength, excellent toughness, and high corrosion resistance regarding to other types of steels. However, these steels are susceptible to form brittle phases during heat treatment associated to manufacture or welding processes, in particular the negative effect of sigma-phase into ferrite-austenitic microstructure has to be pointed out. In this work, a study is carried out about the use of ultrasonic nondestructive technique to establish a relationship between sigma phase signal percentages in 2507 duplex stainless steel and more relevant features of the collected ultrasonic signal in an equation. Thus, a novel method is obtained to perform the inspection of these materials in-situ in order to avoid the loss of mechanical resistance and to increase the susceptibility to localized corrosion in service.

Effect of sigma phase on the wear behavior of a super duplex stainless steel

The effect of sigma phase on the sliding wear behavior of a commercial super duplex stainless steel (sDSS) was investigated. The material was heat treated at temperatures from 875 to 975°C in order to promote the formation of sigma phase. Wear tests were carried out using ball on disc technique at constant velocity and different sliding conditions. Results show that the volume fraction of sigma phase increases the wear resistance under both dry and corrosive media although its detrimental consequences on corrosion resistance. The analysis of wear rate demonstrated that hardness introduced by sigma phase particles reduces significantly the plowing wear mechanism, typical of ductile abrasive wear, and plastic deformation in the subsurface leading to lower fatigue wear.

Effect of The Sigma Phase on the Mechanical Properties of a Cast Duplex Stainless Steel during the Ageing Treatment at 850°C

With about equal amount of austenite and ferrite in volume fraction, duplex stainless steel (DSS) is in advantage of mechanical properties and corrosive behaviors. Hence it is widely applied to the heavy castings for nuclear power plants inshore, such as impellers, pumps and valves. However, lots of cracks usually occur in these castings during manufacturing processes, because it is susceptible to precipitate the brittle intermetallic compound of sigma phase when the castings are exposed from 600 to 1000oC. In this work, the precipitation of sigma phase was observed by optical microscope (OM) and scanning electron microscope (SEM) in a cast DSS named as MAS/6001, which aged at 850oC from 5 to 300 minutes. The effect of sigma phase on the mechanical properties was analyzed by the tensile at room temperature and impact tests at -10°C. The results show that sigma phase in the MAS/6001 steel precipitated simultaneously with the secondary austenite, which obeyed the eutectoid reaction. The interfaces between austenite or secondary austenite and sigma phase were the locations where cracks generated from the void aggregation. Cracks are susceptible to propagate along or cross these interfaces, and to promote the sigma phase breaking-off, which severely deteriorated the mechanical properties.

Effect of sigma phase on near-threshold fatigue crack growth behavior and ultrasonic evaluation in AISI 316L stainless steel

In this work, we examined the influence of microstructural changes, such as an intermetallic sigma (σ) phase, on the fatigue behavior of high-temperature aged AISI 316L stainless steel. Nondestructive ultrasonic test and fatigue crack growth tests were performed to determine the threshold stress intensity factor of these artificially aged specimens. Ultrasonic test results characterizing the microstructural changes were compared with those of the fatigue tests to propose an empirical formula capable of predicting the threshold stress intensity factor by a nondestructive method. We observed a strong correlation between the increase in the volume fraction of the σ phase and the decrease of ΔKth. Ultrasonic velocity increased in response to the coarsening behavior of the σ phase in the vicinity of the grain boundaries.

Magnetic detection of sigma phase in duplex stainless steel UNS S31803

Duplex stainless steels are high strength and corrosion resistant steels extensively used in the chemical and petrochemical industry. The best mechanical properties and corrosion resistance are obtained with a microstructure composed by equal parts of ferrite and austenite and free from tertiary phases. Sigma phase is one of these deleterious tertiary phases. In the present work different amounts of sigma phase were precipitated by heat treatments in a UNS S31803 stainless steel. Some specimens were cold rolled before sigma phase precipitation in order to evaluate the effect of deformation on the magnetic measurements. The amount of sigma phase was precisely determined by microscopy and image analysis for each heat treatment condition. The effects of sigma phase on the steel properties were investigated, confirming the detrimental effects of very small percentages on corrosion resistance and toughness. Two magnetic methods were used to detect sigma phase: magnetization saturation measurements in a Vibrating Sample Magnetometer and ferritoscope testing. Both methods were found to be sensitive to small percentages of sigma phase in the microstructure.

