Showing papers on "Inconel published in 1997"

Assessment of Plasma Enhanced Machining for Improved Machinability of Inconel 718

Abstract: An experimental study has been performed to assess the feasibility of using a hybrid machining system to improve the machinability of Inconel 718. An assembled plasma enhanced machining (PEM) system is described, and experimental results obtained from both conventional and plasma enhanced machining of Inconel 718 are compared. Several advantages of PEM over conventional machining are demonstrated, including improvement of surface roughness, lower cutting forces and extended tool life.

Microprocesses of metal dusting on nickel and Ni-base alloys

Abstract: Microstructural and microchemical investigations were carried out on nickel and Inconel 600 after exposure to strongly carburizing atmospheres at temperatures of about 600 to 650 °C to study their metal dusting behaviour. Contrary to iron and low-alloy steels, where metal dusting proceeds via the formation and disintegration of a metastable carbide M 3 C, both nickel and Inconel 600 directly disintegrate. Inside the metal this disintegration proceeds by formation of thin graphite filaments of nearly 10 nm in diameter, the atomic basal planes of which are oriented perpendicular to the surface thus effecting a high reactivity at the growth front. Subsequently, fine particles or larger parts of the metal surface layer are released, which are buried in the deposited graphite layer on the surface. In case of Inconel 600, containing Cr with mass contents of about 15%, the disintegration can be delayed by the formation of a chromium oxide layer, but no safe protection against metal dusting is obtained.

Structure and tribological properties of plasma nitrided surface films on Inconel 718

Abstract: Inconel 718 specimen was plasma nitrided for microscopic examination and tribological testing. Film structure and composition, depth of film hardness and surface roughness were measured as a function of coating parameters. The film growth rate appears to be diffusion controlled, (i.e. case depth ∝ √treatment time). Nitrogen was detected in the surface of the nitrided layer by Auger electron spectroscopy. The nitrogen was found to react with Cr to form CrN which was detected in the nitrided layer by X-ray diffraction analysis. Friction and wear coefficients as a function of atmosphere, load, sliding speed, and surface temperature were performed. Exterior film structure varied from columnar or pyramidal with high roughness to a smoother spherical structure after nitriding at a lower temperature. The distribution of nitrogen within the film appeared uniform and there was a sharp boundary between the film and substrate. A maximum microhardness of 1290 HK was reached after prolonged nitriding. Nitrided surfaces showed 4 times lower coefficient of friction than a plain Inconel surface until the nitrided layer was worn away. Coarse lamellar wear particles showing signs of extreme plastic deformation were formed during wear of nitrided inconel.

Acoustic harmonic generation due to thermal embrittlement of inconel 718

Abstract: This paper describes an attempt to characterize the deterioration of a structural material's mechanical properties by nonlinear acoustics. In this particular case, the damage was caused by “thermal embrittlement” during which the material, here the nickel-based alloy Inconel 718, loses a significant fraction of its fracture toughness. Harmonic generation was the experimental method used to characterize the microstructural changes in the material as a function of exposure time at elevated temperatures. Tests were performed on two heats of Inconel 718 with slightly different chemistries, with one heat showing particular sensitivity of the fracture toughness to the elevated temperature exposure with corresponding higher changes in the nonlinearity parameter. As a mechanical measure of the fracture toughness deterioration, a small specimen punch test was used in which the ductility of a thin slice of material is determined. A clear difference between the two heats was noted in the metallographic examination, which is reflected in the harmonic generation as well as the punch test data. An explanation for the changes of the harmonic generation during the embrittlement process is speculative at the present time.

Advanced coated ceramic tools for machining superalloys

Abstract: The main limitation on the use of nickel-base superalloys, such as INCONEL 718, is the difficulty in conventional-type machining. The use of high cutting speed to achieve both machining adiabatic conditions and high productivity is necessary for their applications. This non-conventional type machining results in a short life-span of tools, even for those expensive ceramic ones with reinforced SiC whiskers (SiCw) suitable for use at high cutting speeds. The aim of the paper is to present the results of a new idea proposed by the authors to obtain an increase in tool life at high cutting speed by minimizing the temperature effects on composite reinforcement mechanisms. The 2090 SiC whiskers reinforced A12O3 tools were CrN and (Ti,AI)N coated using the PVD technique, and comparative machining tests on INCONEL 718 were carried out using uncoated and coated tools. After machining, the tools were observed with a scanning electron microscope (SEM), and EDAX (X-ray) semiquantitative analyses were performed. The behaviour of the CrN and (Ti,AI)N layers using various cutting conditions was analysed and different wear mechanisms along the tool chip contact length were observed. The cause and the mechanisms of wear were deduced and mathematic models linking tool life with process parameters were suggested.

