US20140294653A1

US20140294653A1 - Martensitic oxide dispersion strengthened alloy with enhanced high-temperature strength and creep property, and method of manufacturing the same

Info

Publication number: US20140294653A1
Application number: US14/191,963
Authority: US
Inventors: Tae Kyu Kim; Sanghoon Noh; Byoung-Kwon Choi; Chang-Hee Han; Ki-Baik Kim; Suk Hoon KANG; Young-Bum Chun; Jinsung JANG; Yong-Hwan Jeong
Original assignee: Korea Atomic Energy Research Institute KAERI; Korea Hydro and Nuclear Power Co Ltd
Current assignee: Korea Hydro and Nuclear Power Co Ltd
Priority date: 2013-03-29
Filing date: 2014-02-27
Publication date: 2014-10-02
Also published as: FR3003875B1; FR3003875A1

Abstract

The present application discloses a martensitic oxide dispersion-strengthened alloy having enhanced high-temperature strength and creep properties. The alloy includes chromium (Cr) of 8 to 12% by weight, yttria (Y₂O₃) of 0.1 to 0.5% by weight, carbon (C) of 0.02 to 0.2% by weight, molybdenum (Mo) of 0.2 to 2% by weight, titanium (Ti) of 0.01 to 0.3% by weight, zirconium (Zr) of 0.01 to 0.2% by weight, nickel (Ni) of 0.05 to 0.2% by weight and the balance of iron (Fe). The application also discloses a method of making the alloy.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 2013-0034720 filed on Mar. 29, 2013, and Korean Patent Application No. 2013-0164341, filed on Dec. 26, 2013, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

1. Field
The present disclosure relates to a martensitic oxide dispersion-strengthened alloy.
2. Discussion of Related Art
Generally, when a Fe—Cr alloy obtained by adding approximately 12% by weight of chromium to iron is normalized and tempered, a tempered, martensite structure is formed. Therefore, the Fe—Cr alloy is used as a structural material for nuclear energy systems (for example, sodium-cooled fast reactors) or coal-fired power generators since it has excellent neutron irradiation resistance and mechanical properties at a high temperature. However, since such an alloy has a problem in that it has a significantly low strength at 650° C. or higher, an oxide dispersion-strengthened alloy manufactured by dispersing an oxide, which is stable at a high temperature, in the structural material has been developed recently.
However, although conventional oxide dispersion-strengthened alloys have an advantage in that they have more excellent strength than other alloys at a high temperature, they have a problem in that they do not satisfy the design requirements.
To solve the above-described problems, research on various methods has been conducted, including a method which includes adding tungsten (W) as a solid-solution hardening element to an iron (Fe)-chromium (Cr)-yttria (Y₂O₃)-based alloy and adding a minor alloying element such as vanadium (V) or niobium (Nb) to the resulting alloy mixture, wherein tungsten (W) is not softened even at a high temperature and not easily abraded due to high hardness (see Korean Patent Publication No. 10-2012-0118312).
However, when tungsten (W) is added as the solid-solution hardening element in the proposed method, tungsten (W) forms a Laves phase such as a brittle (Fe, Cr)₂W phase when it is used under a high-temperature stress atmosphere for a long period of time. Accordingly, the proposed method affects the creep strain rate to be accelerated at a high temperature, which results in manufacture of an alloy having inferior high-temperature creep properties. Therefore, development of a martensitic oxide dispersion-strengthened alloy having enhanced high-temperature strength and creep properties is required.
The foregoing discussion in this section is to provide background information of the invention and does not constitute an admission of prior art.

