JPH04124238A - High corrosion resistant cobalt base alloy - Google Patents

Abstract

PURPOSE:To manufacture a sound precision casting by constituting it of specified wt.% of C, Cr, Ni, W, Ti, Al, Fe, B, Zr and either Ta or Nb and the balance Co. CONSTITUTION:A high corrosion resistant cobalt base alloy is constituted of, by weight, 0.3 to O.7% C, 24.5 to 33% Cr, 8 to 12% Ni, 5 to 9% W, 0.2 to 0.5% Ti, 0.05 to 0.2% Al, <=0.2% Fe, 0.005 to 0.015% B, <=0.1% Zr, 3 to 4.5% of at least either Ta or Nb and the balance Co with inevitable impurities. In this way, the high corrosion resistant cobalt base alloy as the material constituting parts small in the fear of grain boundary oxidation at the time of parts manufacturing and combining sufficient oxidation resistance, corrosion resistance and creep resistance even at a high driving temp. can be offered.

Description

Detailed Description of the Invention [Objective of the Invention] (Industrial Application Field) The present invention relates to a highly corrosion-resistant cobalt-based alloy used as a material for heat-resistant parts of high-temperature equipment, and particularly relates to a highly corrosion-resistant cobalt-based alloy that exhibits less grain boundary oxidation during casting. Relating to corrosion-resistant cobalt-based alloys.

(Prior Art) In recent years, high-temperature combustion has been promoted in combustion power equipment in order to improve its operating economy.

For example, the combustion gas temperature of gas turbines for jet engines has been set at a value exceeding 1,300° C. from the conventional level of about 1,100° C. in some cases. For this reason, the operating temperature of gas turbine stator blades and other components has also increased from 900°C to 1°C.
Temperatures as high as 100°C may be reached. In such a high-temperature usage environment, in addition to the normal oxidation effect, the corrosion effect due to combustion residue becomes noticeable, so we recommend using a high Cr-containing cobalt-based alloy that has excellent oxidation and corrosion resistance. is used.

Conventionally, a typical high Cr-containing cobalt-based alloy used for boat fishing is 24% Cr-Co-based alloy (Mar-M5).
09) is commercially available. This C.

When manufacturing heat-resistant parts using a base alloy, the master alloy ingot must be placed in a crucible! A so-called precision casting method is employed in which a molten alloy is prepared by melting, and then the molten alloy is poured into a mold of a predetermined shape and solidified.

(Problems to be Solved by the Invention) However, in parts cast by precision casting using conventional Co-based alloys as raw materials, carbides precipitated at grain boundaries or dendrite boundaries near the surface, as shown in Figure 4. is subjected to excessive oxidation, so-called grain boundary oxidation, and many wedge-shaped deep oxidation grooves are formed.

These oxidation grooves significantly reduce the creep resistance of the part, so cast parts made using such alloys must be treated to remove the grain boundary oxidation parts on the outer surface by electric discharge machining or machining after casting. It was said that This has the disadvantage of complicating the parts manufacturing process and lowering production efficiency. In particular, it is virtually impossible to process the inner surface of a cast part having a hollow part or to remove the grain boundary oxidized parts, so there is a limit to the reliability of the cast part.

On the other hand, in recent years there has been a growing demand for improved operating efficiency as operating temperatures have increased, and there has been a demand for the development of materials that have sufficient oxidation and corrosion resistance even at these operating temperatures.

The present invention has been made to solve the above problems, and has a structure that reduces the risk of grain boundary oxidation during the manufacturing of parts and has sufficient oxidation resistance, corrosion resistance, and resistance even at higher operating temperatures. The object of the present invention is to provide a highly corrosion-resistant cobalt-based alloy that can be used as a material for parts that have creep properties.

[Structure of the invention]

(Means and effects for solving the problem) In order to achieve the above object, the present inventors prepared alloys having various compositions, and as a result of comparing and examining their properties, the present inventors discovered a cobalt-based alloy having the following composition range. When forming the
An alloy with significantly improved corrosion resistance, oxidation resistance, and creep resistance compared to the conventional one was obtained, and the present invention was completed based on this knowledge.