SAGBO Mechanism on High Temperature Cracking Behavior of Ni-base Superalloys.

Abstract: Many Ni-base superalloys such as Inconel 718 show a time dependent behavior of crack growth at elevated temperatures above 500°C. The environmental interaction at the crack tip plays an important role on the crack growth rate, and the stress assisted grain boundary oxidation (SAGBO) is the acting mechanism for these alloys cracking in air. In this research, the SAGBO effect on the creep crack growth behavior of fatigue pre-cracked single edge-notched (SEN) Inconel 718 specimens tested in air and oxygen-free environments is investigated. High temperature moire interferometry (HTMI) was applied to obtain in-situ crack tip displacement fields of the test specimens at 650°C under constant loading conditions. The diffusion of oxygen along grain boundaries in front of the crack tip governs the size and degree of environmental damage. The formation of a SAGBO-induced damage zone at the crack tip causes the cracking in a brittle mode as verified by our experimental results. The damage zone induces intergranular fracture and accelerates the crack growth rate. The coarsening of the damage zone is a linear function with time under given temperatures, oxygen partial pressure, and stress intensity factor. Superalloys 718,625, 706 and Various Derivatives Edited by E.A. Loria The Minerals, Metals & Materials Society, 1997 679

A New DTA Approach for Verifying Precipitate Solvus in Inconel Alloy 718

Abstract: Using differential thermal analysis (DTA) to determine solvus temperature of precipitated phases in a Nb-hardened superalloy, such as alloy 718, is relatively difficult. The precipitation reaction is not significantly pronounced, and the baseline of a DTA thermogram is not clearly defined. A new approach was employed in this study, which utilized a solution-treated Inconel alloy 7 18 as the reference instead of the conventional alumina. The samples for analysis were preaged at various temperatures to develop specific precipitate phases. The DTA thermograms showed a well-defined baseline because of similar alloy chemistry and heat capacity between the sample and the reference. The offset of thermal peaks associated with precipitate dissolution can be accurately determined. Superalloys 718,625, 706 and Various Derivatives Edited by E.A. Loria The Minerals, Metals & Materials Society, 1997 353

Effects of applied pressure on the brazing of superplastic INCONEL 718 superalloy

Abstract: A superplastic INCONEL 718 superalloy was brazed with a Ni-P and a Ni-Cr-P amorphous filler metal. The effects of applied pressure on the bonding strength, microhardness, and corrosion resistance were studied. The results showed that the brazements with Ni-Cr-P filler metal have higher bonding strength and better corrosion resistance than those with Ni-P filler metal using a conventional brazing method without applied pressure. When brazing was conducted under applied pressure, the bonding strength increased with applied pressure for the brazements with both filler metals. This was caused by a decrease of intermetallic phase through ejection of the liquid phase enriched with melting temperature depressants in the molten filler metals from the joint clearance. Corrosion tests show that these brazements failed at the brazed joint and at the grain boundaries in the region adjacent to the brazement.

Joint strength and interfacial microstructure in silicon nitride/nickel-based Inconel 718 alloy bonding

Abstract: Joining of Inconel 718 alloys to silicon nitrides using Ag–27Cu–3Ti alloys was performed to investigate the microstructural features of interfacial phases and their effect on joint strength. The Si3N4/Inconel 718 alloy joints had a low shear strength in the range 70.4–46.1 MPa on average, depending on joining temperature and time. When the joining time was held for 1.26 ks at 1063 K, shear, tension, and four-point bending strength were 70.4, 129.7, and 326.5 MPa on average. The microstructures of the joints typically consisted of six types of phases. They were TiN and Ti5Si4 between silicon nitride and filler metal, a copper- and silver-rich phase, island-shaped Ti–Cu phase, a Ti–Cu–Ni alloy layer between filler and base metal, and diffusion of titanium into the Inconel 718 alloys. With increasing joining temperature, the thickness increase of the Ti–Cu–Ni alloy layer was much greater than that of the reaction layer. Thus the diffusion rate of titanium into the base metal was much greater than the reaction rate with silicon nitride. This behaviour of titanium results in the formation of a Ti–Cu–Ni alloy layer in all the joints. The formation of these layers was the cause of the strength degradation of the Si3N4/Inconel 718 alloy joints. This fact was supported by the analyses of fracture path after four-point bending strength tests.

