321 stainless steel coiled tube chemical composition Mechanical properties and corrosion behavior of a duplex stainless steel weld with a new electrode

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Stainless Steel 321 Coil Tube Chemical Composition

The chemical composition of 321 stainless steel coil tubing is as follows:
- Carbon: 0.08% max
- Manganese: 2.00% max
- Nickel: 9.00% min

Stainless Steel 321 Coil Tube Mechanical Properties

According to the Stainless Steel 321 Coil Tube Manufacturer, the mechanical properties of stainless steel 321 coil tubing are tabulated below: Tensile Strength (psi) Yield Strength (psi) Elongation (%)

Applications & Uses of Stainless Steel 321 Coil Tube

In many engineering applications, the mechanical and corrosion properties of duplex stainless steel (DSS) welded structures are the most important factors. The current study investigated the mechanical properties and corrosion resistance of duplex stainless steel welds in an environment simulating 3.5% NaCl using a specially designed new electrode without the addition of alloying elements to the flux samples. Two different types of fluxes with a basic index of 2.40 and 0.40 were used on electrodes E1 and E2 for welding DSS boards, respectively. The thermal stability of the flux compositions was evaluated using thermogravimetric analysis. The chemical composition as well as the mechanical and corrosion properties of the welded joints were evaluated using emission spectroscopy in accordance with various ASTM standards. X-ray diffraction is used to determine the phases present in DSS welds, and scanning electron with EDS is used to inspect the microstructure of welds. The tensile strength of welded joints made by E1 electrodes was within 715-732 MPa, by E2 electrodes – 606-687 MPa. The welding current has been increased from 90 A to 110 A, and the hardness has also been increased. Welded joints with E1 electrodes coated with basic fluxes have better mechanical properties. The steel structure has high corrosion resistance in a 3.5% NaCl environment. This confirms the operability of welded joints made with newly developed electrodes. The results are discussed in terms of the depletion of alloying elements such as Cr and Mo observed in welds with coated electrodes E1 and E2, and the release of Cr2N in welds made using electrodes E1 and E2.
Historically, the first official mention of duplex stainless steel (DSS) dates back to 1927, when it was used only for certain castings and was not used in most technical applications due to its high carbon content1. But subsequently, the standard carbon content was reduced to a maximum value of 0.03%, and these steels became widely used in various fields2,3. DSS is a family of alloys with approximately equal amounts of ferrite and austenite. Research has shown that the ferritic phase in DSS provides excellent protection against chloride-induced stress corrosion cracking (SCC), which was an important issue for austenitic stainless steels (ASS) in the 20th century. On the other hand, in some engineering and other industries4 demand for storage is growing at a rate of up to 20% per year. This innovative steel with a two-phase austenitic-ferritic structure can be obtained by suitable composition selection, physical-chemical and thermomechanical refining. Compared to single-phase stainless steel, DSS has a higher yield strength and superior ability to withstand SCC5, 6, 7, 8. The duplex structure gives these steels unsurpassed strength, toughness and increased corrosion resistance in aggressive environments containing acids, acid chlorides, sea water and corrosive chemicals9. Due to the annual price fluctuations of nickel (Ni) alloys in the general market, the DSS structure, especially the low nickel type (lean DSS), has achieved many outstanding achievements compared to face centered cubic (FCC) iron10, 11. The main problem of ASE designs is in that they are subjected to various harsh conditions. Therefore, various engineering departments and companies are trying to promote alternative low nickel (Ni) stainless steels that perform as well as or better than traditional ASS with suitable weldability and are used in industrial applications such as sea water heat exchangers and the chemical industry. container 13 for environments with a high concentration of chlorides.
In modern technological progress, welded production plays a vital role. Typically, DSS structural members are joined by gas shielded arc welding or gas shielded arc welding. The weld is mainly affected by the composition of the electrode used for welding. Welding electrodes consist of two parts: metal and flux. Most often, electrodes are coated with flux, a mixture of metals that, when decomposed, release gases and form a protective slag to protect the weld from contamination, increase the stability of the arc, and add an alloying component to improve the quality of welding14. Cast iron, aluminum, stainless steel, mild steel, high strength steel, copper, brass, and bronze are some of the welding electrode metals, while cellulose, iron powder, and hydrogen are some of the flux materials used. Sometimes sodium, titanium and potassium are also added to the flux mixture.
