Topologically Close-Packed μ Phase Precipitation in Creep-Exposed Inconel 617 Alloy

Two creep-exposed Inconel 617 alloy samples [923 K (650 °C) for 45,000 hours and 973 K (700 °C) for 4000 hours] have been studied using analytical electron microscopy and X-ray diffraction techniques. The thermodynamically predicted equilibrium phases in Inconel 617 alloy were compared with the phases observed which are molybdenum-enriched, topologically close-packed μ-phase, along with precipitates of gamma-prime (γ′), M₂₃C₆ and Ti(C,N). The μ-phase precipitates were in the size range 60 to 500 nm (with some larger agglomerates); they were situated both within the grains, along twin and grain boundaries, and near intra- and intergranular carbides. The stacking faults in the μ-phase were observed in high magnification electron microscopy. The precipitation of the μ-phase in these samples is significant for the potential use of this alloy in future generation steam power plants as the appearance of the μ-phase is associated with an increased tendency for cracks and voids to initiate. The μ-phase has not been previously identified in the literature relating to creep or thermal exposure of Inconel 617 alloy.     

Thermal and Irradiation Creep Behavior of a Titanium Aluminide in Advanced Nuclear Plant Environments

Titanium aluminides are well-accepted elevated temperature materials. In conventional applications, their poor oxidation resistance limits the maximum operating temperature. Advanced reactors operate in nonoxidizing environments. This could enlarge the applicability of these materials to higher temperatures. The behavior of a cast gamma-alpha-2 TiAl was investigated under thermal and irradiation conditions. Irradiation creep was studied in beam using helium implantation. Dog-bone samples of dimensions 10 × 2 × 0.2 mm³ were investigated in a temperature range of 300 °C to 500 °C under irradiation, and significant creep strains were detected. At temperatures above 500 °C, thermal creep becomes the predominant mechanism. Thermal creep was investigated at temperatures up to 900 °C without irradiation with samples of the same geometry. The results are compared with other materials considered for advanced fission applications. These are a ferritic oxide-dispersion-strengthened material (PM2000) and the nickel-base superalloy IN617. A better thermal creep behavior than IN617 was found in the entire temperature range. Up to 900 °C, the expected 10⁴ hour stress rupture properties exceeded even those of the ODS alloy. The irradiation creep performance of the titanium aluminide was comparable with the ODS steels. For IN617, no irradiation creep experiments were performed due to the expected low irradiation resistance (swelling, helium embrittlement) of nickel-base alloys.     

Response to thermal cycling of tool materials under steel thixoforming conditions

Abstract

The principal failure mechanism of steel thixoforming dies is thermal fatigue owing to forging pressures much lower than those encountered in conventional forging. This makes a properly designed thermal fatigue test the best method to identify suitable tooling materials for the steel thixoforming environment. Samples of X32CrMoV33 hot work tool steel and CrNiCo alloy were cycled thermally between 450 and 750°C, every 60 s for a total of 1500 cycles. While the thermal stresses generated at the surfaces of the two materials were very similar, their responses to thermal cycling were markedly different. The X32CrMoV33 steel was softened by nearly 40% after only 400 cycles, raising serious concerns over its temper resistance under steel thixoforming conditions. The extensive oxidation and subsequent spalling of oxide scales suffered by the X32CrMoV33 hot work tool steel is also a major shortcoming. The performance of the CrNiCo alloy, on the other hand, was judged to be satisfactory with a much thinner heat affected zone and a much better oxidation resistance. Lack of evidence for heat checking in this alloy after 1500 cycles is an encouraging sign.     

A CrNiCo Alloy as a Potential Tool Material in Semi‐solid Processing of Steels

Semisolid processing of aluminium and magnesium alloys has matured to become a well established manufacturing route for the production of intricate, thin‐walled parts with mechanical properties as good as forged grades. However, this innovative forming technology faces a major challenge in the case of steels. The tool materials must withstand the complex load profile and relatively higher forming temperatures which promote chemical interaction with steel slurries. Thixoforming tools ought to last thousands of forming cycles for industrial application to be attractive. Hot work tool steel dies proved to be entirely inadequate when thixoforming steels. In spite of extensive research on tool materials for the semisolid processing of steels, there is yet no material to fulfil this critical role. The present work was undertaken to explore the potential of a novel CrNiCo alloy as the tooling material in semisolid processing of steel.     

