US20060266450A1

US20060266450A1 - Cerium-containing austenitic nickel-base alloy having enhanced intergranular attack and stress corrosion cracking resistances, and preparation method thereof

Info

Publication number: US20060266450A1
Application number: US11/328,711
Authority: US
Inventors: Oh Kwon; Yong-Sun Yi
Original assignee: Korea Atomic Energy Research Institute KAERI
Current assignee: Korea Atomic Energy Research Institute KAERI; Korea Hydro and Nuclear Power Co Ltd
Priority date: 2005-05-24
Filing date: 2006-01-10
Publication date: 2006-11-30

Abstract

The present invention relates to a cerium-containing austenitic nickel-base alloy having enhanced IGA(intergranular attack) and SCC(stress corrosion cracking) resistances, and a preparation method thereof. The cerium-containing alloy according to the present invention is prepared by adding cerium to an austenitic nickel-base alloy, which is used as a material of steam generator tube in a nuclear power plant. The cerium-containing alloy has a refiner grain size than the conventional alloy without cerium, a semi-continuous distribution of intergranular carbides, and improved intergranular attack and stress corrosion cracking resistances in the environment of high temperature and base.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation in part of co-pending application Ser. No. 11/135,797 filed on May 24, 2005 entitled CERIUM-CONTAINING AUSTENITIC NICKEL-BASED ALLOY HAVING ENHANCED INTERGRANULAR ATTACK AND STRESS CORROSION CRACKING RESISTANCES AND PREPARATION METHOD THEREOF, By Oh Chul Kwon et al.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a cerium-containing austenitic nickel-base alloy having enhanced IGA(intergranular attack) and SCC(stress corrosion cracking) resistances, and a preparation method thereof.
2. Description of the Prior Art
The term of “austenite” means a solid solution of iron and carbon, subsisting in a metal alloy of steel having a composition of iron and carbon as a primary alloying material, above the critical temperature of 1333° F. (about 723° C.). By adding specific metals such as manganese, nickel or chromium to the austenite, it is possible to obtain an austenitic structure sustaining at room temperature and prepare austenitic steel.
Commercial atomic reactors currently running worldwide may generally be classified into four types of: a pressurized water reactor and a boiling water reactor developed in U.S., a high temperature gas-cooled reactor developed in U.K., and a pressurized heavy water reactor developed in Canada. In Korea, all the nuclear power plants except Wolsung Nuclear Power Plant are utilizing the pressurized water reactor. The pressurized water reactor (PWR) uses slightly enriched uranium containing 2˜5% of uranium-235 as a nuclear fuel and water (light water) as a coolant and moderator.
One of the accidents occurring frequently in nuclear power plants utilizing the pressurized water reactor is a leakage from steam generator tube. The leakage from the heat tubes of the steam generator is mainly caused from intergranular attack and stress corrosion cracking in the steam generator. Such intergranular attack and stress corrosion cracking occurring in the heat tubes of the steam generator cause an accident of leakage from primary cooling water and a stoppage of power plant operation, and further cause repairing of damaged tubes or replacement of the steam generator itself, which results in a lot of economical loss.