Some researchers have tried to study the effect of electrode configuration on the mechanical and corrosion integrity of welded steel structures. Singh et al. 15 investigated the effect of flux composition on the elongation and tensile strength of welds welded by submerged arc welding. The results show that CaF2 and NiO are the main determinants of tensile strength compared to the presence of FeMn. Chirag et al.16 investigated SMAW compounds by varying the concentration of rutile (TiO2) in an electrode flux mixture. It was found that the properties of microhardness increased due to an increase in the percentage and migration of carbon and silicon. Kumar [17] studied the design and development of agglomerated fluxes for submerged arc welding of steel sheets. Nwigbo and Atuanya18 investigated the use of potassium-rich sodium silicate binders for the production of arc welding fluxes and found welds with a high tensile strength of 430 MPa and an acceptable grain structure. Lothongkum et al.19 used a potentiokinetic method to study the volume fraction of austenite in duplex stainless steel 28Cr–7Ni–O–0.34N in an air-saturated NaCl solution at a concentration of 3.5% wt. under pH conditions. and 27°C. Both duplex and micro duplex stainless steels show the same effect of nitrogen on corrosion behavior. Nitrogen did not affect the corrosion potential or rate at pH 7 and 10, however, the corrosion potential at pH 10 was lower than at pH 7. On the other hand, at all pH levels studied, the potential began to increase with increasing nitrogen content. Lacerda et al. 20 studied pitting of duplex stainless steels UNS S31803 and UNS S32304 in 3.5% NaCl solution using cyclic potentiodynamic polarization. In a 3.5 wt.% solution of NaCl, signs of pitting were found on the two investigated steel plates. UNS S31803 steel has a higher corrosion potential (Ecorr), pitting potential (Epit) and polarization resistance (Rp) than UNS S32304 steel. UNS S31803 steel has a higher repassivity than UNS S32304 steel. According to a study by Jiang et al. [21], the reactivation peak corresponding to the double phase (austenite and ferrite phase) of duplex stainless steel includes up to 65% of the ferrite composition, and the ferrite reactivation current density increases with increasing heat treatment time. It is well known that the austenitic and ferritic phases exhibit different electrochemical reactions at different electrochemical potentials21,22,23,24. Abdo et al.25 used potentiodynamic measurements of polarization spectroscopy and electrochemical impedance spectroscopy to study the electrochemically induced corrosion of laser-welded 2205 DSS alloy in artificial seawater (3.5% NaCl) under conditions of varying acidity and alkalinity. Pitting corrosion was observed on the exposed surfaces of the tested DSS specimens. Based on these findings, it was established that there is a proportional relationship between the pH of the dissolving medium and the resistance of the film formed in the process of charge transfer, which directly affects the formation of pitting and its specification. The purpose of this study was to understand how a newly developed welding electrode composition affects the mechanical and wear-resistant integrity of welded DSS 2205 in a 3.5% NaCl environment.
The flux minerals (ingredients) used in the electrode coating formulations were Calcium Carbonate (CaCO3) from Obajana District, Kogi State, Nigeria, Calcium Fluoride (CaF2) from Taraba State, Nigeria, Silicon Dioxide (SiO2), Talc Powder ( Mg3Si4O10(OH) )2) and rutile (TiO2) were obtained from Jos, Nigeria, and kaolin (Al2(OH)4Si2O5) was obtained from Kankara, Katsina State, Nigeria. Potassium silicate is used as a binder, it is obtained from India.
As shown in Table 1, the constituent oxides were independently weighed on a digital balance. It was then mixed with a potassium silicate binder (23% by weight) in an electric mixer (model: 641-048) from Indian Steel and Wire Products Ltd. (ISWP) for 30 minutes to obtain a homogeneous semi-solid paste. The wet mixed flux is pressed into a cylindrical shape from the briquetting machine and fed into the extrusion chamber at a pressure of 80 to 100 kg/cm2, and from the wire feed chamber is fed into the 3.15mm diameter stainless wire extruder. The flux is fed through a nozzle/die system and injected into the extruder to extrude the electrodes. A coverage factor of 1.70 mm was obtained, where the coverage factor is defined as the ratio of the electrode diameter to the strand diameter. Then the coated electrodes were dried in air for 24 hours and then calcined in a muffle furnace (model PH-248-0571/5448) at 150–250 °C\(-\) for 2 hours. Use the equation to calculate the alkalinity of the flow. (1) 26;
The thermal stability of flux samples of compositions E1 and E2 was determined using thermogravimetric analysis (TGA). A sample of approximately 25.33 mg of flux was loaded into the TGA for analysis. The experiments were carried out in an inert medium obtained by a continuous flow of N2 at a rate of 60 ml/min. The sample was heated from 30°C to 1000°C at a heating rate of 10°C/min. Following the methods mentioned by Wang et al.27, Xu et al.28 and Dagwa et al.29, thermal decomposition and weight loss of the samples at certain temperatures were assessed from TGA plots.
Process two 300 x 60 x 6 mm DSS plates to prepare for soldering. The V-groove was designed with a 3mm root gap, 2mm root hole and a 60° groove angle. The plate was then rinsed with acetone to remove possible contaminants. Weld the plates using a shielded metal arc welder (SMAW) with direct current electrode positive polarity (DCEP) using coated electrodes (E1 and E2) and a reference electrode (C) with a diameter of 3.15 mm. Electrical Discharge Machining (EDM) (Model: Excetek-V400) was used to machine welded steel specimens for mechanical testing and corrosion characterization. Table 2 shows the example code and description, and Table 3 shows the various welding operating parameters used to weld the DSS board. Equation (2) is used to calculate the corresponding heat input.
