Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
1999-10-22
2001-05-29
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S244000, C075S246000, C419S011000, C419S028000, C419S049000
Reexamination Certificate
active
06238455
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to precipitation hardenable stainless steel, and in particular to a powder metallurgy steel article formed of a sulfur-containing, precipitation-hardenable stainless steel that provides a unique combination of strength, processability, ductility, and machinability. The invention also relates to a method of making the powder metallurgy stainless steel article.
BACKGROUND OF THE INVENTION
Sulfur is used in many types of stainless steels to provide improved machinability. However, significant amounts of sulfur have typically not been used to enhance the machinability of high-strength, precipitation-hardenable stainless steels because such levels of sulfur adversely affect the processability of such steels and their ductility in the age-hardened condition. Here and throughout this application the term “processability” refers to the capability of a steel to be hot worked and/or cold worked to a desired cross-sectional dimension without sustaining significant damage (i.e., cracking, tearing, etc.). A need has arisen for a high-strength, precipitation-hardenable stainless steel that provides better machinability than the known grades of such steels, but which also provides sufficient processability to permit it to be formed into small diameter wire. It is also desired that the steel provide a combination of strength and ductility that is at least comparable to the known grades of high-strength, precipitation hardenable, stainless steels.
SUMMARY OF THE INVENTION
The disadvantages of the known cast-and-wrought grades of high-strength, precipitation hardenable, stainless steels are overcome to a large degree by a powder metallurgy article in accordance with one aspect of the present invention. In this aspect of the invention, a powder metallurgy article is provided that is formed of a precipitation hardenable stainless steel alloy powder having the broad, intermediate, and preferred weight percent compositions set forth in Table 1 below.
TABLE 1
Inter-
Intermediate
Preferred
Preferred
Broad
mediate A
B
A
B
C
0.03
max.
0.03 max.
0.03
max.
0.015
max.
0.015
max.
Mn
1.0
max.
1.0 max.
1.0
max.
0.30
max.
0.15
max.
Si
0.75
max.
0.75 max.
0.75
max.
0.30
max.
0.15
max.
P
0.040
max.
0.040
0.040
max.
0.010
max.
0.010
max.
max.
S
0.010-0.050
0.020-
0.020-0.040
0.020-0.030
0.020-0.030
0.040
Cr
10-14
10-13
10-13
11.0-12.0
11.0-12.0
Ni
6-12
8-10
10.5-11.6
8.0-8.8
10.8-11.3
Ti
0.4-2.5
1.0-1.5
1.5-2.0
1.0-1.4
1.5-1.8
Mo
6
max.
0.50 max.
0.25-1.5
0.30
max.
0.8-1.1
B
0.010
max.
0.010
0.010
max.
0.0035
0.0015-
max.
max.
0.0035
Cu
4
max.
1.5-2.6
0.75
max.
1.8-2.5
0.10
max.
Co
9
max.
0.75 max.
0.75
max.
0.10
max.
0.10
max.
Nb
1
max.
0.10-0.50
0.3
max.
0.20-0.30
0.10
max.
Al
1
max.
0.25 max.
0.25
max.
0.05
max.
0.05
max.
Ta
2.5
max.
0.3 max.
0.3
max.
0.10
max.
0.10
max.
N
0.03
max.
0.03 max.
0.03
max.
0.010
max.
0.010
max.
The balance of the alloy powder composition is essentially iron and the usual impurities found in the same or similar grades of steels intended for the same or similar service. The powder metallurgy article according to this invention is formed by consolidating the metal powder to substantially full density and is characterized by a fine dispersion of sulfide particles not greater than about 5 &mgr;m in major dimension.
In accordance with another aspect of the present invention, there is provided a method of making precipitation-hardenable, stainless steel wire from metal powder. The process includes the step of melting a precipitation hardenable stainless steel alloy having a weight percent composition as set forth above. The molten alloy is then atomized to form a fine alloy powder. The alloy powder is hot consolidated to form an intermediate article and the intermediate article is mechanically worked to form wire.
The foregoing tabulation is provided as a convenient summary and is not intended thereby to restrict the lower and upper values of the ranges of the individual elements for use in combination with each other, or to restrict the ranges of the elements for use solely in combination with each other. Thus, one or more of the ranges can be used with one or more of the other ranges for the remaining elements. In addition, a minimum or maximum for an element of a broad, intermediate, or preferred composition can be used with the minimum or maximum for the same element in another preferred or intermediate composition. Here and throughout this application, the term “percent” or the symbol “%” means percent by weight, unless otherwise indicated.
DETAILED DESCRIPTION
The precipitation hardenable, stainless steel alloy used in the powder metallurgy article according to this invention contains at least about 10% chromium, and preferably at least about 11.0% chromium to benefit corrosion resistance. Too much chromium adversely affects the phase balance of the alloy and can lead to the formation of an undesirable amount of ferrite and to an excessive amount of retained austenite when the alloy is solution treated. Therefore, chromium is limited to not more than about 14%, better yet to not more than about 13%, and preferably to not more than about 12.0%.
At least about 6% and preferably at least about 8% nickel is present in the alloy used in the powder metallurgy article of this invention. Up to about 4%, preferably at least about 1.5% and better yet at least about 1.8% copper can be present in conjunction with nickel. Both nickel and copper contribute to the formation of a stable austenitic structure during solution treating prior to quenching the alloy to form martensite. Nickel and copper also contribute to the toughness and corrosion resistance of the alloy, and copper benefits the age hardening response of the alloy. Nickel is limited to not more than about 12% and copper to not more than about 2.6% because too much nickel and copper adversely affect the desired phase balance of the alloy and result in the formation of excessive retained austenite when the alloy is solution treated. Preferably, nickel is restricted to not more than about 10% and better yet to not more than about 8.8% in the alloy powder used in this invention, and copper is restricted to not more than about 2.5%.
Up to about 6% molybdenum can be present in the alloy because it contributes to the ductility and toughness of the alloy. Molybdenum also benefits the alloy's corrosion resistance in reducing media and in environments which promote pitting attack and stress-corrosion cracking. Molybdenum is restricted to not more than about 0.50% and preferably to not more than about 0.30% in the alloy powder because too much adversely affects the phase balance of the alloy, i.e., it leads to the undesirable formation of ferrite and to an excessive amount of retained austenite.
At least about 0.4% and preferably at least about 1.0% titanium is present in the alloy to provide hardness and strength by combining with available nickel to form a nickel-titanium-rich precipitate during age-hardening of the alloy. Titanium also combines with sulfur to form fine titanium sulfides that benefit the machinability of the powder metallurgy article in accordance with this invention. Too much titanium adversely affects the toughness and ductility of the alloy. Therefore, titanium is restricted to not more than about 2.5%, better yet to not more than about 1.5%, and preferably to not more than about 1.4% in a powder metallurgy article according to the present invention.
Up to about 1% niobium can be present in the alloy used in this invention to benefit toughness and age hardening response. For this purpose, the alloy contains at least about 0.10% and preferably at least about 0.20% niobium. Too much niobium adversely affects the phase balance of the alloy, producing retained austenite. Therefore, niobium is restricted to not more than about 0.50% and preferably to not more than about 0.30%.
In addition to the desirable combination of machinability and processability provided by the powder metallurgy article according to this invention, a unique combination of
Brown Robert S.
Del Corso Gregory J.
Kosa Theodore
Martin James W.
CRS Holdings, Inc.
Dann Dorfman Herrell and Skillman, P.C.
Mai Ngoclan
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