不同温度时效后Ni-20Cr-18W-1Mo合金晶界偏聚及力学性能研究

采用扫描电镜(SEM)、透射电镜(TEM)、电子探针(EPMA)和高温力学试验机等手段,研究了不同时效温度(200-800 °C)对Ni-20Cr-18W-1Mo高温合金的元素晶界偏聚和力学性能的影响。结果表明,硫、磷元素的晶界偏聚临界时间随时效温度升高而降低;时效温度对元素在晶界和晶内的成分分布有显著的影响;实验合金的抗拉强度和延伸率随时效温度升高而降低。分析发现,硫、磷元素在晶界中的含量随时效温度升高而增大直至两者分别在650和400 °C时达到峰值,是合金在200-600 °C区间内力学性能降低的重要原因。     

Ni-20Cr-18W基合金的锻态组织及拉伸性能

采用拉伸试验机研究了固溶强化Ni-20Cr-18W基热变形高温合金锻造后的室温拉伸行为,用光学显微镜(OM)和扫描电子显微镜(SEM)分析了锻造前后的微观组织及拉伸断口形貌。利用X射线衍射(XRD)确定了合金中碳化物的类型。结果表明,合金铸态组织为典型的枝晶结构;锻态组织晶粒尺寸为60~70μm,晶粒度为ASTM 5级,M6C型碳化物半连续分布在晶界上。M6C型碳化物钉扎晶界,提高了合金的强度;拉伸变形使晶界上的M6C型碳化物进一步破碎,造成外载荷分布均匀性下降,影响合金的塑性。合金的室温拉伸断口为韧性等轴韧窝断裂。     

Ni-20Cr-18W基高温合金热轧组织演变和力学性能

采用力学试验机、扫描电子显微镜(SEM)研究了热轧态固溶强化型Ni-20Cr-18W基变形高温合金的室温拉伸行为、微观组织演变及断口形貌,结合X射线衍射(XRD)确定了合金的相结构。结果表明,合金热轧态组织由再结晶后的细小等轴晶粒构成,晶粒尺寸为20~30μm,可达ASTM 7级,组织中可见大量退火孪晶,M6C型碳化物颗粒主要弥散分布在晶界上。热轧态合金由于组织细化使抗拉强度升高,断裂方式以穿晶断裂为主,碳化物颗粒成为导致拉伸断裂的裂纹源,使合金塑性下降。合金的室温拉伸断口呈韧性等轴韧窝,韧窝中心为M6C型碳化物颗粒。     

Al、Si及Y多元合金化对Cr-20Nb合金高温氧化行为的影响

针对软第二相Cr稍微降低Laves相NbCr2合金的1200℃抗氧化性,采用Al、Si及Y多元合金化来提高Cr-20Nb合金的高温抗氧化性能。结果表明,多元合金化的Cr-20Nb合金1100℃及1200℃抗氧化性均好于加入单一合金化的及纯Cr-20Nb合金,并随着Si合金元素含量增加,Cr-20Nb合金的氧化增重变小,抗氧化性变好;SEM结果表明,添加合金元素后,氧化膜与基体的粘附性得到了明显提高。     

Al对Fe-20Cr-35Ni-0.6Nb合金显微组织和抗氧化性的影响

采用增量法研究了不同Al含量(0.5%、1.5%、2.5%,质量分数)的Fe-20Cr-35Ni-0.6Nb含Nb合金在1000 ℃空气条件下的抗氧化。采用SEM、EDS、TEM、拉曼光谱等手段研究了合金的显微组织和氧化膜特性。结果表明,3种含Nb合金组织为单相奥氏体,基体中存在少量弥散分布的NbC沉淀相,氧化前后沉淀相含量和晶粒大小保持不变。添加0.5%和1.5%的Al后,含Nb合金的表面形成多层结构的氧化膜,最外层和第三层为Cr₂O₃,次表层主要为NiCr₂O₄、NiFe₂O₄和Fe₂O₃,最内层为Al₂O₃内氧化层。基体中的NbC析出相和氧化膜中少量Nb的氧化物(Nb₂O₅)加剧了氧化膜的疏松。当Al含量增加到2.5%时,含Nb合金表面形成连续致密的Al₂O₃氧化膜,降低了Fe-20Cr-35Ni-0.6Nb合金的氧化速率,提高了抗氧化性。     

