Effects of transient carbon deposition on the sodium sulfate-induced corrosion of nickel-base alloys

The role of solid carbon in accelerating the Na₂SO₄-induced corrosion of nickel-base alloys at high temperature has been investigated. Corrosion rates of salt-coated IN 738 alloy in an oxidizing atmosphere containing 0.1 pct SO₂ were much increased when the alloy was initially exposed to a low concentration of methane for a short time at 1273 K (1000°C). Crucible tests carried out with Ni-5 Al, Ni-30 Al, Ni-20Cr-15 Al and IN 738 specimens immersed in Na₂SO₄-carbon mixtures showed that corrosion of these alloys was dramatically enhanced if the carbon persisted long enough to form a Na₂S phase by reaction with the Na₂SO₄. In an oxidizing environment a graphitic carbon had a more pernicious effect on alloy corrosion resistance than a finely dispersed carbon black which burnt away more readily. Corrosion rates in air were also accelerated in the presence of carbonized crude oil residue such as might be formed as a result of incomplete fuel combustion in a gas turbine.     

Mechanisms for the hot corrosion of nickel-base alloys

The Na₂SO₄-induced accelerated oxidation of nickel-base alloys containing elements such as Cr, Al, Mo, W, and V has been studied in 1.0 atm O₂ in the temperature range of 650° to 1000°C. It has been found that the hot corrosion behavior of these alloys can usually be characterized according to one of two types of attack: 1) Na₂SO₄-induced accelerated oxidation; 2) Na₂SO₄-induced catastrophic oxidation. In both types of hot corrosion, accelerated oxidation occurs as a result of the formation of a liquid flux based on Na2SC>4 which dissolves the normally protective oxide scales. Catastrophic, or self-sustaining rapid oxidation can occur in alloys which contain Mo, W, or V, because solutions of oxides of these elements with Na₂SO₄ decrease the oxide ion activity of the molten salts, producing melts which are acidic fluxes for oxide scales. The accelerated oxidation type of attack which was observed with most alloys which did not contain Mo, W, or V, was more severe than for normal oxidation, but much less severe than catastrophic oxidation. Na₂SO₄-induced accelerated oxidation occurs because the oxide ion activity of the Na₂SO₄ increases to the point where oxide scales can partially dissolve in the basic melt. Generally, this basic fluxing results from the diffusion of sulfur from the Na₂SO₄ into the alloy. In some alloys, the formation of sulfides during basic fluxing is a sufficient condition to cause accelerated oxidation. In other alloys, changes in the oxidation mechanism occur because of depletion of the alloy surface, concomitant with basic fluxing, of those elements needed for protective oxide scales, such as aluminum and chromium.     

The thermodynamic behavior of sulfur in molten nickel and nickel-base alloys

The thermodynamic properties of dilute solutions of sulfur in pure liquid nickel were investigated at 1500, 1550, and 1575°C for sulfur concentrations up to 0.7 wt pct. Based on the infinitely dilute, wt pct standard state, the equilibrium data obtained for the reaction: H₂(g) + S = H₂S(g) were fitted by the equations: logK = − 1489/T − 1.772, and ΔG° = 6812 + 8.11T, cal/mole. For the solution ofS ₂(g) in pure Ni according to the reaction: 1/2S ₂(g) = S (in Ni), the standard free energy of solution is found to be: ΔG° = - 28,342 + 3.62T, cal/mole. For the very dilute solutions of sulfur normally encountered in nickel-base melting, the activity coefficient of sulfur in pure Ni at 1575°C is given by: log f_S= -0.035 (pct S). The effects of alloying elements normally used in nickel-base alloys on the activity coefficient of sulfur in molten nickel were investigated. The activity coefficient of sulfur is increased by all of the alloying elements studied, as evidenced by the interaction parameters: e_S ^fe = +0.005, e_S ^Cr = +0.030, e_S ^Mo = +0.053, e_S ^Ti = +0.160, and e_S ^A1 = +0.133. Measured values of the activity coefficient of sulfur in the quaternary system Ni-S-Cr-Fe agreed reasonably well with those predicted from binary and ternary data. This work constitutes a portion of the work performed by W. F. VENAL for the Ph.D. degree from the University of Illinois at Chicago Circle. Formerly Professor of Metallurgical Engineering at UICC.     

