22Cr High-Mn-N Low-Ni Economical Duplex Stainless Steels

 A new family of economical duplex stainless steels in which N or Mn was substituted for Ni with composition of 22Cr-80Mn-xNi-10Mo-07Cu-07W-03N (x=05-20) have been developed by examining the microstructure, mechanical and corrosion properties of these alloys. The results show that these alloys have a balanced ferrite-austenite relation. In addition, the alloys are free of precipitation of sigma phase and Cr-nitride when solution-treated at 750 to 1300 ℃ for 30 min. The yield strength, tensile strength and fracture elongation values of experimental alloys solution-treated at 1050 ℃ for 30 min are about 500, 750 MPa and 400%, respectively. Low-temperature impact properties can be improved distinctly with the increase of nickel content. Among the designed DSS alloys, the alloy with Ni of 20% is found to be an optimum alloy with proper phase proportion, better low-temperature impact properties and higher pitting corrosion resistance compared with those of other alloys. The mechanical and corrosion properties and lower production cost of the designed DSSs are better than those of AISI 304.     

铜对19Cr-1.6Mo铁素体不锈钢力学性能和腐蚀行为的影响

 采用力学性能测试、时效处理、电化学测试、显微硬度以及TEM微观分析等分析手段,研究了19Cr-1.6Mo和19Cr-1.6Mo-0.5Cu两种超纯铁素体不锈钢的力学性能和在3.5%NaCl腐蚀介质中的耐腐蚀性能。试验结果表明：合金元素铜的添加,提高了试验用钢的强度,同时降低了Δr值;随着时效时间的增加,铜析出相尺寸在不断的增加且均匀分布,基体的显微硬度由HV148增加到HV162;合金元素铜的添加降低了试验用钢在氯离子溶液下的耐点蚀能力,尤其是随着时效时间的增加,点蚀电位值由390mV降低到290mV,耐点蚀能力呈明显的下降趋势。     

Isothermal Grain Growth Studies on Nanostructured 9Cr-1Mo and 9Cr-1W Ferritic Steels Containing Nano-sized Oxide Dispersoids

Analysis of isothermal grain growth kinetics of nanocrystalline Fe-9Cr-1Mo and Fe-9Cr-1W-based ferritic oxide dispersion strengthened alloys is reported. Fe-9Cr-1Mo-0.25Ti-0.5Y₂O₃ alloy exhibited ~900 and ~250 pct enhancement in grain-coarsening resistance at 1073 K (800 °C) in comparison with Fe-9Cr-1Mo-0.5Y₂O₃ alloy and Fe-9Cr-1W-0.5Y₂O₃ alloy, respectively. Comparison of grain growth time exponents also revealed that addition of Ti and Y₂O₃ to nanocrystalline Fe-9Cr alloy has significantly enhanced the grain growth resistance. This is attributed to the possible presence of Y-Ti-O-based nanoclusters (<5 nm).     

Effect of Zn addition on corrosion resistance of Cu-10Ni alloy in clean and sulphide contaminated seawater

Cu-10Ni alloy has an outstanding resistance to corrosion in seawater due to formation of protective Cu₂O film. However, in presence of S^2? ions, it suffers accelerated corrosion. The present paper investigates the corrosion behaviour of Cu-10Ni, Cu-10Ni-6Zn and Cu-10Ni-12Zn alloys using weight loss, electrochemical impedance spectroscopy and potentiodynamic polarisation technique. The experiments were performed in clean seawater and sulphide contaminated seawater. The Cu-10Ni-6Zn and Cu-10Ni-12Zn alloys were found to exhibit lower corrosion rate than Cu-10Ni alloy in clean and sulphide contaminated seawater. Lower corrosion rate of Zn containing alloys in clean seawater is attributed to the incorporation of Zn²⁺ ions in Cu₂O lattice. Lower corrosion rate of Zn containing alloys sulphide contaminated seawater is attributed to formation of ZnS in the film.     