Diffusion alloying – a new manufacturing method for PM tool steels

Abstract

To increase the wear resistance of tool steels, high contents of super-hard MC type carbides formed by Ti, V or Nb are desirable. Unfortunately, these carbides precipitate primarily from the melt at high temperatures. Thus, atomising such steels is not feasible with current technology because carbides precipitating in the melt may clog the nozzle. This problem can be avoided by atomising a melt that contains high amounts of carbide forming elements but no carbon. Subsequently the powder is mixed with graphite to provide the carbon necessary to form carbides and for the hardenability of the matrix. During hipping, graphite dissolves and the carbon is distributed evenly in the material. The phase distribution of a candidate cold work tool steel consisting of a stainless steel matrix with fine, well distributed niobium carbide precipitates is reported and compared with equilibrium calculations.     

Thermal Fatigue Testing of Tool Materials for Thixoforging of Steels

Semisolid processing, already a well established manufacturing route for the production of intricate, thin‐walled aluminium and magnesium parts with mechanical properties as good as forged grades, faces a major challenge in the case of steels. The tool materials must withstand complex load profiles and relatively higher forming temperatures for thousands of forming cycles for industrial application to be attractive. Since the forming pressures are much lower than those encountered in conventional forging, the principle die failure mechanism in steel thixoforging is expected to be thermal fatigue. Hence, suitable materials able to withstand the steel thixoforming environment for an economically acceptable life, can be best identified with a thermal fatigue test. Such a test is described in the present work. A novel CrNiCo and a nickel‐base superalloy, reported to exhibit superior thermal fatigue resistance in demanding tooling applications, was tested under thermal fatigue conditions encountered in the thixoforming of steels.     

中国超超临界电站锅炉关键材料用钢及合金的研究现状

 综述了近几年国内用于超超临界火电机组锅炉的马氏体耐热钢、奥氏体耐热钢、镍基耐热合金的研究情况。国内学者们对马氏体和奥氏体耐热钢高温强化机理和生产工艺进行了系统研究,并实现了马氏体/铁素体耐热钢T/P91、T/P92与奥氏体耐热钢S31042、S30432、TP347HFG的国产化,对新型9%Cr马氏体耐热钢展开了研究。对镍基合金的投入和研究还不够,研究内容不够系统,但表现出较好的势头,学者们正对Inconel 740、Alloy 617及其改型合金和Alloy 263合金进行较广泛的研究。对用于超超临界火电机组关键材料的研究提出了建议,展望了国内耐热钢及合金的研究趋势。     

700℃以上超超临界电站锅炉过热器管材用典型镍基合金的平衡析出相规律

利用热力学计算软件Thermo-Calc及镍基合金数据库,计算了三种700℃以上超超临界电站用过热器管道材料Inconel740、Inconel617和GH2984合金的热力学平衡相图,并对比了三种材料主要析出相的析出行为.计算结果表明:三种合金主要的析出相包括γ、γ'、碳化物、σ、η、δ、μ及α-Cr等,凝固过程中Mo、Nb和Ti元素偏析严重,会降低合金的初熔点,因此后期均匀化退火处理十分重要.另一方面,750℃时Inconel740合金γ'相析出量大于另外两种合金,并且Al和Ti含量对γ'相和η相析出行为有较大影响.碳化物的计算表明,Inconel617合金一次碳化物与另两种合金不同,并且其二次碳化物的析出温度范围最大.GH2984合金中Fe含量较大时会导致σ相出现,对合金的性能产生不利影响.      

