Metal treatment – Stock – Nickel base
Reexamination Certificate
2002-05-31
2004-06-22
Sheehan, John P. (Department: 1742)
Metal treatment
Stock
Nickel base
C148S427000, C420S445000, C420S446000, C420S447000, C420S448000, C420S449000, C420S450000, C420S451000, C420S452000, C420S453000, C420S454000
Reexamination Certificate
active
06752883
ABSTRACT:
RELATED APPLICATION
This application claims the priority of Japanese Patent Application NO. 2001-167940 filed on Jun. 4, 2001 which is incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates to a free-cutting Ni-base heat-resistant alloy having an excellent machinability.
BACKGROUND OF THE INVENTION
An excellent high temperature strength is demanded for exhaust valves and bolts for engines since they are used under high temperature environment. There is an additional demand of corrosion resistance against exhaust gas for exhaust pipes and valves in chemical plants as well as the demand of high temperature strength. It has thus been a general practice to use, as a structural material for composing such parts, nickel (Ni)-base heat-resistant alloys excellent in strength and corrosion resistance in high temperature ranges.
A problem of poor machinability has, however, resided in the conventional Ni-base heat-resistant alloy, although being excellent in the strength and corrosion resistance. Structural steel or stainless steel will successfully be improved in the machinability by being added with so-called machinability improving elements such as Pb, Bi, S, Se or Te, but the Ni-base heat-resistant alloy will considerably be ruined in the hot workability by containing such machinability improving elements. So that almost no approach has been made for Ni-base heat-resistant alloy to intentionally improve the machinability, which has inevitably pushed up machining costs of such material in the product making.
It is therefore an object of the present invention to provide a free-cutting Ni-base heat-resistant alloys excellent in strength and corrosion resistance in high temperature ranges and in machinability.
SUMMARY OF THE INVENTION
To solve the foregoing problems, a free-cutting Ni-base heat-resistant alloy of the present invention contains Ni as a major component;
contains C in an amount of 0.01 to 0.3 wt % and Cr in an amount of 14 to 35 wt %;
contains at least one element selected from Ti, Zr and Hf in a total amount of 0.1 to 6 wt %, and S in an amount of 0.015 to 0.5 wt %;
has dispersed in the matrix thereof a machinability improving compound phase, where such phase contains any one of Ti, Zr and Hf as a major constituent of the metal elements, essentially contains C and either S or Se as a binding component for such metal elements; and
satisfies the relations of:
W
Ti
+0.53W
Zr
+0.27W
Hf
>2W
C
+0.75W
S
; and
W
C
>0.37W
S
where W
Ti
represents Ti content (wt %), W
Zr
represents Zr content (wt %), W
Hf
represents Hf content (wt %), W
C
represents C content (wt %) and W
S
represents S content (wt %).
It is to be noted that “major component” in the context of this specification means a component having a largest content on the weight basis in a target texture (the same will apply to other expressions such as “mainly” or “mainly comprises”).
By containing at least one of Ti, Zr and Hf, together with C, and also with either S or Se, the Ni-base heat-resistant alloy will have generated in the matrix thereof a compound (machinability improving compound phase) based on such composition. The present inventors found that the Ni-base heat-resistant alloy was significantly improved in the machinability by having generated in the matrix thereof such machinability improving compound phase, which led us to propose the present invention.
A reason why the machinability of the Ni-base heat-resistant alloy can be improved by the formation of such machinability improving compound phase is supposed as follows. That is, when the alloy is subjected to processing such as cutting or grinding in order to remove a portion thereof, the machinability improving compound phase finely dispersed in the matrix can act just like a perforation to thereby facilitate formation of the sectional plane, which is supposed as being responsible for the improved machinability. Any way, the machinability improving compound phase can be responsible for a machinability equivalent to or superior to that attainable by the foregoing machinability improving elements which have previously been used, while successfully avoiding degradation of other characteristics inherent to the heat-resistant alloy and retaining a good hot workability.
In the conventional Ni-base heat-resistant alloy, it has been considered as necessary to intentionally control the content of sulfur (S) in order to keep a good hot workability, and in some cases even an effort has been made to use a high-purity Ni material containing almost no S. On the contrary in the present invention, such S content will be in an allowable range since the S will be incorporated into such machinability improving compound phase as one constituent thereof. So that S contained in the Ni-base heat-resistant alloy of the present invention will not heavily affect the hot workability of the alloy. This makes it possible to use a source material containing a relatively large amount of S, which is expected to result in an improved productivity.
A reason why the hot workability of the conventional Ni-base heat-resistant alloy degraded due to the addition of S can be explained by the formation of (Ni, S) compound, in particular Ni
3
S
2
in the alloy. In the present invention, S contained in the alloy is incorporated into the machinability improving compound phase during the growth thereof, which suppresses the formation of Ni
3
S
2
and thus successfully prevent the hot workability from being degraded for its S content.
Another advantage of the formation of the machinability improving compound phase relates to that it hardly affects the strength and corrosion resistance at high temperature ranges, which are properties most critical for the Ni-base heat-resistant alloy. In this case, properties such as strength and corrosion resistance at high temperature ranges are defined by residual constituents in the matrix other than such machinability improving compound phase. So that the heat-resistant alloy will be obtained with desired properties by properly adjusting the composition of the matrix other than the machinability improving compound phase.
In the Ni-base heat-resistant alloy of the present invention, the machinability improving compound phase can be generated so as to be dispersed within the matrix. In particular, finer dispersion of such compound phase within the matrix will result in better machinability of the Ni-base heat-resistant alloy. In order to raise the improving effect of the machinability, it is preferable to control an average size of the machinability improving compound phase observed in the polished sectional microstructure of the Ni-base heat-resistant alloy (maximum width between two parallel tangential lines which are drawn in some different directions so as to circumscribe the outer contour of the compound grain) within a range from 1 to 5 &mgr;m or around.
An area ratio of the machinability improving compound phase observed in a polished surface of the material is preferably 0.1 to 10%. For the purpose of obtaining improving effect of the machinability by forming such machinability improving compound phase, such phase must be contained in an amount of 0.1% or more in terms of an area ratio in the polished sectional microstructure. Excessively large area ratio will however be no more effective due to saturation of such effect, or may rather adversely affect other characteristics inherent to the heat-resistant alloy (i.e., strength and corrosion resistance at high temperature ranges). So that the area ratio in the polished sectional microstructure of the Ni-base heat-resistant alloy is preferably set to 10% or below.
The machinability improving compound phase can typically be such that mainly comprising a compound expressed by a composition formula M
4
Q
2
C
2
(where M represents the metal element containing any one of Ti, Zr and Hf as a major constituent, and Q represents either S or Se) It is to be noted now that in this specification the compound expressed by such formula may occasionally be abbreviated
Ebata Takashi
Ishida Kiyohito
Noda Toshiharu
Oikawa Katsunari
Ueta Shigeki
Ishida Kiyohito
Sheehan John P.
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