Using a Bruker Q8 MAGELLAN optical emission spectrometer (OES) with a wavelength of 110 to 800 nm and SQL database software, the chemical composition of weld joints of electrodes E1, E2 and C, as well as samples of the base metal, was determined. uses the gap between the electrode and the metal sample under test Generates electrical energy in the form of a spark. A sample of the components is vaporized and sprayed, followed by atomic excitation, which subsequently emits a specific line spectrum31. For qualitative analysis of the sample, the photomultiplier tube measures the presence of a dedicated spectrum for each element, as well as the intensity of the spectrum. Then use the equation to calculate the equivalent pitting resistance number (PREN). (3) Ratio 32 and the WRC 1992 state diagram are used to calculate the chromium and nickel equivalents (Creq and Nieq) from the equations. (4) and (5) are 33 and 34 respectively;
Note that PREN only takes into account the positive impact of the three main elements Cr, Mo and N, while the nitrogen factor x is in the range of 16-30. Typically, x is selected from the list of 16, 20, or 30. In research on duplex stainless steels, an intermediate value of 20 is most commonly used to calculate PREN35,36 values.
Welded joints made using different electrodes were tensile tested on a universal testing machine (Instron 8800 UTM) at a strain rate of 0.5 mm/min in accordance with ASTM E8-21. Tensile strength (UTS), 0.2% shear yield strength (YS), and elongation were calculated according to ASTM E8-2137.
DSS 2205 weldments were first ground and polished using different grit sizes (120, 220, 320, 400, 600, 800, 1000 and 1200) before hardness analysis. Welded specimens were made with electrodes E1, E2 and C. Hardness is measured at ten (10) points from the center of the weld to the base metal with an interval of 1 mm.
X-ray diffractometer (D8 Discover, Bruker, Germany) configured with Bruker XRD Commander software for data collection and Fe-filtered Cu-K-α radiation with an energy of 8.04 keV corresponding to a wavelength of 1.5406 Å and a scan rate of 3 ° Scan range (2θ) min-1 is 38 to 103° for phase analysis with E1, E2 and C and BM electrodes present in DSS welds. The Rietveld refinement method was used to index constituent phases using the MAUD software described by Lutterotti39. Based on ASTM E1245-03, a quantitative metallographic analysis of microscopic images of the weld joints of electrodes E1, E2 and C was carried out using Image J40 software. The results of calculating the volume fraction of the ferrite-austenitic phase, their average value and deviation are given in Table. 5. As shown in the sample configuration in fig. 6d, optical microscopy (OM) analysis was performed on PM and welded joints with electrodes E1 and E2 to study the morphology of the samples. The samples were polished with 120, 220, 320, 400, 600, 800, 1000, 1200, 1500, and 2000 grit silicon carbide (SiC) sandpaper. The samples were then electrolytically etched in a 10% aqueous oxalic acid solution at room temperature at voltage of 5 V for 10 s and placed on a LEICA DM 2500 M optical microscope for morphological characterization. Further polishing of the sample was performed using 2500 grit silicon carbide (SiC) paper for SEM-BSE analysis. In addition, the welded joints were examined for microstructure using an ultra-high resolution field emission scanning electron microscope (SEM) (FEI NOVA NANOSEM 430, USA) equipped with an EMF. A 20 × 10 × 6 mm sample was ground using various SiC sandpapers ranging in size from 120 to 2500. The samples were electrolytically etched in 40 g of NaOH and 100 ml of distilled water at a voltage of 5 V for 15 s, and then mounted on a sample holder , located in the SEM chamber, for analyzing samples after purging the chamber with nitrogen. An electron beam generated by a heated tungsten filament creates a grating on the sample to produce images at various magnifications, and EMF results have been obtained using the methods of Roche et al. 41 and Mokobi 42 .
An electrochemical potentiodynamic polarization method according to ASTM G59-9743 and ASTM G5-1444 was used to evaluate the degradation potential of DSS 2205 plates welded with E1, E2 and C electrodes in a 3.5% NaCl environment. Electrochemical tests were performed using a computer-controlled Potentiostat-Galvanostat/ZRA apparatus (model: PC4/750, Gamry Instruments, USA). Electrochemical testing was carried out on a three-electrode test setup: DSS 2205 as working electrode, saturated calomel electrode (SCE) as reference electrode and graphite rod as counter electrode. The measurements were carried out using an electrochemical cell, in which the area of ​​action of the solution was the area of ​​the working electrode 0.78 cm2. Measurements were made between -1.0 V to +1.6 V potentials on a pre-stabilized OCP (relative to OCP) at a scan rate of 1.0 mV/s.
Electrochemical pitting critical temperature tests were carried out in 3.5% NaCl to evaluate the pitting resistance of welds made with E1, E2, and C electrodes. clearly on the pitting potential in the PB (between the passive and transpassive regions), and welded specimens with E1, E2, Electrodes C. Therefore, CPT measurements are performed to accurately determine the pitting potential of welding consumables. CPT testing was conducted in accordance with duplex stainless steel weld reports45 and ASTM G150-1846. From each of the steels to be welded (S-110A, E1-110A, E2-90A), samples with an area of ​​1 cm2 were cut, including the base, weld, and HAZ zones. The samples were polished using sandpaper and a 1 µm alumina powder slurry in accordance with standard metallographic sample preparation procedures. After polishing, the samples were ultrasonically cleaned in acetone for 2 min. A 3.5% NaCl test solution was added to the CPT test cell and the initial temperature was adjusted to 25°C using a thermostat (Neslab RTE-111). After reaching the initial test temperature of 25°C, the Ar gas was blown for 15 min, then the samples were placed in the cell, and the OCF was measured for 15 min. The sample was then polarized by applying a voltage of 0.3 V at an initial temperature of 25°C, and the current was measured for 10 min45. Start heating the solution at a rate of 1 °C/min to 50 °C. During the heating of the test solution, the temperature sensor is used to continuously monitor the temperature of the solution and store time and temperature data, and the potentiostat/galvanostat is used to measure the current. A graphite electrode was used as the counter electrode, and all potentials were measured relative to the Ag/AgCl reference electrode. Argon purge was performed throughout the test.
