Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Reexamination Certificate
2002-03-26
2003-10-07
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
C148S325000, C148S326000, C148S327000
Reexamination Certificate
active
06630103
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to precipitation-hardenable, martensitic stainless steel alloys, and in particular to a Cr—Co—Ni—Mo—Al martensitic stainless steel alloy, and a useful article made therefrom, having a unique combination of high strength, notch ductility, fracture toughness, and corrosion resistance.
BACKGROUND OF THE INVENTION
Hitherto, many industrial applications, particularly in the aerospace industry, have utilized structural components manufactured from steel alloys that provide very high strength together with high toughness and ductility. Some of those applications also require good corrosion resistance for components that are exposed to corrosive or oxidizing media in their service environments. More recently, a need has arisen in the aerospace industry for a corrosion resistant steel alloy that provides higher levels of tensile strength (i.e., greater than about 260 ksi) together with high toughness and ductility.
Another field which has generated a great demand for very high strength materials is the golf club industry. In recent years there has been an unprecedented development in golf club design and technology. The new designs have generated a need for ever stronger materials. Because golf is played in the outdoors, it is desirable that any material used for golf club heads be corrosion resistant. Among the early materials used for this application were aluminum and precipitation-hardenable stainless steel. However, as club head design has evolved in recent years, manufacturers have developed new demands for strength and ductility. Among the newer technologies for golf clubs is the multi-material design in which the golf club head is fabricated from multiple pieces each made from a different material. In those designs the material used to form the face of the club has very high strength and hardness. However, because it is formed from strip material, it should also be reasonably malleable so that it can be readily processed to strip form.
Among the known high strength, high toughness steel alloys are the 300M alloy and the AERMET® 100 alloy. Both of those alloys are capable of providing tensile strength levels well in excess of 260 ksi, together with good fracture toughness. However, because those alloys contain relatively low amounts of chromium (i.e., less than about 5% by weight), they lack the corrosion resistance afforded by stainless steels. Consequently, in order to use these very high strength, high toughness steels in environments containing even the mildest corrosive media, the parts must be coated or plated with a corrosion resistant material.
Stainless steels which provide a combination of high strength and corrosion resistance are known. In particular, precipitation-hardenable stainless steels are known which can provide a tensile strength in excess of 260 ksi as well as resistance to corrosion in most types of corrosive media. The precipitation-hardenable stainless steels achieve high hardness and strength through an age-hardening heat treatment in which a strengthening phase is formed in the ductile matrix of the alloy.
One of the known age-hardenable stainless steels is capable of providing good notch ductility (NTS/UTS≧1) and good tensile ductility at a tensile strength of up to about 260 ksi. However, the notch ductility of that alloy leaves something to be desired when the alloy is processed to provide a tensile strength in excess of 260 ksi. Another known age-hardenable stainless steel is capable of providing good ductility and toughness at a tensile strength of 260 ksi and higher. However, in order to achieve strength levels much in excess of 260 ksi, for example, up to about 300 ksi, the alloy must undergo strain hardening (i.e., cold working) prior to the aging heat treatment.
A further type of stainless steel that is designed to provide relatively high strength is the so-called “straight” martensitic stainless steel. Such steels achieve high strength when they are quenched from a solution or austenitizing temperature and then tempered. One such steel is designed to provide a tensile strength in excess of 260 ksi in the quenched and tempered condition. However, the utility of that steel is limited by the fact that it has a relatively large spread between its 0.2% offset yield strength and its ultimate tensile strength. For example, at a tensile strength of about 260 ksi, the attainable yield strength is only about 200 ksi.
Given the foregoing, it would be desirable to have an alloy which provides an improved combination of very high strength and corrosion resistance, without sacrificing much in the way of toughness and ductility, and which does not require special thermomechanical processing to achieve the desired mechanical properties.
SUMMARY OF THE INVENTION
The need for a corrosion resistant alloy that provides a superior combination of strength, notch ductility, and toughness compared to the known high strength stainless steels is essentially fulfilled by the precipitation hardenable, martensitic stainless steel alloy in accordance with the present invention. The alloy according to the present invention is an ultra-high strength, precipitation hardenable stainless steel that provides a unique combination of high strength, notch ductility, fracture toughness, and corrosion resistance, without the need for special thermomechanical processing. The broad, intermediate, and preferred compositional ranges of the steel alloy of the present invention are as follows, in weight percent:
Broad
Intermediate
Preferred
C
0.030 max.
0.020 max.
0.015 max.
Mn
0.5 max.
0.25 max.
0.10 max.
Si
0.5 max.
0.25 max.
0.10 max.
P
0.040 max.
0.015 max.
0.010 max.
S
0.025 max.
0.010 max.
0.005 max.
Cr
9-13
10-12
10.5-11.5
Ni
7-9
7.5-9
7.5-8.5
Mo
3-6
4-5.25
4.75-5.25
Cu
0.75 max.
0.50 max.
0.25 max.
Co
5-11
7-11
8-9
Ti
1.0 max.
0.1 max.
0.005-0.05
Al
1.0-1.5
1.0-1.4
1.1-1.3
Nb
1.0 max.
0.3 max.
0.20 max.
B
0.010 max.
0.001-0.005
0.0015-0.0035
N
0.030 max.
0.015 max.
0.010 max.
O
0.020 max.
0.005 max.
0.003 max.
The alloy according to this invention optionally contains a small amount of one or more rare earth elements (REM), up to about 0.025% max., or a small amount of calcium or magnesium, up to about 0.010% max., for reducing phosphorus and/or sulfur in the alloy. The balance of the alloy is essentially iron, except for the usual impurities found in commercial grades of precipitation-hardenable stainless steels and minor amounts of other elements which may vary from a few thousandths of a percent up to larger amounts that do not objectionably detract from the desired combination of properties provided by this alloy.
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 of the alloy of this invention 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 element ranges of the broad composition can be used with one or more of the other ranges for the remaining elements in the preferred composition. In addition, a minimum or maximum for an element of one preferred embodiment can be used with the maximum or minimum for that element from another preferred embodiment. Throughout this application, percent or the symbol % shall mean percent by weight, unless otherwise indicated.
In accordance with another aspect of the present invention, there is provided a useful article such as an aircraft structural component or a golf club head that is formed, at least in part, from the aforesaid alloy.
In accordance with a further aspect of the present invention, there is provided an elongated strip formed from the aforesaid and a method of making such strip material.
DETAILED DESCRIPTION
The precipitation-hardenable, stainless steel alloy according to this invention contains at least about 9% chromium, bet
Kosa Theodore
Martin James W.
CRS Holding, Inc.
Dann Dorfman Herrell and Skillman, P.C.
Yee Deborah
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