Microstructure and properties of as-cast Cu-Cr-Zr alloys with lanthanum addition

Cu-0.45Cr-0.2Zr-xLa (x = 0–0.48) alloys were prepared by vacuum casting. The effects of La addition and orientation on the microstructure and properties of the as-cast alloy were investigated by an optical microscope, a scanning electron microscope with an energy dispersive X-ray spectrometer, a tensile testing machine and an electrical conductivity tester. The result shows that the addition of La significantly refines the columnar grainsize and decreases the secondary dendrite arm spacing. Trace addition of La improves the room temperature ultimate tensile strength, elongation and electrical conductivity mainly by purifying during melting and casting. The ultimate tensile strength, elongation and electrical conductivity significantly decrease with the increase of La content due to formation of coarse particles and oxides, which severely harm the performance of the Cu-0.45Cr-0.2Zr-xLa alloys. The Cu-0.45Cr-0.2Zr-0.13La alloy possesses a good combination of room temperature ultimate tensile strength, elongation and electrical conductivity. In addition, room temperature ultimate tensile strength and electrical conductivity along transverse direction of the ingot are higher than that along longitudinal direction, which is mainly ascribed to different distribution of grain boundary and grain orientation.     

Studies on Nb Microalloying of 13Cr Super Martensitic Stainless Steel

The effect of Nb microalloying on microstructure, mechanical properties, and pitting corrosion properties of quenched and tempered 13?pct Cr-5?pct Ni-0.02?pct C martensitic stainless steels with different Mo and N contents was investigated. The microstructure, density, and dispersion of high-angle boundaries, nanoscale precipitates, and amount of retained austenite were characterized by using electron backscattered diffraction, transmission electron microscopy, and X-ray diffraction to correlate with properties. The results show that the combined effects of lowering nitrogen content in 13?pct Cr-5?pct Ni-1~2?pct Mo-0.02?pct C steels to 0.01?wt pct, and adding 0.1?pct Nb are to decrease the amount of Cr-rich precipitates, as Nb preferentially combines with residual carbon and nitrogen to form carbonitrides, suppressing the formation of Cr₂N and Cr₂₃C₆. Austenite grain refinement can be achieved by Nb microalloying through proper heat treatment. If the nitrogen content is kept high, then Cr-rich precipitates would occur irrespective of microalloying addition. The NbN would also occur at high temperature, which will act as substrate for nucleation of coarse precipitates during subsequent tempering, impairing the toughness of the steel. It was shown that the addition of Nb to low interstitial super martensitic stainless steel retards the formation of reversed austenite and results in the formation of nanoscale precipitates (5 to 15?nm), which contribute to a significant increase in strength. More importantly, the pitting corrosion resistance was found to increase with Nb addition. This is attributed to suppression of Cr-rich precipitates, which can cause local depletion of Cr in the matrix and the initiation of pitting corrosion.     

Corrosion behavior of plasma-sprayed coatings on a Ni-base superalloy in Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-60 Pct V<Subscript>2</Subscript>O<Subscript>5</Subscript> environment at 900 °C

The shrouded plasma spray process was used to deposit NiCrAlY, Ni-20Cr, Ni₃Al, and Stellite-6 metallic coatings on a Ni-based superalloy (62Ni-23Cr-1.48Al-0.80Mn-0.37Si-0.10Cu-0.025C-bal Fe). NiCrAlY was used as a bond coat in all cases. Hot corrosion studies were conducted on uncoated as well as plasma-spray-coated superalloy specimens after exposure to molten salt at 900 °C under cyclic conditions. The thermogravimetric technique was used to establish the kinetics of corrosion. X-ray diffraction (XRD), scanning electron microscopy/energy-dispersive X-ray analysis (SEM/EDAX) and electron-probe microanalysis techniques were used to analyze the corrosion products. The uncoated superalloy suffered accelerated corrosion in the form of intense spalling of the scale. The NiCrAlY coated specimen showed a minimum weight gain, whereas the Stellite-6 indicated a maximum weight gain among the coatings studied. All the coatings were found to be successful in developing resistance against hot corrosion, which may be attributed to the formation of oxides, and spinels of nickel, aluminum, chromium, or cobalt.     