The followings are reported as methods to improve the intergranular attack and stress corrosion cracking resistances: development of a system enabling prediction of defects occurring in heat tubes, a thermal treatment method to improve a corrosion characteristic by changing precipitation behavior of intergranular carbides through mill annealing (MA) followed by thermal treatment at about 700° C. for 10˜15 hours (Korea Patent No. 250810), a method to improve physical and mechanical properties of material by thermomechanical processing (TMP) to modify microstructure of crystal boundaries (U.S. Pat. No. 4,481,043 & 5,244,515), and a method to improve passive layer stability by adding corrosion inhibitor, such as TiO₂, TiB₂, CeB₆or LaB₆, to the secondary side environment of the steam generator (Korea Patent No. 261665).
As a more substantial solution to improve resistance to degradation threatening safety of the heat tubes, a study on developing a new alloy has been started according to the requirement of alloy having excellent resistance to the stress corrosion cracking (U.S. Pat. No. 4,689,279 & 4,715,909). As a result, Alloy 690 has been developed and utilized as an alternative heat tube material for the steam generator since 1985. Alloy 690 is a high-chromium nickel alloy having excellent resistance to a variety of corrosive aqueous media and high-temperature atmospheresis. High content of chromium in Alloy 690 provides high resistance to oxidizing acids, especially to aqueous corrosion by nitric acid and sulfidation at a high temperature. Besides the above corrosion resistance, Alloy 690 has high strength, good metallurgical stability and favorable fabrication characteristics. Alloy 690 contains about 60% of nickel, about 30% of chromium, and about 10% of iron as major components, and further contains manganese (0.50% max.), sulfur (0.015% max.), silicon (0.50% max.), copper (0.50% max.), carbon (0.05% max.) and cobalt (0.05% max.). The stress corrosion cracking of Alloy 690 heat tubes has not been reported yet. However degradation of the Alloy 690 heat tubes may possibly happen due to the stress corrosion cracking, considering that it takes quite a long time until problems related to the corrosion can be reported after using a new material, and considering the study result that Alloy 690 is possibly more sensitive to lead-induced stress corrosion cracking (PbSCC). Accordingly, a study for developing an alloy having an excellent resistance to the intergranular attack and stress corrosion cracking is still continued (Korea Patent No. 261665).
It has been reported that cerium improves high-temperature oxidation characteristic when a small amount of cerium is added to a Fe—Cr—Al alloy as an alloying element and a corrosion characteristic in a basic solution when cerium is adsorbed to the surface of a commercial steel material. Additionally, it has been reported that micronization of grain in a surface molten layer is obtained by laser clad coating of stainless steel type 316 with powder containing CeO₂and, in the stainless steel type 316 containing a trace of cerium, cerium atoms can be distributed more efficiently in grain boundaries having an irregular atomic arrangement, because the atomic radius of cerium is very larger than those of nickel, chromium, or iron, and thereby high-energy sites required for precipitation of intergranular carbides may be provided.
In a study for developing an alternative alloy, the inventors completed the present invention by clarifying that an alloy, having a more improved resistance to the intergranular attack and stress corrosion cracking comparing to conventional alloys, may be prepared by adding cerium to an austenitic nickel-base alloy, and thereby used as a material of heat tubes for a steam generator in a nuclear power plant.

