JP2002003970A - Ni HEAT RESISTANT ALLOY - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an Ni heat resistant alloy having excellent creep characteristics even when used at a high temperature of 1,100 to 1,150 deg.C under the conditions in which carburization, oxidation and temperature variation are repeated and having excellent carburization resistance and coking resistance. SOLUTION: This Ni-based heat resistant alloy has a composition containing, by mass, >0.1 to 0.7% C, <=5% Si, 0.05 to 5% Mn, <=0.04% P, <=0.01% S, 10 to 25% Cr, 1 to 12% Al and one or more kinds of 0.01 to 15% Mo and 0.01 to 9% W by 2.5 to 15% in total.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a Ni-base heat-resistant alloy having high creep rupture strength and excellent carburization resistance.
In particular, it is used to crack naphtha, propane, ethane, gas oil, and other raw materials together with water vapor at a high temperature of 800 ° C or higher to produce cracking furnace tubes for ethylene plants that produce petrochemical basic products such as ethylene and propylene. It relates to a suitable Ni-based heat-resistant alloy.

[0002]

2. Description of the Related Art The use temperature of a cracking furnace tube for an ethylene plant is increasing in temperature from the viewpoint of improving the ethylene yield.

[0003] Such a material for a cracking furnace tube is required to have carburizing resistance as well as high temperature strength such as creep rupture strength because the inner surface is exposed to a carburizing atmosphere. On the other hand,
During operation, carbon is deposited on the inner surface of the cracking furnace tube (this phenomenon is called coking), and an increase in the amount of deposition causes adverse effects on operation such as an increase in pressure in the tube and a decrease in heating efficiency.

[0004] Therefore, in actual operation, a so-called decoking operation for periodically removing carbon deposited with air or water vapor is performed, and the operation stoppage and the number of man-hours during the operation become a serious problem. Such coking and the associated problems become more serious as the size of the cracking furnace tube becomes smaller, which is advantageous for improving the yield.

As a conventional technique for preventing caulking, for example, Japanese Patent Application Laid-Open No. 2-8336 discloses that
Strong and stable C on the alloy surface by containing 8% or more of Cr
A method is disclosed in which an r ₂ O ₃ film is formed, Fe and Ni, which are catalytic elements that promote carbon deposition, are prevented from being exposed to the surface, and coking is suppressed.

On the other hand, in order to improve carburization resistance, for example, as disclosed in JP-A-57-23050,
It is effective to increase the Si content in the alloy. However, the above-described prior art has the following problems.

To prevent caulking, see Japanese Patent Application Laid-Open No. 2-833
In the case of using a high Cr alloy as disclosed in Japanese Patent Publication No. 6 as a high-temperature strength member, it is necessary to increase the amount of Ni in the alloy to austenitize the metal structure, but the high-temperature strength is higher than that of the conventional alloy. Therefore, it is difficult to apply it alone as a high-temperature strength member. Japanese Patent Laid-Open No. 2-8336 discloses that a double tube is used in combination with another high-temperature strength member, but the double tube has many problems in terms of manufacturing cost and reliability.

[0008] The inventors of the present invention increased the amount of Al in a Ni-based alloy to 4.5 to 12%, and formed a strong and dense Al ₂ O ₃ film on the metal surface. The carburization resistance and coking resistance are significantly improved, and in such a high Al alloy, the γ ′ phase is finely precipitated in the matrix during use at high temperature by increasing the Ni content,
They found that the creep rupture strength was also greatly improved, and filed a patent application earlier (JP-A-5-239577, JP-A-6-259577).
-207235).

However, such alloys are not suitable when large hot working is required, as in the case of producing a cracking furnace tube for an ethylene plant. Further, as described above, the operating temperature of the cracking furnace tube for an ethylene plant tends to increase to 1100 ° C. to 1150 ° C. from the viewpoint of improving the ethylene yield. In such a high temperature range, higher strength is required. You.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to use an ethylene plant at a high temperature of 1100 to 1150 ° C. in an environment where a cracking furnace tube for an ethylene plant is placed, that is, an environment where carburization, oxidation and temperature fluctuation are repeated. Has excellent creep rupture strength, excellent carburization resistance and coking resistance, and also has excellent hot workability.
An object of the present invention is to provide a base heat-resistant alloy.

