Field of the Invention This invention relates to precipitation-hardenable stainless steels, and in particular, to a precipitation-hardenable stainless steel which is suitable for casting in air and which provides a unique combination of high strength and good toughness compared to the lcnown grades of such steels. The invention also relates to a cast article made from a precipitation-hardenable stainless steel, in particular, a cast golf club head.

Background of the Invention Precipitation-hardenable stainless steels have been used to make cast articles such as golf club heads. The most economical way to cast such articles is under air, rather than under a protective atmosphere such as vacuum or an inert gas. High strength, i. e., a yield strength of at least 190 ksi (1310 MPa) at room temperature, is desired for a precipitation-hardenable stainless steel for golf club heads. The known grades of high-strength, precipitation-hardenable stainless steels contain highly reactive elements such as titanium or aluminum to develop the desired strength levels during the age-hardening heat treatment. Those elements have a strong affinity for oxygen and nitrogen. Therefore, when the molten alloy is exposed to air, the alloys become"dirty"through the formation of oxides and nitrides of titanium and/or aluminum. Furthermore, the loss of the strengthening elements titanium and aluminum to oxide and/or nitride inclusions, prevents the alloy from developing maximum strength during age hardening.

Also, the presence of a relatively large amount of nitrides and/or oxides impairs the castability of the alloy.

An alloy sold under the registered trademark CUSTOM 450 by Carpenter Technology Corporation has the following weight percent composition.

Carbon 0.05 max.

Manganese 1.00 max.

Silicon 1.00 max.

Phosphorus 0.030 max.

Sulfur 0.030 max.

Chromium 14.00-16.00 Nickel 5.00-7.00 Molybdenum 0.50-1.00 Copper 1.25-1.75 Columbium 8 x % C min.

The balance of the alloy is iron and the usual impurities. The CUSTOM 450 alloy is a martensitic, age-hardenable stainless steel that is typically cast and wrought, but which can be used for air casting of shapes. However, it does not provide a room temperature yield strength of at least 190 ksi (1310 MPa), even in the wrought and age-hardened condition.

In view of the foregoing, it would be desirable to have a stainless steel alloy that provides high strength and that is readily castable in air.

Summary of the Invention The need for a high strength, castable, stainless steel alloy is essentially fulfilled by the stainless steel according to the present invention. The alloy according to this invention is an age-hardenable stainless steel alloy having the following broad, intermediate, and preferred weight percent ranges.

Broad Intermediate Preferred C 0.040 max. 0.030 max. 0.025 max.

Mn 1.00 max. 0.75 max. 0.50 max.

Si 1.00 max. 0.75 max. 0.50 max.

P 0.050 max. 0.040 max. 0.030 max.

S 0.015 max. 0.010 max. 0.005 max.

Cr 12.00-15.50 13.00-15.25 14.00-15.00 Ni 5.75-7.25 6.00-7.00 6.25-6.75 Mo 0.50-1.50 0.50-1.25 0.70-1. 00 Cu 1.00-4.00 1.25-3.00 1.25-2.00 Cb 1.00-2.00 1.00-1.75 1.15-1.50 Cb+Ta 2.00 max. 1.75 max. 1.50 max.

N 0.040 max. 0.030 max. 0.025 max.

The balance of the alloy is essentially iron and the usual impurities found in commercial grades of age-hardenable stainless steels intended for similar use or service. A cast article made from this alloy provides a yield strength of at least about 190 ksi (1310 MPa) together with good toughness and ductility.

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 intermediate or preferred compositional ranges. In addition, a minimum or maximum for an element of the broad, intermediate, or preferred range can be used with the maximum or minimum for that element from one of the other ranges. Throughout this application, the term"percent"or the symbol"%"means percent by weight, unless otherwise indicated.

Detailed Description A small but effective amount of silicon, up to about 1.00%, is present in this alloy for deoxidation and because it benefits the fluidity of the alloy during casting. Too much silicon leads to the formation of an undesirable amount of ferrite which adversely affects the strength provided by this alloy. Therefore, silicon is preferably restricted to not more than about 0.75%, and better yet to not more than about 0.50%.

