Super-austenitic stainless steel S32654 sheets with 2.4mm thickness were successfully welded by friction stir welding (FSW) at the rotational speeds of 300 and 400rpm with a constant traverse speed ...of 100mm/min using W-Re tool. The sound joints with almost no nitrogen loss were successfully produced. The microstructure evolution was characterized by optical digital microscope (ODM), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), electron backscatter diffraction (EBSD) and transmission electron spectroscopy (TEM). The results suggest that the grain structure evolution in stir zone (SZ) is dominated by continuous dynamic recrystallization (CDRX). The strain rate plays a dominated effect on obvious grain refinement. The band structures containing W and Re are generated due to the wear between tool probe and steel in SZ. Furthermore, the microhardness measurements and transverse tensile tests indicate that the grain refinement combining with high density dislocations and substructures improves the hardness and strength, but greatly reduces the plastic deformation capacity of joints. The more suitable welding parameters are determined as 300rpm and 100mm/min for this steel.
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•Friction stir welding was successfully applied to super-austenitic stainless steel S32654.•Grain structure evolution in stir zone is dominated by continuous dynamic recrystallization.•Strain rate plays a dominated effect on grain refinement.•Grain refinement, high density dislocations and substructures together improve hardness and strength, but reduce elongation.•The more suitable welding parameters are determined as 300 rpm and 100 mm/min for this steel.
The viscosity of 30%CaO-30%SiO2-15%Al2O3-5%MgO-10%Na2O-10%CaF2-xCe2O3 (mass%, x = 0, 5, 10, 15) were measured by the rotating column method, and then the viscosity and the Tbr (break temperature) of ...the mold flux were analyzed based on the results. Meanwhile, the structural characteristics of the mold flux were investigated using Raman spectroscopy and XRD (X-ray diffraction). The results show that Ce2O3 predominantly destroys the silicate network structure at high temperature, reduces the polymerization degree of the mold flux, simplifies the high-temperature structure of the mold flux, and reduces the friction resistance of viscous flow. From the apparent phenomenon, the viscosity of the mold flux decreases at high temperature. In addition, with the increase of Ce2O3 content in the mold flux, Ca4Si2O2F changes to Ce9.33(SiO4)6O2, which enhances the crystallization ability of the mold flux and increases the Tbr of the mold flux.
In this work, time temperature transformation (TTT) diagram, thermodynamic calculations and microstructure observations were performed to investigate the precipitation behavior of hyper duplex ...stainless steel UNS S32707 aged at nose temperature for various aging time up to 48h. The corresponding secondary phase transformation mechanisms were proposed. The results demonstrate that the precipitation kinetics of σ phase in UNS S32707 is very fast due to its higher Cr and Mo contents at nose temperature of 950°C. At the initial stage of aging, σ phase preferentially formed along the α/α and α/γ phase boundaries and then penetrated into α phase, resulting from the eutectoid reaction α→γ2+σ. Meanwhile, a few σ phases nucleated at the γ/γ phase boundaries. With the aging time prolonging, some σ phases nucleated within γ phase due to the decomposition of the supersaturated γ phase. In addition, a cellular structure consisting of lamellar σ and γ2 phase was detected during short term aging, and it gradually grew with aging time via cell boundary migration. After 30min and 6h aging, the lamellar σ phases in the cellular structure broke into small lamellas or into bending bars and evolved into blocky shapes. Most of rounded Cr2N particles formed at the σ/γ2 interfaces and were surrounded by σ and γ2 phase. A few rod-like Cr2N particles nucleate at γ/γ phase boundaries due to the low energy interfaces between Cr2N and γ phase. No Cr2N was detected in α phase, which was attributed to the fact that σ phase has much faster precipitation kinetics than that of Cr2N.
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•Precipitation behavior and phase transformation of S32707 at 950°C are investigated.•Precipitation kinetics of S32707 is fast due to high alloy contents.•σ phase nucleate α/α, α/γ and γ/γ phase boundaries and within primary γ phase.•The transformation mechanism of cellular structure containing σ and γ2 is discussed.•Rounded Cr2N forms at σ/γ2 interfaces and rod-like Cr2N nucleate at γ/γ interfaces.
