JPS6153423B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a shroud for a gas turbine. [Prior Art] As illustrated in FIG. 1, a gas turbine shroud 1 is disposed on the inner peripheral surface of a turbine casing 2 at a portion facing the tips of blades 3.
It has an appearance as shown in FIG. In Figure 1,
4 is a turbine disk to which the blade 3 is attached. Such a shroud for a gas turbine has a thick wall shape and is repeatedly exposed to high temperature corrosive gas, so that high thermal stress is repeatedly generated. [Problems to be solved by the invention] On the other hand, high-efficiency gas turbines have been developed in recent years, and the metal temperature of the shroud has come to reach 700-900°C. Although 25Cr-20Ni-based materials equivalent to SUS310SS are used in such shrouds, there is a problem that thermal fatigue cracking occurs due to higher thermal stress, resulting in a shortened component life. [Object of the Invention] An object of the present invention is to provide a shroud for a gas turbine that has excellent thermal fatigue resistance, high-temperature strength, and high-temperature corrosion resistance. [Means for solving the problem] The inventors of the present invention conducted a detailed investigation into cracks in SUS310SS material, and found that the cracks were caused by thermal fatigue.
This is a mixture of cracking along the σ phase and intergranular cracking, and the causes are (1) a large amount of acicular, brittle σ phase precipitates during heating, and the σ phase itself cracks, as well as intragranular deformation due to the σ phase. (2) Film-like carbides are formed continuously at the grain boundaries, and are destroyed by high thermal stress, causing intergranular cracks to propagate all at once. 3) It was discovered that grain boundary erosion occurs due to high-temperature corrosion, and cracking is accelerated by wedge action. The present invention was made based on this knowledge, and it is based on the following: C0.3-0.5%, Cr20-35%,
Ni20~40%, Ti0.1~0.5%, Nb0.1~0.31%,
It is made of a cast material of 2% or less Mn, 2% or less Si, and the balance is Fe, and is characterized by having a σ phase content of 5% or less and an austenite structure. The present invention can contain 0.1 to 1% of rare earth elements. Further, at least one of W and Mo can be contained together with a rare earth element. W and
The total amount of Mo shall be 7% or less. Furthermore, Co can be contained together with at least one of W and Mo and a rare earth element. The amount of Co is 5 to 20%. Next, the reason for limiting the components will be explained first. However, in this specification, % indicates weight % unless otherwise specified. C: C plays a very important role in improving thermal fatigue resistance and high temperature strength. If the C content is less than 0.25%, the σ phase tends to precipitate, and at the same time, film-like carbides precipitate continuously at the grain boundaries, which is not preferable. In addition, when the content of C is high, the amount of brittle eutectic carbides and secondary carbides at cell grain boundaries increases,
Thermal fatigue properties are reduced. Considering these, C is
0.3-0.5% is most preferred. Cr: 20% or more of Cr is required to suppress grain boundary erosion due to high-temperature corrosion of shroud materials, and 35% or more is required to prevent excessive precipitation of carbides and embrittlement due to precipitation of σ phase during use at high temperatures. It is not advisable to exceed it. Therefore, the Cr content is 20 to 35
limited to %. Of these, 25 to 30% is most suitable. Ni: Ni has a value of making the base austenite and improving high-temperature strength, stabilizing the structure and preventing precipitation of the σ phase, but for this purpose, 20% or more is required. Also, from the viewpoint of high temperature corrosion resistance, it is better to have more Ni. However, if the amount exceeds 40%, the amount of eutectic carbide increases and the thermal fatigue resistance decreases. accordingly
The Ni content is 20 to 40%, especially 25 to 40%.
35% is suitable. Ti, Nb: These elements are
When TiC and Nb are used alone, NbC is added, and when Nb and Ti are added in combination, an MC type carbide is formed, such as (Ti, Nb)C. Although precipitation strengthening cannot be expected much due to its amount, the precipitation and growth of secondary Cr carbides, which have a large effect on precipitation strengthening, can be appropriately suppressed.
Suppresses the decline in high temperature strength over a long period of time. It also suppresses continuous precipitation of Cr carbides at grain boundaries. If the content of these elements is small, the effect will be small, and if the content is large, secondary Cr carbides will decrease due to an increase in these MC carbides, resulting in a decrease in high-temperature strength.
M/C with Atomic Ratio
(M is the sum of metal elements that form MC type carbides) is 0.2
~0.3 is most preferred. Ti and Nb each have Ti
0.1-0.5%, Nb 0.1-0.31%. Rare earth elements: Rare earth elements are added to help the functions of Ti and Nb.If there is too little, there will be no effect, and if there is too much, casting cracks will occur.
It is preferably 0.1 to 1%, particularly 0.1 to 0.5%. W, Mo: W and Mo are added for the purpose of solid solution strengthening of the base, and the higher the amount added, the higher the high temperature strength. However, the total amount of W and Mo is 7
%, a large amount of eutectic carbides will crystallize and the thermal fatigue resistance will decrease. Co: Co is added in an amount of 5% or more for the purpose of solid solution strengthening to strengthen the base, but even if it exceeds 20%, the effect is small, so 5 to 20% of Co is appropriate. Note that if the σ phase exceeds 5%, the effect on thermal fatigue becomes significant, so it is preferable that the σ phase is 5% or less. To do this, v shown in the following equation must be 2.5
It is recommended that the composition be as follows. v={0.66Ni+1.71Co+2.66Fe+ 3.66Mn+4.66(Cr+Mo+W)+6.66Si}/100 However, in the formula, Ni, Co, Fe, Mn, Cr, Mo,
W and Si are the weight content (percentage) of each element. Note that Si and Mn are added as deoxidizing agents, but according to the above formula, it is preferable to reduce the amount of these added, and each is suitably 2% or less. In the present invention, a shroud is obtained by molding an alloy having such a composition into a predetermined shape, for example, by precision casting. It is also preferable to perform heat treatment after casting from the viewpoint of improving properties. Examples of such heat treatment include a method in which solution treatment is followed by aging treatment. [Example] Hereinafter, the present invention will be specifically explained with reference to Examples. Performance tests were conducted on shroud materials using test materials whose compositions (wt%) are shown in Table 1. However, in Table 1, No. 1 is a conventional material (SUS310SS) for comparison, and Nos. 2 to 5, 7, and 8 are test materials according to the present invention. After casting the test materials shown in Nos. 1 to 8 in Table 1 into rods of 15 mmφ x 100 mm by precision casting, solution treatment (1175°C x 2h → air cooling) and aging treatment (850°C x 4h) were performed. → air cooling). A thermal fatigue test piece of 10 mmφ x 10 mm was obtained from the obtained heat-treated product, held at 850°C for 6 minutes, and then water-cooled 300 times.
Thermal fatigue resistance was evaluated based on the length of the crack that occurred in the cross section of the test piece. Figure 3 shows the crack length (mm) that occurred in each test piece. In addition, No.7
The results of testing the thermal fatigue resistance of the sample material in the same manner without heat treatment as cast are also shown. From Figure 3, the shroud material of the present invention has a shorter crack length than the No. 1 conventional material.
It can be seen that both exhibit excellent thermal fatigue resistance.
In particular, Mitsushi metal was added as a rare earth element.
The thermal fatigue resistance of Nos. 3 to 5 and No. 7 with added Co was significantly improved. Here, Mitsushi Metal is 50% lanthanum, neodymium and other similar elements.
It is an alloy of rare earth metals with plasticity of 50% cerium. No. 6 with 9% W added has a large amount of W added, so the improvement in thermal fatigue resistance is small.
Furthermore, from a comparison between the heat-treated material No. 7 and the non-heat-treated material, the shroud material of the present invention shows sufficiently excellent thermal fatigue resistance even when it is a non-heat-treated material, but it is even more excellent when heat-treated as described above. It is clear that it exhibits thermal fatigue resistance. The effect of improving thermal fatigue resistance by heat treatment is greater for shroud materials with fewer eutectic carbides. In addition, No. 1 (conventional material) shroud and No. 3
Figure 4 shows the structure of the shroud made of the material of the present invention after one year of use. In Fig. 4, a and b are shrouds made of conventional materials;