At least about 12.00%, better yet at least about 13.00%, and preferably at least about 14.00% chromium is present in this alloy to benefit the corrosion resistance of the alloy. However, chromium is a strong ferrite forming element and therefore is limited to not more than about 15.50%, better yet to not more than about 15.25%, and preferably to not more than about 15.00% in this alloy.

Nickel is included in this alloy to provide the austenitic structure needed at the solution treating temperature, so that the alloy will transform fully or substantially fully to martensite when quenched from the solution treating temperature. Nickel also benefits the corrosion resistance and toughness of the alloy. For those reasons, the alloy contains at least about 5.75%, better yet at least about 6.00%, and preferably at least about 6.25% nickel. Too much nickel leads to an undesirable amount of retained austenite in the alloy which adversely affects the strength provided by this alloy. Therefore, nickel is limited to not more than about 7.25%, better yet to not more than about 7.00%, and preferably to not more than about 6.75% in order to avoid or substantially avoid the formation of retained austenite.

At least about 0.50% and preferably at least about 0.70% molybdenum is present in this alloy to benefit the corrosion resistance and toughness of the alloy.

Molybdenum is limited to not more than about 1.50%, better yet to not more than about 1.25%, and preferably to not more than about 1.00% because, like chromium, it is a ferrite forming element. As described above, too much ferrite is detrimental to the strength provided by this alloy.

Copper contributes to the corrosion resistance of this alloy and is beneficial for the strengthening of the alloy during the aging heat treatment.

Therefore, the alloy contains at least about 1.00% and preferably at least about 1.25% copper. Copper is restricted to not more than about 4.00%, better yet to not more than about 3.00%, and preferably to not more than about 2.00% because too much copper adversely affects the phase balance of this alloy by causing the formation of retained austenite. Furthermore, exceeding the solubility limit of copper in this alloy can result in the precipitation of copper at the grain boundaries, thereby adversely affecting the strength provided by the alloy.

Columbium combines with some of the nickel in this alloy to form an intermetallic strengthening phase during the age-hardening heat treatment.

Accordingly, at least about 1.00% and preferably at least about 1.15% columbium is present in this alloy to benefit the high strength provided by the alloy.

Tantalum may be substituted for some of the columbium on a 2 for 1 weight percent basis. Too much columbium and/or tantalum adversely affects the tensile ductility and notch tensile strength provided by this alloy. Moreover, columbium and tantalum are strong ferrite-forming elements. Therefore, the combined amount of columbium and tantalum in this alloy is limited to not more than about 2.00%, better yet to not more than about 1.75%, and preferably to not more than about 1.50%.

The balance of the alloy is essentially iron and the usual impurities found in commercial grades of age-hardenable stainless steels intended for similar use or service. Carbon, nitrogen, manganese, phosphorus, and sulfur are inevitably present in this alloy. However, the amounts of those elements are controlled because the presence of too much of them, either individually or in combination, adversely affects the strength and toughness provided by the alloy. Accordingly, carbon and nitrogen are each restricted to not more than about 0.040%, better yet to not more than about 0.030%, and preferably to not more than about 0.025% in this alloy. Manganese is limited to not more than about 1.00%, better yet to not

more than about 0.75%, and preferably to not more than about 0.50%.

Phosphorus is restricted to not more than about 0.050%, better yet to not more than about 0.040%, and preferably to not more than about 0.030%. Sulfur is limited to not more than about 0.015%, better yet to not more than about 0.010%, and preferably to not more than about 0.005%.

The alloy according to this invention is preferably melted in air and cast into desired shapes, such as golf club heads. The cast alloy is then heat treated to obtain the desired level of strength. Preferably, the alloy is solution treated by heating at about 1800-2000°F (980-1095°C) for at least about one-half hour and then quenched in water. The alloy is then aged to final strength by heating at about 800-1000°F (425-540°C), preferably at about 900-950°F (482-510°C) for about 2 to 8 hours, followed by cooling in air. When thus solution treated and aged, a cast article made from this alloy provides a room temperature yield strength of at least about 190 ksi (1310 MPa), with acceptable toughness and ductility.

The terms and expressions that have been employed herein are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions to exclude any equivalents of the features described or any portions thereof. It is recognized, however, that various modifications are possible within the scope of the invention claimed.