高Cr镍基合金的高温内氧化和内氮化行为

通过对镍基合金进行不同温度的恒温氧化试验、横断面的组织形貌观察及微区成分分析,研究一种高Cr镍基合金在恒温氧化期间的氧化物分布特征、内氧化及内氮化行为。结果表明:该高Cr镍基合金在850、900、950和1 000℃空气中氧化100 h期间,合金表层发生元素的外氧化,且在外氧化膜中出现分层结构,由表及里各层中的主要氧化物分别为Al2O3、Cr2O3、NiCr2O4和NiAl2O4、Cr2O3、CrTaO4和Al2O3;合金在900℃以下为完全抗氧化级,900~1 000℃为抗氧化级。该合金在各温度的恒温氧化期间,均发生元素Al的内氧化和内氮化;与外氧化膜相邻的区域为元素Al的内氧化区,远离外氧化膜的基体内部形成元素Al的内氮化区;随恒温氧化温度的升高,内氧化区和内氮化区的深度增加,内氧化物和内氮化物的尺寸增大。其中,在内氧化物、内氮化物周围形成元素Al的贫化区,在贫化区内发生-′相的分解及贫乏。     

再结晶晶界对合金钢锭Cr元素均匀化扩散的影响

从晶界扩散的角度对含Cr钢锭锻后扩散加快的现象进行了实验验证和理论解释。首先由粗、细两种晶粒的Cr元素扩散偶的均匀化退火数据,得到细晶晶界加快了Cr元素的体扩散。基于锻造变形再结晶细化晶粒的作用,推断锻后再结晶晶界对Cr原子的扩散具有加速作用。在此基础上,对锻后扩散退火的钢锭中Cr元素的分布进行了测定,得出锻后Cr元素的扩散系数大于锻前。此与晶界加快扩散的结果相一致,证明了锻后细晶加速了Cr原子的扩散。     

Intergranular fracture in high chromium heat-resistant steel

The grain boundary segregation of impurities, especially phosphorus, and the fracture behavior of 12Cr heatresistant steel have been investigated. Temper embrittlement caused by phosphorus at grain boundaries was observed at a temperature below 750°C. Phosphorus at the grain boundaries increased as tempering temperature decreased, which changed the fractrure mode from trans- to intergranular. The segregation free energy of phosphorus was evaluated asδG _segrδH_segr- TδS_segr=- 24,900-48.2T (Jmol^-1). The enhanced concentration of phosphorus at the grain boundaries is due to the decrease in carbon activity by the chromium addition rather than the synergistic cosegregation of chromium and phosphorus.     

Volatilization kinetics of chromium oxide,manganese oxide,and manganese chromium spinel at high temperatures in environments containing water vapor

Performance degradation of solid oxide fuel cells due to chromium volatilization is a well‐investigated issue in the literature. Therefore, retention coatings were developed to distinctly reduce the chromium volatilization. One approach was by alloying with manganese to ferritic steels to form manganese chromium spinel which is reported to decrease chromium volatilization by 61–75%. In the present paper, the volatilization rates of pure manganese chromium spinel ceramics were examined as well as those of the two oxides forming this spinel—pure chromium oxide and pure manganese oxide—in synthetic air containing 10% water vapor (high p(O₂)) and argon/hydrogen containing 10% water vapor (low p(O₂)) at 850°C, 950°C, and 1,050°C. Chromium oxide showed higher volatilization rates in high p(O₂), whereas manganese oxide demonstrated higher volatilization rates in low p(O₂). Contradictory to the literature, manganese chromium spinel displayed the highest volatilization rates in both atmospheres and nonlinear kinetics behavior. This deviation from linear behavior can be attributed to diffusion‐controlled volatilization in high p(O₂).     

Experimental analysis and computer‐based simulation of nitridation of Ni‐base alloys—the effect of a pre‐oxidation treatment

Nickel‐base superalloys are commonly used for high‐temperature applications in power‐generation industry, e.g., gas‐turbine blades or heat exchangers. They are designed to resist high creep loading and severe corrosion attack during operation. Nitridation is one of these corrosion processes, in particular when the alloys need to be exposed to a N₂ atmosphere. Based on past assumptions, a dense oxide layer should be an efficient barrier against N₂ ingress. But is this really the case? This work is focused on the nitridation behavior of commercial Ni‐base alloys and the influence of a pre‐oxidation treatment. To model the growth of the internal‐nitridation zone, the diffusion processes were solved using the numerical implicit finite‐difference method in combination with the subroutine ChemApp for thermodynamic calculations.     

