Case-hardened stainless foundry alloy and methods for making the same

Abstract

 A case-hardened stainless steel part and method of making the same are disclosed. The part comprises a case defining a first layer proximate the outer surface of the part and a core defining a second layer adjacent the case and distal from the outer surface. The core has a hardness of from about 80 on the Rockwell Hardness B Scale to about 50 on the Rockwell Hardness C Scale. The case has a hardness of not less than about 30 equivalence on the Rockwell Hardness C Scale.

Description

FIELD OF THE INVENTION

[0001] The present application relates to hardenable stainless steels and, more particularly, to case hardened stainless steels suitable for the manufacture of golf club heads.

BACKGROUND OF THE INVENTION

[0002] Many steels suffer from a number of drawbacks when used to manufacture irons and wedges for golf club heads. For example, golf professionals generally like clubs with a balance of corrosion resistance and "feel," which relates to the balance of surface hardness and core hardness, as well as a high degree of polish. The core hardness is generally responsible for the feel of the club, while the surface hardness is generally responsible for the aesthetic appearance of the club, as well as its corrosion resistance and scratch resistance. The lack of stainless casting alloys with the desired properties has forced golf club designers to use chrome plated alloys for such applications. However, chrome plating is expensive and environmentally undesirable. Thus, a need has arisen for an alloy and method of manufacture which addresses the foregoing problems.

Summary of the Preferred Embodiments

[0003] In accordance with one aspect of the present invention, a case-hardened stainless steel part is provided. The part is formed from a stainless-steel alloy, wherein the alloy generally comprises chromimum in an amount of from about 15.5 to about 18.5 percent by weight of the alloy and preferably includes carbon in an amount of from about 0.13 to about 0.29 percent by weight of the alloy, manganese in an amount of up to about 1.5 percent by weight of the alloy, silicon in an amount of from about 0.25 to about 1.5 percent by weight of the alloy and molybdenum in an amount up to about 1.0 percent by weight of the alloy, with iron present in a remaining amount based on the total weight of the alloy.

[0004] The cast part preferably has an outer surface and a case that defines a first layer proximate the outer surface. The cast part further comprises a core that defines a second layer distal from the outer surface. The core preferably has a hardness from about HRB 80 (Rockwell Hardness B Scale) to about HRC 50 (Rockwell Hardness C Scale) and more preferably has a hardness from about HRC 20 to about HRC 34. The case preferably has a hardness of not less than about HRC 30 equivalence, and more preferably has a hardness of not less than about HRC 50 equivalence. It is also preferred that the case and core each have a carbon concentration, with the carbon concentration in the case being higher than that in the core.

[0005] In another aspect of the present invention, a method of making a case-hardened stainless steel part is provided. The method comprises providing a cast part formed from a stainless steel alloy, wherein the alloy comprises chromium, generally in an amount of from about 15.5 to about 18.5 percent by weight of the alloy, and carburizing the part. Preferably the carburization is conducted at a temperature of from about 1600°F to about 1900°F for a period of from about 1 hour to less than about 2 hours.

[0006] In another aspect of the present invention, a method of making a golf club head is provided. According to the method, a steel alloy comprising chromium in an amount from about 15.5 to about 18.5 percent by weight of the alloy is formed into a part having a shape of a, golf club head. Preferably, the part is preheated, carburized, quenched, cooled, and tempered. More preferably, it includes an outer surface and has a case defining a first layer proximate the outer surface and a core defining a second layer adjacent the first layer and distal from the outer surface, wherein the core has a hardness which is preferably from about HRB 80 to about HRC 50, and more preferably from about HRC 20 to about HRC 34. The case has a hardness which is preferably not less than about HRC 30 equivalence and is more preferably not less than about HRC 50 equivalence.

Brief Description of the Drawings

[0007] The invention may be more readily understood by referring to the accompanying drawing in which:

Figure 1 is a perspective view of a golf club head in accordance with a preferred embodiment of the present invention.

Detailed Description of the Preferred Embodiments

[0008] The present invention is directed to stainless steel alloy parts and methods of making the same. One particular part for which the invention is well-suited is in the manufacture of golf club heads.