SUMMARY

According to an aspect of the present invention, there is provided a martensitic oxide dispersion-strengthened alloy having enhanced high-temperature strength and creep properties, which includes chromium (Cr) of 8 to 12% by weight, yttria (Y₂O₃) of 0.1 to 0.5% by weight, carbon (C) of 0.02 to 0.2% by weight, molybdenum (Mo) of 0.2 to 2% by weight, titanium (Ti) of 0.01 to 0.3% by weight, zirconium (Zr) of 0.01 to 0.2% by weight, nickel (Ni) of 0.05 to 0.2% by weight, and the balance of iron (Fe).
According to one exemplary embodiment of the present invention, titanium (Ti), zirconium (Zr) and nickel (Ni) may be included at a total content of 0.5% by weight or less.
According to another exemplary embodiment of the present invention, the martensitic oxide dispersion-strengthened alloy may be used as a material for core structure parts including nuclear fuel claddings, wires, end plugs and ducts of a fast reactor.
According to another aspect of the present invention, there is provided a method of manufacturing a martensitic oxide dispersion-strengthened alloy having enhanced high-temperature strength and creep properties. Here, the method includes:
(a) mixing an yttria (Y₂O₃) powder with a metal powder including carbon (C), iron (Fe), chromium (Cr), molybdenum (Mo), titanium (Ti), zirconium (Zr) and nickel (Ni) and manufacturing an alloy powder by mechanically alloying the resulting mixture;
(b) charging a can-shaped container with the mechanically alloyed alloy powder and degassing the alloy powder;
(c) manufacturing an oxide dispersion-strengthened alloy by hot-working the degassed alloy powder; and
(d) cold-working the hot-wrought oxide dispersion-strengthened alloy.
According to one exemplary embodiment of the present invention, in step (a) the alloy powder may include chromium (Cr) of 8 to 12% by weight, yttria (Y₂O₃) of 0.1 to 0.5% by weight, carbon (C) of 0.02 to 0.2% by weight, molybdenum (Mo) of 0.2 to 2% by weight, titanium (Ti) of 0.01 to 0.3% by weight, zirconium (Zr) of 0.01 to 0.2% by weight, nickel (Ni) of 0.05 to 0.2% by weight, and the balance of iron
(Fe), wherein titanium (Ti), zirconium (Zr) and nickel (Ni) are included at a total content of 0.5% by weight or less.
According to another exemplary embodiment of the present invention, the hot working in step (c) may be performed using at least one process selected from the group consisting of a hot isostatic pressing process, a hot forging process, a hot rolling process, a hot extrusion process, and a combination thereof.
According to still another exemplary embodiment of the present invention, the cold working in step (d) may be performed using at least one process selected from the group consisting of a cold rolling process, a cold drawing process, a cold pilgering process, and a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the accompanying drawings, in which:

FIG. 1 is a diagram showing the experimental results obtained by comparing room-temperature and high-temperature strength properties of a martensitic oxide dispersion-strengthened alloy according to the present invention and a conventional martensitic oxide dispersion-strengthened alloy; and

FIG. 2 is a diagram showing the experimental results obtained by comparing high-temperature creep properties of the martensitic oxide dispersion-strengthened alloy according to the present invention and the conventional martensitic oxide dispersion-strengthened alloy.

