JP2006152412A - Corrosion resistant and oxidation resistant cast alloy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a corrosion resistant and oxidation resistant cast alloy whose durability in the temperature range of ≥800°C is improved. <P>SOLUTION: The corrosion resistant and oxidation resistant cast alloy has a composition comprising, by weight, 40 to 50% Cr, 10 to 35% Fe and 0.32 to 0.5% Si, and the balance substantially Ni. In the alloy, desirably, the content of Cr is controlled to 46 to 49%, and the content of Si is controlled to 0.35 to 0.45%. By setting the content of Si to the prescribed range, the generation of a σ phase in the temperature of ≥800°C is suppressed, and further, the fluidity of molten metal at the time of casting is secured. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a corrosion-resistant and oxidation-resistant casting alloy used for, for example, a boiler burner nozzle, a high-temperature pipe, and the like. This relates to a castable alloy.

  Conventionally, an alloy disclosed in Japanese Patent Publication No. 45-2822 (Patent Document 1) is known as a corrosion-resistant and oxidation-resistant casting alloy used for a burner nozzle of a boiler, high-temperature piping, and the like. This alloy has a composition of Cr: 35 to 45 wt%, Fe: 10 to 35 wt%, C: 1 wt% or less, and the balance Ni. This corrosion-resistant and oxidation-resistant cast alloy can have both excellent heat resistance, corrosion resistance and weldability by preventing the crystallization of the α-chromium phase and the precipitation of the σ-phase and providing a stable austenite base, and It is known that there is an advantage that it is inexpensive because the Ni content is relatively low. For this reason, this alloy has many uses for boiler burner nozzles, high-temperature piping, and the like.

Japanese Patent Publication No. 45-2822

The alloy disclosed in Patent Document 1 has excellent heat resistance, corrosion resistance, and weldability as described above. However, as a result of studies by the present inventors, the alloy disclosed in Patent Document 1 is insufficient in terms of durability when exposed to a temperature of 800 ° C. or higher for a long time. In this alloy, due to precipitation of the σ phase, the decrease in tensile strength and elongation becomes remarkable in the temperature range of 800 ° C. or higher. Recently, along with the improvement of the performance of the combustor, the tensile strength and the durability of elongation in the temperature range of 800 ° C. or higher have become particularly important.
The present invention has been made based on such a technical problem, and provides a corrosion-resistant and oxidation-resistant casting alloy having improved tensile strength and elongation durability in a temperature range of 800 ° C. or higher. Objective.

  In order to improve the tensile strength and elongation, the Cr content may be increased. However, in Patent Document 1, as the reason for setting the upper limit of the Cr content to 45 wt%, when the Cr content exceeds 45 wt%, the Ni amount decreases, so that precipitation of the σ phase occurs and the toughness becomes poor. This is disclosed. In other words, as long as the disclosure of Patent Document 1 is followed, it is not possible to expect an improvement in corrosion resistance and heat resistance by increasing the Cr content. Therefore, the present inventor has studied to increase the Cr content. As a result, it was found that by controlling the amount of Si contained in the alloy disclosed in Patent Document 1, it is possible to increase the Cr content while avoiding the precipitation of the σ phase. The present invention is based on this knowledge, Cr: 40-50 wt%, Fe: 10-35 wt%, Si: 0.32-0.5 wt%, the balance consisting essentially of Ni, the corrosion resistance, It is an oxidation resistant casting alloy.

  The corrosion-resistant and oxidation-resistant casting alloy according to the present invention prevents the deterioration of toughness due to precipitation of σ phase even if the Cr content in the alloy is increased by setting the Si content to 0.32 to 0.5 wt%. Became possible. In the present invention, Cr is allowed to be contained in the range of 40 to 50 wt%, but the Cr content is preferably 46 to 49 wt% from the viewpoint of improving the tensile strength and the durability of elongation. Moreover, the desirable range of Si content is 0.35-0.45 wt%.

  According to the present invention, by increasing the Cr content, the tensile strength and the durability of elongation are improved, and by setting the Si content to a predetermined value, the precipitation of the σ phase that causes a decrease in toughness is caused. Can be prevented.