【table】

[Table] Many deep cracks have occurred in the material, and film-like carbides can be seen at the grain boundaries, and large amounts of windman-like acicular carbides can be seen within the grains. Shrouds made of such conventional materials suffer from numerous cracks and require repair by welding. On the other hand, the shroud material of the present invention shown in c has almost no σ phase, grain boundary carbides are discontinuous, and cracking is slight, so it can be used continuously without repair. . Note that the magnification is 100 times for a and 400 times for b and c. The shroud made of the material of the present invention also had significantly superior creep rupture strength, especially on the long-term side.
This means that the material of the present invention is less susceptible to heat embrittlement. [Effects of the Invention] As explained above, the present invention has excellent thermal fatigue resistance, high-temperature strength, and high-temperature corrosion resistance, and the life of the member is significantly extended.

[Brief explanation of the drawing]

FIG. 1 is a schematic view showing the arrangement of a gas turbine shroud, and FIG. 2 is a sketch showing the appearance of the gas turbine shroud. Figure 3 is a graph showing the length of cracks that occurred in thermal fatigue tests of conventional materials and materials of the present invention, and Figure 4 is a microscopic photograph of the metal composition of a gas turbine shroud after one year of use. c is the conventional material and c is the inventive material (No. 3). 1... Shroud, 2... Turbine casing, 3... Blade, 4... Turbine disc.

Claims (1)

[Claims] 1. C0.3-0.5%, Cr20-35%, Ni20-40 by weight
%, Ti 0.1-0.5%, Nb 0.1-0.31%, Mn 2% or less, Si 2% or less, and the balance is Fe, the σ phase content is 5% or less, and it has an austenitic structure. Shroud for gas turbines. 2 C0.3-0.5%, Cr20-35%, Ni20-40 by weight
%, Ti0.1~0.5%, Nb0.1~0.31%, rare earth elements
A shroud for a gas turbine, characterized in that it is made of a cast material of 0.1 to 1%, Mn 2% or less, Si 2% or less, and the balance Fe, has a σ phase content of 5% or less, and has an austenitic structure. 3 C0.3-0.5%, Cr20-35%, Ni20-40 by weight
%, Ti0.1~0.5%, Nb0.1~0.31%, rare earth elements
0.1~1%, at least one of W and Mo total 7%
A shroud for a gas turbine, characterized in that it is made of a cast material of 2% or less Mn, 2% or less Si, and the balance is Fe, has a σ phase content of 5% or less, and has an austenitic structure. 4 C0.3-0.5%, Cr20-35%, Ni20-40 by weight
%, Ti0.1~0.5%, Nb0.1~0.31%, rare earth elements
0.1~1%, at least one of W and Mo total 7%
A shroud for a gas turbine, characterized in that it is made of a cast material of 5 to 20% Co, 2% or less Mn, 2% or less Si, and the balance is Fe, has a σ phase content of 5% or less, and has an austenitic structure.