DETAILED DESCRIPTION OF EMBODIMENTS

Certain embodiments of the present invention will be described in detail below with reference to the accompanying drawings. While the present invention is shown and described in connection with exemplary embodiments thereof, it will be apparent to those skilled in the art that various modifications can be made without departing from the scope of the invention.
Unless specifically stated otherwise, all the technical and scientific terms used in this specification have the same meanings as generally understood by a person skilled in the related art to which the present invention belongs. In general, the nomenclatures used in this specification and the experimental methods described below are widely known and generally used in the related art.
The present inventors have conducted research on a martensitic oxide dispersion-strengthened alloy having enhanced strength and creep properties at a high temperature, and found that the martensitic oxide dispersion-strengthened alloy has more enhanced high-temperature strength and creep properties than conventional martensitic oxide dispersion-strengthened alloys when molybdenum (Mo) is added as a solid-solution hardening element and titanium (Ti), zirconium (Zr) and titanium (Ti) are also added as minor alloying elements. Therefore, the present invention has been completed based on these facts.
In one embodiment, a martensitic oxide dispersion-strengthened alloy has enhanced high-temperature strength and creep properties and includes chromium (Cr) of 8 to 12% by weight, yttria (Y₂O₃) of 0.1 to 0.5% by weight, carbon (C) of 0.02 to 0.2% by weight, molybdenum (Mo) of 0.2 to 2% by weight, titanium (Ti) of 0.01 to 0.3% by weight, zirconium (Zr) of 0.01 to 0.2% by weight, nickel (Ni) of 0.05 to 0.2% by weight and the balance of iron (Fe), wherein the titanium (Ti), zirconium (Zr) and nickel (Ni) are included at a total content of 0.5% by weight or less.
When the content of chromium (Cr) is less than 8% by weight, corrosion resistance may be degraded, whereas a martensite phase may not be easily formed when the content of chromium (Cr) is greater than 12% by weight. Therefore, the content of chromium (Cr) is preferably in a range of 8 to 12% by weight, and more preferably in a range of 9 to 11% by weight.
When the content of yttria (Y₂O₃) is less than 0.1% by weight, a dispersion-strengthening effect may be poor, whereas processability may be degraded due to an increase in dispersion-strengthening effect by residual dispersed particles when the content of yttria (Y₂O₃) is greater than 0.5% by weight. Therefore, the content of yttria (Y₂O₃) is preferably in a range of 0.1 to 0.5% by weight, and more preferably in a range of 0.3 to 0.4% by weight.
When the content of molybdenum (Mo) is less than 0.2% by weight, high-temperature strength may be poorly enhanced, whereas economic feasibility may be reduced due to the presence of a large amount of expensive molybdenum (Mo) when the content of molybdenum (Mo) is greater than 2% by weight. Therefore, the content of molybdenum (Mo) is preferably in a range of 0.2 to 2% by weight, and more preferably in a range of 0.7 to 1.5% by weight. That is, when molybdenum (Mo) is added instead of tungsten (W), high-temperature strength may be more enhanced than in conventional oxide dispersion-strengthened alloys, and formation of a Laves phase may also be inhibited under high-temperature stress conditions exposed to a neutron irradiation atmosphere, which results in enhancement of long-term creep properties.
The content of titanium (Ti) is preferably in a range of 0.01 to 0.3% by weight, and more preferably in a range of 0.1 to 0.3% by weight. Such titanium (Ti) is bound to yttria (Y₂O₃) in a heating process to form a Y-Ti-O-based complex oxide such as Y₂Ti₂O₇or Y₂TiO₅, which contributes to high-density fine dispersion, thereby enhancing a strength property.
The content of zirconium (Zr) is preferably in a range of 0.01 to 0.2% by weight, and more preferably in a range of 0.1 to 0.2% by weight. Such zirconium (Zr) is also bound to yttria (Y₂O₃) in a heating process to form a Y—Zr—O-based complex oxide so that zirconium (Zr) can be uniformly dispersed in a base, and the remaining zirconium (Zr) may also be formed into ZrC or dissolved in a solid solution, thereby further enhancing a high-temperature strength property.