That is, the highly corrosion-resistant cobalt-based alloy according to the present invention contains 0.3 to 0.7% carbon and 24.5 to 33% chromium by weight.
%, nickel 8-12%, tungsten 5-9%,
0.2-0.5% titanium, 0.05-0.5% aluminum
0.2%, iron less than 0°2%, boron 0.005-0
．． 0.15% of zirconium, 0.1% or less of zirconium, 3 to 4.5% of at least one of tantalum and niobium, and the remainder cobalt and inevitable impurities.

The reason for limiting the composition of the highly corrosion resistant cobalt-based alloy according to the present invention will be described below.

Carbon is an element that combines with titanium, tantalum, niobium, zirconium, and tungsten to form carbides as a dispersion-strengthening phase and maintains tensile strength and creep strength. is necessary. However, when the amount of carbon increases beyond 0.7%, the carbides precipitated at the dendrite boundaries are connected in a network, resulting in a decrease in ductility in the high-temperature region and a decrease in creep strength. The upper limit is 7% by weight.

Chromium is a basic element that maintains and improves oxidation and corrosion resistance.
4.5% by weight or more is required.

However, if the content exceeds 33% by weight, the ductility in the high temperature region will decrease, so the content should be 24.5 to 3% by weight.
It is set within a range of 3%.

Nickel is an element that, together with cobalt and chromium, forms a cobalt-nickel-chromium alloy base that is stable at high temperatures.In order to form a stable alloy base within the above range of chromium, its content is 8 to 12% by weight. The range is set to .

Tungsten forms carbides with some carbon and contributes to strengthening the matrix, but most of it melts into the cobalt-nickel-chromium matrix and strengthens the alloy structure.

In order to fully obtain the effect, at least 5% by weight is required, but if the content exceeds 9%, embrittlement will occur, so the upper limit is set at 9% by weight.

Titanium is an element that combines with carbon to form carbides as a dispersion-strengthening phase.
．． 2% by weight or more is required, but if the amount is too high, high temperature ductility will be reduced, so the upper limit is set at 0.5% by weight.

Aluminum has the effect of increasing high-temperature strength even when added in a small amount, preferably 0.05% by weight or more, or ductility decreases as the amount added increases, so 0.2
The upper limit is % by weight.

Zirconium has conventionally been added to improve high-temperature strength and ductility, but since it accelerates the oxidation of carbides formed during casting solidification, it is preferable to reduce it as much as possible, and the upper limit of the content is 0. It is set at 1% by weight.

Boron has the effect of concentrating in grain boundaries and dendrite boundaries to strengthen the alloy matrix, or at least 0.005% by weight of boron must be added to obtain this effect. However, if it is added in a large amount exceeding 0.015% by weight, a brittle phase will be formed in the concentrated part and the strength will decrease.
．． The upper limit of the amount added is 0.015% by weight.

Both tantalum and niobium act as reinforcing elements that form carbides, and also have the effect of increasing high-temperature strength and corrosion resistance by being solid-solved in the cobalt-nickel-chromium matrix. In order to obtain this effect, it is necessary to add 3% by weight or more, but if it is added in excess of 4.5% by weight, segregation will occur. Therefore, to obtain a homogeneous casting, the upper limit is 4.5% by weight. Since the above-mentioned strengthening effects of tantalum and niobium can be considered to be almost equivalent, either tantalum or niobium may be used alone, or both may be used at the same time. In either case, the total amount of one or both is set within a range of 53 to 4.5% by weight.

Iron is one of the inevitable impurities for the alloy of the present invention, but its content is set at 0.20% by weight or less, especially in order to ensure the stability of the cobalt-nickel-chromium matrix. Ru.