Strength and wear resistance of mechanical components

Abstract: The strength and wear resistance of a steel component is improved by adhering to it a material of higher strength compatible metal alloy, such as Inconel (RTM) 725 or like precipitation or age hardenable alloy. The higher strength alloy may be adhered by welding, plasma spraying, dip coating or electroplating. The component may be subjected to post-deposition heat treatment which preferably simultaneously softens a heat affected zone in the component and hardens the higher strength material.

Characterization of low-cycle fatigue damage in inconel 718 by laser light scanning

Abstract: A technique for in situ laser light scanning (LLS) was developed to monitor surface damage on nickel-base superalloy specimens under low-cycle fatigue conditions. This technique characterizes the surface state with a parameter called the defect frequency which minimizes memory requirements and data processing time since it does not involve image processing. As a result, the present technique is capable of scanning speeds that are substantially greater than those achieved with image processing methods. Cylindrical Inconel 718 specimens were tested using an automated servo-hydraulic machine at ambient temperature under fully reversed strain control conditions for constant strain amplitudes ranging from 0.3% to 1%. The fatigue damage was monitored by scanning a laser beam along the gauge section of the specimens during periodic interruptions of the cyclic loading. Acetate replicas of the gauge section surface were also made on some of the specimens to characterize the damage using SEM and image analysis techniques. Comparisons of the results demonstrate the capabilities of the present light-scanning technique for characterizing fatigue damage on the surface of the Inconel 718 specimens. In particular, a rapid rise in the mean defect frequency is shown to correspond to an initial increase in microcrack density that saturates at approximately 20% of the fatigue life. This transient behavior is followed by a plateau in defect frequency which corresponds to crack propagation and interlinkage until failure occurs. The number of cycles to microcrack density saturation as indicated by the defect frequency is found to be linearly related to the number of cycles to failure. Accordingly, the present system provides a characterization of microcrack damage that may be used to predict the low-cycle fatigue life of Inconel 718 specimens long before failure occurs.

Selection of materials for heat exchangers

Abstract: 1 his paper provides a U-J;P,G work for selecting heat exchangers materials especially those used in nuclear power plants. Typical examples of materials selection for heat exchanger tubing of nuclear power plants and condensers are presented. The paper brings out also, the importance of continued intensive R & D in materials in order to enhance the reliability and reduce cost by improving upon the existing materials by minor additions of alloying elements or new materials. The properties of Cr Mo alloys with minor additions of W, V, Nb and N are discussed in view of their use at elevated temperatures in the power industry. These alloys were found to provide considerable operation flexibility due to their low expansion coefficient and high thermal conductivity in comparison with the austenitic stainless steels. Also, the Ni base alloy Inconel 617. could be selected for his excellent combination of creep and hot corrosion resistance up to a temperature of a50 °C.

Effect of Heat Treatment Variations on the Hardness and Mechanical Properties of Wrought InconeI 718

Abstract: The effect of heat treat variations within current specification tolerances on the hardness of wrought Inconel 7 18 was evaluated. The statistically designed experiment identified cooling rate from solution heat treatment as having the greatest impact on hardness with increasing cooling rate increasing fully heat treated hardness. A specimen test program was then initiated to determine the effect of fast and slow cooling rates on mechanical properties. Slower cooling rates resulted in reduced creep rupture and yield strengths with no effect on low cycle fatigue and fatigue crack growth properties. After thermal exposure (100 hours at 732°C) both sets of material exhibited comparable strength and stress rupture properties. Superalloys 718,625, 706 and Various Derivatives Edited by E.A. Loria The Minerals, Metals &Materials Society, 1997 43

The Effect of B Segregation on Heat-Affected Zone Microfissuring in EB Welded Inconel 718

