JP2006503980A - Alloy heat treatment method - Google Patents

Abstract

The present invention relates to heat treatment of alloys having elements for improving grain boundary strength. Parts immediately after casting often exhibit low transverse grain boundary strength or no transverse grain boundary strength at all, resulting in cracks and reduced yield. The present invention does not lead to low lateral grain boundary strength, maintains effective lateral grain boundary strength, and increases the yield of crack-free parts. A second phase to dissolve is present and heat treated to only partially dissolve the second phase.

Description

  The present invention relates to a heat treatment method for an alloy, particularly a nickel-base superalloy, and a casting having a columnar grain microstructure.

  U.S. Pat. No. 4,597,809 describes 9.5-14 wt% Cr, 7-11 wt% Co, 1-2.5 wt% Mo, 3-6 wt% W, 1-4 wt% Nickel-base superalloy having a matrix of the following composition: Ta, 3-4 wt% Al, 3-5 wt% Ti, 6.5-8 wt% Al + Ti, 0-1 wt% Nb and the balance nickel The matrix comprises 0.05 to 0.15 wt% C and an additional amount of Ta equal to 1 to 17 times the content of C in the alloy. As a result of inclusion, it contains 0.4 to 1.5% by volume of a phase based on tantalum carbide.

  The transverse grain boundary strength exhibited by the single crystal casting produced from the nickel-base superalloy described above is insufficient. The inventors have attempted to produce unidirectionally solidified (DS) columnar grain castings of nickel-base superalloys. However, the produced unidirectionally solidified (DS) columnar grain castings essentially have transverse grain boundary strength and elongation when the castings are tested at a temperature of 750 ° C. and a stress of 660 Mpa (95.7 Ksi). Therefore, it was not suitable as a DC casting. The transverse grain boundary strength and elongation were insufficient so that a DS cast particle casting made from the nickel-base superalloy would be inappropriate for use as a gas turbine engine turbine blade.

  WO 99/67435 discloses a nickel-base superalloy casting to which boron is added to improve the transverse stress rupture strength and elongation of the DS casting. The casting is heat-treated at 1250 ° C. for 4 hours to completely dissolve the second phase (γ′-phase). Due to the grain boundary cracking after the complete melting heat treatment, the production rate is insufficient so that the DS cast particle casting produced from the nickel-base alloy described above becomes unsuitable for use as a turbine blade of a gas turbine engine.

  It is an object of the present invention to heat treat an alloy, particularly an as-cast alloy, for example a DS columnar grain cast based on the aforementioned single crystal nickel-base superalloy, wherein the DS cast is a turbine of a gas turbine engine. It is to have transverse stress rupture strength and elongation and production rate substantially improved to the extent that they can be used as high temperature applications such as blades.

  The present invention is significant in terms of transverse stress rupture strength and elongation of unidirectionally solidified (DS) columnar grain castings that are produced by a heat treatment that only partially dissolves the second phase, for example, without a complete solution heat treatment. Heat treatment of a cast alloy, such as a superalloy, having at least one additive such as boron in the nickel-base superalloy described hereinabove, which improves the grain boundary strength so that the improvement can be seen.

  In many cases, boron is added to the superalloy composition in an amount effective to substantially improve the transverse stress rupture strength and elongation of unidirectionally solidified columnar grain castings made from boron modified superalloys. To do. Therefore, the boron concentration is preferably adjusted within the range of 0.003 to 0.0175% by weight of the superalloy composition.

  In connection with the addition of boron to the superalloy composition, the carbon concentration may be adjusted within the range of 0.05 to 0.11% by weight of the superalloy composition.

  A preferred nickel-base superalloy according to one embodiment of the present invention is 11.6-12.70 wt% Cr, 8.5-9.5 wt% Co, 1.65-2.15 wt% Mo. 3.5 to 4.10 wt% W, 4.80 to 5.20 wt% Ta, 3.40 to 3.80 wt% Al, 3.9 to 4.25 wt% Ti, 0 0.05 to 0.11 wt% C, 0.003 to 0.0175 wt% B and the balance Ni. This superalloy can be cast as a DS columnar crystal grain casting according to a normal DS casting technique such as the well-known Bridgeman mold release method.

  The DS casting produced in this way typically has a main stress axis of the cast and a number of columnar grains extending in the direction of the <001> crystal axis that is generally parallel to the main stress axis. The DS columnar grain casting according to the present invention preferably exhibits a stress rupture life of at least about 100 hours and an elongation of at least about 2.5% when tested at a temperature of 750 ° C. and a stress of 660 Mpa (95.7 Ksi). , Turbine blades, vanes, external air seals and other parts of industrial and aviation gas turbine engines.