On fig. 1 shows the composition (in weight percent) of the flux components F1 and F2 used for the production of alkaline (E1) and acidic (E2) electrodes, respectively. The flux basicity index is used to predict the mechanical and metallurgical properties of welded joints. F1 is the component of the flux used to coat the E1 electrodes, which is called alkaline flux because its basic index is > 1.2 (i.e. 2.40), and F2 is the flux used to coat the E2 electrodes, called acid flux due to its basicity index < 0.9 (i.e. 2.40). 0.40). It is clear that electrodes coated with basic fluxes in most cases have better mechanical properties than electrodes coated with acidic fluxes. This characteristic is a function of the dominance of the basic oxide in the flux composition system for electrode E1. On the contrary, the slag removal (separability) and low spatter observed in joints welded with E2 electrodes are characteristic of electrodes with an acidic flux coating with a high content of rutile. This observation is consistent with the findings of Gill47 that the effect of rutile content on slag detachability and the low spatter of acid flux coated electrodes contributes to rapid slag freezing. Kaolin in the flux system used to coat electrodes E1 and E2 was used as a lubricant, and talc powder improved the extrudability of the electrodes. Potassium silicate binders in flux systems contribute to better arc ignition and performance stability, and, in addition to their adhesive properties, improve slag separation in welded products. Since CaCO3 is a net breaker (slag breaker) in the flux and tends to generate a lot of smoke during welding due to thermal decomposition into CaO and about 44% CO2, TiO2 (as a net builder / slag former) helps to reduce the amount of smoke during welding . welding and thus improve slag detachability as suggested by Jing et al.48. Fluorine Flux (CaF2) is a chemically aggressive flux that improves solder cleanliness. Jastrzębska et al. 49 reported the effect of the fluoride composition of this flux composition on weld cleanliness properties. Typically, flux is added to the weld area to improve arc stability, add alloying elements, build up slag, increase productivity, and improve the quality of the weld pool 50.
The TGA-DTG curves shown in Figs. 2a and 2b show a three-stage weight loss upon heating in the temperature range of 30–1000°C in a nitrogen atmosphere. The results in Figures 2a and b show that for basic and acidic flux samples, the TGA curve drops straight down until it finally becomes parallel to the temperature axis, around 866.49°C and 849.10°C respectively. Weight loss of 1.30% and 0.81% at the beginning of the TGA curves in Fig. 2a and 2b is due to moisture absorbed by the flux components, as well as evaporation and dehydration of surface moisture. The main decompositions of samples of the main flux at the second and third stages in fig. 2a occurred in the temperature ranges 619.45°C–766.36°C and 766.36°C–866.49°C, and the percentage of their weight loss was 2.84 and 9.48%. , respectively. While for the acidic flux samples in Fig. 7b, which were in the temperature ranges of 665.23°C–745.37°C and 745.37°C–849.10°C, their percentage weight loss was 0.81 and 6.73%, respectively, which was attributed to thermal decomposition. Since the flux components are inorganic, the volatiles are limited to the flux mixture. Therefore, reduction and oxidation are terrible. This is consistent with the results of Balogun et al.51, Kamli et al.52 and Adeleke et al.53. The sum of the mass loss of the flux sample observed in fig. 2a and 2b is 13.26% and 8.43%, respectively. Less mass loss of flux samples in fig. 2b is due to the high melting points of TiO2 and SiO2 (1843 and 1710°C respectively) as the main oxides that make up the flux mixture54,55, while TiO2 and SiO2 have lower melting points. melting point Primary oxide: CaCO3 (825 °C) in the flux sample in fig. 2a56. These changes in the melting point of primary oxides in flux mixtures are well reported by Shi et al.54, Ringdalen et al.55 and Du et al.56. Observing continuous weight loss in Fig. 2a and 2b, it can be concluded that the flux samples used in the E1 and E2 electrode coatings undergo one-step decomposition, as suggested by Brown57. The temperature range of the process can be seen from the derivative curves (wt%) in fig. 2a and b. Since the TGA curve cannot accurately describe the specific temperature at which the flux system undergoes phase change and crystallization, the TGA derivative is used to determine the exact temperature value of each phenomenon (phase change) as an endothermic peak to prepare the flux system.
TGA-DTG curves showing thermal decomposition of (a) alkaline flux for E1 electrode coating and (b) acidic flux for E2 electrode coating.