Effects of Aluminization Via Thermo-Chemical Diffusion on the Wear Behavior of Structural Materials for High-Temperature Gas-Cooled Reactors

We explore the effects of thermo-chemical aluminization on wear resistance of candidate alloys Incoloy 800HT and Inconel 617 for high-temperature gas-cooled reactor (HTGR) components. Aluminized samples were conditioned for 22 days in a once-through helium loop to simulate oxidizing conditions of HTGRs. The tribological performance of unconditioned and conditioned samples was tested using pin-on-disk tribometer in an air environment at 750 °C for 800HT and 900 °C for 617. Aluminized 800HT exhibited superior wear resistance compared to that of the as-received 800HT due to harder Fe-Al intermetallic compounds that formed at the surface of the alloy. Aluminized 617 exhibited wear resistance superior to that of the as-received 617 at higher loads but inferior resistance at lower loads. Conditioned 800HT as well as aluminized and then conditioned 800HT exhibited negligible wear when the protective oxide remained intact during tribotesting. When the oxide wore through, aluminized and then conditioned 800HT exhibited superior wear resistance compared to that of conditioned 800HT due to the increased hardness of intermetallic zones. Aluminized and then conditioned 617 exhibited inferior wear resistance compared to that of conditioned 617 due to significant ceramic wear of the aluminum oxide. Connections between surface hardness, chemical composition, initial friction coefficients, and wear resistance are established.

     

SOME EXPERIMENTS IN THE PRODUCTION OF LOW-ALLOY STEEL BY POWDER METALLURGY

Abstract

The properties of alloy steels produced by powder-metallurgy techniques, as compared with those of conventional cast or wrought alloy steels, are reviewed. Factors affecting the production of alloy steels by powder-metallurgy techniques are enumerated, and the importance of adequate mechanical properties coupled with ease of processing is emphasized. The paper covers the preparation and testing of a wide range of low-alloy steels including copper steels, nickel steels, and complex steels; these being made up either from elemental powders or from pre-alloyed or complex powders. The influence of various alloying elements upon the mechanical properties of the steels produced is indicated, and the merits of the various compositions investigated, are discussed.

The response of powder-metallurgy alloy steels to quenching is next determined, and the effects of the elements carbon, manganese, nickel, copper, and molybdenum upon the hardcnability of alloy steels is summarized in the form of depth-hardening curves. The use of precipitation-hardening as a means of improving the properties of low-alloy steels is also investigated for a wide range of copper-containing steels. It is demonstrated that by employing this technique it is possible to obtain very significant improvements in properties, together with some processing advantages.     

Selection of Suitable Tool Materials and Development of Tool Concepts for the Thixoforging of Steels

This paper describes the results of the European project “THIXOCOMP” within the 5th framework programme concerning the material selection and development of tools for the thixoforging of steels. Due to high process temperatures, special requirements are necessary regarding the tool material and the tool concept. Special tool coatings (High Velocity Oxi Fuel, HVOF and Plasma Spray, CAPS) with high strength, high corrosion and oxidation resistance were deposited on substrate materials 1.2367, 1.4841 and 2.4631 to improve the properties of the tool materials. Different laboratory tests were performed to investigate the suitability of the tool systems. Pull tests, micro‐hardness measurements, high temperature corrosion tests and spelling resistance tests were performed to investigate the adhesion of the coatings. The investigation of the thermal shock loading on the coated samples indicated a clear dependence on the base material. Both coatings on the base materials 1.2367 and 1.4841 were removed completely in the spelling resistance tests, so that even in the coated state, they are not suitable as tool materials for the thixoforming of steel. The combination of 2.4631+HVOF provided the best results. Afterwards, thixoforging trials were performed with the nickel‐based alloy, 2.4631. After 30 trials at 1290°C (HS6‐5‐3), the tool showed no macroscopic damages, whereas a deformation of the press channel was already visible after eight forming trials at 1430°C (100Cr6).     