  Objects and advantages of the present invention will become apparent with reference to the drawings, together with the following detailed description.

  Specific examples of the alloy include 9.5 to 14% by weight of Cr, 7 to 11% by weight of Co, 1 to 2.5% by weight of Mo, 3 to 6% by weight of W, and 1 to 6% by weight of Ta. 3-4 wt% Al, 3-5 wt% Ti, 0-1 wt% Nb, DS substantially improves transverse stress rupture strength compared to similar castings without boron A nickel-base superalloy consisting of B present in an effective amount and the balance Ni is selected.

  As an additive to improve the grain boundary strength of the alloy, the substantial transverse stress rupture strength and elongation of the DS columnar grain cast produced from the alloy compared to a similar cast without boron. The inclusion of boron in an amount found to be effective to achieve is selected.

  For this purpose, the nickel-base superalloy is modified by containing boron B within a range of 0.003 to 0.0175% by weight, preferably 0.010 to 0.015% by weight of the superalloy composition. Good.

  In connection with the addition of boron to the superalloy composition, the carbon C concentration is adjusted within a preferred range of 0.05 to 0.11% by weight of the superalloy composition. Silicon Si, zirconium Zr and hafnium Hf can also be used as additives.

  Furthermore, all combinations of B, C, Si, Zr, and Hf are possible.

  The transverse stress rupture strength and elongation and production rates of DS castings made from nickel-base superalloys with modified heat treatment make it possible to use them as turbine blades and other parts of gas turbine engines It is realized to the extent of making.

  A particularly preferred boron-modified nickel-base superalloy casting composition is 11.6-12.70% Cr, 8.5-9.5% Co, 1.65-2.15% by weight. Mo, 3.5-4.10% W, 4.80-5.20% Ta, 3.4-3.80% Al, 3.9-4.25% Ti, 0.05- 0.11% C, 0.003 to 0.0175% B and the balance Ni and can be cast to achieve a DS columnar grain microstructure.

  The DS microstructure of the columnar grain casting includes 0.4 to 1.5 volume percent of a phase based on tantalum carbide.

  While not wishing to be bound by any theory, at high service temperatures, such as 816 ° C. typical of gas turbine engine blades, boron and carbon migrate to the grain boundaries in the DS microstructure and It is thought to add strength and elongation. Typically, a DS columnar grain cast made from the boron modified nickel-base superalloy has a <001> crystal axis parallel to the principal stress axis of the cast and a temperature of 750 ° C. and When tested at a stress of 660 Mpa (95.7 Ksi) applied perpendicular to the <001> crystal axis of the casting, it exhibits a stress rupture life of at least about 100 hours and an elongation of at least about 2.5%.

  For example, the following DS casting test was performed. These tests provide further explanation of the invention and do not limit the invention.

  Sample # 1 having the nickel-base superalloy composition of the aforementioned US Pat. No. 4,597,809, and samples # 1A and # 2 and # 3 of boron-modified nickel-base superalloys were prepared as shown in Table I below. (Unit:% by weight).

  Each sample was cast to form a DS columnar grain non-hollow casting having a square body shape for a transverse stress rupture test according to ASTM E-139 test procedure.

  The DS casting was produced using, for example, the usual Bridgman mold release unidirectional solidification method.

  For example, each sample was melted in a crucible of a normal casting furnace under a vacuum of 1 micron and heated to 1427 ° C. The heated melt was poured into a casting precision casting mold with a surface coating comprising zircon lined with a further slurry / stucco layer comprising zircon / alumina. The mold was preheated to 1482 ° C., a chill plate was placed, and unidirectional heat removal from the molten alloy in the mold was performed. The mold and chill plate filled with the melt were removed from the furnace at a removal speed of 15-40 cm / hour under a vacuum of 1 micron and transferred to the solidification chamber of the casting furnace.

  The DS columnar grain casting is cooled to room temperature in the chamber under vacuum, removed from the mold in the usual manner using a mechanical extrusion procedure, and the dissolution of the second phase in the matrix is partially It was heat-treated at such a temperature and time that it can only be performed.

  This nickel-base superalloy has a γ'-phase as the second phase.

