Effects of Post Weld Heat Treatment on Microstructures of Alloy 617 and 263 Welds for Turbines of HSC Power Plants
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Recently nickel based superalloys are extensively being regarded as the materials for the steam turbine parts for hyper super critical (HSC) power plants working at the temperature over 700°C, since the materials have excellent strength and corrosion resistance in high temperature.In this paper, alloy 617 of solution strengthened material and alloy 263 of γâ-precipitation strengthened material were prepared as the testing materials for HSC plants each other. Post weld heat treatment (PWHT) was conducted with the gas tungsten arc (GTA) welded specimens. The microstructure of the base metals and weld metals were investigated with Electron Probe Micro-Analysis (EPMA) and Scanning Transmission Electron Microscope (STEM).The experimental results revealed that Ti-Mo carbides were formed in both of the base metals and segregation of Co and Mo in both of the weld metals before PWHT and PWHT leaded to precipitation of various carbides such as Mo carbides in the specimens. Furthermore, fine particles, that were not precipitated in the specimens before PWHT, were observed in base metal as well as in the weld metal of alloy 263 after PWHT. Key words: Hyper super critical power plant, Ni-based Superalloy, γâ (Gamma prime) Precipitation, Steam turbine rotorKeywords:
Base metal
This paper presents hot corrosion results carried out systematically on the selected nickel based superalloys such as IN 738 LC, GTM-SU-718 and GTM-SU-263 for marine gas turbine engines both at high and low temperatures that represent type I and type II hot corrosion respectively. The results were compared with advanced superalloy under similar conditions in order to understand the characteristics of the selected superalloys. It is observed that the selected superalloys are relatively more resistant to type I and type II hot corrosion when compared to advanced superalloy. In fact, the advanced superalloy is extremely vulnerable to both types of hot corrosion. Subsequently, the relevant reaction mechanisms that are responsible for slow and faster degradation of various superalloys under varied hot corrosion conditions were discussed. Based on the results obtained with different techniques, a degradation mechanism for all the selected superalloys as well as advanced superalloy under both types of hot corrosion conditions was explained. Finally, the necessity as well as developmental efforts with regard to smart corrosion resistant coatings for their effective protection under high temperature conditions was stressed for their enhanced efficiency.
Degradation
High-Temperature Corrosion
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It has been established that carbides in superalloys serve three functions. Fine carbides precipitated in the matrix give strengthening results. Carbides also can tie up certain elements that would otherwise promote phase instability during service. Grain boundary carbides prevent or retard grain-boundary sliding and strengthen the grain boundary, which depends significantly on carbide shape, size and distribution. Various types of carbides are possible, depending on superalloy composition and processing. In the paper optical and scanning electron microscopy investigations of carbides occurring in specimens of the polycrystalline nickel-base superalloy MAR-M247 were carried out. Conditions of carbides revealing and microstructure images acquisition have been described. Taking into consideration distribution and morphology of the carbides in matrix a method of quantitative description of Chinese script-like and blocky primary carbides on the basis of image analysis was proposed.
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In turbine engines, Ni or Co based alloys are used at high temperature, either as base materials, superalloys, or deposited on the surface of superalloys, as coatings. In the present study, two different MCrAlY overlay coatings, Ni and Co based, on a Ni based superalloy IN792 were aged for different times in air at three temperatures, 900°C, 1000°C and 1100°C. The aging processes were simulated by using DICTRA software by focusing on the interdiffusion behavior in the superalloy-coating systems. The results of simulation captured the main microstructural features observed and were used to analyze the diffusion behavior of alloying elements and the corresponding microstructure development. It was found that coating composition and temperature affected significantly the microstructure near the superalloy-coating interface, and their relations were mapped as a summary.
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The effects of welding heat input on toughness and microstructure characteristics of coarse-grained heat-affected zone(CGHAZ) in X80 grade pipeline steel and inside weld in welded pipe were investigated by welding thermal simulation technology,microscopic analysis method and mechanical property testing.The results show that with the increase of welding heat input,the toughness distribution of CGHAZ of X80 pipeline steel base metal and inside weld in welded pipe is different.For the steel,when the heat input is 10～20 kJ/cm,CGHAZ get the best toughness and toughness values remain the same level with the base metal.When the welding heat input is greater than 20 kJ/cm,the toughness presents downward trend;when the welding heat input is greater than 35 kJ/cm,the toughness drops sharply.But for the inside weld in welded pipe,when the welding heat input is 17～35 kJ/cm,inside weld CGHAZ can get the best toughness level;when the welding heat input is less than 17 kJ/cm or greater than 35 kJ/cm,the toughness levels of X80 inside weld CGHAZ entirely decline.