Microstructure and mechanical behavior of spray-deposited High-Li Al-Li alloys

High-Li alloys, with the composition Al-3.8Li-XCu-1.0Mg-0.4Ge-0.2Zr, were synthesized using a spray deposition technique (wt. pct, X=0∼1.5). The microstructure of the spray-deposited Al-Li alloys consisted of equiaxed grains with an average grain size in the range from 20 to 50 μm. The grain-boundary phases were fine and discrete. The spray-deposited and thermomechanically processed materials were isothermally heat treated at 150 °C and 170 °C to investigate the age-hardening kinetics. It was noted that the spray-deposited Al-3.8Li-XCu-1.0Mg-0.4Ge-0.2Zr alloys exhibited relatively sluggish aging behavior. The peak-aged condition was achieved at 170 °C in the range from 20 to 90 hours. It was noted that Cu increases the hardness of alloys during aging. Moreover, the influence of Cu on age-hardening kinetics is marginal. The mechanical properties of the spray-deposited and extruded Al-Li alloys were studied in the underaged, peak-aged, and overaged conditions. For example, the peak-aged yield strength, tensile strength, and ductility of Al-3.8Li-1.0Cu-1.0Mg-0.4Ge-0.2Zr are 455 MPa, 601 MPa, and 3.1 pct, respectively. Moreover, an increase in the Cu content of the alloy led to improvements in strength, with only slight changes in ductility, for Cu contents up to 1.0 wt pct. Beyond this range, an increase in Cu content led to decreases in both strength and ductility.     

Carbide Formation and Dissolution in Biomedical Co-Cr-Mo Alloys with Different Carbon Contents during Solution Treatment

The microstructures of as-cast and heat-treated biomedical Co-Cr-Mo (ASTM F75) alloys with four different carbon contents were investigated. The as-cast alloys were solution treated at 1473 to 1548 K for 0 to 43.2 ks. The precipitates in the matrix were electrolytically extracted from the as-cast and heat-treated alloys. An M₂₃C₆ type carbide and an intermetallic σ phase (Co(Cr,Mo)) were detected as precipitates in the as-cast Co-28Cr-6Mo-0.12C alloy; an M₂₃C₆ type carbide, a σ phase, an η phase (M₆C-M₁₂C type carbide), and a π phase (M₂T₃X type carbide with a β-manganese structure) were detected in the as-cast Co-28Cr-6Mo-0.15C alloy; and an M₂₃C₆ type carbide and an η phase were detected in the as-cast Co-28Cr-6Mo-0.25C and Co-28Cr-6Mo-0.35C alloys. After solution treatment, complete precipitate dissolution occurred in all four alloys. Under incomplete precipitate dissolution conditions, the phase and shape of precipitates depended on the heat-treatment conditions and the carbon content in the alloys. The π phase was detected in the alloys with carbon contents of 0.15, 0.25, and 0.35 mass pct after heat treatment at high temperature such as 1548 K for a short holding time of less than 1.8 ks. The presence of the π phase in the Co-Cr-Mo alloys has been revealed in this study for the first time.     

A New Economical Sigma-Free Duplex Stainless Steel 19Cr-6Mn-1.0Mo-0.5Ni-0.5W-0.5Cu-0.2N

A new resource-saving duplex stainless steel with composition of 19Cr-6Mn-1.0Mo-0.5Ni-0.5W-0.5Cu-0.2N has been developed,and the microstructure,mechanical properties and corrosion properties have been investigated.The results show that the alloy has a balanced ferrite-austenite relation,and the ferrite content rises with the solution treatment temperature.The designed alloy is free of precipitation of sigma phase when aged at 650,750 and 850 ℃ for 3 h,respectively,whereas a few Cr23C6 precipitates are found...     

Microstructural size effects in high-strength high-conductivity Cu-Cr-Nb alloys

Microstructural refinement to further improve the strength and stability of high-strength high-conductivity Cu-Cr-Nb alloys was attained by mechanical milling (MM). Mechanically milled Cu-4Cr-2Nb and Cu-8Cr-2Nb exhibited an increase in hot-pressed Vickers hardness of 122 and 96 pct, respectively. Mechanical milling produced a corresponding decrease in electrical conductivity of ∼33 pct for both alloys. The increase in hardness was more due to Cu grain-size refinement than to second-phase particle-size refinement. The drop in conductivity was due to second-phase particle-size refinement, which both increased particle/matrix interfacial area and solute solubility. Mechanically processed Cu-4Cr-2Nb displayed an enhanced thermal stability. Hot-pressed 4-hour milled Cu-4Cr-2Nb experienced a 30 pct increase in conductivity with only a 22 pct drop in hardness when annealed at 1273 K for 50 hours. Such changes were largely due to an increase in dispersed-particle size (i.e., a decrease in solute and interfacial electron scattering) and Cu grain size (reduced Hall-Petch effect), respectively. The optimum hardness and conductivity combination was found in 4-hour milled and hot-pressed Cu-4Cr-2Nb material.     

