KR930004478B1 - Nickel base super alloy - Google Patents

Abstract

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Description

Nickel-Based Superalloy

The present invention relates to a compositional improvement to make a special superalloy known as waspaloy weldable.

The alloy on which the present invention is based is an alloy called waspallo developed in the early 1950s for use in gas turbine engines. The nominal composition of this alloy is 19.5% Cr, 13.5% Co, 3.75% Mo, 3.0% Ti, 1.12% Al, 0.005% B, 0.06% C and balance Ni in weight percent. In addition, some impurity may be present in the alloy in limited amounts. The most notable of these impurities is iron, which is allowed up to 2% by weight (some early references suggest that iron is an essential component of Waspalloy in an amount of about 3%).

Waspalloy's "balance" element nickel is an expensive material, typically several dollars per pound. Iron, on the other hand, is well under $ 1 per pound. Therefore, it is economically beneficial for Waspalloy manufacturers to add iron to reduce raw material costs. Another reason for considering iron is the fact that many superalloy scraps are contaminated with iron, an element that is difficult to remove from scrap. Considering the availability of such a high iron content, the iron content in the Waspalloy became more natural.

Applicants analyzed the 13 recently received Waspalloy material iron contents. The average iron content was 1.16%, and the minimum iron content was 0.70%. These data support the above theory that iron is present in amounts of about 1% or more in almost all commercial wasploy materials.

Waspaloys are widely applied in the gas turbine industry, usually in the form of forgings and often in the form of castings. Waspalloy is stronger than most of its alternative compositions, especially in the range of 538 ° C to 816 ° C.

In the prior art, the alloys have been used in the form of forgings which can be bent or formed into useful shapes or in the form of castings in which the essential shape of the part is set by the casting process. Complex shapes and precise tolerance dimensions in both the forged and cast parts can be achieved by machining. Modern gas turbine engines contain quite complex parts. In addition, considerable efforts have been made to reduce the weight of the engine. As a result, excess material of the casting is permitted in conventional engines, while it is desirable to remove all excess material in current engines. At the same time, significant progress has been made in casting technology, resulting in parts that increase complexity, which can be cast into near-final or net shape, requiring only minimal finishing machining. In addition, because both the operating temperature and the applied stress are high, a stronger material is required due to its driving force. Both of these factors have led to gas turbine engine designers seeking to use complex castings of compositions such as Waspalloy. The main problem faced in manufacturing these parts is that the alloy is susceptible to some degree of cracking during the casting process. As a result, Waspalloy's castings have produced relatively few crack-free parts to this day. Thus, the practical application of the cast portion of the waspalloy composition depends on the ability to weld repair the casting exhibiting microcracks and similar defects. It may also be useful to have the ability to repair minor cracks, etc. due to machining in insignificant parts of the components by filling the material by welding when the material is accidentally excessively removed. In other respects, the preferred waspal has suffered from the conventional lack of weldability.

Waspalloy type compositions are described in numerous US patents to date starting from US Patent No. 2,570,193, filed in 1946. Some of these patents are, for example: 2,570,193; 2,977,222; 3,166,412; 3,207,599; 3,512,963; 3,625,678; 3,723,107; 3,850,624; 4,110,110; 4,213,026; And 4,456,481.

Given practically the various US patents filed within this limited alloy range, it is not surprising that many elements are added to or removed from the base composition to achieve various purposes. Patent, US Pat. No. 3,166,412, deals with castings made from Waspaloy type compositions and in part should not be present at least 0.5%, preferably 0.25%, since iron is an undesirable element in the alloy. Explain. However, the nature of the problems arising from the presence of iron and / or the benefits that can be obtained by limiting iron to such a range have not been described. In addition, the patent does not mention welding or weldability.

Similarly, several patents have proposed adding manganese to superalloys for the purpose of improving weldability. For example, US Pat. No. 4,213,026 improves the welding process and provides for cracking in heat-affected zones. It was suggested to add 0.5% -3% manganese to the nickel-based superalloy welding wire for the purpose of improving the generation rate. However, Figures 2 and 3 of the patent and Table 4 suggest that the weldability of waspalloy is not necessarily sufficient even when improved to manganese.

Waspalloy type compositions inevitably limit the high iron content to about 0.2% or less, preferably 0.1% or less, add 0.3-0.5% manganese, limit the zirconium content, sulfur, By limiting bismuth and lead as low as possible and preferably keeping aluminum and molybdenum at the lower end of the conventional composition range of waspalloy, it is possible to make weldable in an improved and cast form.