[0009] An embodiment of a case-hardened stainless steel part in accordance with the present invention will now be described. According to this embodiment, the part is formed from a stainless steel alloy that generally comprises chromium in an amount of from about 15.5 to about 18.5 percent by weight of the alloy, carbon in an amount of from about 0.13 to about 0.29 percent by weight of the alloy, manganese in an amount of up to about 1.5 percent by weight of the alloy, silicon in an amount of from about 0.25 to about 1.5 percent by weight of the alloy and molybdenum in an amount up to about 1.0 percent by weight of the alloy, with iron present in a remaining amount of the alloy, except for trace impurities typically found in commercial grades of stainless steels. For example, the alloy may include trace elements of up to about 0.03 weight percent sulfur, up to about 0.02 weight percent phosphorous, and up to about 0.03 wight percent nitrogen.

[0010] The alloy more preferably comprises chromium in an amount of from about 16.5 to about 18.5 percent by weight of the alloy, carbon in an amount of from about 0.13 to about 0.24 percent by weight of the alloy, manganese in an amount of from about 0.3 to about 1.0 percent by weight of the alloy, molybdenum in an amount of from about 0.25 to about 0.5 percent by weight of the alloy, and silicon in an amount of from about 0.3 to about 1.0 percent by weight of the alloy. It is especially preferred that the amount of chromium is from about 16.5 to about 18.0 percent by weight of the alloy, the amount of carbon is from about 0.13 to about 0.21 percent by weight of the alloy, the amount of manganese is from about 0.3 to about 0.6 percent by weight of the alloy, the amount of molybdenum is from about 0.25 to about 0.5 percent by weight of the alloy and the amount of silicon is from about 0.3 to about 0.8 percent by weight of the alloy.

[0011] According to this embodiment, the cast part has an outer surface and a case that defines a first layer proximate the outer surface. The cast part further includes a core that defines a second layer distal from the outer surface.

[0012] The layers comprising the case and core are preferably defined in the part as a result of carburizing the part at a temperature of from about 1600°F to about 1900°F for a period of from about 1 hour to less than about 2 hours. A carburization temperature of about 1800 °F and a carburization period of about 1 hour are especially preferred. According to this embodiment, the part is preferably homogenized prior to carburization at a temperature of from about 1900°F to about 2100°F for a period of about 1 to about 4 hours. A homogenization temperature of about 2100 °F and a homogenization period of about 1.5 hours are especially preferred. The carburization process will modify the carbon content of the part by providing a locally higher carbon concentration closer to the outer surface of the part. However, the weight percentages of carbon recited herein refer to the condition of the cast steel part before it is carburized.

[0013] Prior to carburizing it, the part is preferably preheated at a temperature of from about 1100°F to about 1400°F for a period commensurate with the thickness of the part, most preferably between about 30 minutes and about 1 hour. More preferably, the preheating step follows the homogenization step and precedes the carburization step. A preheat temperature of about 1200°F is especially preferred.

[0014] The part is also preferably quenched from the preheated temperature in a fluid, which is preferably oil. The quenching more preferably follows the carburization step and is conducted at a fluid temperature of from about 100°F to about 200°F for a period of about 5 minutes to about 15 minutes. It is especially preferred to conduct the quenching at a fluid temperature of about 200°F for a period of about 10 minutes.

[0015] In addition, the part is preferably cooled. More preferably, the cooling follows the quenching and is conducted at a temperature of not more than about -100°F for a period of about 1 to about 3 hours. A temperature of about -100°F and a period of about 2 hours are especially preferred. It is especially preferred that the cooling be performed using known refrigeration processes.

[0016] According to this embodiment, the part is also preferably tempered. More preferably, the tempering follows the cooling step and is conducted at a temperature of from about 200 to about 600°F for a period of from about 1 to about 3 hours. A temperature of about 300°F and a period of about 2 hours are especially preferred. If necessary to achieve the desired core and case hardness, this tempering step can be repeated by again tempering the part at about 600°F, and if necessary, at about 1400°F.

[0017] The part is preferably polished after tempering. The polishing generally removes from about 0.002 to about 0.006 inches, preferably removes from about 0.002 to about 0.004 inches and more preferably removes from about 0.002 to about 0.003 inches from the surface of the casting.