The reasons for limiting the components of the corrosion-resistant and oxidation-resistant cast alloy according to the present invention will be described below.
Cr: 40-50 wt%
In the present invention, Cr is an element necessary for imparting corrosion resistance and oxidation resistance to the alloy and obtaining necessary mechanical properties. In the present invention, the lower limit of the Cr content is set to 40 wt%, in particular, in order to improve the durability of tensile strength and elongation. By containing this amount of Cr, corrosion resistance and oxidation resistance are also good. However, when the Cr content exceeds 50 wt%, precipitation of the σ phase occurs in an environment of 800 ° C. Moreover, when Cr content exceeds 50 wt%, the fluidity | liquidity of the molten metal at the time of casting will be inferior. Therefore, in the present invention, the upper limit of the Cr content is set to 50 wt%. A desirable Cr content is 46 to 49 wt%.

Fe: 10 to 35 wt%
The Fe content is the same as that of the alloy disclosed in Patent Document 1. That is, if Fe is less than 10%, the Ni content increases and the cost of the alloy increases. On the other hand, when the Fe content exceeds 35 wt%, a large amount of σ phase precipitates due to the correlation between Cr and Ni and becomes brittle and hard. Desirable Fe content is 15-30 wt%, and more desirable Fe content is 18-25 wt%.

Si: 0.32-0.5 wt%
Si is contained in the alloy for the purpose of ensuring the fluidity of the molten metal during casting. However, according to the study by the present inventors, the Si content affects the precipitation of the σ phase. In particular, even when the σ phase does not precipitate under an environment of less than 800 ° C., the σ phase easily precipitates at a temperature range of 800 ° C. or higher.
Here, the fluidity of the molten metal (Fe: 18 to 28 wt%, Ni: balance) when the Cr content and the Si content were varied was evaluated. In the evaluation, the molten metal is poured into a 200 mm × 200 mm × 10 mm cavity and the properties thereof are observed. In addition to evaluating the fluidity of the molten metal, the precipitation of σ phase by aging treatment was observed. The evaluation criteria are shown below, and the results are shown in Table 1.
○: Molten metal is filled in the cavity, and there is no generation of hot water (Yujiwa). △: The molten metal is almost filled in the cavity, but molten metal is generated on the surface. The gap is not filled with molten metal in the cavity. ●: The molten metal is filled in the cavity, but the σ phase is precipitated in the aging test at 800 ° C x 2500 hours.

As shown in Table 1, when the Si content is 0.30 wt% or less, the fluidity of the molten metal is inferior, and when the Si content is 0.60 wt% or more, precipitation of the σ phase is observed. The above experimental results support that the Si content of the present invention is defined as 0.32 to 0.5 wt%. A desirable Si content in the present invention is 0.35 to 0.45 wt%.
The results in Table 1 also give suggestions for Cr content. That is, Table 1 shows that the fluidity of the molten metal tends to decrease as the Cr content increases.

  By the way, Patent Document 1 has a description “Generally, about 0.3% silicon is included.” However, from the results of Table 1, the above description of Patent Document 1 is just a general theory, and the alloy disclosed in Patent Document 1 specifically contained 0.3 wt%. It does not serve as a basis for On the other hand, the mechanical properties (tensile strength, elongation, Vickers hardness) shown in Patent Document 1 are measured at room temperature, and in a temperature range of 800 ° C. or higher to which the alloy of the present invention is applied. It does not suggest that the σ phase precipitates.

Ni: balance In the cast alloy of the present invention, the balance other than the above elements is substantially made of Ni. The impurities contained in the production do not affect the present invention within the following ranges.
C: 0.1 wt% or less, desirably 0.08 wt% or less, more desirably 0.05 wt% or less Mn: 1.0 wt% or less, desirably 0.8 wt% or less, more desirably 0.5 wt% or less P: 0.5 wt% or less, desirably 0.05 wt% or less, more desirably 0.03 wt% or less S: 0.1 wt% or less, desirably 0.01 wt% or less, more desirably 0.008 wt% or less

  The cast alloy according to the present invention is cast into a predetermined shape, or is further processed into a predetermined shape after casting and is used. As a use, it is suitable for boiler burner nozzles, high-temperature pipes, etc., but it can be used for other purposes as well. Moreover, it is assumed that the cast alloy of this invention is applied to the use in the high temperature range of 800 degreeC or more and also 900 degreeC or more. Therefore, when used in a combustor such as a boiler, the operating temperature can be increased, which contributes to improving the performance of the combustor.