Nickel (Ni) is an austenite-forming element that serves to enhance strength of a base structure due to martensite strengthening. In this case, the content of such nickel (Ni) is preferably in a range of 0.05 to 0.2% by weight, and more preferably in a range of 0.1 to 0.2% by weight.
Meanwhile, when titanium (Ti) or zirconium (Zr) is present in a large amount, the strength may rather be reduced due to formation of a coarse oxide such as TiO₂or ZrO₂and inhibition of grain refinement. Therefore, titanium (Ti), zirconium (Zr) and nickel (Ni) may be included at a total content of 0.5% by weight or less.
According to another aspect of the present invention, the present invention provides a method of manufacturing a martensitic oxide dispersion-strengthened alloy having enhanced high-temperature strength and creep properties. Here, the method includes:
(a) mixing an yttria (Y₂O₃) powder with a metal powder including carbon (C), iron (Fe), chromium (Cr), molybdenum (Mo), titanium (Ti), zirconium (Zr) and nickel (Ni) and manufacturing an alloy powder by mechanically alloying the resulting mixture;
(b) charging a can-shaped container with the mechanically alloyed alloy powder and degassing the alloy powder;
(c) manufacturing an oxide dispersion-strengthened alloy by hot-working the degassed alloy powder; and
(d) cold-working the hot-wrought oxide dispersion-strengthened alloy.
In step (a), the alloy powder is prepared by mixing the yttria (Y₂O₃) powder with the metal powder including carbon (C), iron (Fe), chromium (Cr), molybdenum (Mo), titanium (Ti), zirconium (Zr) and nickel (Ni) and mechanically alloying the resulting mixture. In this case, the metal powder includes chromium (Cr) of 8 to 12% by weight, carbon (C) of 0.02 to 0.2% by weight, molybdenum (Mo) of 0.2 to 2% by weight, titanium (Ti) of 0.01 to 0.3% by weight, zirconium (Zr) of 0.01 to 0.2% by weight, nickel (Ni) of 0.05 to 0.2% by weight and the balance of iron (Fe), wherein titanium (Ti), zirconium (Zr) and nickel (Ni) are preferably included at a total content of 0.5% by weight or less. A mixed powder obtained by mixing the metal powder with 0.1 to 0.5% by weight of the yttria (Y₂O₃) powder is mechanically alloyed using a mechanical alloying machine such as a horizontal ball mill to prepare an alloy powder.
In step (b), the alloy powder prepared in step (a) is degassed under a vacuum condition. More particularly, a can-shaped container made of carbon steel or stainless steel is charged with the mechanically alloyed alloy powder prepared in step (a), and sealed. Thereafter, the alloy powder is degassed at a temperature of 400 to 650° C. and a degree of vacuum of 10⁻⁴torr for 1 to 4 hours.
In step (c), the alloy powder degassed in step (b) is hot-wrought. More particularly, an oxide dispersion-strengthened alloy is manufactured using at least one selected from the group consisting of a hot isostatic pressing process, a hot forging process, a hot rolling process, a hot extrusion process, and a combination thereof.
In step (d), the oxide dispersion-strengthened alloy manufactured in step (c) is cold-wrought. More particularly, the cold working may be performed using at least one selected from the group consisting of a cold rolling process, a cold drawing process, a cold pilgering process, and a combination thereof.
According to one exemplary embodiment of the present invention, martensitic oxide dispersion-strengthened alloys including chromium (Cr) of 11% by weight, yttria (Y₂O₃) of 0.35% by weight, carbon (C) of 0.15% by weight, molybdenum (Mo) of 1% by weight, titanium (Ti) of 0.1% by weight, zirconium (Zr) of 0.2% by weight, nickel (Ni) of 0.1% by weight and the balance of iron (Fe) were prepared (see Example 1), and high-temperature tensile strength and creep properties were compared with those of a conventional martensitic oxide dispersion-strengthened alloy. As a result, the martensitic oxide dispersion-strengthened alloy according to the present invention was found to have an excellent strength property (see Example 2) and creep property (see Example 3) at room temperature and a high temperature, especially 700° C., compared with the conventional martensitic oxide dispersion-strengthened alloy.
Hereinafter, certain examples will be described in order to aid in understanding the present invention. However, it should be understood that the description set forth herein is merely exemplary and illustrative of exemplary embodiments for the purpose of describing the present invention, and is not intended to limit the present invention.