The foregoing and other features and advantages of the present invention will become more apparent from the following description.

Commercial specifications for Waspaloy allow up to 2% iron. Although iron has been tolerated in various superalloys, in particular in earlier developed superalloys, the waspalloy composition is rare among the major superalloys currently used in that it allows the iron content to be at this level.

According to the present invention it has been found that at least about 0.2% iron by weight in the waspalloy composition has a significant impact on the weldability of such materials. Table 1 shows a sample of the various cast waspalloy components welded and repaired at the Applicant's facility. Swatches 1 to 6 were found to be essentially weldable, while specimens 7 to 12 formed according to the invention were found to be reliably weldable. Weldability was evaluated by evaluating welds made with GTA welding equipment for cracks and other defects. The most obvious difference between weldable and non-welded specimens was iron content. That is, it was found that all of the weldable specimens contained less than 0.2% iron, whereas the non-weldable specimens were all obtained commercially by heating materials that contained more than 1% by weight iron. Accordingly, the present invention finds, in a major aspect, that the weldability of a waspalloy type material can be dramatically improved by limiting iron to 0.2% or less, preferably 0.1% or less by weight.

Preferably, in addition to limiting the iron content, intentionally add manganese in an amount of 0.3-0.5% by weight. It is also desirable to limit the zirconium content to 0.02% or less by weight, and preferably it is not intentionally added to zirconium at all. Zirconium has conventionally been added under the belief that it will improve stress rupture characteristics, but extensive testing is directed to the stress rupture characteristics between waspalloy materials containing conventional zirconium amounts and intentionally free of zirconium. It didn't show any difference.

In addition, in the prior art there are elements which are generally regarded as detrimental to weldability, ie sulfur, bismuth and lead. Such components are known to segregate at grain boundaries and are inherently low melting materials or cause significant cracking by forming low melting phases at grain boundaries. Sulfur forms a low melting nickel sulfide phase. While the above-described manganese reduces the influence of sulfur by forming manganese sulfides at the grain boundaries having significantly higher melting points than iron sulfides that would have been formed without manganese, however, it is desirable to limit the sulfur content as low as possible anyway. In addition, since the bismuth and lead are intrinsically low melting point elements, it is desirable to limit such elements to the lowest possible level. Control of the amount of such elements can be achieved by forming the alloy from virgin material (ie, the pure form of the alloying component) rather than using scrap or reclaimed material.

The final alloy improvement that can be made to improve the weldability of the material is to limit the aluminum and molybdenum content to about the lower half of the normal range for such components in the Waspalloy specification. Both aluminum and molybdenum are reinforcing components, but limiting such components to the lower half of their normal range (e.g., 1-1.12% Al and 3.5-4.25% Mo) does not significantly affect the strength of the weldability. I found it to improve. Thus, in the most preferred form of the invention such components are limited to the lower half of their normal range. Table 2 shows the general composition for conventional (cast) and spalloy materials and the composition range for the present invention.

For the benefit obtainable according to the invention, the first limitation of iron is the most important factor for improving weldability, the addition of manganese is the next most important factor, the removal of zirconium is the third important factor, sulfur, bismuth and Restriction of lead is the fourth most important factor, and I think the fifth important factor is to limit aluminum and molybdenum to the lower end of their normal range.

While the invention has been described in detail with respect to embodiments, it will be understood by those skilled in the art that various changes in form and detail of the invention may be made without departing from the scope of the following claims.

[Chart I]

* Not analyzed

[Chart II]

Claims (4)

18-21% Cr, 12-15% Co, 2.75-3.25% Ti, 0.002-0.008% B, 0.03-0.1% C, 0.1% Cu or less, 0.015% or less P, 0.5 ppm or less Se, 0.2 ppm or less Te, 0.2 ppm or less Th, 0.2% or less Fe, 0.3-0.5% or less Mn, 0.12% or less Zr, 0.03% or less S, 1.0-1.5% or less Al, 3.5 A nickel-based superalloy characterized by substantially improved weldability of castings produced by the alloy as an alloy consisting of -5.0% Mo and nickel balance. The nickel-based superalloy according to claim 1, wherein the weldability is further improved by not intentionally adding Zr but limiting the Zr content to 0.02% or less. 3. The nickel-based superalloy according to claim 2, wherein weldability is further improved by lowering the content of S and very small amounts of Bi and Pb, which are inevitably added by forming the alloy with virgin material. The nickel-based superalloy according to claim 3, wherein the weldability is further improved by limiting the Al content to 1.0% to 1.125% and the Mo content to 3.5% to 4.25%.