•The MIC behavior of nickel-free HNS in the presence of Pseudomonas aeruginosa is investigated.•The pitting corrosion of HNS is greatly accelerated by the P. aeruginosa biofilm.•The probability ...analysis confirms the pitting acceleration of HNS by the P. aeruginosa biofilm.
High nitrogen nickel-free stainless steel (nickle-free HNS) has been widely used in recent years because of its strong corrosion resistance. However, little is known on its resistance to microbiologically influenced corrosion (MIC). This study investigated the MIC behavior of nickel-free HNS in the presence of marine aerobe Pseudomonas aeruginosa. A 14-day corrosion test demonstrated that the HNS had a largest pit depth of 16.2μm, over three times deeper than that of the abiotic control coupon (4.7μm). Electrochemical tests and surface morphology examination confirmed that HNS was susceptible to MIC.
A V-Nb microalloyed spring steel was investigated through a series of austempering heat treatments. Optimal properties were obtained after austempering at 240 °C for 2 hours because of the good ...coordination of multiphase martensitic/bainite, filmy retained austenite and fine carbides. The tensile strength was 2239 MPa and the total elongation was 16.1 pct. With an increase in temperature, the matrix changed from martensite to bainite. The volume fraction of retained austenite decreased from 13.7 pct at 240 °C to 9.6 pct at 280 °C and then increased to a maximum of 19.2 pct at 360 °C. The tensile strength remained above 1500 MPa and the impact energy increased to 30 J. MC carbides formed in martensite lathes and
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- or
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-type carbides with acicular nanoscale were precipitated in bainite lathes at a low temperature. MC- and M
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C-type precipitated in bainite lathes at high temperature. Samples that were subjected to a low isothermal temperature had a high strain-hardening rate before necking, whereas samples at a high isothermal temperature could exert a TRIP effect for a long time. After stretching to fracture, a hierarchical nanotwinned structure and many shear cells formed, which enhanced the austempered sample ductility and toughness at 360 °C.
To optimize CaF2 content in highly basic CaO-18%Al2O3-SiO2-10%MgO-CaF2 (%CaO/%SiO2=6, denoted as C/S=6) refining slags used for the production of Al-killed duplex stainless steel with high ...cleanliness demand, the effect of CaF2 content on the viscosity and refining ability of the slags were studied and compared with typical CaF2-free highly basic CaO-30%Al2O3-SiO2-10%MgO (C/S=6) slag. The effect of CaF2 addition in decreasing slag viscosity becomes less obvious with increasing temperature and CaF2 content. When CaF2 content exceeds 10%, slag viscosity only marginally decreases with further increasing CaF2 content. Both monoxide-CaO and monoxide-MgO phases are precipitated in all the CaF2-bearing slags. CaF2 addition slightly increases monoxide-MgO precipitation, but dramatically decreases monoxide-CaO precipitation. Viscosities of the CaF2-bearing slags were also theoretically calculated and good agreement with the measured values was observed. Moreover, the 6% CaF2-bearing slag has very close viscosities above 1833 K but much lower viscosities below 1833 K, compared with the CaF2-free highly basic slag. Further evaluation of the 6% CaF2-bearing slag on steel cleanliness confirms that 6% CaF2 addition is sufficient for the highly basic CaO-18%Al2O3-SiO2-10%MgO-CaF2 (C/S=6) slag. The mechanism of CaF2 in decreasing the viscosity of CaF2-bearing slags was discussed from the viewpoints that CaF2 behaves as a network breaker and that CaF2 suppresses the precipitation of solid phases. The first aspect was identified to play a much greater role in decreasing slag viscosity.
During the refining process of Incoloy825 nickel-based alloy, the reaction between the refining slag and Al and Ti in the alloy can result in the oxidation loss. Therefore, the effect of the TiO2 and ...Al2O3 contents in the refining slag (CaO–SiO2–Al2O3–MgO–CaF2–TiO2) on the final Al and Ti contents in Incoloy825 alloy was investigated by slag-metal reaction experiment in a 1-kg MoSi2 resistance furnace. The thermodynamic model was established based on ion and molecular coexistence theory and the kinetic model was established based on two-film theory. The experimental results show that an increase in TiO2 (Al2O3) content in the slag leads to an increase (decrease) in Ti content and a decrease (increase) in Al content in the final alloy. The thermodynamic model demonstrates that the activities of SiO2, Al2O3, and TiO2 in the slag are negatively correlated with the content of CaO and positively correlated with SiO2, Al2O3, and TiO2 in the slag. The kinetic model indicates that the slag-metal reaction reaches equilibrium within 10 min, and the model has good applicability for the reaction process of Incoloy825 alloy and slag refining.