Resistance against Abrasive Wear and Corrosion of Laser Powder Bed Alloyed High Chromium Tool Steels

Additive manufacturing by laser-based powder bed fusion of metals (PBF-LB/M) enables the production of complex shaped components. High-carbon tool steels tend to cracking during PBF-LB/M due to internal stresses caused by the rapid solidification. Expensive atomization and long lead times for powder generate high costs in this processing route. In situ alloying during PBF-LB/M of powder blends from conventionally available powders enables a more flexible approach of alloy design. For industrial use, the mechanical properties of in situ alloyed parts must be comparable to those of conventionally manufactured parts. In some cutting and forming applications, high wear resistance and corrosion resistance are required simultaneously. High alloyed cold work tool steels with sufficient chromium solved in the metal matrix fulfill these demands. Herein, AISI H13 is modified by Cr₃C₂ and elemental Cr to suit these requirements. Two novel alloys are modeled thermodynamically and processed by PBF-LB/M. In-depth microstructural investigations by backscatter electron imaging and diffraction in combination with abrasive wear tests and potentiodynamic polarization curves allow microstructure property correlations for different heat-treated conditions. Partial crack-free processing, hardenability, formation of Cr-rich carbides, and residual Cr-rich inclusions are observed and their influence on the wear and corrosion resistance is discussed.     

Thermal Fatigue Testing of Plasma Transfer Arc Stellite Coatings on Hot Work Tool Steels under Steel Thixoforming Conditions

The thermal fatigue performance of Stellite 12 coating deposited on X32CrMoV33 hot work tool steel via the plasma transfer arc (PTA) process was investigated under steel thixoforming conditions. Stellite 12 coating has made a favorable impact on the thermal fatigue performance of the X32CrMoV33 hot work tool steel. The latter survived steel thixoforming conditions lasting much longer, for a total of 5000 cycles, when coated with a PTA Stellite 12 layer. This marked improvement is attributed to the higher resistance to oxidation and to temper softening of the Stellite 12 alloy. The Cr-rich oxides, which form during thermal cycling, provide adequate protection to high-temperature oxidation. In contrast to hot work tool steel, Stellite 12 alloy enjoys hardening upon thermal exposure under steel thixoforming conditions. This increase in the strength of the coating is produced by the formation of carbides and contributes to the superior thermal fatigue resistance of the Stellite 12 alloy. When the crack finally initiates, it propagates via the fracture of hard interdendritic carbides. The transformation of M₇C₃ to M₂₃C₆, which is more voluminous than M₇C₃, promotes crack propagation.     

Corrosion behaviors of inconel 617 in hydrogen base gas mixture

The corrosion behavior of Inconel 617, a candidate for the structural material of heat exchanger in the high temperature gas-cooled reactor (HTGR), has been investigated at elevated temperatures in the hydrogen base gas mixture (80 pct H₂ + 15 pct CO + 5 pct CO₂). This gas mixture simulates the reducing gas in the direct steel making system that uses heat from HTGR in Japan. This gas has relatively high oxidizing and carburizing potentials. In the temperature range of 650 to 1000 °C Inconel 617 oxidized to form a Cr₂O₃ scale containing titanium oxide. The activation energy for this process is estimated to be 50 to 60 kcal/mol. The time dependence of the growth of the surface oxide scale was parabolic. The aluminum in Inconel 617 was internally oxidized. The time dependence of the internal oxidation was noticed to obey a 0.4 power rate law. Carburization was noticed at 650 and 900 °C. At 900 °C, carbides containing Si, Ti, and Mo precipitated beneath the oxide scale for gas exposure times up to 200 h. After 200 h, the formation and growth of the surface scale suppresses carburization. The thermodynamic analysis of gas atmosphere proposed by Gurry could be applied successfully to the experimental results. Some inconsistency existed mainly because of the scale formation and direct gas-metal interactions.     