Chromia-Assisted Decarburization of W-Rich Ni-Based Alloys in Impure Helium at 1273 K (1000 °C)

The microstructure and surface stability of two experimental W-rich Ni-based alloys have been studied at 1273 K (1000 °C) in an impure-He environment containing only CO and CO₂ as impurities. The alloy Ni-2.3Al-12Cr-12W contained 0.08 wt pct carbon in solution, whereas the second alloy Ni-2.3Al-3Mo-12Cr-12Co-12W contained M₆C carbides at the same carbon level. Both alloys, which were preoxidized with ~2.3 μm Cr₂O₃ layer, were decarburized completely within 50 hours of exposure to the helium gas mixture at 1273 K (1000 °C) via the following chromia-assisted decarburization reaction: Cr₂O₃ (s) + 3C_alloy (s) → 2Cr (s) + 3CO (g). Microstructural observations, bulk carbon analysis, and microprobe measurements confirmed that the carbon in solid solution reacted with the surface chromium oxide resulting in the simultaneous loss of chromia and carbon. The Cr produced by the decomposition of the Cr₂O₃ diffused back into the alloy, whereas CO gas was released and detected by a gas chromatograph. Once the alloy carbon content was reduced to negligible levels, subsequent exposure led to the uninterrupted growth of Cr₂O₃ layer in both alloys. In the preoxidized alloys, chromia-assisted decarburization rates were slower for an alloy containing carbides compared with the alloy with carbon in solid solution only. The formation of Cr₂O₃ is shown to be the rate-limiting step in the chromia-assisted decarburization reaction. Exposure of as-fabricated alloys to the impure-He environment led to the formation of a thin layer of Al₂O₃ (<1 μm) between the substrate and surface Cr₂O₃ oxide that inhibited this decarburization process by acting as a diffusion barrier.     

Hydrogen embrittlement of ultra-pure alloys of the inconel 600 type: Influence of the additions of elements (C,P, Sn,Sb)

The mechanical behavior of very high purity nickel base alloys of the Inconel 600 type that were simultaneously charged with hydrogen and deformed in tension was investigated. Experimental results show that this procedure decreases markedly the fracture strain of the pure 76 pct Ni-16 pct Cr-8 pct Fe alloy; cracks are observed after two to four pct elongation, and the fracture is completely intercrystalline. Hydrogen embrittlement appears as an intrinsic property of the Ni-Cr-Fe system in the sense that the grain boundary cohesion decreases when the purity of the alloy increases. The presence of carbon or phosphorus in the alloys increases grain boundary cohesion. The addition of metallic elements such as antimony or tin has relatively little effect on intergranular embrittlement.     

Carbon diffusion and phase transformations during gas carburizing of high-alloyed stainless steels: Experimental study and theoretical modeling

Gas carburizing of high-alloyed stainless steels increases surface hardness, as well as the overall mechanical characteristics of the surface. The growth of chromium-rich carbides during carbon transfer into the steel causes precipitation hardening in the surface, but decreases the chromium content in solid solution. In order to maintain a good corrosion resistance in the carburized layer, the stainless steel composition and the carburizing process need to be optimized. To limit the experimental work, a methodology using software for modeling the thermodynamic and kinetic properties in order to simulate carbon diffusion and phase transformations during gas carburizing is presented. Thermodynamic calculations are initially used to find the optimum parameters (T, carbon wt pct, etc.) in order to maintain the highest Cr and Mo contents in the austenitic solid solution. In a second step, kinetic calculations using the diffusion-controlled transformations (DICTRA) software are used to predict how the amount of the different phases varies and how the carbon profile in the steel changes as a function of time during the process. Experimental carbon profiles were determined using a wavelength-dispersive spectrometer for electron-probe microanalysis (WDS-EPMA), while carbide compositions were measured by energy-dispersive spectroscopy_X (EDS_X) analyses. A good agreement between calculated and experimental values was observed for the Fe-13Cr-5Co-3Ni-2Mo-0.07C and the Fe-12Cr-2Ni-2Mo-0.12C (wt pct) martensitic stainless steels at 955 °C and 980 °C.     