SUMMARY OF THE INVENTION

The present invention provides an austenitic nickel-base alloy containing 0.02˜0.09 wt % of cerium.
The austenitic nickel-base alloy preferably contains 58˜80 wt % of nickel, 14˜31 wt % of chromium, 6˜11 wt % of iron, and 0.01˜0.15 wt % of carbon as main components, and may further contain 0.05˜0.15 wt % of silicon, 0.001˜0.004 wt % of manganese, 0.010˜0.03 wt % of phosphor, 0.005˜0.01 wt % of cobalt, 0.003˜0.007 wt % of titanium, 0.001˜0.005 wt % of copper, 0.005˜0.05 wt % of aluminum, and 0.001˜0.1 wt % of boron.
Additionally, the austenitic nickel-base alloy preferably contains 0.02˜0.09 wt % of cerium, and more preferably 0.03˜0.09 wt % of cerium, most preferably 0.04 wt % of cerium.
Additionally, grain size of the austenitic nickel-base alloy containing cerium is preferably 25˜60 μm, and more preferably 35 μm.
Additionally, intergranular carbide coverage of the austenitic nickel-base alloy containing cerium is preferably 60˜70%.
Additionally, the austenitic nickel-base alloy containing cerium has the mechanical properties of: 200˜350 MPa of yield strength, 550˜750 MPa of ultimate tensile strength, and above 30% of elongation.
Hereinafter, a role and a preferable content of each alloying element used in the preparation of the alloy according to the present invention will be described in detail.
Cerium is harder than tin and softer than zinc. It is well known that mechanical properties and ductility of alloy are improved by adding cerium into the alloy. Cerium oxide is used as an abrasive material or catalyst. In the present invention, cerium is used as an alloying element to improve resistance of alloy to intergranular attack and stress corrosion cracking. In the present invention, cerium content added to the alloy is equal to or more than 0.02 wt % because intragranular carbides precipitate more when cerium is added less than 0.02 wt %. Also, cerium is added to the alloy equal to or less than 0.09 wt % because problems such as difficulties during preparation of alloy, and deterioration of behavior of carbides and resistance to degradation begin to occur when cerium is added more than 0.09 wt %. Also, minimum cerium content that intragranular carbides don't precipitate and intergranular carbides advantageously precipitate is 0.03 wt %, and the cerium content of sufficiently contributing to precipitation of intergranular carbides is 0.04 wt %. Thus, the austenitic nickel-base alloy according to the present invention contains preferably 0.02˜0.09 wt %, more preferably 0.03˜0.09 wt %, most preferably 0.04 wt %. Nickel and chromium reinforce oxide film, and thereby improve corrosion resistance of alloy. Especially, nickel is an austenite stabilizing element, which accelerates deformation from BCC (body-centered cubic) to FCC (face-centered cubic). Nickel delays formation of an intermetallic compound in austenite-stainless at the room temperature, and thereby stabilizes the structure of the austenite. Nickel and chromium respectively form precipitates such as Ti2Ni and Ti2Cr and reduce over-potential of hydrogen, and thereby reinforce titanium oxide film. Additionally, such precipitates in the oxide film reduce a current density required for maintaining a passive state.
It is reported that nickel and chromium give a great influence in reinforcing and stabilizing the passive state of titanium, if added together with an element of platinum family. Additionally, addition of nickel and chromium reduces the addition of an expensive platinum family element, and thereby enables manufacturing an economical alloy. The alloy containing cerium in accordance with the present invention preferably contains 58˜80 wt % of nickel and 14˜31 wt % of chromium.
Iron is added as a major element like chromium to improve the corrosion resistance of the alloy, and the cerium-containing alloy in accordance with the present invention preferably contains 6˜11 wt % of iron.
Carbon and silicon reduce absorption of hydrogen and delay transition time of corrosion, and the cerium-containing alloy in accordance with the present invention preferably contains 0.01˜0.20 wt % (max.) of carbon and 0.05˜0.15 wt % of silicon, as impurity elements related to the corrosion resistance.
Manganese enables an improvement of alloy strength without decreasing the corrosion resistance, and further reduces or inhibits an adverse reaction of iron. Additionally, manganese is effective for micronization of grain size and reduction of electric conductivity, and generally used as an important metal component of alloy. The cerium-containing alloy in accordance with the present invention preferably contains 0.001˜0.004 wt % of manganese.
Phosphor is a very active element forming a compound with copper and behaves as a very strong oxidant for oxygen. When phosphor melts, a wide, thin and strong film is formed on a fusion surface and an alloy containing both phosphor and copper shows a high tensile strength. The cerium-containing alloy in accordance with the present invention preferably contains 0.010˜0.03 wt % of phosphor.
Cobalt is one of the important elements of alloy and improves oxidation and corrosion resistances. The cerium-containing alloy in accordance with the present invention preferably contains 0.005˜0.01 wt % of cobalt.