Abstract: In a previous study on cast Inconel 718 it was observed that B can segregate on the grain boundaries during heat treatment and contribute to the HAZ microfissuring. In the wrought Inconel 718, however, the HAZ microfissuring has been mainly attributed to the segregation of S at the grain boundaries. Therefore, a research project was started to study the effect of B on the HAZ microfissuring of wrought Inconel 718. Two alloys with identical composition, except for the concentration of B were prepared. The alloys were given a final solution heat-treatment at 1050°C to produce a single phase material. Half of the solution treated samples were waterquenched and the other half were air cooled. Although, quantitative analysis could not be performed by SIMS, qualitatively it was observed that: (1) In the same alloy significantly more B was present at the grain boundaries of the air-cooled specimen as compared to the waterquenched specimen. (2) The segregation of B was significantly higher in the higher B specimen as compared to the low B specimens. The solution treated specimens were EB welded and their susceptibility to HAZ cracking was measured. It was found that the higher boron alloy specimens were more susceptibility to cracking. Also the material that was air cooled from this solution treatment temperature was more susceptible to HAZ cracking as compared to the water-quenched specimen. Superalloys 718,625,706 and Various Derivatives Edited by E.A. Loria The Minerals, Metals &Materials Society, 1997 743

Phase Formation Modeling of an Alloy 706 Casting Using Computational Thermodynamics

Abstract: Computational thermodynamics to perform alloy and process modeling for Ni-based superalloys is a new technology which has become available to industry. This paper presents an application of the technology to a superalloy of current interest. The objective of this paper is to show how results obtained from the computational program Thermo-Calc ‘1 using a recently developed thermodynamic database for Ni-based superalloy9 can be applied to help understand solidification in INCONEL’ alloy 706. The ability of the computational method to predict both stable and metastable phase formation is explored. INCONEL@ alloy 706 was chosen because of its importance in the manufacture of large land-based gas turbine disks. In addition, the alloy’s complex structure tested the viability of the computational method. The phases examined were: Laves, MC carbide, and the y matrix. Quantitative microstructure evaluation and X-ray dispersive analysis were performed on an as-cast, vacuum induction melted (VIM) ingot. Differential Thermal Analysis (DTA) was performed on selected samples of the alloy. The experimental data was compared to the data generated via the computational approach. TM Thermo-Calc, trade name of the Division of Computational Thermodynamics, Royal Institute of Technology, Stockholm, Sweden. 8 INCONEL is a trademark of the INCO group of companies.

Microstructural and Mechanical Property Characterization of Aged Inconel Alloy 625LCF

Abstract: Industrial and aerospace applications for INCONEL" alloy 625LCF" sometimes require thermal exposure in the temperature range of 593OC to 650°C for extended periods of time. The alloy has the potential to precipitate both y" and M,,C, carbides in this temperature regime with concomitant changes in mechanical properties. This paper correlates observedmicrostructural changes as a function of time and temperature with selected properties for the alloy. Microstructural features as defined by microscopy, SEM and X-ray identification of extracted phases were utilized to characterize the microstructure after exposure at 593OC and 650°C for times to 7500 hrs (11 months). The same material was evaluated for its room and high temperature tensile properties. Observations from this study were compared to historical reports of microstructure and properties. @ INCONEL and 625LCF are registered trademarks of the Into group of companies. Superalloys 718,625, 706 and Various Derivatives Edited by E.A. Loria The Minerals, Metals &Materials Society, 1997 447

Environmentally enhanced crack growth in nickel-based alloys at elevated temperatures

Abstract: A recent understanding of environmentally enhanced sustained-load crack growth in nickel-based superalloys at elevated temperatures is presented. This understanding is based on the results of coordinated studies of crack growth kinetics, surface chemistry, and microstructure in a commercial Inconel 718. The results suggest that environmental enhancement of sustained-load crack growth in Inconel 718 is associated with the formation and rupture of niobium oxides at grain boundary surfaces and is controlled mainly by the rate of oxidation and decomposition of niobium carbides at the grain boundaries. Data on other nickel-based alloys in the literature appear to support this suggested role of niobium. Initial results from a study of a niobium-free Ni-18Cr-18Fe alloy (its base composition is identical to Inconel 718) confirm the possible influence of niobium and the proposed mechanism. Some open issues for further investigation are discussed.