The interaction of titanium and titanium alloys with nitrogen at elevated temperatures. II. The nitridation behavior of alloys containing 5 weight percent of aluminum or chromium

The nitridation behavior of a Ti-5Al and a Ti-5Cr alloy has been examined at 1000°C and compared with the performance of pure titanium, involving gravimetric, metallographic, and microhardness studies. All of the materials corrode in accordance with a parabolic overall weight gain-time regime, pure titanium nitriding faster than either of the alloys. The reduced rates of nitridation of the alloys relative to pure titanium are reflected in the smaller relative depths of nitrogen dissolution in the substrates and may be accounted for by the corresponding decreased diffusivity values for nitrogen. The addition elements appear to play no part in the linear scaling reactions.     

高温扩散对2.25Cr-1Mo-0.25V钢锭组织与性能的影响

研究了高温扩散工艺对2.25Cr-1Mo-0.25V钢锭枝晶组织、锻造纤维状组织以及锻件冲击韧性的影响.结果表明,提高加热温度、增加保温时间均可提高组织均匀程度.从枝晶组织消除程度判断,2.25Cr-1Mo-0.25V钢锭至少应在1200 ℃保温20 h,但保温时间不宜过长.锻前进行高温扩散可有效减轻锻后的纤维状组织.锻造比较大时,横向冲击韧性随保温时间增大;而发达的纤维状组织以及高温扩散引起成分的均匀化可提高纵向冲击韧性.     

Metal dusting resistance of high chromium alloys

Samples of 5 high Cr‐alloys were discontinuously exposed for 10,000 hours under severe metal dusting conditions, i. e. in flowing 49%CO‐49%H₂‐2%H₂O at 650°C. After each of the 11 exposure periods the mass change was determined and any coke removed and weighed. Metallographic cross sections were prepared after about 4,000 h and 10,000 h. The high Cr‐alloys: 1. PM 2000 (Fe‐19%Cr‐5.5%Al‐0.5%Ti‐0.5%Y₂O₃), 2. Cr‐44%Fe‐5%Al‐0.4%Ti‐0.5%Y₂O₃, 3. Cr‐50%Ni, 4. Cr‐5%Fe‐1%Y₂O₃ and 5. porous chromium showed no or only minute metal dusting attack. Compared to the attack on reference samples of Alloy 601 (Ni‐23%Cr‐14%Fe‐1.4%Al), the metal dusting symptoms were negligible on the 5 high Cr‐alloys, minor coking and pitting and no internal carburization was observed. Because of the high Cr‐content, carbon solution and ingress should be minute, and in addition are inhibited by the formation of a chromia scale, as confirmed for four of the Cr‐rich alloys, and formation of an alumina scale on PM 2000. These alloys could be used for parts exposed to severe metal dusting conditions, and in fact, 50Cr‐50Ni has been applied successfully under such conditions.     

Nitriding of chromium in nitrogen gas at high temperatures

Parabolic rate constants of the reaction of chromium with nitrogen gas under oxygen-free conditions have been determined over a range of temperature (1000–1250°C) and nitrogen pressure (0.265–101.33 kPa). The growth rate of the subnitride was measured by a thermogravimetric technique using a single specimen. Wagner's oxidation theory is used to calculate the self-diffusivity and intrinsic diffusivity of nitrogen in the subnitride from a theoretical analysis of the parabolic rate constant. The calculated diffusivities varied with the composition of the subnitride, having minimum values at intermediate compositions of the nonstoichiometric chromium nitride Cr₂N.     

Effect of Ti-V-Nb-Mo addition on microstructure of high chromium cast iron

The effects of trace additions of multi-alloying elements (Ti,Nb,V,Mo) on carbides precipitation and ascast microstructure of eutectic high chromium cast iron containing 2.85wt.％C and 31.0wt.％Cr were i...     

Sulfur segregation during the high-temperature oxidation of chromium

A TEM and STEM examination has been carried out of cross-sectioned specimens of convoluted chromia scale formed by oxidizing chromium at 950° C. Sulfur was detected at the oxide/metal interface and the oxide grain boundaries (apart from low-angle grain boundaries), but not within the oxide grains. These results are consistent with the sulfur effect theory.     