Titanium is a metal with a silver white color and has a light weight next to magnesium and aluminum. Titanium has a high ductility and excellent mechanical properties. Specific strength of titanium is 2 times higher than that of iron and 6 times higher than that of aluminum. Titanium has an excellent thermal resistance, and titanium has a high yield point up to 500° C. Titanium also has a very excellent resistance to acid or seawater, and is thereby used as an alloying element. The cerium-containing alloy in accordance with the present invention preferably contains 0.003˜0.007 wt % of titanium.
Copper is added as a major component element like iron or chromium to improve the corrosion resistance of alloy, and has an excellent effect especially when a very small amount is added. Accordingly, the cerium-containing alloy in accordance with the present invention preferably contains 0.001˜0.005 wt % of copper.
Aluminum has a chemical characteristic of forming a stable oxide film in the air through corrosion caused by inorganic acid or salts, and is thereby used in manufacturing an alloy together with various alloying elements.
Boron has the radius less than 1 Å and improves the hardness of alloy by penetrating into crystal grains of alloy. The cerium-containing alloy in accordance with the present invention preferably contains 0.001˜0.1 wt % of boron.
Grain size of the cerium-containing austenitic nickel-base alloy prepared by the above composition in accordance with the present invention is preferably 25˜60 μm, and more preferably 35 μm. The grain size is indicated with an average diameter of the grain or grain fineness number, which is a very important factor influencing various mechanical properties of metal material. The grain sizes may be classified into two. One is a ferrite grain size, which indicates a grain size in pure iron or low-carbon steel and gives a great influence to hardness and deep drawing property of the alloy. The other is an austenitic grain size existing in a high temperature before transforming to a grain size in high-carbon steel or low alloy steel.
Additionally, the cerium-containing austenitic nickel-base alloy in accordance with the present invention preferably has the intergranular carbide coverage of 60˜70%. The intergranular carbide coverage is calculated by dividing the length of grain boundary into the total length of the intergranular carbide, and is used as a factor of measuring a primary water intergranular stress corrosion cracking (PWSSC) resistance of Alloy 600. Therefore, the intergranular carbide coverage has been applied as a factor of measuring a SCC resistance in a base environment, in the present invention.
Additionally, the cerium-containing austenitic nickel-base alloy in accordance with the present invention preferably has mechanical properties of: 200˜350 MPa of yield strength, 550˜750 MPa of maximum tensile strength, and above 30% of elongation. In a tensile test of a material, when a stress is increased above the limit of forgebility, deformation of the material is rapidly increased resulting in plastic deformation. Stress and strength of the material at this point are called respectively yield stress and yield strength. The maximum tensile strength is also called ultimate tensile strength, and means the maximum stress that the material can sustain.
In a graph showing correlation between tensile load and deformation by stretching a material, the deformation of the material initially increases in a linear form as the tensile load increases until reaching a yield point, and plastic deformation starts after passing the yield point. An increase rate of the deformation then slows down and, after passing a maximum point, the tensile load is reduced due to the decrease of material cross-section by stretching, and the material finally breaks down. A stress at the point where the tensile load becomes a maximum is the tensile strength of the material, which is distinguished from fracture strength as a stress when the material breaks down. Additionally, the elongation means an extent that a material is stretched in a tensile test.
The present invention also provides a preparation method of a cerium-containing austenitic nickel-base alloy comprising the steps of:
preparation of an ingot by adding cerium to an austenitic nickel-base alloy composition;
homogenizing heat treatment of the ingot after a surface grinding work;
preparation of a hot rolled material by hot rolling (hot working);
removal of surface oxide film by acid pickling the hot rolled material;
preparation of a cold rolled material by cold rolling (cold working);
solution annealing; and
thermal treatment.
Hereinafter, the preparation method of a cerium-containing austenitic nickel-base alloy will be described in more detail.
Firstly, an ingot is prepared after mixing and melting compositions of nickel, chromium, iron, carbon, and cerium according to a specific ratio. Vacuum induction melting provides a uniform structure of the ingot by a self-mixing action of inductive heating. The vacuum induction melting has the following advantages. Degasification is increased as an effect of vacuum heating, and may be further increased by maintaining the vacuum state for a specific period after melting. The degasification is performed without adding antioxidant, and excellent cleanliness is thereby maintained.