  For a test article (eg, nickel-base superalloy) having the composition of claim 21, the heat treatment of the present invention is at least 1213 ° C. after casting, which is not the melting temperature of the second phase (eg, y ′ phase) of the alloy Perform for 1 hour.

  As long as the second phase is not completely dissolved in the matrix, the 1250 ° C. temperature normally used for complete dissolution processing (referred to as the complete dissolution temperature) can also be used.

  A second phase that does not dissolve in the matrix, depending on its geometry and manufacturing rate after heat treatment to avoid grain boundary cracking, increase the yield of the specimen and increase the desired mechanical properties of the specimen Is less than 90, 70, 50 or 30% by volume.

  This alloy takes either a single crystal structure or a structure having only particles along one direction.

  If necessary, the composition is subjected to an aging heat treatment at 1080 ° C. for at least 2 hours after the solution heat treatment. Thereafter, a secondary aging heat treatment can be performed at 870 ° C. for at least 12 hours.

  In particular, the heat treatment of the present invention is used for hollow specimens, especially wings, blades or liners, because cracks occur more frequently on walls, especially thin walls, after the heat treatment normally performed after casting, than on solid specimens. .

  The heat treatment of the present invention results in an increase in grain boundary strength during this heat treatment and increases the yield after heat treatment (parts without cracks).

  The transverse stress rupture of the part as a final product also increases during use of the part under the conditions of use due to the increased grain boundary strength. The method of the present invention also produces good results, for example for solid parts of gas turbines.

  The casting was also analyzed for chemical properties and machined into test specimen shapes.

  The stress rupture test was conducted in air at a temperature of 750 ° C. and a stress of 660 Mpa (95.7 Ksi) applied perpendicular to the <001> crystal axis of the test product.

  The results of the stress rupture test are listed in Table II below. Here, the life (unit: time) indicates the time until the test article breaks, the elongation is the elongation of the test article until the break, and the area red indicates the decrease in the area of the test article until the break. The baseline data corresponds to the test data for sample # 1, and the data for # 1A, # 2 and # 3 correspond to the test data for samples # 1A, # 2 and # 3, respectively. The baseline data represents the average of two stress rupture test specimens, while the data for # 1A, # 2 and # 3 represent the specimen for a single stress rupture test.

  DS columnar grain test product produced from Sample # 1 has essentially no transverse grain boundary strength when tested at a temperature of 750 ° C. and a stress of 660 Mpa (95.7 Ksi) (eg, stress rupture life 0 hours) It is clear from Table II that That is, these specimens failed immediately and produced a near zero stress rupture life. Furthermore, the elongation and area reduction data were also almost zero. These stress rupture properties are insufficient to render the DS columnar grain cast produced from Sample # 1 unfit for use as a turbine blade of a gas turbine engine.

  In contrast, when tested at a temperature of 750 ° C. and a stress of 660 Mpa (95.7 Ksi), the DS columnar grain specimen produced from Sample # 1A has a 275 hour stress rupture life, 3.1% elongation, and 4 Table II shows that the specimen manufactured from Sample # 2 showed an area reduction of .7, and exhibited a stress rupture life of 182 hours, an elongation of 2.6% and an area reduction of 6.3%. I understand. These stress rupture properties of the present invention represent an unexpected and surprising improvement over the specimens made from sample # 1, and the stress rupture properties of the present invention also allow samples # 1A, # 2 and # 3. DS columnar grain castings made from are more suitable for use as turbine blades and other parts of gas turbine engines.

  The present invention is effective to produce DS columnar grain castings having substantial transverse stress rupture strength and elongation. These properties are achieved without adversely affecting other mechanical properties such as tensile strength, creep strength, fatigue strength, and corrosion resistance of the DS casting. The present invention has an alloy composition as described above for imparting substantial transverse stress rupture strength and elongation to a casting used throughout multiple stages of a turbine of a stationary industrial gas turbine engine. It is particularly useful for making large DS columnar grain industrial gas turbine (IGT) blade castings having lengths of ˜60 cm and above (such as about 90 cm). The boron-modified nickel-base superalloy casting composition described above can be cast as DS columnar grain parts or single crystal parts.

  Although the present invention has been described in terms of specific embodiments of the present invention, the present invention is not limited thereto but only by the description of the scope of claims.

