Metal treatment – Process of modifying or maintaining internal physical... – Treating consolidated metal powder – per se
Reexamination Certificate
1999-04-06
2002-11-26
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Treating consolidated metal powder, per se
C148S605000, C148S621000, C148S325000, C148S334000
Reexamination Certificate
active
06485584
ABSTRACT:
TECHNOLOGICAL FIELD OF THE INVENTION
This invention relates to a method of manufacturing an improved ferritic or martensitic alloy, including chromium and which is strengthened by a dispersion of oxides, commonly called an ODS (Oxide Dispersion Strengthened) alloy, and more particularly, to a method of manufacturing a ferritic or martensitic ODS alloy with large grains based on iron and chromium which has single phase ferritic or martensitic matrix having an isotropic microstructure and a grain size that is sufficient to guarantee mechanical strength compatible with a use of this alloy at high temperature and/or under neutron irradiation.
ODS alloys are made up of a metal matrix having a body-centered cubic crystal structure. This structure is strengthened by a dispersion of oxides of the type Y
2
O
3
, TiO
2
etc. which gives it excellent mechanical and chemical properties at medium and high temperatures.
The resistance to oxidation of these alloys is due particularly to the presence of chromium. This resistance is only effective when the concentration of chromium is greater than 8% by weight in the alloy. However, when this concentration is greater than 12% by weight, the alloy becomes brittle.
Furthermore, thanks to their crystal structure, these alloys have good resistance to swelling and to creep under neutron irradiation.
These alloys can be used, for example, as structural materials for components in the core of a nuclear power station since these components must have a high mechanical strength at high temperature, for example from 400 to 700° C., must be resistant to neutron radiation, must be compatible with use in a sodium environment and resistant to oxidation etc.
In a general way, these alloys are also useful in the manufacture of components subject to high mechanical and thermal stresses such as components of thermal power stations, components used in the glass, gas or aeronautical industries etc.
PRIOR ART
Many types of ODS alloys that include chromium have already been developed in the prior art. These have chromium concentrations between 13 and 20%, variable contents of Mo, W, Al and Ti, and a small quantity of carbon, generally less than 0.02% by weight (200 ppm). In this type of alloy, the matrix is totally ferritic whatever the heat treatment temperature.
Hence, American U.S. Pat. No. 4,075,010 describes an alloy having a composition Fe-14 Cr-1 Ti-0.3 Mo-0.25 Y
2
O
3
.
This alloy displays a very good compromise between strength and ductility in a direction parallel to the axis of forming of the alloy. However the grains that make it up are elongated in the direction of forming which leads to a high degree of anisotropy in its mechanical properties. This anisotropy leads to too low a mechanical strength along directions perpendicular to the direction of forming. Such an alloy can therefore not be used, for example to make cladding tubes for nuclear reactors, since the radial direction is the main direction of mechanical stress in these tubes in a reactor. In addition, this alloy contains a high level of chromium which causes it to become brittle under neutron radiation through the precipitation of phases rich in this element.
This type of alloy generally produced by mechanical alloying from its constituents starting with elemental or pre-alloyed powders. In this type of alloy, mechanical alloying is a method that allows one to introduce into the metal matrix, the fine and homogeneous distribution of oxides that confers a very high hot strength on the alloy. The powders thus provided are compacted and drawn at high temperature and pressure.
This method of production however produces an alloy in which the mean grain size is generally too small, that is to say less than 1 &mgr;m and which has an anisotropic microstructure when the initial chemical composition of the matrix means it has a ferritic structure. Under these conditions, too small a grain size causes a reduction in the mechanical strength of the alloy, particularly at high temperatures greater than 500° C. Furthermore, the anisotropy of the grain size leads to anisotropy in the mechanical properties of the alloy.
In particular, the initial ferritic structure is inevitable in examples that contain more than 12% of chromium.
In order to avoid these problems of anisotropy, a man skilled in the art has been drawn into using a martensitic material less rich in Cr, but in this case, control of the mean grain size has proved to be impossible. In effect, in this type of material, no variation in grain size whatsoever has been observed after traditional heat treatment even at temperatures as high as 1250° C.
Patent application GB-A-2 219 004 describes an ODS alloy with a tempered martensitic matrix having a chromium concentration of from 8 to 12% by weight and concentrations of (Mo+W) and of carbon respectively between 0.1 and 4% and 0.05 and 0.25% by weight. In addition, the alloy described is strengthened by a dispersion of Y
2
O
3
and TiO
2
oxide particles at a concentration of from 0.1 to 1% by weight. The application examples described in this document include a chromium concentration greater than 10% by weight and a concentration of Mo and W between 2 and 4% by weight. The preparation method of the alloy described comprises mechanical alloying of the alloy in an attritor, compaction of the alloy under vacuum and hot drawing at a temperature ranging from 900 to 1200° C. This procedure is followed by a normalization treatment at a temperature ranging from 950 to 1200° C. and a tempering at a temperature ranging from 750 to 820° C.
However the method described does not allow one to control the grain size of the alloy.
The prior art methods therefore all have one or more of the following disadvantages
they do not allow one to obtain an isotropic microstructure of the formed alloy,
they do not allow one to specify and control the grain size of the alloy
they lead to a grain size that remains too small.
As a consequence, the prior art alloys all have one or more of the following disadvantages:
insufficient mechanical strength at high temperature due to the anisotropy of its microstructure,
embrittlement at high temperature and/or under neutron irradiation through precipitation of embrittling phases in the alloy due particularly to an excess of chromium, and
a mechanical strength that is not always compatible with use at high temperature and/or under neutron irradiation due particularly to having no control over the grain size of the alloy and to the grain size being too small.
DESCRIPTION OF THE INVENTION
The precise purpose of this invention is to provide a method of manufacturing a ferritic ODS alloy that includes chromium, and a method of manufacturing a martensitic ODS alloy that includes chromium, which does not have the disadvantages mentioned above and which in particular, allows one to specify and control the grain size of the alloy produced by exercising control over successive phase transformations.
The method, according to the invention, of manufacturing an alloy with a ferritic ODS structure that includes chromium, comprises preparation of a martensitic ODS blank that includes chromium and a step consisting of subjecting the martensitic ODS blank to at least one thermal cycle comprising an austenitization of the martensitic ODS blank at a temperature greater or equal to the AC3 point of this alloy in such a way as to obtain an austenite, followed by cooling of this austenite at a slow cooling rate that is less than or equal to the critical cooling rate for transformation of this austenite into ferrite in such a way as to obtain an alloy with a ferritic structure, said slow critical cooling rate being determined from a phase transformation diagram for this austenite under continuous cooling.
The alloy manufactured by the method of the invention is notably improved since it has a single phase ferritic matrix having an isotropic microstructure and a grain size that is sufficient to guarantee mechanical strength compatible with use of this alloy at high temperature and/or under neutron irradiat
Alamo Ana
Lambard Véronique
Seran Jean-Louis
Anderson Kill & Olick
Combs Morillo Janelle
Commissariat a l'Energie Atomique
Lieberstein Eugene
Meller Michael N.
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