Secondly, the ingot is processed by a surface grinding work and homogenizing heat treatment to remove impurities on the surface of the ingot. After the surface grinding work, homogenizing heat treatment is performed for a long period at the temperature range of austenite in order to remove segregation by diffusion of atoms and to homogenize the components. The homogenizing heat treatment is generally performed for 1 hour per inch (2.54 cm) of thickness, and performed preferably at 1100˜1300° C. for 60˜180 min., and more preferably 1250° C. for 135 min. to get a satisfactory result.
Thirdly, after the homogenizing heat treatment, the ingot is hot rolled above the recrystallization temperature of the metal material.
Fourthly, the ingot is acid pickled to remove oxide film on the surface of the hot rolled material. In the acid pickling process, a solution mixed with H2O, HNO3, HCl and H2O2 is preferably used, and a solution mixed with the volume ratio of H2O:HNO3:HCl:H2O2=3:2:2:1 is used more preferably.
Fifthly, a cold rolled material is prepared by cold rolling the hot rolled material. The purpose of the cold rolling is to increase the strength of the material, which is different from the purpose of the hot rolling to form the material. The cold rolling increases hardness and strength of material, accompanying a decrease of ductility.
Sixthly, the cold rolled material is solution annealing in a proper condition so that the grain size of the material becomes uniform. The alloy used for the heat tubes of the steam generator requires the average grain size above 32 μm approximately. Therefore, it is important to set a solution annealing condition so that carbides do not exist in intragranule and intergranule by removing cold rolling structure and solubilizing carbides completely into the matrix. Accordingly, the cerium-containing alloy in accordance with the present invention is preferably solution annealing at 900˜1200° C. for 5˜60 min. to obtain the uniform grain size above 32 μm, and more preferably at 1010° C. for 10 min.
Lastly, thermal treatment (TT) is applied to the alloy in order to increase the SCC resistance. It is reported that the thermal treatment generally performed to increase the SCC resistance of Alloy 600, which is used as heat tubes of the steam generator improves the SCC resistance of material due to precipitating optimum intergranular carbides and distributing the intergranualr carbides semi-continuously [J. R. Crum, Corrosion, 38, 40-45, 1982], and is most effective when final thermal treatment is performed at 704° C. for 15 hours. Accordingly, the final thermal treatment of the cerium-containing austenitic nickel-base alloy in accordance with the present invention preferably is applied at 700˜720° C. for 10˜20 hours, and more preferably at 704° C. for 15 hours.
The cerium-containing austenitic nickel-base alloy in accordance with the present invention has more carbide precipitates distributed semi-continuously than the comparative example without cerium, and may thereby have more improved IGA and SCC resistances. Additionally, the SCC resistance of heat tubes in an operating condition of the steam generator of the nuclear power plant may be improved. Accordingly, the cerium-containing austenitic nickel-base alloy in accordance with the present invention may be used effectively as a material of heat tubes in the steam generator of the nuclear power plant. By using this material for the heat tubes of the steam generator, breakages of the heat tube due to the intergranular attack and stress corrosion cracking may be decreased, and thereby the safety and economical efficiency may also be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a trend of grain sizes of a comparative example and an alloy containing cerium (Ce) according to solution annealing condition.
FIG. 2 is a micrograph showing microstructures of the comparative example and alloy containing cerium after the solution annealing in the present invention.
FIG. 3 is a graph showing a trend of intergranular carbide coverage of the comparative example and alloy containing cerium after the thermal treatment at 704° C. for 15 hours.
FIG. 4 is a micrograph of SEM showing the intergranular carbides of the comparative example and alloy containing cerium after the thermal treatment at 704° C. for 15 hours.
FIG. 5 is a micrograph of TEM showing selective area diffraction pattern of the intergranular carbides of comparative example and alloy containing cerium after the thermal treatment at 704° C. for 15 hours.
FIG. 6 is a graph showing a trend of chromium concentration in the intergranule of the comparative example and alloy containing cerium after the thermal treatment at 704° C. for 15 hours.
FIG. 7 is a graph showing stress-strain curves of the comparative example and alloy containing cerium after the thermal treatment at 704° C. for 15 hours.
FIG. 8 is a graph showing a test result of stress corrosion cracking of the comparative example and alloy containing cerium in the environment of 40% NaOH solution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a preferred example embodiment and a comparative example will be described in detail to help the understanding of the present invention. However, the following example embodiment and comparative example are provided for an easier understanding of the present invention, and this invention should not be construed as limited to the example embodiment set forth herein or in the accompanying drawings.