[0011]

The gist of the present invention is as follows.

(1) In mass%, C: more than 0.1% and 0.7
%, Si: 5% or less, Mn: 0.05 to 5%, P:
0.04% or less, S: 0.01% or less, Cr: 10 to 2
5%, Al: 1 to 12% and Mo: 0.01 to 15%
And W: one or more of 0.01 to 9% in total of 2.5
Ni-base heat-resistant alloy containing up to 15%.

(2) The Ni-base heat-resistant alloy according to (1), wherein the N content is 0.05% or less.

The inventor of the present invention has, in a high temperature range of 1100 ° C. to 1150 ° C., not only a creep characteristic that does not break even when used for a long time, but also excellent carburization resistance and coking resistance, and excellent hot workability. In order to develop a Ni-based alloy, Ni alloys having variously changed chemical compositions were melted and experimentally studied, and as a result, the following findings were obtained.

A) In a Ni-based alloy, Al lowers the hot workability, and particularly when the content is as large as 4.5% or more, the hot workability remarkably decreases. To improve hot workability, strengthening of grain boundaries is important and effective. S and P, which are usually mixed into steel as impurities, segregate at the grain boundaries to weaken the bonding strength of crystal grains and deteriorate hot workability.
Must be reduced to 0.01% or less, and P must be reduced to 0.04% or less. In particular, P forms a Cr phosphide at a temperature of 1000 ° C. or less and precipitates at the grain boundaries to weaken the grain boundaries, so that the effect of reducing P is large.

B) Even if the Al content is reduced to 1% in order to ensure hot workability, a protective oxide film can be formed on the alloy surface, and good carburization resistance and coking resistance can be obtained. Can be granted.

C) However, when the Al content is as low as 1 to less than 4.5%, precipitation of the Ni-Al intermetallic compound is reduced at a high temperature, and the creep rupture strength is reduced.

C) When the C content is increased to 0.1% or more,
A large amount of carbide mainly composed of Cr is formed in the grains and at the grain boundaries, and the grain boundaries are covered with the carbide, so that the Al content is 1%.
Even if it is as low as ~ 4.5%, the creep rupture time can be remarkably shifted to a longer time side. When the Al content is 4.5% or more, the creep rupture strength is not particularly problematic, but the strength is further increased by increasing the C content to 0.1% or more.

D) N generally has the effect of increasing the high-temperature strength by solid solution strengthening in heat-resistant steel, but its effect cannot be expected in Ni-based alloys containing a large amount of Al, but rather forms Al-based nitrides. And hinder hot workability,
In addition to reducing the contents of S and P, if N is limited to 0.05% or less, hot workability is further improved.

E) Mo and W generally have the effect of increasing the high-temperature strength by solid solution strengthening in heat-resistant steel, but the effect is particularly large at high temperatures.
And W or more must be contained in a total amount of 2.5 to 15%.

The present inventors investigated the effect of C content on the creep rupture time of Ni-based alloys by melting alloys having various C contents and producing creep rupture test pieces under the following conditions. The test was performed. The Al content in the alloy used was 1 to 10%.

Test temperature: 1150 ° C. Load stress: 9.8 MPa FIG. 1 shows the results of creep rupture test. As is clear from the figure, even when the Al content is relatively low, when the C content is 0.1% or more, the rupture time is as long as about 500 hours or more.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the chemical composition of the alloy of the present invention and the function and effect will be described. In addition,% display of an alloy element shows mass%. C: C is an important element in the present invention.
A carbide mainly composed of Cr is formed at the grain boundary, and has an effect of improving the tensile strength and creep rupture strength required for heat-resistant steel. In order to exert such precipitation strengthening by carbides, a content exceeding 0.1% is necessary. However, if it exceeds 0.7%, the ductility and toughness of the alloy are greatly reduced, so the upper limit was made 0.7%. A desirable range is more than 0.15% and 0.5% or less, more preferably more than 0.2% and 0.5% or less.