Behavior of oxide scales on 12Cr-1Mo steel during thermal cycling

Soot-blower operation leads to thermal-cyclic-oxidation conditions of heat-resistant steels in conventional power stations. The consequence may be failure of the protective oxide scales and increased corrosive attack. The behavior of protective oxide scales on 12Cr-1Mo steel was investigated under isothermal conditions at 650°C and under thermal cycling conditions between 650 and 300°C (200°C). The tests were performed in air, air + 0.5%SO₂, simulating the fire side, and Ar-5% H₂-50% H₂O, simulating the steam side. Complete heat-exchanger tubes were used as specimens. The main instrument for the detection of scale failure was acoustic-emission analysis. In air and air + 0.5% SO₂ the M₂O₃ scales with M = Fe, Cr were very thin and did not show significant failure either during isothermal or during cyclic oxidation. The thicker scales formed in Ar-5% H₂-50% H₂O, consisting of several partial layers, failed even during isothermal oxidation due to geometrically-induced growth stresses in the scale. Thus, in the thermal-cycling cooling periods there was only very little additional scale cracking. The scale behavior can be explained consistently by applying the existing quantitative models.     

Mn含量对Fe-x Mn-9Al-8Ni-1C合金微观组织及氧化行为的影响

采用真空电弧熔炼的方法制备Fe-x Mn-9Al-8Ni-1C合金(x=10,15,20,25,wt%),研究了该系列合金的物相、微观组织、力学性能及合金在600℃时的氧化行为。结果表明：该系列合金主要含有奥氏体相、NiAl化合物相和κ-碳化物相,随Mn含量的增加,奥氏体相逐渐增多,NiAl相逐渐减少;与此同时,合金的塑性提高,硬度值有所降低;当Mn含量为10%时,硬度值最大为HRC 40.7。氧化行为测试结果表明,随着Mn含量的增加,合金的抗氧化性增强,合金表面形成的氧化膜分为两层,外氧化层由Fe₂O₃和少量Mn₂O₃组成,内氧化层主要为Al₂O₃与Mn₂O₃。相较于Mn含量为10%时的合金,Mn含量为25%时,合金氧化性能大幅改善,其氧化增重减少了127.8%,氧化膜厚度降低了102.7%。     

Cu-20Ni-30Cr合金在700℃和800℃纯氧气中的氧化

研究了三元复相合金Cu-20Ni-30Cr（at%）在700℃和800℃纯氧气中的氧化行为，合金由三相组成，具有最大Cu浓度和最小Cr浓度的α相为合金的基体，中间浓度的Ni和Cr的β相和富Cr的γ相以颗粒状态分布在合金基体中。合金在2个温度下的氧化动力学曲线偏离抛物线规律，其氧化增重随时间延长而降低，合金氧化速率随温度升高而加快，合金形成了复杂的氧化膜结构，外层为富Cu氧化物，中间层为尖晶石层，最内层为不规则但连续的Cr2O3膜，合金中的复相组织限制Cr在合金中的扩散，抑制了外氧化膜的形成。     

Effect of External Stress on the Behavior of Oxide Scales on 9Cr-1Mo Steel

Tensile specimens of 9Cr-1Mo steel weresubjected to oxidation in air at a temperature of 973 Kfor periods of 25, 47, 70, 97, 120, and 140 hr. Theintegrity of the oxide scale was examined by an in-situ technique which involved the recording of theacoustic-emission activity associated with the breakingof the scale. Specimens were also subjected to oxidationfor the same times after subjecting them to an external stress of 40 MPa. The applicationof the external stress was found to alter the integrityof the scale. Spalling of the scale occurred to a lesserextent when external stress was applied due to partial release of growth stresses becauseof the elongation of the specimen. The scale buckledbefore spalling when no external stress was applied. Onthe other hand, the scale developed wedge-type cracks before spalling when subjected toexternal stress during oxidation. Postoxidationexamination of the stressed specimen revealedsubstantial segregation of silicon and chromium at thegrain boundaries (oxide ridges). The specimens, which wereoxidized without external stress, revealed a reductionin the concentration of chromium on the surface. Thisbehavior was attributed to enhanced spalling in the case of the latter specimen compared to theformer.     