EXAMPLE

Preparation of Cerium-containing Austenitic Nickel-base Alloy

A 30 kg ingot is prepared by melting and tapping a mixture without cerium and a mixture with a trace of cerium in vacuum induction melting, using nickel of 99.90% purity, chromium of 99.48% purity, iron of 99.93% purity, carbon and high purity cerium. Hot rolled plates with the thickness of 7 mm are prepared by cutting the ingot into half pieces, surface grinding, keeping at 1250° C. for 135 min., and hot rolling. Acid pickling of the hot rolled plates are applied to remove surface oxide film, using a solution mixed with the volume ratio of H2O:HNO3:HCl:H2O2=3:2:2:1, and cold rolled plates with the final thickness of 1.5 mm are prepared by cold rolling. The compositions of the prepared alloy are shown in Table 1.
As a subsequent process, the prepared alloy is solution annealing so that the grain size of the alloy becomes about 35 μm. In order to analyze an effect of the solution annealing temperature and time influencing the grain size of the alloy, microstructure of the alloy is observed after the solution annealing at 900˜1100° C. for 30 min. Based on the above result of the experiment, the comparative example and cerium-containing alloy are solution annealing respectively at 975° C. for 20 min. and at 1010° C. for 10 min.
As the last step, in order to investigate an influence of thermal treatment time to the precipitation of the carbides, the thermal treatment of the solution annealing alloy is applied at 704° C. for 1˜96 hours, and distribution of the carbides is observed, and finally the thermal treatment of the solution annealing alloy is applied at 704° C. for 15 hours, based on the above result of the experiment.

COMPARATIVE EXAMPLE

Preparation of Austenitic Nickel-base

Alloy without Cerium

Another alloy is prepared with the same method as the above example embodiment except the step of adding cerium. The compositions of the prepared alloy are shown in Table 1.

TABLE 1


Composition of austenitic nickel-base alloy with and
without cerium (wt %)

		Comparative	Embodiment
		Example	Example

C	0.03	0.03
Si	0.07	0.12
Mn	0.003	0.002
P	0.024	0.018
Cr	16.12	16.15
Ni	74.42	74.36
Fe	9.06	9.04
Co	0.007	0.007
Ti	0.005	0.005
Cu	0.003	0.002
Al	—	0.02
B	0.006	0.03
Ce	0.003	0.04