Oxidation kinetics of Y‐doped FeCrAl‐alloys in low and high pO2 gases

A model Fe‐20Cr‐5Al‐0.05Y alloy was oxidized in Ar‐20%O₂ and Ar‐4%H₂‐7%H₂O at 1200–1300 °C. Two‐stage oxidation experiments using oxygen isotope tracers showed that inward oxygen diffusion was predominant in both gases, but more isotope exchange was observed in the H₂/H₂O gas reaction. The alumina scales formed in both gases were composed of columnar grains, the lateral size of which increased linearly with depth beneath the scale surface. Thermogravimetric measurement of oxygen uptake revealed kinetics which were intermediate to parabolic and cubic kinetic rate laws. A model based on grain boundary diffusion control coupled with competitive oxide grain growth accounts satisfactorily for the results when the requirement for a divergence‐free flux within the scale is imposed. This treatment shows that the oxide grain boundary diffusion coefficient is lower when H₂O is the oxidant. It is concluded that hydrogen slows the grain boundary diffusion process by altering the nature of the diffusing species.     

Effect of chromium on the oxidation resistance of TiAl intermetallics

The effect of 10 at.%Cr on the oxidation resistance of TiAl intermetallic compound at 800–1100°C in air was investigated. The results indicated that 10 at.%Cr equally substituting for Ti and Al in TiAl alloy had duplex effects on the isothermal kinetics of DAL At lower temperatures (800–900°C), Cr increased the oxidation rates as a result of the doping effect of Cr in the scale and at higher temperatures (1000–1100°C), especially at 1100°C, Cr significantly reduced the oxidation rates as a result of the formation of a continuous Al₂O₃ film on the surface. 10 at.%Cr only substituting for Ti in TiAl alloy remarkably reduced the oxidation rates at all temperatures by about two orders of magnitude. Moreover, 10 at%Cr significantly improved the cyclic-oxidation rsistance of TiAl alloy.     

The interaction of titanium and titanium alloys with nitrogen at elevated temperatures. I. The kinetics and mechanism of the titanium-nitrogen reaction

The nitridation behavior of titanium has been examined in an atmosphere of nitrogen over the temperature range 900–1000° C. In addition to determining the reaction kinetics, microhardness and metallographic studies have been carried out on the nitrided substrates and nitride scales. Overall parabolic kinetics were exhibited, while a linear scaling reaction (associated principally with Ti₂N formation) was observed. Extensive dissolution of nitrogen in the substrate was evident from the microhardness studies, and at 950 and 1000° C the growth of the nitrogen-stabilized Ti phase layer followed a parabolic law. Consideration of all the observed processes and derived data suggested that diffusion of nitrogen in the metal substrate was probably the rate-controlling mechanism.     

镀铬Inconel690合金管激光熔凝层中Cr的扩散行为

采用电镀铬+激光辐照表面改性方法对Inconel 690合金管进行表面处理,获得激光熔凝层。用SEM观察镀铬层及熔凝层的微观形貌,用EDX方法研究了Cr元素在熔凝层中的扩散行为。结果表明,在较浅的熔凝层中,从管壁的外缘至管的内部,Cr浓度梯度较大,呈逐渐降低趋势;随熔凝层深度的增加,Cr扩散深度增加,分布趋于均匀。     

Effects of water vapour and oxygen partial pressures on low carbon steel oxidation in N2–H2–H2O mixtures

A low carbon, low silicon steel was oxidised at temperatures of 900–1000 °C in flowing N₂–H₂–H₂O gas mixtures in which oxygen and water vapour partial pressures were varied independently. Scales of dense, single‐phase, coarse grained wüstite grew rapidly, according to parabolic kinetics. Both the scaling rate and the oxide grain growth increased with $p_{{\rm O}_{2} } $ at constant $p_{{\rm H}_{2} {\rm O}} $ , and also with $p_{{\rm H}_{2} {\rm O}} $ at constant $p_{{\rm O}_{2} } $ . An inert marker experiment showed that significant oxygen transport but majority metal transport supported scale growth. Gas composition effects are interpreted using point defect models involving formation of hydroxyl species on anion sites as well as cation vacancies.