Claims (28)

A method for heat treatment of a cast alloy comprising at least one additive for improving grain boundary strength,
The alloy has a second phase that melts at a melting temperature in the alloy matrix after casting;
A method in which the heat treatment is performed so that the second phase is only partially dissolved.   The method of claim 1, wherein at least one aging treatment is performed after the heat treatment.   The method according to claim 1, wherein a temperature of the heat treatment is lower than a complete dissolution temperature.   The method according to claim 1 or 3, wherein the time for the heat treatment is selected so that the second phase is not completely dissolved.   The method according to claim 1, wherein the heat treatment is performed on a nickel or cobalt base superalloy.   The method according to claim 1, wherein the heat treatment is performed on a hollow part.   The method according to claim 6, wherein the heat treatment is performed on a part having a length of at least 200 mm.   The method according to claim 6, wherein the heat treatment is performed on a hollow part having an outer wall thickness of 8 mm or less.   The method of claim 5, wherein the second phase is a γ′-phase.   The method according to claim 1, wherein the heat treatment is performed on an alloy containing boron as an additive.   The method according to claim 1, wherein the heat treatment is performed on an alloy containing carbon as an additive.   The method according to claim 1, wherein the heat treatment is performed on an alloy including unidirectionally solidified columnar grains.   The method according to claim 1, wherein the heat treatment is performed on an alloy including a single crystal structure.   The method of claim 1, wherein the heat treatment parameters are selected such that the amount of the second phase to be dissolved is less than 90% by volume.   The method of claim 1, wherein the heat treatment parameters are selected such that the amount of the second phase to be dissolved is less than 70% by volume.   The method of claim 1, wherein the heat treatment parameters are selected such that the amount of the second phase to be dissolved is less than 50% by volume.   The method of claim 1, wherein the heat treatment parameters are selected such that the amount of the second phase to be dissolved is less than 30% by volume. 9.5 to 14% by weight of Cr,
7-11% by weight of Co,
1 to 2.5 wt% Mo,
3-6 wt% W,
1 to 6 wt% Ta,
3-4% by weight of Al,
3-5 wt% Ti,
0-1 wt% Nb,
B and balance Ni present in an effective amount to substantially improve the transverse stress rupture strength of the casting compared to a similar casting without boron.
The method according to claim 1, wherein the heat treatment is performed on a unidirectionally solidified columnar grain nickel-base alloy casting.   The method according to claim 18, wherein the heat treatment is performed on an alloy in which B is present in a range of 0.003 to 0.018% by weight.   When the heat-treated alloy is tested at a temperature of 750 ° C. and a stress of 660 Mpa (95.7 Ksi) applied perpendicular to the <001> crystal axis of the casting, a stress rupture life of at least about 100 hours and at least The method of claim 18, having an elongation to failure of 2.5%. 11.6-12.70 wt% Cr,
8.5-9.5 wt% Co,
1.65 to 2.15 wt% Mo,
3.5-4.10 wt% W,
4.8-5.20 wt% Ta,
3.4-3.80 wt% Al,
3.9 to 4.25 wt% Ti,
0.05 to 0.11% by weight of C,
0.003 to 0.0175 wt% B and the balance Ni
2. The method of claim 1 wherein the heat treatment is performed on a unidirectionally solidified columnar grain nickel-base alloy casting comprising: and having superior transverse stress rupture strength compared to a similar casting without boron.   When the heat-treated alloy is tested at a temperature of 750 ° C. and a stress applied perpendicular to the <001> crystal axis of the cast, 660 Mpa (95.7 Ksi), a stress rupture life of at least 120 hours and at least 2.5% The method of claim 21 having an elongation of. 12.00% by weight of Cr,
9.00% by weight Co,
1.85 wt% Mo,
3. 70 wt% W,
5. 10 wt% Ta,
3. 60% by weight of Al,
4.00 wt% Ti,
0.0125 wt% B,
0.09 wt% C, and the balance Ni
And a stress rupture life of at least about 100 hours and at least 2 when tested at a temperature of 750 ° C. and a stress of 660 Mpa (95.7 Ksi) applied perpendicular to the <001> crystal axis of the casting. The method of claim 1, wherein said heat treatment is performed on a unidirectionally solidified columnar grain nickel-base alloy casting having an elongation to failure of 5%.   The method according to claim 1, wherein the heat treatment is performed after casting.   The process according to claim 4, wherein a complete dissolution temperature is used.   The method according to claim 6, wherein the hollow part is one of a blade, a wing, and a liner.   The method according to claim 1, wherein the heat treatment is performed on a solid part.   The method of claim 1, wherein the heat treatment is performed using an alloy comprising an additive selected from the group consisting of zircon, silicon, and hafnium.