KR920000035B1 - Weldable cast nickel base superalloy - Google Patents

Abstract

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Description

Welding product that does not cause cracks

1 is a micrograph of grain boundaries in the heat affected zone.

2 is a scanning electron micrograph at 1000 magnifications of pores associated with grain boundaries.

3 shows M.C. Scanning electron micrograph at 1000x magnification of carbide.

The present invention relates to changing the composition of Inconel ^R 718 (Inconel ^R 718, manufactured by International Nickel), which is a nickel-based superalloy in order to have good weldability.

Inconel 718 is a super alloy based on nickel developed in the 1960s, especially for gas turbine engines.The nominal composition of superalloys is 5.12% niobium and tantalum, 19% chromium, 52.5% nickel and molybdenum. 3.05%, titanium 0.9%, aluminum 0.6%, the rest is iron, the content of impure elements is limited to silicon up to 0.3%, carbon up to 0.8%, sulfur up to 0.015%, manganese up to 0.35%. Inconel 718 is described in US Pat. No. 3,046,108 as a medium strength material suitable for use in the temperature range of about 538-816 ° C. (1000-1500 ° F.).

Conventionally, the alloy has been used in a forged form that is bent or formed into other useful forms or in a cast form cast in a predetermined shape, in which case a complex shape or a precise margin is completed through machining.

Modern gas turbine engines contain more complex shaped parts than previous engines, and much effort has been made to reduce the weight of the engine. At the same time, many advances in casting technology allow casting to near-final products or even finer shapes that require minimal machining, and increase the operating temperature and stress to create driving forces to become stronger materials.

These factors make gas turbine engine designers use complex shaped cast parts with the same composition as Inconel 718. The main problem in manufacturing such complex shaped parts is the defect in casting of Inconel 718. And cracks), and in the castings of Inconel 718, relatively few defect-free parts of complex shape have been produced. Thus, the practical application of complex cast parts made of Inconel 718 depends on the technique of welding repair the casting to reduce cracks or other defects. It is also useful to have the ability to repair machining defects. However, the Inconel 718 has been a problem of the lack of good weldability.

As a result, it was observed that cracks (or more precisely, microcracks) generated during welding of Inconel 718 occur at grain boundaries in the heat affected zone. The heat affected zone is an area adjacent to the welding area and is a region which does not melt itself upon welding.

A manual GTA (gas-tungsten-arc) welding method was used to evaluate the present invention, but it is believed that the microcracks are reduced even when other types of welding methods are used. The material of the heat affected zone is heated to a high temperature during welding, and the microcracks are shown to be generated by the local melting of the grain boundaries of the heat affected zone and by the mechanical confinement upon cooling.

The crack is related to the presence of Laves type phases at the grain boundaries that melt and resolidify the heat affected zone and the presence of MC carbide at the grain boundaries. Melting of grain boundaries is also associated with high concentrations of melting point dropping impurities such as sulfur, boron and phosphorus. FIG. 1 is a 1000-fold micrograph showing the shape of the grain boundaries in the heat affected zone Inconel 718 that has been melted and resolidified after welding, with an unusual phase clearly visible in the center of the molten grain boundary and within the grain boundaries associated with this abnormal phase. Cracks and / or voids are visible. FIG. 2 is a scanning electron micrograph 1000 times larger than the cracks or voids in the center of the grain boundary in the heat affected zone. The fact that the cracks were formed from molten metal, as the surface of the cracks were wave or round, In other words, it can be seen that the crack was formed before solidification was completed.

Table 1 shows the composition of the base alloy material, the grain boundary separated from the weld, and the grain boundary near the weld where cracks were observed. Reference symbols (A, B, and C) used here refer to FIG. It corresponds to a symbol. In addition, the grain boundary near the welded part contains a phase called the Laves phase described above. As can be seen from Table 1, the composition of the grain boundary near the region where the weld was solidified is about 10 times that of silicon and about 6 times that of niobium, compared to the composition of the base metal. 3 shows a scanning electron micrograph in which the technique of increasing the contrast of MC carbide is employed, where a marked concentration of MC carbide in the grain boundaries of the heat affected zone can be clearly seen.

TABLE 1

From the above factors, it was concluded that the concentrations of niobium, silicon and carbon were reduced, and the total content of niobium tube tantalum (here, tantalum is usually present as an impurity in niobium and has the same alloying effect as niobium) It reduces the niobium content from its standard content of 4.75-5.5% to 4.75-5.125%, suggests a maximum allowable silicon content of up to 0.05% up to 0.35%, and the carbon content is 0.03 from 0.1% based on its commercial maximum content. By varying by -0.06%, the microcracks associated with welding can be significantly reduced. It is also desirable to reduce the content of sulfur, zirconium, boron and phosphorus to a commercial minimum.

Successful removal of the weld cracks observed in the alloy of the modified composition according to the invention will be the result of two series of points. Firstly, the melting of the grain boundaries in the heat affected zone is substantially reduced by reducing boron, sulfur, zirconium and phosphorus (all of which tend to concentrate at grain boundaries and are melting points lowering elements), and second, in modified compositions By reducing niobium and silicon in order to reduce the amount of melting of the grain boundaries occurring, the formation of harmful Laves phases is substantially eliminated. In addition, the amount of MC carbide formed by the limitation of the carbon content is also reduced.

The above described features and other advantages of the present invention will become more apparent from the following description with reference to the accompanying drawings. % In this specification is weight%.

The best aspect of the present invention is to limit the total content of niobium + tantalum in the Inconel 718 composition to 4.75-5.125%, limit the content of silicon up to 0.05%, and change the carbon content to 0.03-0.06%. In addition, it is highly desirable to limit the content of sulfur, zirconium, boron and phosphorus in as small amounts as possible to meet commercial standards. The reduction of sulfur, zirconium, boron and phosphorus is well achieved by using the best method of using virgin material as starting material, i.e., using individual pure starting elements rather than recycled or scraped material as starting material.

Table 2 shows the composition and weld test results for 10 specimens of Inconel 718. Seven of the ten specimens are test specimens specially prepared by adjusting the chemical composition from the virgin material according to the present invention, and the remaining three specimens are commercial engine parts, one of which is the virgin material or the scrip. It is made from ash.

TABLE 2

(1) commercial engine parts; Virgin material

(2) commercial engine parts; Recyclables

It can be seen from Table 2 that the amount of microcracks increases when silicon, niobium and / or carbon are in excess. In addition, it can be seen that the use of the virgin material is preferable because the two samples made of the recycled material or the scrap material have a greater fine crack than the specimen made of the virgin material. The niobium content of specimen Nos. 3 and 5 is higher than or equal to the upper limit of the value defined in the present invention, and the degree of cracking is higher than that of the sample made of virgin material. It is the upper limit. Thus, from the above data, the influence of carbon, silicon and niobium and the weldability of the Inconel 718 alloy are demonstrated, and the weldability of Inconel 718 is substantially improved by limiting the elements as described above.

The foregoing is merely a description of specific embodiments of the present invention, and various changes may be made without departing from the spirit and scope of the present invention.

Claims (1)

17-21% Cr, 50-55% Ni, 2.8-3.0% Mo, 0.65-1.15% Ti, 0.2-0.8% Al, 4,75-1.1% (Nb + Ta), 0.03-0.06% C, 0 Welding products with a weld-free crack with a nominal composition of Si, residual iron of -0.05%.

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