Table 4 shows the results of spectrophotometric analysis and SEM-EDS analysis of DSS 2205 base metal and welds made using E1, E2 and C electrodes. E1 and E2 showed that the content of chromium (Cr) decreased sharply to 18.94 and 17.04%, and the content of molybdenum (Mo) was 0.06 and 0.08%, respectively. the values ​​of welds with electrodes E1 and E2 are lower. This is slightly in line with the calculated PREN value for the ferritic-austenitic phase from the SEM-EDS analysis. Therefore, it can be seen that pitting begins at the stage with low PREN values ​​(welds from E1 and E2), basically as described in Table 4. This is indicative of depletion and possible precipitation of the alloy in the weld. Subsequently, the reduction in the content of Cr and Mo alloying elements in welds produced using electrodes E1 and E2 and their low pitting equivalent values ​​(PREN) are shown in Table 4, which creates a problem for maintaining resistance in aggressive environments, especially in chloride environments. -containing environment. The relatively high nickel (Ni) content of 11.14% and the allowable limit of manganese content in the welded joints of the E1 and E2 electrodes may have had a positive effect on the mechanical properties of weldments used in conditions simulating sea water (Fig. 3). were made using the work of Yuan and Oy58 and Jing et al.48 on the effect of high nickel and manganese compositions on improving the mechanical properties of DSS welded structures under severe operating conditions.
Tensile test results for (a) UTS and 0.2% sag YS and (b) uniform and full elongation and their standard deviations.
The strength properties of the base material (BM) and welded joints made from the developed electrodes (E1 and E2) and a commercially available electrode (C) were evaluated at two different welding currents of 90 A and 110 A. 3(a) and (b) show UTS, YS with 0.2% offset, along with their elongation and standard deviation data. The UTS and YS offset results of 0.2% obtained from Figs. 3a show the optimal values ​​for sample no. 1 (BM), sample no. 3 (weld E1), sample no. 5 (weld E2) and sample no. 6 (welds with C) are 878 and 616 MPa, 732 and 497 MPa , 687 and 461 MPa and 769 and 549 MPa, respectively, and their respective standard deviations. From fig. 110 A) are samples numbered 1, 2, 3, 6 and 7, respectively, with minimum recommended tensile properties in excess of 450 MPa in tensile test and 620 MPa in tensile test proposed by Grocki32. The elongation of welding specimens with electrodes E1, E2 and C, represented by samples No. 2, No. 3, No. 4, No. 5, No. 6 and No. 7, at welding currents of 90 A and 110 A, respectively, reflects plasticity and honesty. relation to base metals. The lower elongation was explained by possible welding defects or the composition of the electrode flux (Fig. 3b). It can be concluded that BM duplex stainless steel and welded joints with E1, E2 and C electrodes in general have significantly higher tensile properties due to their relatively high nickel content (Table 4), but this property was observed in welded joints. Less effective E2 is obtained from the acidic composition of the flux. Gunn59 demonstrated the effect of nickel alloys on improving the mechanical properties of welded joints and controlling phase equilibrium and element distribution. This again confirms the fact that electrodes made from basic flux compositions have better mechanical properties than electrodes made from acidic flux mixtures, as suggested by Bang et al.60. Thus, a significant contribution has been made to the existing knowledge about the properties of the welded joint of the new coated electrode (E1) with good tensile properties.
On fig. Figures 4a and 4b show the Vickers microhardness characteristics of experimental samples of welded joints of electrodes E1, E2 and C. 4a shows the hardness results obtained from one direction of the sample (from WZ to BM), and in fig. 4b shows the hardness results obtained on both sides of the sample. The hardness values ​​obtained during welding of samples Nos. 2, 3, 4 and 5, which are welded joints with electrodes E1 and E2, can be due to the coarse-grained structure during solidification in welding cycles. A sharp increase in hardness was observed both in the coarse-grained HAZ and in the fine-grained HAZ of all samples Nos. 2-7 (see sample codes in Table 2), which can be explained by a possible change in the microstructure of the weld as a result of chromium-weld samples are rich in emissions (Cr23C6) . Compared with other welding samples 2, 3, 4 and 5, the hardness values ​​of the welded joints of samples No. 6 and 7 in Figs. 4a and 4b above (Table 2). According to Mohammed et al.61 and Nowacki and Lukoje62, this may be due to the high ferrite δ value and induced residual stresses in the weld, as well as depletion of alloying elements such as Mo and Cr in the weld. The hardness values ​​of all considered experimental samples in the area of ​​BM seem to be consistent. The trend in the results of hardness analysis of welded specimens is consistent with the conclusions of other researchers61,63,64.
Hardness values ​​of welded joints of DSS specimens (a) half-section of welded specimens and (b) full section of welded joints.
The various phases present in the welded DSS 2205 with E1, E2 and C electrodes were obtained and the XRD spectra for the diffraction angle 2\(\theta\) are shown in Fig. 5. Peaks of austenite (\(\gamma\)) and ferrite (\(\alpha\)) phases were identified at diffraction angles of 43° and 44°, conclusively confirming that the weld composition is two-phase 65 stainless steel. that DSS BM shows only austenitic (\(\gamma\)) and ferritic (\(\alpha\)) phases, confirming the microstructural results presented in Figures 1 and 2. 6c, 7c and 9c. The ferritic (\(\alpha\)) phase observed with DSS BM and the high peak in the weld to electrode C are indicative of its corrosion resistance, since this phase aims to increase the corrosion resistance of the steel, as Davison and Redmond66 have stated , the presence of ferrite stabilizing elements, such as Cr and Mo, effectively stabilizes the passive film of the material in chloride-containing environments. Table 5 shows the ferrite-austenitic phase by quantitative metallography. The ratio of the volume fraction of the ferrite-austenitic phase in the welded joints of the electrode C is achieved approximately (≈1:1). The low ferrite (\(\alpha\)) phase composition of weldments using E1 and E2 electrodes in the volume fraction results (Table 5) indicates a possible sensitivity to a corrosive environment, which was confirmed by electrochemical analysis. confirmed (Fig. 10a,b)), since the ferrite phase provides high strength and protection against chloride-induced stress corrosion cracking. This is further confirmed by the low hardness values ​​observed in the welds of electrodes E1 and E2 in fig. 4a,b, which are caused by the low proportion of ferrite in the steel structure (Table 5). The presence of unbalanced austenitic (\(\gamma\)) and ferritic (\(\alpha\)) phases in welded joints using E2 electrodes indicates the actual vulnerability of steel to uniform corrosion attack. On the contrary, the XPA spectra of two-phase steels of welded joints with E1 and C electrodes, along with the results of BM, usually indicate the presence of austenitic and ferritic stabilizing elements, which makes the material useful in construction and the petrochemical industry, because argued Jimenez et al.65; Davidson & Redmond66; Shamant and others67.