Behavior of oxide scales on 2.25Cr-1Mo steel during thermal cycling. II. Scales grown in water vapor

The acoustic emission (AE) technique has been applied to identify scale cracking during thermal cycling of tubes of 2.25Cr-1Mo steel. The scale morphology and failure mode were investigated by light and electron optical methods. The scale formed at 600°C in water vapor consists of an outer magnetite and an inner, chromium-containing spinel layer. Cooling leads to tensile stresses in the scale that cause macro and microcrack formation in the scale. At constant-cycle parameters, a characteristic set of crack length and crack density is established. Changes in the cycle parameters also change the crack length and crack density. The experimental results can be described by a model developed by Hasselmann assuming a large number of noninteracting microcracks in a ceramic plate.     

Effect of cold work on the oxidation resistance of2\tfrac{1}{4} Cr-1 Mo steel

The effect of cold work on the oxidation rate of 21/4 Cr-1 Mo steel in pure oxygen at 1 atm pressure at temperatures ranging from 400 to 950C has been studied for short exposure periods (max. 4 hr). The specimens had been cold worked up to 90% by cold rolling. The results indicate negligible effect of cold work on the oxidation kinetics up to 700C, beyond which there is general reduction in oxidation. The effect was pronounced at 900C. The increased resistance to oxidation has been attributed to the faster diffusion of chromium in the cold-worked material compared to the annealed one, leading to the formation of a chromium-rich spinel which helps in slowing down the oxidation of the alloy. The findings have been corroborated by the examination of all samples oxidized at 900C by optical, EPMA, SEM, and EDAX analyses.     

组织形态对2.25Cr-1Mo钢长期时效稳定性的影响

通过力学性能检测、微观组织分析、相分析等方法,研究了不同组织形态2.25Cr-1Mo钢在530 ℃×2000 h长期时效过程中的组织与力学性能稳定性。结果表明:随着正火冷速下降,组织中大尺寸等轴状铁素体含量增加,贝氏体含量下降。组织中贝氏体含量高时,钢的强韧性匹配良好,时效过程第二相碳化物粗化不明显,组织稳定性更高。大尺寸等轴状铁素体基体强度低,时效过程界面碳化物粗化与聚集速率快,且沿晶界呈链状分布,导致组织稳定性较差。铁素体含量＞40%,时效脆化现象显著。     

Effect of Trace Yttrium Oxide on High and Low Temperature Properties of 9Cr-1Mo Weld Joint for Super Critical Unit

研究了稀土元素钇对超临界发电机组用9Cr-1Mo焊缝组织和性能的影响。采用熔滴过渡的方式将氧化钇从焊条药皮中添加到焊缝金属中,通过力学性能、相变点测试、扩散氢含量测量等试验确定了最佳综合性能的钇含量,在此基础上进一步对稀土含量相对比(RRY)为1.5实际焊接接头和未含稀土元素的商用焊条进行了室温拉伸、系列冲击、高温拉伸、硬度试验。结果表明,含稀土元素组的性能均高于未含稀土元素的试样,通过金相、SEM、XDR等分析,发现了第二相粒子是提高性能,特别是低温冲击性能的主要原因。     

Surface Chemistry of Oxide Scale on IN-738LC Superalloy: Effect of Long-Term Exposure in Air at 1173 K