Si: Si is an element having a deoxidizing effect on molten steel and also contributing to an improvement in oxidation resistance and carburization resistance. From the viewpoint of hot workability, the lower the Si, the better, and the upper limit is 5%.
%. A desirable Si content is 4% or less, and a more desirable Si content is 0.01% to 3%.

P: Regulation of the P content is of utmost importance in the present invention. P is an extremely harmful element that segregates at the grain boundaries, weakens the bonding strength of the grain boundaries, and deteriorates hot workability.
Furthermore, Cr phosphides are formed and precipitated at the grain boundaries, thereby significantly weakening the grain boundaries. Therefore, it is preferable to reduce P as much as possible. 0.04 to improve hot workability
% Or less is effective. Preferably it is 0.025% or less, more preferably 0.015% or less.

S: S is an extremely harmful element that segregates at the grain boundaries to weaken the bonding strength of the grain boundaries and deteriorates hot workability, and the upper limit is extremely important. Since grain boundary strengthening is important in an Al-containing Ni-based alloy, it is preferable to reduce S as much as possible, particularly in an alloy having a high Al content. 0.01% or less is effective for improving hot workability. Preferably it is 0.005% or less, more preferably 0.003% or less.

Cr: Cr is an element effective for improving oxidation resistance, carburization resistance and coking resistance, and has an effect of uniformly forming at an early stage of film formation. It also forms carbides and contributes to improvement in creep rupture strength. Further, the alloy having the chemical composition specified in the present invention contributes to improvement of hot workability. To achieve these effects,
It must be contained at 0% or more. On the other hand, when Cr is excessively contained, the toughness is significantly reduced. Therefore, in the present invention, the Cr content is set to 10 to 25%. Desirably 12 ~
23%.

Al: Al is an element that is extremely effective in improving carburization resistance and coking resistance, and also in improving high-temperature strength, but in order to exert its effect, it is necessary to form an alumina oxide film. . On the other hand, the γ 'phase [N
i ₃ (Al, Ti) intermetallic compound], and a precipitation strengthening effect can be expected. To obtain these effects, an Al content of at least 1% is required. On the other hand, when the Al content exceeds 12%, the hot workability is extremely reduced, and P, S
However, even if the N content is regulated, it cannot be improved.
Therefore, the Al content is set to 1 to 12%. Preferably, it is 2 to 10%, more preferably 4.5 to 10%.

Mo and W: Mo and W are mainly effective as solid solution strengthening elements, and increase the creep rupture strength by strengthening the austenite phase of the matrix. In order to exhibit this effect, Mo: 0.01 to
It is necessary to contain at least one of 15% and W: 0.01 to 9% in total of 2.5 to 15%. However, if it is contained excessively, not only the intermetallic compound which causes a decrease in toughness is precipitated, but also the carburization resistance and the coking resistance are deteriorated. Therefore, the total amount needs to be 15% or less.

As described above, the Al content is 1 to 4.5%.
Even when the effect of precipitation strengthening by the γ ′ phase is small, the creep rupture strength is improved by increasing C. However, when the content of Mo and W is further increased, the synergistic effect of these is exerted, and the superiority is obtained. The resulting creep characteristics are obtained.

Ni: The alloy of the present invention consists essentially of Ni except for the above-mentioned elements and the following necessary elements. Ni is an element that is indispensable for obtaining a stable austenite structure and ensuring carburization resistance. In particular, Ni is more desirable to enhance the effect of precipitation strengthening by the γ 'phase. Preferably 5
It is 0.1% or more, more preferably 60.1% or more.