Optical micrographs of welded joints of E1 electrodes with different weld geometries: (a) HAZ showing the fusion line, (b) HAZ showing the fusion line at higher magnification, (c) BM for the ferritic-austenitic phase, (d) weld geometry, ( e) Shows the transition zone nearby, (f) HAZ shows the ferritic-austenitic phase at higher magnification, (g) Weld zone shows the ferritic-austenitic phase Tensile phase.
Optical micrographs of E2 electrode welds at various weld geometries: (a) HAZ showing the fusion line, (b) HAZ showing the fusion line at higher magnification, (c) BM for the ferritic-austenitic bulk phase, (d) weld geometry , (e) ) showing the transition zone in the vicinity, (f) HAZ showing the ferritic-austenitic phase at higher magnification, (g) welding zone showing the ferritic-austenitic phase.
Figures 6a–c and, for example, show the metallographic structure of DSS joints welded using an E1 electrode at various welding geometries (Figure 6d), indicating where the optical micrographs were taken at different magnifications. On fig. 6a, b, f – transition zones of welded joints, demonstrating the phase equilibrium structure of ferrite-austenite. Figures 7a-c and for example also show the OM of a DSS joint welded using an E2 electrode at various welding geometries (Figure 7d), representing the OM analysis points at different magnifications. On fig. 7a,b,f show the transition zone of a welded joint in ferritic-austenitic equilibrium. OM in the welding zone (WZ) is shown in fig. 1 and fig. 2. Welds for electrodes E1 and E2 6g and 7g, respectively. OM on BM is shown in Figures 1 and 2. In fig. 6c, e and 7c, e show the case of welded joints with electrodes E1 and E2, respectively. The light area is the austenite phase and the dark black area is the ferrite phase. Phase equilibria in the heat-affected zone (HAZ) near the fusion line indicated the formation of Cr2N precipitates, as shown in the SEM-BSE micrographs in Figs. 8a,b and confirmed in fig. 9a,b. The presence of Cr2N observed in the ferrite phase of the samples in Figs. 8a,b and confirmed by SEM-EMF point analysis and EMF line diagrams of welded parts (Fig. 9a-b), is due to the higher welding heat temperature. Circulation accelerates the introduction of chromium and nitrogen, since high temperature in the weld increases the diffusion coefficient of nitrogen. These results support studies by Ramirez et al.68 and Herenyu et al.69 showing that, regardless of nitrogen content, Cr2N is usually deposited on ferrite grains, grain boundaries, and α/\(\gamma\) boundaries, as also suggested by other researchers. 70.71.
(a) spot SEM-EMF analysis (1, 2 and 3) of a welded joint with E2;
The surface morphology of representative samples and their corresponding EMFs are shown in Figs. 10a–c. On fig. Figures 10a and 10b show SEM micrographs and their EMF spectra of welded joints using electrodes E1 and E2 in the welding zone, respectively, and in fig. 10c shows SEM micrographs and EMF spectra of OM containing austenite (\(\gamma\)) and ferrite (\(\alpha\)) phases without any precipitates. As shown in the EDS spectrum in Fig. 10a, the percentage of Cr (21.69 wt.%) and Mo (2.65 wt.%) compared to 6.25 wt.% Ni gives a sense of the corresponding balance of the ferrite-austenitic phase. Microstructure with a high reduction in the content of chromium (15.97 wt.%) and molybdenum (1.06 wt.%) compared with a high content of nickel (10.08 wt.%) in the microstructure of the welded joint of electrode E2, shown in fig. 1. Compare. EMF spectrum 10b. The acicular shape with finer-grained austenitic structure seen in the WZ shown in fig. 10b confirms the possible depletion of the ferritizing elements (Cr and Mo) in the weld and the precipitation of chromium nitride (Cr2N) – the austenitic phase. The distribution of precipitation particles along the boundaries of the austenitic (\(\gamma\)) and ferritic (\(\alpha\)) phases of DSS welded joints confirms this statement72,73,74. This also results in its poor corrosion performance, since Cr is considered to be the main element for forming a passive film that improves the local corrosion resistance of steel59,75 as shown in Fig. 10b. It can be seen that the BM in the SEM micrograph in Fig. 10c shows strong grain refinement as its EDS spectrum results show Cr (23.32 wt%), Mo (3.33 wt%) and Ni (6.32 wt). %) good chemical properties. %) as an important alloying element for checking the equilibrium microstructure of the ferrite-austenitic phase of the DSS76 structure. The results of the compositional EMF spectroscopic analysis of the welded joints of the E1 electrode justify its use in construction and slightly aggressive environments, since the austenite formers and ferrite stabilizers in the microstructure comply with the DSS AISI 220541.72 standard for welded joints, 77.