The oxidation kinetics of IN-738LC at 1173 K in dry air up to 1500 hr followed parabolic law. Surface morphology and the oxide phases present in the scale were characterized by SEM, XRD, EDS, FIB, and XPS. FIB investigation exhibited a compact and adherent oxide layer. XRD analysis revealed the presence of NiO, NiAl₂O₄, NiCr₂O₄ spinel, and Al₂O₃ on the top scale surface formed at 1173 K in dry air. Extensive XPS analyses revealed the presence of Cr₂O₃, CrO₂, and CrO₃ on the top scale surface formed on IN-738LC after 10 hr of exposure. The presence of TiO₂, Al₂O₃, Cr₂O₃, NiO, and NiAl₂O₄ and NiCr₂O₄ spinels along with the oxides of Ta at the top surface of the scale was observed after 100 hr of oxidation. The TiO₂ content was high on the surface and the entire scale cross section was composed mostly of Cr₂O₃, NiO, TiO₂, and Al₂O₃ after 100 hr of exposure to dry air at 1173 K. The concentration of Al₂O₃ on the surface of the oxide scale was found to increase after 100 hr of exposure and remained constant thereafter. After 300 and 1500 hr of exposure, the surface oxide was mainly Al₂O₃ along with oxides of Ni, Ti, and Cr. The oxide scale cross section consisted mostly of Al₂O₃ along with other oxides such as Cr₂O₃, NiO, and TiO₂. The oxide-scale composition was found to vary significantly with the duration of exposure to dry air at 1173 K.     

Behavior of oxide scales on 2.25Cr-1Mo steel during thermal cycling. I. Scales formed in oxygen and air

The acoustic-emission (AE) technique has been applied to study scale-damage processes during thermal cycling of a tube, preferentially between 600 and 300°C in air, oxygen, and air + 0.5% SO₂. The AE measurements were accompanied by optical and electron-optical investigations on tube rings exposed to the same cycling conditions. During the first period of cycling, a scale rich in hematite is formed. It suffers compressive stresses during cooling. The result is a buckled multilayered scale with separated lamellae. The scaling rate is lower than under isothermal conditions. AE signals start after 175°C cooling. After longer exposure times, the scale contains an increasing amount of magnetite and becomes more compact. The scaling rate increases and is comparable to that under isothermal conditions. AE signals are already observed after 50°C cooling and are correlated with crack formation in the magnetite caused by tensile stresses there. The addition of SO₂ to air enhances the crack-healing process due to higher Fe diffusion in FeS. The scale is more compact.     

Effect of Y2O3 dispersoids in 80Ni-20Cr alloy on the early stages of oxidation at low-oxygen potential

Specimens of a 80Ni-20Cr type alloy, with and without Y₂O₃ dispersoid particles, were oxidized at 1000°C in H₂/H₂O mixtures where the partial pressure of oxygen (P _{O 2}) was varied between 10₃ and 10₂₄ atm. Oxide particles nucleated homogeneously on both alloys, and preferential nucleation on dispersoid particles at the surface was not observed. Continuous Cr₂O₃ films formed slightly faster at aP _{O 2} of 10^–21 atm on the alloy containing the dispersoid, but the difference was negligible at higher pressures. Oxidation atP _{O 2}=10^-19 and 10^–21 atm involved both the formation of Cr₂O₃ and the evaporation of chromium. Thin films of -Al₂O₃ were observed on both alloys after oxidation atP _{O 2}.     

Changes of an Outer β-NiAl and Inner α-Cr Coating on Ni–40 at% Cr Alloy during Oxidation at 1373K in Air

Formation mechanisms of a coating with a duplex layer, outer β-NiAl(Cr) and inner α-Cr(Ni) layer structure on a Ni–40.2 at% Cr alloy were proposed and change in the coating structure was investigated during high temperature oxidation. The Ni–40.2 at% Cr alloy was electro-plated with about 12μm Ni followed by a high Al activity pack cementation at 1073K to form a coated layer with an outer δ-Ni₂Al₃ and an inner layer containing Al more than 70at% which grew with an inward diffusion of Al. The coated Ni–40.2at% Cr alloy was oxidized at 1373K in air for up to 2592ks. It was found that at the initial stage of oxidation the as-coated layer structure changed to a two-layer, outer β-NiAl(Cr) and inner α-Cr(Ni), structure. Al contents in the α-Cr(Ni) layer was less than 0.3at%. With long term oxidation an intermediate γ-Ni(Cr, Al) layer formed between the outer and inner layers, whereas the inner α-Cr(Ni) layer became thinner and then disappeared after the 2592ks oxidation at 1373K. Coating processes and changes in the coating structure during high temperature oxidation were discussed based on diffusion and composition paths plotted on a Ni–Cr–Al phase diagram