EXAMPLE 1

Manufacture of Martensitic Oxide Dispersion-Strengthened Alloy

Martensitic oxide dispersion-strengthened alloys having compositions as listed in the following Table 1 were manufactured.

TABLE 1

Fe	C	Cr	W	Mo	Ni	Ti	Zr	Y₂O₃

Reference alloy 1	Bal.	0.15	10	2			0.25		0.35
Reference alloy 2	Bal.	0.15	11	2		0.1	0.25		0.35
Novel alloy	Bal.	0.15	11		1	0.1	0.1	0.2	0.35

Units: % by weight

That is, a high-purity source powder (Fe, Cr, Mo, Ti, Zr and Ni: a grain size of 200 mesh or less and a purity of 99% or more) and Y₂O₃powder (a particle size of 50 nm or less and a purity of 99.9%) were mixed at respective weight ratios, and then mechanically alloyed at 240 rpm for 48 hours under an ultra-high purity Ar atmosphere using a horizontal ball mill to manufacture an alloy powder. Thereafter, a stainless can was charged with the alloy powder and sealed, and the alloy powder was then degassed at 500° C. for 3 hours under a degree of vacuum of 10⁻⁴torr, or less. The can charged with the manufactured alloy powder was subjected to a hot isostatic pressing process under conditions of 1,150° C. and 100 MPa for 3 hours to manufacture an oxide dispersion-strengthened alloy. Subsequently, a hot rolling process was performed by heating the oxide dispersion-strengthened alloy at 1,150° C. for an hour. In this case, a reduction rate corresponding to one cycle of rolling was maintained at 5 to 10% of the thickness of the alloy, and the hot rolling was performed until the reduction rate reached a reduction in thickness of 80% or more by repeatedly performing several cycles of the hot rolling. A temperature of the alloy during a rolling process was maintained in a range of 950 to 1,150° C., and intermediate heat treatment was performed at 1,150° C. for more than 5 minutes. Finally, the alloy manufactured by the hot rolling was cooled in the air.

EXAMPLE 2

Comparison Test of Room-Temperature and High-Temperature Strength Properties

The three martensitic oxide dispersion-strengthened alloys (i.e., reference alloys 1 and 2 and the novel alloy) manufactured in Example 1 were measured for yield strength (YS), ultimate tensile strength (UTS) and total elongation (TE) at room temperature and 700° C. The results are shown in FIG. 1.
As shown in FIG. 1, it could be seen that the reference alloy 1 to which tungsten (W) and titanium (Ti) were added had yield strengths of 748 MPa and 195 MPa at room temperature and 700° C., respectively, and the reference alloy 2 to which nickel (Ni) was further added at a content of 0.1% by weight had yield strengths of 1,309 MPa and 162 MPa at room temperature and 700° C., indicating that the reference alloy 2 had enhanced tensile strength, compared with the reference alloy 1. In particular, it could be seen that the alloy (i.e., the novel alloy) according to the present invention in which tungsten (W) was replaced with molybdenum (Mo) and to which titanium (Ti), zirconium (Zr) and nickel (Ni) were added together had significantly enhanced tensile strength at room temperature and a high temperature.
From these results, it could be seen that the martensitic oxide dispersion-strengthened alloy according to the present invention had enhanced tensile strength at room temperature and a high temperature, especially around 700° C., compared with the conventional martensitic oxide dispersion-strengthened alloy.