(Example) Next, the highly corrosion-resistant cobalt-based alloy according to the present invention will be specifically explained using examples. A molten alloy having the composition shown in Table 1 below was prepared using an ordinary vacuum melting furnace, and poured into a mold in vacuum to prepare a sample piece for material testing.

Table 1 (Unit: Weight %) Of the component compositions shown in Table 1, Examples 1, 2 and 3 are alloys according to the present invention, and Comparative Example 1 is the commercially available Co-based alloy (Ma
r-M509). As a mold for pouring molten metal, S t O2A j! 20a Z r
A mold made of a general-purpose material consisting of 02 was used.

Incidentally, the amount of grain boundary oxidation that occurs during casting varies depending on the composition of the raw material alloy, but is also influenced by the material of the mold. Therefore, in order to suppress the oxidation reaction on the alloy surface, the above-mentioned disadvantages can be prevented by changing the composition ratio between the main components of the mold material and the molten alloy. However, the purpose of the present invention is to prevent grain boundary oxidation equally effectively regardless of whether a mold made of a general-purpose material is used and no particular mold material is specified. In this embodiment, a mold made of a general mold material is used. Therefore, in the present invention, the mold material is not limited to that of this example.

FIG. 1 is a micrograph showing the micrometallic structure of the surface of the sample piece according to Example 1 cast as described above, and shows the cross-sectional structure in the as-cast state.

On the other hand, FIG. 4 is a microscopic photograph showing the micrometallic structure of a sample piece according to Comparative Example 1. As shown in FIG. 4, in Comparative Example 1, many deep grain boundary oxidation layers with an average depth of 0.3 mm are observed.

On the other hand, in Example 1 shown in FIG. 1, no grain boundary oxidation layer was observed, indicating that it had extremely good corrosion resistance. This tendency was observed in Examples 2 to 3 and Comparative Example 2 as well.

Figure 2 shows the results of cross-sectional micro-observation of each of the alloy sample pieces of Examples 1 to 3 and Comparative Examples 1 to 2 prepared as described above and measurement of the grain boundary oxidation depth distribution in the alloy. It is a graph expressed in relation to the zirconium content.

As is clear from the graph of FIG. 2, it was confirmed that grain boundary oxidation does not substantially occur by setting the zirconium content to 0.1% by weight or less as specified in the present invention.

Generally, the mold material used for casting super heat-resistant alloys contains zirconium oxide (ZrO2), so when molten metal is poured into the mold, the zirconium contained in the mold is removed by the reaction between the molten metal and the mold. may be eluted into the molten metal, increasing the zirconium content in the cast product. This amount of increase varies depending on the temperature of the molten metal and the amount of zirconium oxide in the mold, and is approximately 0.02 to 0.03% by weight at most. Therefore, grain boundary oxidation can be almost completely prevented by limiting the zirconium content to a value lower than the limit silconium r'7 content that does not cause grain boundary oxidation, as predicted from Figure 2. .

Next, a corrosion test was conducted on each sample piece prepared in Examples 1 to 3 and Comparative Examples 1 to 2 to investigate the corrosion resistance properties. Corrosion test: 90% Na, SO4 + 10% NaCJ
The artificial synthetic ash consisting of ash was applied to the surface of each sample piece, heated at a temperature of 900°C for 5 hours, and the maximum erosion depth of the applied surface was measured. In addition, in the sample pieces of Comparative Examples 1 and 2, the sample surface is covered with a grain boundary oxidation layer generated during casting, so in order to eliminate this effect, all cast alloy samples were pre-machined to a diameter of 20-1. height 20
- A sample piece was prepared by cutting it into a cylindrical shape and removing the oxide layer on the surface of the cast product, and was subjected to a corrosion test.

Figure 3 shows the results of the corrosion test under the above conditions.

The corrosion status is expressed as the maximum corrosion depth among the depths to which corrosion has progressed to the grain boundaries of each sample piece. As shown in FIG. 3, the maximum erosion depth of the cobalt-based alloys prepared in Examples 1 to 3 is about half or less compared to the conventional cobalt-based alloys prepared in Comparative Examples 1 to 2. It has been demonstrated that it exhibits excellent corrosion resistance. It is thought that this corrosion resistance mainly depends on the amount of Cr added to the alloy, and in the composition range shown in the present invention, corrosion resistance superior to that of conventional alloys can be ensured.