[0018] The methods and compositions disclosed herein will generally produce a core having a hardness that is preferably from about HRB 80 to about HRC 50, and more preferably from about HRC 20 to about HRC 34. After tempering and polishing, the methods and compositions disclosed herein will generally produce a case having a hardness that is preferably not less than about HRC 30 equivalence, and more preferably not less than about HRC 50 equivalence. Parts having a core hardness in the foregoing ranges will typically have a yield strength of not more than about 125,000 psi. Without wishing to be bound by any theory, it is believed that the composition, the degree of carburization and the tempering conditions all contribute to the core hardness and yield strength. The post-polishing thickness of the case as measured from the outer surface of the part is preferably from about 0.002 to about 0.035 inches, more preferably from about 0.002 to about 0.030 inches, and is preferably uniform throughout the part.

[0019] As mentioned above, the alloys and processing methods described herein are especially suited for making golf club heads. An embodiment of a method for making a golf club head in accordance with the present invention will now be described. According to this embodiment, a steel alloy comprising chromium in an amount generally ranging from about 15.5 to about 18.5 percent of the alloy is formed into the shape of a golf club head, preferably one with a wedge-shaped striking surface. Preferably, an investment casting process is used to form a mold for shaping the alloy into the shape of a golf club head.

[0020] More preferably, the investment casting process comprises providing a temporary preform--for example, a wax preform-shaped in the form of a golf club head and surrounding the preform with a refractory slurry and refractory stucco. Several layers of slurry and stucco are preferably applied until the required thickness is obtained. The completed shell is then placed into an autoclave for removal of the wax preform. The shell is then placed into a preheat oven and its temperature is raised until the required temperature for pouring the steel alloy is reached. The steel alloy is then melted and poured into the preheated mold. After sufficient cooling, the part is then removed from the hardened slurry mold.

[0021] Following removal from the mold, the part is case-hardened by carburizing it, preferably after preheating it. Following carburization, the part is preferably quenched from its preheated temperature with a fluid, preferably an oil, and then cooled. The part is preferably tempered and polished following the cooling step to produce the final part. Although a variety of temperatures and times can be used for each of the foregoing steps, the temperatures and times set forth above for the steps of preheating, carburizing, quenching, cooling and tempering are preferred. It is further preferred to homogenize the golf club head, more preferably prior to the preheating step and using the temperatures and periods set forth above.

[0022] Referring to Figure 1, a golf club head prepared in accordance with a preferred embodiment of the present invention is depicted. Golf club 10 generally comprises a head 30 and a shaft 20 (only a portion of which is depicted in Figure 1). Head 30 includes an outer surface 40. Face 35 comprises a portion of outer surface 40 used to strike a golf ball. Golf club heads prepared according to this embodiment of the invention will preferably comprise a case 50 defining a first layer proximate the outer surface 40. They will also preferably comprise a core 50 defining a second layer adjacent the case 50 and distal from the outer surface 40. The core 60 will preferably have a hardness of from about HRB 80 to about HRC 50, and more preferably from about HRC 20 to about HRC 34. The case 50 will preferably have a hardness of not less than about HRC 30 equivalence and more preferably not less than about HRC 50 equivalence. Preferably, head 30 will have a yield strength of not more than about 125,000 psi. The post-polishing thickness of the case 50 as measured from the outer surface 40 of the part is preferably from about 0.002 to about 0.035 inches, more preferably from about 0.002 to about 0.030 inches, and is preferably uniform throughout head 30.

[0023] As mentioned above, the steel alloy used for manufacturing golf club heads in accordance with this embodiment generally comprises chromium in an amount of from about 15.5 to about 18.5 percent by weight of the alloy. A chromium content of about 15.5 to about 18 percent by weight of the alloy is more preferred, and a chromium content of about 16.5 to about 18 percent by weight of the alloy is especially preferred. Preferably, the alloy further comprises carbon in an amount of from about 0.13 to about 0.29 percent by weight of the alloy. An amount of from about 0.13 to about 0.24 percent by weight of the alloy is more preferred, and an amount from about 0.13 to about 0.21 percent by weight of the alloy is especially preferred.