Effect of Sigma Phase in Co-29Cr-6Mo Alloy on Corrosion and Mechanical Properties

Effect of the sigma (σ) phase in Co-29Cr-6Mo alloy on corrosion and mechanical behavior has been investigated. The area fraction of the σ phase varies depending on the aging time at 1023 K. The area fraction of the sigma phase increases with increasing aging time and reaches 0.6 % after aging at 1023 K for 21.6 ks. Inductively coupled plasma atomic emission spectrometer (ICP-AES) analysis revealed the quantity of released Co ion shows almost the same values regardless of different area fraction of the σ phase. In addition, plastic elongation and 0.2% proof strength exhibit almost the same values, although the alloys have different area fraction of the σ phase. These results suggest that a small amount of the σ phase (<0.6%) hardly affect the corrosion and mechanical properties in the Co-29Cr-6Mo alloy.

Effect of Sigma Phase in Co-29Cr-6Mo Alloy on Corrosion Behavior in Saline Solution

Effect of the sigma (a) phase in Co-29Cr-6Mo alloy on corrosion behavior in saline solution has been investigated. The area fraction of the a phase contained in Co-29Cr-6Mo alloy varies depending on the aging time at 1023 K. The a phase is mainly observed at grain boundaries. The area fraction of the σ phase increases with increasing aging time and reaches 0.6% after aging at 1023 K for 21.6 ks. The Co-29Cr-6Mo alloys aged at 1023 K for various aging times were immersed in saline solution at 310K for 1 week and metal ions released from the alloys were examined by inductively coupled plasma atomic emission spectrometer (ICP-AES). The quantity of a released Co ion does not depend on the aging time and shows almost the same value regardless of the different area fraction of the σ phase, while that of released Cr, Mo and Ni ions increases with increasing area fraction of the σ phase. Results of polarization test in saline solution at 310 K revealed that passive current density and breakdown potential of Co-29Cr-6Mo alloys aged at 1023 K for various aging times exhibit almost the same values, although the alloys have different area fraction of the σ phase. These results suggest that a small amount of the a phase (<0.6%) hardly affect the formation and breakdown of passive film in the Co-29Cr-6Mo alloy aged at 1023 K.

Effects of Sigma Phase on the Initiation and Propagation of Pitting Corrosion of Duplex Stainless Steel

Abstract Effects of σ phase on the initiation and propagation of pitting corrosion of duplex stainless steel were examined using critical pitting temperature (CPT) and electrochemical noise measurements in a ferric chloride solution, and the results were discussed with microstructural variations with aging time. Power spectral density (PSD) and corrosion admittance (Ac) spectrum as measures of the severity of localized corrosion were used to analyze the electrochemical noise quantitatively. With aging at 850°C, the passive film of the alloy becomes significantly unstable due to the initiation of metastable pitting corrosion, which is represented by the increase in PSD. In the admittance spectrum, the value of negative Ac (the state for pitting corrosion) was found to increase with aging. This may be attributed to the precipitation of σ phase, which depletes Cr and Mo around the α/σ and/or γ/σ phase boundaries.

二相ステンレス鋼溶接金属における水素とシグマ相の相互作用

The effect of sigma phase on hydrogen embrittlement in 329J3L duplex stainless weld metal was studied. In addition, in order to clarify the mechanism, hydrogen behavior was also investigated by means of internal friction and fractography technique. The internal friction measurements were conducted over the temperature range from 77K to 373K by using an inverted torsion pendulum for a frequency of about 1.5Hz. After hydrogen charging, almost hydrogen can be diffused easily into the austenite through the ferrite and ferrite/austenite interfaces. Consequently, a sharp significant peak due to hydrogen in austenite was detected at 245K in the as-welded specimen. In the case of as-welded specimen a little reduction of strength was recognized, while the significant loss of strength was observed in the weld metal containing significant sigma phase after hydrogen charging.The sigma phase clearly accelerated hydrogen embrittlement. In the hydrogen-charged specimen containing significant sigma phase, the internal friction analysis revealed that hydrogen entered in the sigma phase lattice and hydrogen was also trapped sigma/austenite phase boundaries. Furthermore, on the fracture surface, the many secondary cracks associated with interaction between sigma phase and hydrogen were observed. Therefore, it could be concluded that sigma phase itself and sigma/widmanstatten austenite phase boundaries were preferential hydrogen cracking sites. It was found that the reduction of notch tensile strength was mainly attributed to many secondary cracks associated with hydrogen and sigma phase.