Next, in order to compare the creep characteristics of the highly corrosion resistant cobalt-based alloy according to the present invention with conventional materials, each sample piece was subjected to a high temperature creep test.

That is, sample pieces for the creep rupture test were prepared from the round rod-shaped test pieces cast in Examples 1 to 3 and Comparative Examples 1 to 2. In Comparative Examples 1 and 2, the sample was prepared by cutting it into a predetermined shape in order to remove the influence of the oxidized layer generated on the sample surface during casting. The prepared specimens were divided into two groups, and one group was subjected to the creep test as cast without heat treatment, while the other group was subjected to solution treatment by heating at a temperature of 1230°C for 4 hours. It was subjected to a creep test.

And 10.5kgf/In for each sample piece! The rupture time until creep failure of the sample piece was measured while the tensile stress of A was applied and the temperature was maintained at 1000°C, and the results shown in Table 2 below were obtained.

Table 2 As is clear from the results shown in Table 2, the cobalt-based alloys according to Examples 1 to 3 have a creep rupture strength equivalent to or higher than that of Comparative Examples 1 to 2. It is confirmed. In particular, in Examples 1 and 3, the rupture time can be extended by about 10% by performing solution treatment in advance.

Furthermore, as examples of heat treatment conditions, the case of as-cast without heat treatment and the case of solution treatment of heating at 1230°C for 4 hours are exemplified, but the presence or absence of heat treatment can be set as appropriate depending on the required characteristics of the material. However, in any case, it is not limited to the above conditions. Further, the solution treatment (heat treatment) conditions are not limited to the condition values shown in Table 2, and can be appropriately selected depending on the composition. Commercially available alloy Mar-M509 (Comparative Example 1)
A high creep strength of carbon, tantalum, and zirconium can be achieved by a suitable combination of carbon, tantalum, and zirconium. However, in the present invention, from the viewpoint of preventing grain boundary oxidation, the zirconium content is limited within the range of the present invention, while the grain boundary By increasing the boron content for reinforcement purposes, high creep rupture strength is ensured. Furthermore, as shown in Table 2, the effects of tantalum and niobium on improving creep strength are almost the same, and even when either tantalum or niobium is contained, or both are used simultaneously. Even when it is contained, as long as it is within the scope of the present invention, high Loveture strength can be achieved.

〔Effect of the invention〕

As explained above, according to the highly corrosion-resistant cobalt-based alloy according to the present invention, there is little oxidation reaction between the molten alloy and the mold during precision casting, and it is possible to produce a sound precision cast product without grain boundary oxidation. In addition to maintaining high creep rupture strength, the corrosion resistance has been greatly improved compared to conventional materials, making it suitable for high-temperature materials such as stator blades of gas turbine engines, which are used under more severe and high-temperature conditions than before. Demonstrates excellent performance when used as a material for parts. Therefore, it has excellent effects such as increasing the reliability of high-temperature equipment and enabling long-term operation of the equipment.

4,

[Brief explanation of the drawing]

Figure 2 is a graph showing the relationship between grain boundary oxidation depth and zirconium content, Figure 3 is a graph showing the maximum corrosion depth measured by corrosion tests, and Figure 4 is a graph showing the relationship between grain boundary oxidation depth and zirconium content.

Claims (1)

[Claims] In weight%, carbon 0.3-0.7%, chromium 24.5%
~33%, nickel 8~12%, tungsten 5~
9%, titanium 0.2-0.5%, aluminum 0.
05-0.2%, iron 0.2% or less, boron 0.00
5 to 0.015%, 0.1% or less of zirconium, 3 to 4.5% of at least one of tantalum and niobium, and the remainder cobalt and inevitable impurities.