Rare earth metals and magnesium are important purifying elements in steel. However, most scholars mainly focused on the cleanliness of molten steel by adding single rare earth or adding single ...magnesium. There are few reports on the simultaneous addition of rare earth and magnesium metals to steel. In this paper, rare earth‐magnesium alloy is added to a hot work die steel in industrial production. Combined with thermodynamic calculation, the effect of rare earth‐magnesium alloy on the evolution of inclusions was studied. The results show that composite inclusions of Ce2O2S and CeS attached or wrapped by MgO are formed after adding rare earth‐magnesium alloy. With the help of low‐density MgO, the composite inclusions containing high‐density Ce inclusions are promoted to float up. Thermodynamic calculation indicates that the addition of magnesium changes the formation tendency of rare earth inclusions. With the increase of magnesium content in steel, the stability area of Ce inclusions will move from the Ce2O2S stability area slightly close to the Ce2O3 stability area to the direction of the CeS stability area, which will improve further the desulfurization ability. The thermodynamic analysis is in good agreement with the test results.
After adding rare earth–magnesium alloy, composite inclusions of Ce2O2S and CeS attached or wrapped by MgO are formed. Mg addition obviously inhibits the formation ability of Ce deoxidization products, so that the generation of rare earth sulfide is promoted. Therefore, compared with adding single rare earth or single magnesium, rare earth–magnesium alloy can improve further desulfurization ability.
Mo-rich precipitation in super-austenitic stainless steels has significant effects on their hot workability and corrosion resistance. The occupation tendencies of Mo and other alloying elements at ...Σ3(112), Σ5(210), and Σ9(114) grain boundaries of fcc-Fe were calculated through the first-principles method, and the influence of B and Ce on the grain boundary segregation of alloying elements was analyzed. Then, microscopic mechanism for the precipitation in super-austenitic stainless steels containing B and B + Ce was also discussed. The results show that the segregation tendencies of Cr and Ni to grain boundaries are extremely weak, while those of Mo and Ce are materially stronger, and they are apt to segregate at Σ5(210) and Σ9(114) grain boundaries. Mo, in particular, exhibits a wider segregation area and tends to segregate to the whole grain boundary regions. However, after being located at grain boundaries, B and B + Ce make the segregation of Mo at these grain boundaries more difficult, which is beneficial to adjusting the grain boundary segregation of Mo. Moreover, the calculated results agree well with the experimental results. Through microstructure characterization, the micro-alloying of B and B + Ce can indeed inhibit the Mo-rich precipitation in super-austenitic stainless steels, especially for B + Ce, reducing a great number of large precipitates along grain boundaries to discontinuous and finer precipitates.
The occupation tendencies of Cr, Cu, Mo, Nb, Ni, Ti and V in Fcc-Fe stacking fault containing vacancy were studied by the first principles calculations. The effects of C, N and Nb, Ti co-segregation ...in the region of vacancy is discussed. The results are as follows: the substitutional elements Cr, Cu, Mo, Ni, Nb, Ti and interstitial elements C, N have no segregation tendency at the stacking fault of Fcc-Fe (111) plane. When the stacking fault contains vacancy, the above elements have the tendency to segregate around the vacancy, and C, N tend to occupy vacancy and their segregation tendency is the stronger than that of substitutional elements. C, N are easily co-segregated with Nb, Ti in the region of vacancy. It is difficult for Mo to aggregate around the carbonitride precipitates, which is conducive to the use of nano-scale carbide and nitride precipitation strengthening in high Mo austenitic steel, and simultaneously improves corrosion resistance. Nb and Ti tend to aggregate around the vacancy where occupied by C, N.
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