SEM micrographs of welded joints, where (a) electrode E1 of the welding zone has an EMF spectrum, (b) electrode E2 of the welding zone has an EMF spectrum, (c) OM has an EMF spectrum.
In practice, it has been observed that DSS welds solidify in a fully ferritic (F-mode) mode, with austenite nuclei nucleating below the ferritic solvus temperature, which is mainly dependent on the chromium to nickel equivalent ratio (Creq/Nieq) (>1.95 constitutes mode F) Some researchers have noticed this effect of steel due to the strong diffusing ability of Cr and Mo as ferrite-forming elements in the ferrite phase8078,79. It is clear that DSS 2205 BM contains a high amount of Cr and Mo (showing higher Creq), but has a lower Ni content than the weld with E1, E2 and C electrodes, which contributes to a higher Creq/Nieq ratio. This is also evident in the current study, as shown in Table 4, where the Creq/Nieq ratio was determined for DSS 2205 BM above 1.95. It can be seen that welds with electrodes E1, E2 and C harden in austenitic-ferritic mode (AF mode), austenitic mode (A mode) and ferritic-austenitic mode, respectively, due to the higher content of bulk mode (FA mode). ), as shown in Table 4, the content of Ni, Cr and Mo in the weld is less, indicating that the Creq/Nieq ratio is lower than that of BM. The primary ferrite in the E2 electrode welds had a vermicular ferrite morphology and the determined Creq/Nieq ratio was 1.20 as described in Table 4.
On fig. 11a shows Open Circuit Potential (OCP) versus time for an AISI DSS 2205 steel structure in 3.5% NaCl solution. It can be seen that the ORP curve shifts towards a more positive potential, indicating the appearance of a passive film on the surface of the metal sample, a drop in potential indicates generalized corrosion, and a nearly constant potential over time indicates the formation of a passive film over time. , The surface of the sample is stable and has a Sticky 77. The curves depict the experimental substrates under stable conditions for all samples in an electrolyte containing 3.5% NaCl solution, with the exception of sample 7 (weld joint with C-electrode), which shows little instability. This instability can be compared to the presence of chloride ions (Cl-) in solution, which can greatly accelerate the corrosion reaction, thereby increasing the degree of corrosion. Observations during OCP scanning without applied potential showed that Cl in the reaction can affect the resistance and thermodynamic stability of the samples in aggressive environments. Ma et al. 81 and Lotho et al. 5 confirmed the claim that Cl- plays a role in accelerating the degradation of passive films on substrates, thereby contributing to further wear.
Electrochemical analysis of the studied samples: (a) evolution of the RSD depending on time and (b) potentiodynamic polarization of the samples in 3.5% NaCl solution.
On fig. 11b presents a comparative analysis of the potentiodynamic polarization curves (PPC) of welded joints of electrodes E1, E2 and C under the influence of a 3.5% NaCl solution. Welded BM samples in PPC and 3.5% NaCl solution showed passive behavior. Table 5 shows the electrochemical analysis parameters of the samples obtained from the PPC curves, such as Ecorr (corrosion potential) and Epit (pitting corrosion potential) and their associated deviations. Compared to other samples No. 2 and No. 5, welded with electrodes E1 and E2, samples No. 1 and No. 7 (BM and welded joints with electrode C) showed a high potential for pitting corrosion in NaCl solution (Fig. 11b). The higher passivating properties of the former compared to the latter are due to the balance of the microstructural composition of the steel (austenitic and ferritic phases) and the concentration of alloying elements. Due to the presence of ferrite and austenitic phases in the microstructure, Resendea et al. 82 supported the passive behavior of DSS in aggressive media. The low performance of samples welded with E1 and E2 electrodes can be associated with depletion of the main alloying elements, such as Cr and Mo, in the welding zone (WZ), since they stabilize the ferrite phase (Cr and Mo), act as passivators Alloys in the austenitic phase of oxidized steels. The effect of these elements on pitting resistance is greater in the austenitic phase than in the ferritic phase. For this reason, the ferritic phase undergoes passivation faster than the austenitic phase associated with the first passivation region of the polarization curve. These elements have a significant impact on DSS pitting resistance due to their higher pitting resistance in the austenitic phase compared to the ferritic phase. Therefore, the fast passivation of the ferrite phase is 81% higher than that of the austenite phase. Although Cl- in solution has a strong negative effect on the passivating ability of the steel film83. Consequently, the stability of the passivating film of the sample will be greatly reduced84. From Table. 6 also shows that the corrosion potential (Ecorr) of welded joints with E1 electrode is somewhat less stable in solution compared to welded joints with E2 electrode. This is also confirmed by the low values ​​of the hardness of welds using electrodes E1 and E2 in fig. 4a,b, which is due to the low content of ferrite (Table 5) and the low content of chromium and molybdenum (Table 4) in the steel structure made of. It can be concluded that the corrosion resistance of steels in the simulated marine environment increases with decreasing welding current and decreases with low Cr and Mo content and low ferrite content. This statement is consistent with a study by Salim et al.85 on the effect of welding parameters such as welding current on the corrosion integrity of welded steels. As chloride penetrates the steel through various means such as capillary absorption and diffusion, pits (pitting corrosion) of uneven shape and depth are formed. The mechanism is significantly different in higher pH solutions where the surrounding (OH-) groups are simply attracted to the steel surface, stabilizing the passive film and providing additional protection to the steel surface25,86. The best corrosion resistance of samples No. 1 and No. 7 is mainly due to the presence in the steel structure of a large amount of δ-ferrite (Table 5) and a large amount of Cr and Mo (Table 4), since the level of pitting corrosion is mainly present in steel, welded by the DSS method, in the austenitic-phase structure of the parts. Thus, the chemical composition of the alloy plays a decisive role in the corrosion performance of the welded joint87,88. In addition, it was observed that the specimens welded using the E1 and C electrodes in this study showed lower Ecorr values ​​from the PPC curves than those welded using the E2 electrode from the OCP curves (Table 5). Therefore, the anode region starts at a lower potential. This change is mainly due to the partial stabilization of the passivation layer formed on the surface of the sample and the cathodic polarization that occurs before full stabilization of OCP89 is achieved. On fig. 12a and b show 3D optical profiler images of experimentally corroded specimens under various welding conditions. It can be seen that the pitting corrosion size of the specimens increases with the lower pitting corrosion potential created by the high welding current of 110 A (Fig. 12b), comparable to the pitting corrosion size obtained for welds with a lower welding current ratio of 90 A. (Fig. 12a ). This confirms Mohammed90′s claim that slip bands are formed on the surface of the sample to destroy the surface passivation film by exposing the substrate to a 3.5% NaCl solution so that the chloride begins to attack, causing the material to dissolve.
The SEM-EDS analysis in Table 4 shows that the PREN values ​​of each austenitic phase are higher than those of ferrite in all welds and BM. The initiation of pitting at the ferrite/austenite interface accelerates the destruction of the passive material layer due to the inhomogeneity and segregation of elements occurring in these areas91. Unlike the austenitic phase, where the pitting resistance equivalent (PRE) value is higher, pitting initiation in the ferritic phase is due to the lower PRE value (Table 4). The austenite phase seems to contain a significant amount of austenite stabilizer (nitrogen solubility), which provides a higher concentration of this element and, therefore, higher resistance to pitting92.
On fig. Figure 13 shows critical pitting temperature curves for E1, E2, and C welds. Given that the current density increased to 100 µA/cm2 due to pitting during the ASTM test, it is clear that the @110A weld with E1 showed a minimum pitting critical temperature of 27.5°C followed by E2 @ 90A soldering shows a CPT of 40°C, and in the case of C@110A the highest CPT is 41°C. The observed results are in good agreement with the observed results of polarization tests.
The mechanical properties and corrosion behavior of duplex stainless steel welds were investigated using the new E1 and E2 electrodes. The alkaline electrode (E1) and the acidic electrode (E2) used in the SMAW process were successfully coated with a flux composition with an overall coverage ratio of 1.7 mm and an alkaline index of 2.40 and 0.40, respectively. The thermal stability of fluxes prepared using TGA in an inert medium has been evaluated. The presence of a high content of TiO2 (%) in the flux matrix improved the slag removal of weldments for electrodes coated with acidic flux (E2) compared to electrodes coated with basic flux (E1). Although the two coated electrodes (E1 and E2) have a good arc starting ability. Welding conditions, especially heat input, welding current and speed, play a critical role in achieving the austenite/ferrite phase balance of DSS 2205 welds and the excellent mechanical properties of the weld. The joints welded with the E1 electrode showed excellent tensile properties (shear 0.2% YS = 497 MPa and UTS = 732 MPa), confirming that the basic flux coated electrodes have a high basicity index compared to the acid flux coated electrodes. Electrodes exhibit better mechanical properties with low alkalinity. It is obvious that in the welded joints of electrodes with a new coating (E1 and E2) there is no equilibrium of the ferrite-austenitic phase, which was revealed using OES and SEM-EDS analysis of the weld and quantified by the volume fraction in the weld. Metallography confirmed their SEM study. microstructures. This is mainly due to the depletion of alloying elements such as Cr and Mo and the possible release of Cr2N during welding, which is confirmed by EDS line scanning. This is further supported by the low hardness values ​​observed in welds with E1 and E2 electrodes due to their low proportion of ferrite and alloying elements in the steel structure. The Evidence Corrosion Potential (Ecorr) of the welds using the E1 electrode proved to be slightly less resistant to solution corrosion compared to the welds using the E2 electrode. This confirms the effectiveness of the newly developed electrodes in welds tested in 3.5% NaCl environment without flux mixture alloy composition. It can be concluded that the corrosion resistance in the simulated marine environment increases with decreasing welding current. Thus, the precipitation of carbides and nitrides and the subsequent decrease in the corrosion resistance of welded joints using E1 and E2 electrodes was explained by an increased welding current, which led to an imbalance in the phase balance of welded joints from dual-purpose steels.
Upon request, data for this study will be provided by the respective author.
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