Base metal
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Ni-base superalloy is a kind of the most complex alloy,it was used widely as higher temperature components.Its relative applied temperature is the highest in common alloy series.From the developing history of superalloy,it experienced four steps:wrought superalloy、C.C.superalloy、D.S. superalloy、S.C.superalloy.The development and research method of the alloy design was presented.In this paper,and the effect of material physical properties due to elements deflection was discussed too.
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Purpose To characterise the high temperature oxide scales for some plasma sprayed NiCrAlY coated Ni‐ and Fe‐based superalloys. Design/methodology/approach Ni‐22Cr‐10Al‐1Y metallic coatings were deposited on two Ni‐based superalloys; Superni 601 and Superni 718 and one Fe‐based superalloy; Superfer 800H by the shrouded plasma spray process. Oxidation studies were conducted on uncoated as well as plasma spray coated superalloys in air at 900°C under cyclic conditions for 50 cycles. Each cycle consisted of 1 h heating followed by 20 min of cooling in air. The thermogravimetric technique was used to approximate the kinetics of oxidation. X‐ray diffraction, SEM/EDAX and EPMA techniques were used to analyse the oxide scales. Findings All of the coated, as well as the uncoated, superalloys followed an alnost‐parabolic rate of oxidation. The NiCrAlY coating was found to be successful in maintaining its continuous contact with the superalloy substrates in all the cases. The oxide scales formed on the exposed NiCrAlY coated superalloys were found to be intact and spallation‐free. The main phases analysed for the coated superalloys were oxides of nickel, chromium and aluminium and spinel of nickel and chromium, which are expected to be useful for developing oxidation resistance at high temperatures. Practical implications The coated superalloys showed remarkable cyclic oxidation resistance under simulated laboratory conditions. However, it is suggested that these coated superalloys also should be tested in actual industrial environments of boilers and gas turbines, etc. so as to obtain more practical and reliable oxidation data. Originality/value The knowledge of the reaction kinetics and the nature of the surface oxide scales formed during oxidation is important for evaluating the alloys for their use and degradation characteristics in high temperature applications such as steam boilers, furnace equipment, heat exchangers and piping in chemical industry, reformer, baffle plates/tubes in fertilizer plants, jet engines, pump bodies and parts.
Thermogravimetric analysis
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The carbides in a conventionally cast and rapidly solidified IN—100 typenickel-based superalloy were investigated by TEM,SEM,EDX and metallography.Theresults indicated that the obvious changes in the morphology,size,composition and distri-bution of carbides appeared in the alloy after rapid solidification.It is possible that thechange in MC-type carbides affects strongly the mechanical properties of a superalloy athigh temperature.
Metallography
Morphology
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The importance of understanding and controlling the basic solidification process in high temperature alloy technology as applied to gas turbine engine production is discussed. Resultant tailoring of the superalloy macro- and microstructure offers significant potential for continued advances in superalloy use temperatures in turbine engines. Atomized superalloy powders, rapidly solidified superalloys, microstructural control, and advanced superalloys are discussed.
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In this paper superalloys, their processing and application areas have been researched. The superalloys are widely used in the industrial production elds such as aircraft, nucleer, space industry and so on due to superior properties at high temperature and resistance to metallurgical and structural variations. The most important groups of the superalloys is Ni, Fe and Co-based superalloys. Also processing of the superalloys are investigated and another goal of the present paper is to investigate microstructure and mechanical properties of IN718 subjected to strength ening heat treatment.
Inconel
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Superalloys are indispensable for the superior perfomance and efficie n of jet engine turbine disks,turbine blades,combustors,roket shells and many other devices and components used in various industries, i n Cluding nuclear, space and aeronautics. Over the last thirty years,nickel-base superalloys has undergone more structural studies than any other superalloy as a result of attempts to correlate its properties to microstructure and heat treatment. To avoid contradictory or inconsistent findings, it is also necessary a carefully microstructural observation of the employed starting materials. In the present paper,we report morphologies relating to the as cast , forged and heat-treated Ni-base superalloy by transmission and scanning electron microscopy. This superalloy contain intermettallic compounds and carbides. The alloy in study was an as cast and also forged Ni-Fe based superalloy with the following chemical composition:
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