Example 3

Comparison Test of High-Temperature Creep Property

A creep test was performed at 700° C. on the three martensitic oxide dispersion-strengthened alloys prepared in Example 1. The results are shown in FIG. 2.
As shown in FIG. 2, it could be seen that the alloy (i.e., a novel alloy) according to the present invention in which tungsten (W) was replaced with molybdenum (Mo) and to which titanium (Ti), zirconium (Zr) and nickel (Ni) were added together had a significantly increased creep rupture time under stresses of 100 and 120 MPa, compared with the reference alloys 1 and 2 containing tungsten (W) and titanium (Ti).
From these results, it could be seen that the martensitic oxide dispersion-strengthened alloy according to the present invention had a more excellent high-temperature creep property than the conventional martensitic oxide dispersion-strengthened alloy.
The martensitic oxide dispersion-strengthened alloy according to the present invention includes chromium (Cr) of 8 to 12% by weight, yttria (Y₂O₃) of 0.1 to 0.5% by weight, carbon (C) of 0.02 to 0.2% by weight, molybdenum (Mo) of 0.2 to 2% by weight, titanium (Ti) of 0.01 to 0.3% by weight, zirconium (Zr) of 0.01 to 0.2% by weight, nickel (Ni) of 0.05 to 0.2% by weight and the balance of iron (Fe). Here, titanium (Ti), zirconium (Zr) and nickel (Ni) are included at a total content of 0.5% by weight or less. Therefore, the martensitic oxide dispersion-strengthened alloy of the present invention has excellent strength and creep properties at a high temperature, especially around 700° C., and thus is expected to be able to be effectively used as a material for core structure parts, including nuclear fuel claddings, wires, end plugs and ducts of a fast reactor such as a sodium-cooled fast reactor.
It will be apparent to those skilled in the art that various modifications can be made to the above-described exemplary embodiments of the present invention without departing from the scope of the invention. Thus, it is intended that the present invention covers all such modifications provided they come within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. A martensitic oxide dispersion-strengthened alloy comprising:

chromium (Cr) of 8 to 12% by weight,

yttria (Y₂O₃) of 0.1 to 0.5% by weight,

carbon (C) of 0.02 to 0.2% by weight,

molybdenum (Mo) of 0.2 to 2% by weight,

titanium (Ti) of 0.01 to 0.3% by weight,

zirconium (Zr) of 0.01 to 0.2% by weight,

nickel (Ni) of 0.05 to 0.2% by weight, and

the balance of iron (Fe).

2. The martensitic oxide dispersion-strengthened alloy of claim 1, wherein the sum of titanium (Ti), zirconium (Zr) and nickel (Ni) in the alloy is 0.5% by weight or less with reference to the total weight of the alloy.

3. The martensitic oxide dispersion-strengthened alloy of claim 1, wherein the martensitic oxide dispersion-strengthened alloy is shaped to form at least one of a a nuclear fuel cladding, a wire, an end plug and a duct of a fast reactor.

4. A method of manufacturing a martensitic oxide dispersion-strengthened alloy having high-temperature strength and creep properties, the method comprising:

mixing yttria (Y₂O₃) powder with powder of carbon (C), iron (Fe), chromium (Cr), molybdenum (Mo), titanium (Ti), zirconium (Zr) and nickel (Ni) to provide alloy powder;

charging alloy powder in a container and degassing the alloy powder;

hot-working the degassed alloy powder to produce an oxide dispersion-strengthened alloy; and

cold-working the hot-wrought oxide dispersion-strengthened alloy.

5. The method of claim 4, wherein the alloy powder comprises:

chromium (Cr) of 8 to 12% by weight,

yttria (Y₂O₃) of 0.1 to 0.5% by weight,

carbon (C) of 0.02 to 0.2% by weight,

molybdenum (Mo) of 0.2 to 2% by weight,

titanium (Ti) of 0.01 to 0.3% by weight,

zirconium (Zr) of 0.01 to 0.2% by weight,

nickel (Ni) of 0.05 to 0.2% by weight and

the balance of iron (Fe),

wherein the sum of titanium (Ti), zirconium (Zr) and nickel (Ni) in the alloy powder is 0.5% by weight or less with reference to the total weight of the alloy powder.

6. The method of claim 4, wherein the hot working is performed using at least one process selected from the group consisting of a hot isostatic pressing process, a hot forging process, a hot rolling process, a hot extrusion process, and a combination thereof.

7. The method of claim 4, wherein the cold working is performed using at least one process selected from the group consisting of a cold rolling process, a cold drawing process, a cold pilgering process, and a combination thereof.