In the present invention, the cerium content added to the alloy is more than 0.02 wt % because intragranular carbides precipitate more when cerium is added less than 0.02 wt %. Also, cerium is added to the alloy equal to or less than 0.09 wt % because problems during preparation of alloy, behavior of carbides and resistance to degradation are initiated when cerium is added more than 0.09 wt %. Also, minimum cerium content that intragranular carbides don't precipitate and intergranular carbides advantageously precipitate is 0.03 wt %. As shown in Table 1, in the present invention, for survey characteristic changes according to the addition of cerium, alloy of Embodiment Example containing 0.04 wt % of cerium and alloy of Comparative Example containing no cerium were used.
Experiment 1: Observation of Microstructure
In order to observe the microstructure of the solution annealing alloy, the alloy of the example embodiment is electrically etched in a nital solution at 4.5V DC for 30 sec., and microstructure of the alloy is observed by an optical microscope. The average grain size is measured by the linear intercept method from the optical micrograph.
The results of the alloys solution annealing at 900˜1100° C. for 30 min. according to the example embodiment are shown in FIG. 1. FIG. 2 shows micrographs of the comparative example solution annealing at 975° C. for 20 min. and the cerium-containing alloy solution annealing at 1010° C. for 10 min.
As shown in Table 1, according to the result of measuring grain size, the grain size of the embodiment example is smaller than that of the comparative example. This may be interpreted as the addition of trace cerium induces refinement of the grain size. Additionally, it is clarified that the comparative example and embodiment example should be solution annealing respectively at 975° C. for 20 min. and at 1010° C. for 10 min., in order to obtain the grain size of about 35 μm, both for the comparative example and embodiment example.
As shown in FIG. 2, it is clarified that carbides of the comparative and embodiment examples are solubilized into a matrix respectively at 975° C. for 20 min. and 1010° C. for 10 min., and thereby microstructure having a uniform grain size may be obtained.
Experiment 2: Analysis of Intergranular Carbides
In order to evaluate the influence of cerium after the final thermal treatment to the distribution of the intergranular carbides, intragranular and intergranular carbides of both alloys after the thermal treatment are investigated and analyzed by a scanning electron microscope (SEM) and a transmission electron microscope (TEM).
2-1: SEM Analysis
The embodiment example and the comparative example etched by 2% bromine solution (2% bromine+98% methanol) are investigated by the SEM and taken micrographs. A trend of intergranular carbide precipitation is investigated by measuring the lengths of the intergranular and the intergranular carbide and quantifying the coverage of the intergranular carbide. The coverage of the intergranular carbides is calculated by dividing the total length of intergranule into the total length of the intergranular carbide. The result is shown in FIG. 3.
As shown in FIG. 3, according to the result of comparing the coverage of the embodiment example with that of comparative example, the coverage of the intergranular carbide of the embodiment example is increased by about 10% comparing to that of the comparative example. Therefore, it is clarified that the embodiment example containing a trace of cerium has higher coverage of the intergranular carbide than that of the comparative example. This indicates that the IGA and SCC resistances may be improved, because the embodiment example has more intergranular carbide distributed semi-continuously than the comparative example.
Additionally, as shown by SEM micrographs in FIG. 4, it is clarified that the intergranualr carbides both for embodiment example and comparative example has the highest coverage and is distributed semi-continuously by the thermal treatment at 704° C. for 15 hours.
2-2: TEM Analysis
In order to analyze the composition and crystal structure of the intragranular and intergranular carbides, only carbides are extracted by a carbon replica method [Y. S. Lim, A study on the microstructure and corrosion characteristics of laser surface melted Ni-base Alloy 600, Ph.D., Thesis, KAIST, 2000] and analyzed with EDS (Energy Dispersive X-ray Spectroscopy, Oxford Link ISIS-5947) of TEM (Transmission Electron Microscope, JEOL JSM-2000FX).
According to the analysis result, the carbide of the comparative example contains 98.4 wt % of chromium and 1.6 wt % of iron, and the carbide of embodiment example contains 96.9 wt % of chromium, 2.0 wt % of iron, and 1.2 wt % of nickel. Referring to FIG. 5, both alloys show selected area diffraction patterns having continuous streaks parallel to the diffraction planes, which typically appear in the carbide of Cr7C3. From the compositions and selected area diffraction patterns, both alloys are identified as the carbides have a pseudo hexagonal structure of Cr7C3. As shown in FIG. 6, according to the result of investigating the change of chromium concentration in the intragranule, both alloys don't show any significant difference.
Experiment 3: Analysis of Mechanical Characteristics
Tensile tests are performed to analyze mechanical characteristics of the alloy prepared by the present invention.
A tensile testing machine of Model Instron 8872 is utilized at room temperature. Cross head speed is set to 2 mm/min, and a specimen is provided with 25 mm of gauge length, 1.5 mm of gauge thickness, and 4 mm of width, using electric discharge machining. The result of the analysis is shown in Table 2 and FIG. 7.
As shown in Table 2 and FIG. 7, according to the result of the tensile tests for the comparative example and the embodiment example, both alloys doesn't show any significant difference in 0.2% offset yield strength (YS), ultimate tensile strength (UTS), and elongation. Accordingly, it is clarified that the trace cerium does not give any significant influence to the mechanical properties of the austenitic nickel-base alloy.