In order to solve the problem of the present invention, it is necessary to use an alloy having at least the above chemical composition.
Further, the following elements can be contained as necessary, and N can be reduced. Mn: Mn is an element that is added as necessary, is effective as a deoxidizing element, and has an adverse effect on hot workability.
Since it has the function of fixing O, it is preferable to contain 0.05% or more in order to obtain these effects. On the other hand, excessive addition promotes generation of a spinel-type oxide and hinders uniform formation of an initial alumina film, so the upper limit is preferably set to 5%. Preferably it is 0.1-5% or less. More preferably, it is 0.2% or more and 3% or less.

Cu and Co: These elements have the effect of stabilizing the austenite structure, and are therefore effective in improving the creep rupture strength. However, when it is contained excessively, hot workability and toughness are reduced. Therefore, when it is contained, it is added at 15% or less. Preferably, it is 10% or less. More preferably, it is set to 8% or less.

Ti: Ti is an element that promotes the precipitation of the γ ′ phase and improves the creep rupture strength. It also contributes to strengthening the grain boundaries. However, when it is contained excessively, the γ 'phase is excessively precipitated, and hot workability and weldability are remarkably deteriorated. Therefore, when it is contained, the content is set to 3% or less.

B, Zr and Hf: These elements are mainly effective for strengthening the grain boundary of the alloy, and when they are contained because they improve hot workability and weldability, they should be contained in one or more kinds. Is good. However, an excessive content causes a decrease in creep rupture strength.
, 0.03% for Zr, 0.9% for Hf, and a total of 1.1%. Desirably 0.8% in Hf
Or less, and 1% or less in total.

Fe: Fe improves the creep ductility, increases the creep rupture strength, and further contributes to the improvement of hot workability and cold workability. However, if it is contained excessively, on the contrary, the creep rupture strength and the hot workability also decrease, so the upper limit is 20.
% Or less. Preferably it is 15% or less, more preferably less than 10%.

Nb, V and Ta: These elements form a solid solution in the austenite phase and also form a solid solution in the γ ′ phase, Cr carbide and nitride, thereby contributing to improvement in creep rupture strength. However, an excessive content causes a decrease in toughness, so that Nb and V are less than 1.0% and Ta is 2.0%.
Less than Even when two or more kinds are used in combination, it is desirable that the total of both is less than 3.0%.

La, Ce and Nd: These elements are:
It mainly prevents peeling of the alumina film under thermal cycling conditions and improves carburization resistance and coking resistance even when used in an environment where the temperature fluctuates. In order to exhibit the effect, each of La, Ce and Nd is 0.0
01% or more is required. However, if it is contained excessively, the workability deteriorates and the effect of preventing the alumina film from peeling is saturated, so the upper limits of La, Ce and Nd are each 0.10%. These elements may be contained singly or in combination of two or more.

Y: Like La, Ce, and Nd, Y mainly prevents peeling of the alumina film under thermal cycling conditions, and exhibits resistance to carburization and coking even when used in an environment where the temperature fluctuates. Improve. However, Fe
, And an intermetallic compound is easily formed, which causes a reduction in hot workability. 0.10% or less may be contained in order to exhibit the effect of improving the carburization resistance and the coking resistance, but if hot workability is important, 0.00% or less may be contained.
It should be less than 1%.

Mg and Ca: These elements mainly fix S, which is harmful to hot workability, as sulfides and increase the grain boundary strength, so that they are contained as necessary when improving hot workability. . When it is contained, both Mg and Ca are set to 0.1.
Effective at 0005% or more. However, if it is contained excessively, it is present in the steel in a solid solution state, and conversely reduces the hot workability and the weldability. Therefore, the upper limit is Mg, C
a is preferably set to 0.01%.

N: N is originally effective for increasing the strength at high temperatures by solid solution strengthening. However, in an Al-containing Ni-based alloy, solid solution strengthening cannot be expected because it precipitates as AlN in steel. It significantly impairs hot workability and weldability. In order to prevent these adverse effects, the content is preferably 0.05% or less. It is preferable to reduce as much as possible, desirably less than 0.02%, more desirably 0.015
%. As described above, the hot workability is remarkably improved by reducing S and P. However, when N is further reduced, the synergistic effect is exerted, and more excellent hot workability is obtained.

