Heat treatment of alloys having elements for improving grain boundary strength

Abstract

Heat treatment of alloys having elements for improving grain boundary strength is disclosed. Components directly after castings often reveal low or no transverse grain boundary strength, so that cracks do appear and decrease the yield rate. The provided measures do not lead to low transverse grain boundary strength but maintains efficient grain boundary strength, so that the yield rate of components without cracks is increased.

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation-in-part of U.S. application Ser. No. 10 / 641,995, filed Aug. 15, 2003, which is the U.S. National Stage of International Application No. PCT / EP02 / 11856, filed Oct. 12, 2002, and claims the benefits of these applications. The International and U.S. Applications are incorporated by reference herein in their entirety.FIELD OF THE INVENTION [0002] The present invention relates to a heat treatment of alloys, especially nickel base superalloy and, more particularly, to castings having a columnar grain microstructure. BACKGROUND OF THE INVENTION [0003] U.S. Pat. No. 4,597,809 describes single crystal castings made from a nickel base superalloy having a matrix with a composition consisting essentially of, in weight %, of 9.5% to 14% Cr, 7%. to 11% Co, 1% to 2.5% Mo, 3% to 6% W, 1% to 4% Ta, 3% to 4% Al, 3% to 5% Ti, 6.5% to 8% Al+Ti, 0% to 1% Nb, and balance essentially nickel with the matrix containing about 0...

AI Technical Summary

Benefits of technology

[0007] The present invention involves a heat treatment of cast alloys, such as superalloys, having at least one addition, which improves grain boundary strength such as boron in the nickel base superalloy described here above in a manner discovered to significantly improve transverse stress rupture strength and ductility of directionally solidified (DS) columnar grain castings produced with a heat treatment, which solves a secondary phase only partly, e.g. no fully solution heat treatment is performed.

[0008] Boron is often added to superalloy compositions in an effective amount to substantially improve transverse stress rupture strength and ductility of directionally solidified columnar grain castings produced from the boron-modified superalloy. The boron concentration preferably is controlled in the range of about 0.003% to about 0.0175% by weight of the superalloy composition to this end.

Problems solved by technology

Single crystal castings produced from the aforementioned nickel base superalloy exhibit inadequate transverse grain boundary strength.

However, the directionally solidified (DS) columnar grain castings produced were unacceptable as DS castings as a result of the castings exhibiting essentially no transverse grain boundary strength and no ductility when tested at a temperature of 750 degrees C.

The transverse grain boundary strength and ductility were so deficient as to render DS columnar grain castings produced from the aforementioned nickel base superalloy unsuitable for use as turbine blades of gas turbine engines.

Due to the occurrence of grain boundary cracks after the fully solution heat treatment the producibility is so deficient as to render DS columnar grain castings produced from the aforementioned nickel base superalloy unsuitable for use as turbine blades of gas turbine engines.