Hereinafter, the present invention will be described based on specific examples.
After casting an alloy having the composition shown in Table 2, aging treatment was performed at 800 ° C. and 900 ° C., and precipitation of σ phase was observed by X-ray diffraction. The results are shown in Table 2. The evaluation criteria in Table 2 are as follows.
○: σ phase does not precipitate, Δ: σ phase micro-precipitation, x: σ phase precipitation In Table 2, sample No. By comparing 2 to 4, it can be seen that the degree of precipitation of the σ phase increases as the Si content increases. On the other hand, sample no. As shown in Fig. 5, even if the Cr content is as high as 48.0 wt%, if the Si content is as low as 0.36 wt%, the σ phase will precipitate even after aging treatment at 900 ° C x 2500 hours. There is no. Sample No. 2 with a Cr content as low as 25.0 wt% was obtained. No. 1 also suppresses the precipitation of the σ phase.

  Next, sample No. About the alloys of 1-6, the fluctuation | variation of the hardness by an aging treatment was observed. The results are shown in Table 3. As for the chemical composition of the alloy, only the contents (wt%) of Si, Cr and Fe are listed in Table 3.

  Next, sample No. For the alloys 1 to 6, the change in tensile strength due to the aging treatment was observed. The results are shown in Table 4 and FIGS. As for the chemical composition of the alloy, only the contents (wt%) of Si, Cr and Fe are listed in Table 4.

  Next, sample No. For the alloys 1 to 6, the variation in elongation due to the aging treatment was observed. The results are shown in Table 5 and FIGS. As for the chemical composition of the alloy, only the contents of Si, Cr and Fe are listed in Table 5.

The following can be understood from Tables 3 to 5 and FIGS.
Sample No. with a low Cr content of 25.0 wt% The alloy No. 1 has a problem that it is easily deformed during use because of its low hardness and tensile strength. This can also be explained by the large elongation value. When such an easily deformable alloy is used, for example, when it is used for a burner, a change in performance is assumed, which is not desirable.
Sample Nos. With different Cr content, substantially equal Cr content, Fe content and Ni content and different Si content. When 2 to 4 are compared, the tensile strength and elongation decrease significantly as the Si content increases. This is understood to be caused by the σ phase precipitation. In contrast to the above, sample nos. 4 and 5 are sample Nos. Compared with 2-4, the fall of tensile strength and elongation is suppressed. In particular, Sample No. with a high Cr content of 48.0 wt% was obtained. No. 5 shows that both the tensile strength and elongation at room temperature are high, and the decrease in aging treatment is low.
From the above results, and further from the evaluation results of the fluidity of the molten metal, the present invention sets the Cr content to 40 to 50 wt%, the Si content to 0.35 to 0.5 wt%, and the Cr content to 46 to 49 wt%. It is recommended to set to. The present invention can suppress the precipitation of the σ phase and suppress the decrease in tensile strength and elongation. In particular, even if the Cr content is increased, it is extremely important to suppress the precipitation of the σ phase by setting the Si content low in order to enjoy the effect of improving the tensile strength and elongation by increasing the Cr content. is there.

It is a graph which shows the relationship between the aging time (heating temperature 800 degreeC) and tensile strength in an Example. It is a graph which shows the relationship between the aging time (heating temperature 900 degreeC) and tensile strength in an Example. It is a graph which shows the relationship between aging time (heating temperature 800 degreeC) and elongation in an Example. It is a graph which shows the relationship between aging time (heating temperature 900 degreeC) and elongation in an Example.

Claims (3)

  Cr: 40-50 wt%, Fe: 10-35 wt%, Si: 0.32-0.5 wt%, the balance consisting essentially of Ni, a corrosion-resistant and oxidation-resistant casting alloy.   The corrosion-resistant and oxidation-resistant casting alloy according to claim 1, wherein Cr: 46 to 49 wt%.   The corrosion-resistant and oxidation-resistant casting alloy according to claim 1 or 2, wherein Si: 0.35 to 0.45 wt%.

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