Properties and application of TRIP‐steel in sheet metal forming

A further development of dual‐phase‐steels are represented by TRIP (transformation induced plasticity) ‐steels. TRIP‐steels contain austenite, which is metastable at room temperature. It transforms to martensite during straining (TRIP effect). This process improves the strength‐ductility balance of these steels. Two types of TRIP‐steels, low alloyed (L‐TRIP) and high alloyed (H‐TRIP), can be applied in sheet forming processes and exhibit different forming characteristics. Basing on results of uniaxial tensile tests and the evaluation of Young's modulus the forming limits in deep drawing processes and the component properties of deep drawn parts are discussed. The Young's modulus decreases significantly with increasing pre‐strain, especially demonstrated for the L‐TRIP material TRIP700. Forming limit curves determined at different forming temperatures indicate its influence on the forming limits. Martensite transformation is suppressed at a temperature of approximately T = 200 °C and therefore the major strain ?₁ decreases significantly. For the investigated stainless steel AISI304 (H‐TRIP) different lubricant types in comparison to chlorinated paraffins have been tested. Lubricants consisting of sulphur additives led to good forming conditions in forming processes, even better than lubricants based on chlorinated paraffins. The evaluation of component properties, compared between L‐TRIP and H‐TRIP, was done based on the analysis of springback and dent resistance. The L‐TRIP material TRIP700 shows higher springback angles than AISI304 resulting from higher yield strength and decreased Young's modulus, resulting from the forming process. The dent resistance of TRIP‐steel was exemplarily demonstrated for AISI304. Uniaxial pre‐strained sheet specimen were analysed to show the dent resistance depending on dent depth. During elastic denting pre‐strain has no influence on dent resistance. Further increasing dent depth lead to increased dent forces for pre‐strained specimens.     

Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through theα →γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta−7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base FeTa-Cr alloy, the rupture and creep strengths were considerably increased. Formerly with Lawrence Berkeley Laboratory.     

Inconel 690合金TT处理后的析出相

 用扫描电子显微镜、透射电子显微镜和相分析研究了3种不同钛含量的Inconel 690合金经不同固溶温度+715 ℃×15 h,TT处理后的析出相。结果表明：合金经TT处理后,均有M23C6析出,并含有一定量的Ti(C,N);钛含量为110%的合金中有γ′相析出,γ′相在晶内弥散分布;M23C6大多在晶界上析出,而且在孪晶界和晶内位错线处也均有析出,M23C6与界面一侧基体保持立方 立方的平行位向关系;钛含量增加,析出的M23C6尺寸变小,间距增大,晶界附近的铬含量明显提高,有利于改善合金的耐腐蚀性能。     

Structure and properties of mechanically alloyed composite material from waste of high purity aluminium production

Abstract

The present investigation has been based on the study of an opportunity of production dispersion strengthened composite material from waste of high purity aluminium production. Such waste represents an alloy with the following chemical composition: 50–60%Al, 25–35%Cu, ～10%Fe and 5–10%Si. The alloy Al–25Cu–10Fe–10Si with a chemical composition typical for industrial anodic sludge has been milled in a high energy planetary mill. The change in morphology and properties of granules of this material during mechanical alloying is estimated. For an assessment of some operational characteristics, a consolidated sample is obtained. The compact sample of the material possesses a high level of hardness, both at room and at higher temperatures, low level of thermal expansion coefficient and high level of wear resistance.     

Thermal property measurements of metal injection moulded Inconel 625 and Inconel 718 using combined thermal analysis techniques

ABSTRACT

The high-temperature thermal properties of powder metallurgy derived superalloys have not been reported in the literature. In this study, the coefficient of thermal expansion (CTE), specific heat and thermal diffusivity of metal injection moulded (MIM) Inconel 625 and Inconel 718 were measured. Measurements of wrought Nickel 200 were also made to verify the methods. These thermal property measurements were made in the range of room temperature to 1000–1200°C using dilatometry, differential scanning calorimetry and laser flash analysis. Thermal conductivity of all three materials was calculated using the measured diffusivity, specific heat and CTE. All the MIM results were compared to published data for the wrought form of these alloys and found to be in close agreement outside of phase transition regions.     

Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through the α→γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta-7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base Fe-Ta-Cr alloy, the rupture and creep strengths were considerably increased. M. Dilip Bhandarkar, formerly with Lawrence Berkeley Laboratory.