Manufacture of powders of nickel-base brazing alloys

Conclusions A process has been developed for the manufacture of a Ni-Cr-Si-Fe-B brazing alloy in powder form, designated as PKh12N75S8R, and also brazing alloy powder No. 6MA, which is a mechanical mixture of 85% of PKh12N75S8R powder and 15% of molybdenum. Using alloy powder No. 6MA (instead of PKh12N75S8R) for brazing high-alloy steels raises the secondary melting (unbrazing) temperature of the brazed joint.Translated from Poroshkovaya Metallurgiya, No. 5 (113), pp. 82–88, May, 1972.     

硼含量对Ni-Cr-Mo合金热腐蚀性能的影响

采用真空液相烧结法制备了4种掺加不同B含量的Ni-Cr-Mo合金,研究了B对其组织与性能的影响。研究结果表明,B与Mo、Cr、Ni等合金元素在烧结时可以形成共晶液相,通过原位化学反应,生成Mo2NiB2、(Mo,Cr)2NiB2陶瓷相。热腐蚀性试验表明,形成的硼化物相具有较好的耐腐蚀性,能够有效提高Ni-Cr-Mo合金的耐热腐蚀性能。     

Effect of segregated sulfur on the stress corrosion susceptibility of nickel

Auger electron spectroscopy has been used to investigate surface segregation on nickel single crystals and grain boundary segregation in Ni 200. Sulfur was found to be the main segregating species in the temperature range 550 to 850°C. Segregated sulfur was shown to catalyze the anodic dissolution kinetics when nickel is exposed to sulfuric acid. Its presence in the grain boundaries of Ni 200 causes susceptiblity to IGSCC. H. CHAUNG, formerly with the Department of Metallurgical Engineering, The Ohio State University R. W. STAEHLE, formerly with the Department of Metallurgical Engineering, The Ohio State University     

The influence of sulfides on the oxidation behavior of nickel-base alloys

The oxidation of presulfidized chromium, Ni?Cr, and Ni?Al alloys, and complex nickel base alloys was studied at 1000°C in 1.0 atm of oxygen. Sulfur-rich surface layers were produced in the pretreatment by using H₂S?H₂ mixtures. Presulfidized chromium oxidized at a rate similar to that of sulfur-free chromium. The oxidation rate of presulfidized Ni?Cr alloys was affected by sulfur only when liquid nickel sulfide was present which accelerated the oxidation rate by creating rapid diffusion paths through the Cr₂O₃ scale. The oxidation behavior of presulfidized Ni?Al alloys, with aluminum contents sufficient for the formation of a protective Al₂O₃ layer in the sulfur-free condition, was influenced by sulfur only when aluminum sulfide was formed in the presulfidation treatment which caused the Al₂O₃ scale to be porous. The oxidation behavior of nickel-base alloys containing both chromium and aluminum was insensitive to the presence of sulfides when the concentration of aluminum in the alloy was such that a protective Al₂O₃ scale was formed during oxidation of the sulfur-free alloy and aluminum sulfide was not formed in the presulfidizing treatment.     

The low cycle fatigue of three wrought nickel-base alloys

The low cycle fatigue behavior of three wrought nickel-base alloys, alloy 901, Waspaloy and alloy 718, has been examined at both room temperature and 811 K (1000°F). The fatigue properties can be rationalized in terms of the Manson-Coffin equation relating total strain range with the number of cycles to failure. For a given total strain range the fatigue life of the alloys at 811 K (1000°F) is substantially less than at room temperature. Microstructural examination revealed that crack nucleation and initial propagation was of classical stage I type. The cracks develop at the surface of test specimens and propagate along slip planes. Evidence is presented to relate the observed fatigue softening with shear and dissolution of γ precipitates.     

Creep transients in the deformation of anisotropic nickel-base alloys

The transient creep behaviour of two anisotropic nickel-base alloys (viz directionally solidified IN738LC and the in situ composite γ-γ-Cr₃C₂) has been examined following a wide range of stress changes. The response following an unloading loading sequence depends critically on the magnitude of the stress change and on the time spent on the reduced load. When a proportion of the stress is removed and immediately replaced delay periods of zero creep. Δt′, before creep recommences are obtained in both alloys although the magnitudes of Δt′ are 3–100 times greater for the composite than for the superalloy. When the reduced stress is maintained for long periods (⪆ 10 h) conventional primary creep, rather than delay periods are observed. A model is proposed that accounts for the transient strains in terms of the stress redistributions between fibre and matrix in the composite and across the dendritic structure in the superalloy. The implications of the results to the cyclic creep behaviour of these materials are discussed.     