[0024] The alloy also preferably comprises manganese in an amount up to about 1.5 percent by weight of the alloy. An amount of from about 0.3 to about 1.0 percent by weight of the alloy is more preferred, and an amount of from about 0.3 to about 0.6 percent by weight of the alloy is especially preferred.

[0025] In addition, the alloy preferably comprises molybdenum in an amount up to about 1 percent by weight of the alloy, and more preferably from about 0.25 to about 0.5 percent by weight of the alloy. It also preferably comprises silicon in an amount of from about 0.25 to about 1.5 percent by weight of the alloy. An amount of from about 0.3 to about 1.0 percent by weight of the alloy is more preferred, and an amount of about 0.3 to about 0.8 percent by weight of the alloy is especially preferred.

[0026] The invention may be better understood by the following specific examples of case-hardened stainless steel parts prepared in accordance with the present invention. Set forth below in Table 1 are experimental results for four samples of alloys that were case-hardened in accordance with an embodiment of the present invention. The samples were homogenized for 1.5 hours at a temperature of 2100°F and then preheated at a temperature of 1200°F for 30 minutes. The samples were then carburized at 1800°F for 1 hour. The carbon potentials were set to obtain a desired case hardness of HRC 62 at a pre-polishing depth of 0.004 inches from the surface. Following carburization, the samples were cooled at -100°F for 2 hours and tempered at 300°F for 2 hours to achieve the desired core and surface hardness. The post-tempering hardnesses are shown as "Core Hardness After Carburization" in Table 1. The parts were subsequently heated to 600°F and 1400°F to further refine the core hardness.

TABLE 1

Element	Alloy Wt %	Core Hardness After Carburization	Core Hardness After Heating at 600°F	Core Hardness After Heating at 1400°F
Sample 1		HRC 29.83	HRC 20.7	HRB 85.9
C	0.16
Si	0.61
Cr	16.97
Mo	0.34
Mn	0.31

Sample 2		HRC 36.2	HRC 30.8	HRB 94.2
C	0.24
Si	0.62
Cr	16.88
Mo	0.39
Mn	0.30

Sample 3		HRC 47.9	HRC 38.8	HRB 96.2
C	0.29
Si	0.62
Cr	16.73
Mo	0.37
Mn	0.30

Sample 4		HRB 97.1	HRB 87.2	HRB 82.9
C	0.13
Si	0.60
Cr	17.15
Mo	0.34
Mn	0.34

[0027] As shown in Table 2, following carburization, the surface hardness of Samples 1 and 4 was HRC 62 at a depth of 0.004 inches from the cast surface as measured using a 500 g load (Knoop Hardness). After carburization, samples 1 and 4 were polished to remove approximately 0.004 inches of the alloy. After polishing, the samples were sectioned and the transverse microhardness of the sample surfaces was measured at various depths from the surface of the sample. Using known visual techniques, the case depth in Sample 1 was determined to be approximately 0.006 inches, and the case depth in Sample 4 was determined to be approximately 0.035 inches. As indicated in Table 2, the samples had a case hardness of greater than HRC 30 equivalence.

[0028] Table 2 includes the transverse microhardness results and their corresponding depths from the surface of the sample. The negative signs are used to indicate that the depths are measured from the surface of the part after polishing.

Table 2

Sample No.	Surface Hardness and Depth (in.) Before Polishing	Surface Hardness After Polishing at Depth	Depth (in.)
4	HRC 62/0.004	HRC 51	-0.002
4		HRC 51	-0.003
4		HRC 38	-0.005
4		HRC 25	-0.009
4		HRB 98	-0.012
4		HRB 98	-0.016
4		HRB 98	-0.035
1	HRC 62/0.004	HRC 32	-0.012
1		HRC 32	-0.016
1		HRC 32	-0.035

[0029] The foregoing embodiments are merely examples of the present invention. Those skilled in the art may make numerous uses of, and departures from, such embodiments without departing from the spirit and the scope of the present invention. Accordingly, the scope of the present invention is not to be limited to or defined by such embodiments in any way, but rather, is defined solely by the following claims.