TABLE 2

0.2% offset Ultimate

yield tensile

strength strength Elongation

(Mpa) (Mpa) (%)

Comparative 212 (±1) 677 (±2) 40.4 (±1.2)

example

Embodiment 212 (±2) 694 (±1) 40.6 (±0.2)

example

Experiment 4: Measurement of SCC Resistance
In order to compare and evaluate the SCC resistances of the comparative example and the embodiment example, SCC tests are carried out using a U-bend specimen in a simulated model of crevice in the secondary side of the steam generator of the nuclear power plant.
The U-bend specimens of alloy are prepared both for the comparative example and the embodiment example after thermal treatment at 704° C. for 15 hours, according to ASTM (American Society for Testing and Materials) G30-79. An environment of the most frequently causing the stress corrosion cracking in the secondary side of heat tube of the steam generator is a highly basic condition at the operating temperature of the steam generator. Therefore, the test is carried out at 315° C. with 40% NaOH solution in an autoclave with 2 liters capacity. Additionally, in order to accelerate the stress corrosion cracking, the test is carried out by applying a voltage 200 mV higher than the corrosion potential of the material. After completing the test, the U-bend specimen is divided into 3 pieces equally, and the shape, depth, and number of cracks were investigated. The result of measurement is shown in FIG. 8.
As shown in FIG. 8, in the case that SCC test time is 30 hours, the average depth of maximum crack of the comparative example is 184 μm, however the embodiment example doesn't show any crack. Additionally, in the case that SCC test time is 60 hours, the average depth of maximum crack in the comparative example is 1,279 μm, whereas that in the embodiment example is 946 μm. From the result of the SCC test, it is verified that the embodiment example delays starting time of cracking and has an excellent SCC resistance comparing to the comparative example.
As described above, it is verified that the grain is refined, more intergranular carbides is distributed semi-continuously, and SCC resistance is improved, by adding cerium into an austenitic nickel-base alloy.
Accordingly, the new alloy having improved IGA and SCC resistances may be applied as a material for steam generator tube in a nuclear power plant, and will contribute to improvement of the safety and economical efficiency in a nuclear power plant by reducing breakages of heat tubes caused from the intergranular attack and stress corrosion cracking.

Claims

1. A cerium-containing austenitic nickel-base alloy having the cerium content of 0.02˜0.09 wt %.

2. The cerium-containing austenitic nickel-base alloy of claim 1, wherein the cerium content is 0.03˜0.09 wt %.

3. The cerium-containing austenitic nickel-base alloy of claim 1, wherein the cerium content is 0.04 wt %.

4. The cerium-containing austenitic nickel-base alloy of claim 1, wherein the alloy contains 58˜80 wt % of nickel, 14˜31 wt % of chromium, 6˜11 wt % of iron, and 0.01˜0.15 wt % of carbon, as major components.

5. The cerium-containing austenitic nickel-base alloy of claim 1, wherein the grain size of the alloy is 25˜60 μm.

6. The cerium-containing austenitic nickel-base alloy of claim 4, wherein the grain size of the alloy is 35 μm.

7. The cerium-containing austenitic nickel-base alloy of claim 1, wherein the alloy has the mechanical properties of: 200˜350 MPa of yield strength, 550˜750 MPa of ultimate tensile strength, and above 30% of elongation.

8. A preparation method of a cerium-containing austenitic nickel-base alloy comprising the steps of:

preparation of an ingot by adding 0.02˜0.09 wt % of cerium to an austenitic nickel-base alloy composition;

homogenizing heat treatment of the ingot after surface grinding;

preparation of a hot rolled material by hot rolling the heat-treated ingot;

removal of surface oxide film by acid pickling the hot rolled material;

preparation of a cold rolled material by cold rolling the material after hot rolling and removal of oxide film;

solution annealing of the cold rolled material; and

thermal treatment of the solution annealing material.

9. The preparation method of a cerium-containing austenitic nickel-base alloy of claim 8, wherein the cerium content added to the alloy is 0.03˜0.09 wt %.

10. The preparation method of a cerium-containing austenitic nickel-base alloy of claim 8, wherein the solution annealing is performed at 900˜1200° C. for 5˜60 min.

11. The preparation method of a cerium-containing austenitic nickel-base alloy of claim 10, wherein the solution treatment is performed at 1010° C. for 10 min.

12. The preparation method of a cerium-containing austenitic nickel-base alloy of claim 8, wherein the thermal treatment is performed at 700˜720° C. for 10˜20 hours.

13. The preparation method of a cerium-containing austenitic nickel-base alloy of claim 12, wherein the thermal treatment is performed at 704° C. for 15 hours.