[0042] The alloy of the present invention can be melted in a usual melting and refining process, and then cast into a product to obtain a product. Further, after casting, the product can be made into a product such as a pipe through a working process such as hot working and cold working. Also,
The product may be manufactured by powder metallurgy. The heat treatment promotes the homogenization of the structure and contributes to the improvement of the performance of the alloy of the present invention. Usually 1
Although the homogenization treatment is performed at 100 to 1300 ° C., it can be used as cast or processed without heat treatment. Further, after casting, after working or after heat treatment, the surface may be subjected to surface adjustment by grinding or pickling with a shot blast or grinder or the like before use.

[0043]

EXAMPLE An alloy having the chemical composition shown in Tables 1 and 2
After melting in a kg vacuum high-frequency furnace, a plate material having a thickness of 15 mm was formed by forging and subjected to a solution heat treatment at 1200 ° C. to obtain a test material. To evaluate carburization resistance, high temperature strength and hot workability,
Each test was performed in the following manner.

[0044]

[Table 1]

[Table 2] (1) Solid carburization test (evaluation of carburization resistance) Test piece: thickness 4 mm, width 20 mm, length 30 mm Test method: Insert the test piece into a carburizing agent, heat to 1150 ° C, hold for 48 hours, and then test piece (2) Creep rupture test (high-temperature strength evaluation) Specimen: Round bar with parallel part diameter of 6 mm, length of 70 mm, distance between gauge marks of 30 mm Test method: holding time 1150 ° C, load stress 9.8MPa
(3) Grieving test (evaluation of hot workability) Test piece: Round bar with parallel part diameter of 10 mm and length of 130 mm Test method: After heating at 1200 ° C for 5 minutes, up to 1000 ° C After cooling at 100 ° C./min, the film was pulled at a strain rate of 5 / s, cooled to room temperature with He gas after breaking, and measured for the aperture value.

[0045]

[Table 3]

[Table 4] In the present invention, each test result is evaluated as follows. Solid carburization test: Carburization resistance is excellent if C increase is 0.2% or less. Creep rupture test: Good high-temperature strength if rupture time is 500 hours or more Grease test: Any drawing value of 50% or more Good hot workability As is clear from Tables 3 and 4, Al content is 1% or more and 1% or more.
The alloy of the present invention containing 2% or less has a higher carburization resistance than the comparative alloy A in which the Al content is smaller than the amount specified in the present invention,
Good creep rupture strength. In addition, it can be seen that the comparative alloy B in which the Al content exceeds the upper limit specified in the present invention has a low grease drawing of 12.5%. Also, S
Comparative alloy C having a high P, comparative alloy D having a high P, comparative alloy E having a low Mn, and comparative alloy F having a high Si also do not satisfy the hot workability.

The comparative alloy G whose Mn is higher than the amount specified in the present invention is inferior in carburization resistance, and the comparative alloy H whose Mn is lower than the amount specified by Cr does not satisfy the targets in terms of carburization resistance and creep rupture strength. You can see that.

[0047]

According to the present invention, an alloy having excellent carburization resistance and coking resistance, having sufficient creep rupture strength to be used as a high-temperature strength member, and having excellent hot workability. And exhibits excellent effects in thermal cracking and thermal cycle environments where carburizing, oxidation and temperature fluctuations of a cracking furnace tube for an ethylene plant and the like are repeated. as a result,
By using the alloy of the present invention, it is possible to operate at a higher temperature, to extend the continuous operation time, and to prolong the replacement span with a new material by improving the durability.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the C content and creep rupture strength.

Claims (2)

[Claims] 1. In mass%, C: more than 0.1% and 0.7% or less, Si: 5% or less, P: 0.04% or less, S: 0.0
1% or less, Cr: 10 to 25%, Al: 1 to 12%, Mo: 0.01 to 15%, and W: 0.01 to 9%, and a total of at least one of 2.5 to 15% A Ni-base heat-resistant alloy comprising: 2. The Ni-base heat-resistant alloy according to claim 1, wherein the N content is 0.05% or less.