镍基合金中元素分析方法研究进展

综述了近年来国内外测定镍基合金中化学元素分析方法的研究进展。从原子发射光谱法、原子吸收光谱法、分光光度法、原子荧光光谱法、X-射线荧光光谱法、红外吸收光谱法、质谱法、电分析化学法和化学分析法等9个方面介绍了目前有关镍基合金材料中化学元素分析方法的基本特点和应用实例,比较了各种方法的优缺点和适用范围。提出了镍基合金中化学元素分析方法的发展方向:从化学法及单一的元素分析到仪器分析及多元素同时分析。     

Ni-Cr-Mo-Cu-Mx合金耐晶间腐蚀及点蚀性能

向镍基耐蚀合金中添加Ti、Fe元素,采用手工电弧炉熔炼制备新型Ni-Cr-Mo-Cu-Mx耐蚀合金,用化学浸泡法、电化学法(极化曲线法、循环伏安法)对其耐晶间腐蚀和耐点蚀能力进行研究.结果表明:在Ni-Cr-Mo-Cu合金中加入Ti元素可以增强其耐晶间腐蚀能力,减弱其耐点蚀的能力;加入Fe元素会降低Ni-Cr-Mo-Cu合金耐晶间腐蚀的能力,但提高该合金耐点蚀的能力;实验合金晶间腐蚀与点蚀的电化学行为和特征与其浸泡腐蚀的结果是吻合的.     

The effects of microstructure on elevated temperature crack growth in nickel-base alloys

The effects of microstructure on the fatigue and creep crack growth of Waspaloy and P/M Astroloy were evaluated at 650°C. In Waspaloy, changes in γ′ size and distribution did not markedly affect fatigue crack growth. An increase in fatigue crack growth rate occurred at low test frequencies and was associated with a transition to intergranular crack propagation. In P/M Astroloy, a coarser grain size lowered the fatigue crack growth rate. Serrated grain boundaries, though beneficial under creep loading, have no effect in fatigue.     

The effects of molybdenum and aluminum on the thermal expansion coefficients of nickel-base alloys

The effects of molybdenum and aluminum on the mean linear thermal expansion coefficients from room temperature to 1050°C were determined for two types of nickel-base alloys. The Solid Solution Alloys were cast and homogenized Ni-Co-Cr-Mo alloys with 0, 312, and 612 nominal wt pct molybdenum concentrations. The Gamma Prime Alloys were wrought and heat-treated Ni-Cr-Mo-Al(Ti) alloys with 0, 2, 5, and 8 nominal wt pct molybdenum in each of four aluminum plus titanium levels (3 pct Al, 412 pct Al, 6 pct Al, or 1 pct Al + 312 pct Ti nominal wt pct). Thermal expansion coefficients were determined on at least two specimens from each alloy. It was found that molybdenum lowers the thermal expansion coefficients of both the cast Ni-Co-Cr solid solutions and the wrought Ni-Cr-Al(Ti) two-phase alloys. Both aluminum and titanium were also observed to decrease expansion coefficients in the two-phase, γ + γ, alloys. Results are discussed in terms of relative melting point effects between solute and solvent elements, and in terms of the volume fraction of the γ phase present.     

The breakdown of single-crystal solidification in high refractory nickel-base alloys

The breakdown of single-crystal solidification has been studied over a wide range of solidification conditions in ten superalloys with large variations in Re, Ta, and W content. Over the range of experimental conditions investigated, grain defect formation was sensitive to local thermaland solutal conditions. For a fixed alloy composition and withdrawal rate, the transition from single-crystal to equiaxed solidification did not occur abruptly. Instead, as thermal gradients were decreased in a series of experiments, isolated, highly misoriented columnar grains with the same composition as that of the base alloy developed in the presence of positive (stabilizing) thermal gradients with increasing frequency until the advance of the single-crystal front was completely blocked. The onset of columnar grain formation occurred when the primary dendrite arm spacing exceeded a critical value, corresponding to a morphological transition in the dendritic array. The onset of “freckling” was observed at the same primary dendrite arm spacing where misoriented columnar grains began to appear. In experiments with varying levels of refractory alloy content, there was also a strong correlation between the onset of grain formation and freckle formation. These observations strongly suggest that in high refractory content superalloys, the breakdown of single-crystal solidification and the formation of misoriented grains as well as freckle-type defects are sensitively dependent on thermosolutal convection processes.