Improvements in microstructure homogenization and mechanical properties of diffusion-alloyed steel compact by the addition of Cr-containing powders

The use of diffusion-alloyed powders for fabricating powder metal parts, despite alleviating the segregation problem of the alloying elements while retaining good compressibility, still cannot attain homogeneous microstructure in as-sintered products. The presence of soft Ni-rich areas and pores causes poor mechanical properties compared to those of wrought steel counterparts. This study investigated the effects of adding 0.5 wt pct Cr, which was introduced in the 316L stainless steel powder form, on the microstructure and mechanical properties of diffusion-alloyed Fe-4Ni-1.5Cu-0.5Mo-0.5C (Metal Powder Industries Federation (MPIF) FD-0405) steels. The results show that weak Ni-rich areas were present in the Cr-free specimen when sintered at 1120 °C and 1250 °C. These areas were lean in carbon because of the strong repelling effect between Ni and C. With the addition of 316L powders, the Cr was uniformly distributed and helped eliminate the soft Ni-rich areas, particularly in specimens sintered at 1250 °C. The distribution of carbon also improved. With a more uniform distribution of Ni and C, and more homogeneous microstructure, which consisted mainly of bainite and martensite, the mechanical properties of the Fe-4Ni-1.5Cu-0.5Mo-0.5C diffusion alloy steels were improved significantly.     

Effect of solution temperature on pitting corrosion resistance of 00Cr20Ni25Mo6Cu3 stainless steels

The effect of solution temperature on pitting resistance of superaustenitic stainless steels 20Cr- 25Ni- 6Mo in concentrated chloride media was studied by potentiodynamic scanning(PDS), chemical immersion test and electrochemical impedance spectroscopy(EIS). The microstructure of the alloy after heat treatment was observed by optical microscope(OM). The results show that with the increase of solution temperature, the secondary phases dissolve in the matrix, the grain grows significantly. The rate of weight loss decreases first, and reaches the minimum at 1200??, and thereafter, it increases with further increasing solution temperature. Meanwhile, the radius of the EIS capacitive loop increases first and then reduces, signifying that the corrosion resistance of the stainless steels is first enhanced and then weakened. The deterioration of the corrosion resistance of the stainless steel is attributed to the presence of secondary phases.     

Tensile strength of thermomechanically processed Cu-9Ni-6Sn alloys

The tensile properties of Cu-9Ni-6Sn alloys with different swaging amounts of 64, 77, and 95 pct, either solutionized and aged (S/A) or directly aged (D/A), were examined as a function of aging time. It was found that the aging response of Cu-9Ni-6Sn alloys varied greatly depending on the prior solution heat treatment before aging and/or different swaging amounts. The swaged S/A Cu-9Ni-6Sn alloys showed a multistage increase in tensile strength with respect to aging time, probably due to the sequential occurrence of spinodal decomposition, formation of metastable γ· precipitates, and recrystallization. The effect of different swaging amounts, ranging from 64 to 95 pct, was minimal on the aging response of S/A specimens. The prior cold working, however, appeared to favor the spinodal strengthening, comparing unswaged and swaged S/A Cu-9Ni-6Sn alloys. In 95 pct swaged D/A Cu-9Ni-6Sn alloys, the level of hardening was much less sensitive to aging time. A complex interaction between the reduction in dislocation density, the formation of equilibrium precipitates, and the reduction of Sn content in the Sn-rich segregates during an aging process is believed to be responsible for such a lean sensitivity. The increases in tensile strength of 64 and 77 pct swaged D/A Cu-9Ni-6Sn alloys were found to be much steeper than that in the 95 pct counterparts in the early and intermediate stages of aging, which is believed to be related to the relative contribution from work hardening and precipitation hardening to the strength level of D/A specimens.