Claims

1. A case-hardened, cast part formed from a stainless steel alloy, the alloy comprising:

chromium in an amount of from about 15.5 to about 18.5 percent by weight of the alloy;

carbon in an amount of from about 0.13 to about 0.29 percent by weight of the alloy;

manganese in an amount up to about 1.5 percent by weight of the alloy;

silicon in an amount of from about 0.25 to about 1.5 percent by weight of the alloy;

molybdenum in an amount up to about 1.0 percent by weight of the alloy; and

iron present in a remaining amount of the alloy;

wherein said cast part has an outer surface and a case, said case defining a first layer of said part proximate said surface, said cast part further comprising a core, said core defining a second layer of said cast part adjacent to said case and distal from said surface, wherein said core has a hardness of from about HRB 80 to HRC 50 and said case has a hardness of not less than about HRC 30 equivalence.

2. The cast part of Claim 1, wherein said case further has a thickness of from about 0.002 inches to about 0.035 inches.

3. The cast part of Claim 1, wherein said part has a yield strength of not more than about 125,000 psi.

4. The cast part of Claim 1, wherein said amount of chromium is from about 15.5 to about 18.0 percent by weight of the alloy.

5. The cast part of Claim 1, wherein said amount of chromium is from about 16.5 to about 18.0 percent by weight of the alloy.

6. The cast part of Claim 1, wherein said amount of carbon is from about 0.13 to about 0.24 percent by weight of the alloy.

7. The cast part of Claim 1, wherein said amount of carbon is from about 0.13 to about 0.21 percent by weight of the alloy.

8. The cast part of Claim 1, wherein said amount of manganese is from about 0.3 to about 1.0 percent by weight of the alloy.

9. The cast part of Claim 1, wherein said amount of manganese is from about 0.3 to about 0.6 percent by weight of the alloy.

10. The cast part of Claim 1, wherein said amount of molybdenum is from about 0.25 to about 0.5 percent by weight of the alloy.

11. The cast part of Claim 1, wherein said amount of silicon is from about 0.3 to about 1.0 percent by weight of the alloy.

12. The cast part of Claim 1, wherein said amount of silicon is from about 0.3 to about 0.8 percent by weight of the alloy.

13. A golf club head formed from the cast part of Claim 1.

14. The golf club head of Claim 13, comprising a wedge-shaped striking surface.

15. The cast part of Claim 1, wherein the case has a carbon concentration and the core has a carbon concentration, said case carbon concentration being higher than said core carbon concentration.

16. A case-hardened, cast part formed from a stainless steel alloy, the alloy comprising:

chromium in an amount of from about 15.5 to about 18.5 percent by weight of the alloy;

wherein said cast part has an outer surface and a case defining a first layer proximate said outer surface, said part further comprising a core defining a second layer of said part adjacent said case and distal from said surface, wherein said core has a hardness of from about HRB 80 to about HRC 50 and said case has a hardness of not less than about HRC 30 equivalence.

17. The cast part of Claim 16, wherein the alloy further comprises carbon in an amount of from about 0.13 to about 0.29 percent by weight of the alloy.

18. The cast part of Claim 17, wherein said amount of carbon is from about 0.13 to about 0.21 percent by weight of the alloy.

19. The cast part of Claim 16, wherein the alloy further comprises manganese in an amount up to about 1.5 percent of the alloy.

20. The cast part of Claim 19, wherein said amount of manganese is from about 0.3 to about 0.6 percent by weight of the alloy.

21. The cast part of Claim 16, wherein the alloy further comprises silicon in an amount of from about 0.25 to about 1.5 percent by weight of the alloy.

22. The cast part of Claim 21, wherein said amount of silicon is from about 0.3 to about 0.8 percent by weight of the alloy.

23. The cast part of Claim 16, wherein the alloy further comprises molybdenum in an amount up to about 1.0 percent by weight of the alloy.

24. The cast part of Claim 23, wherein said amount of molybdenum is from about 0.25 to about 0.5 percent by weight of the alloy.

25. The cast part of Claim 16, wherein said amount of chromium is from about 16.5 to 18 percent by weight of the alloy.

26. A golf club head formed from the stainless steel cast part of Claim 16.

27. The golf club head of claim 26, wherein said golf club head comprises a wedge-shaped striking surface.

28. The cast part of Claim 16, wherein said case has a thickness of from about 0.002 inches to about 0.035 inches.

Drawing