Golf club head

Abstract

A golf club head includes a single component. The unitary part is formed of a stainless steel material. The single component has a variable hardness. The first portion of the component has a first hardness H1 not less than 50 HRC. The second portion of the component has a second hardness H2 that is not greater than 85 HRB.

Description

[0001] Cross - Reference to Related Applications

[0002] This application is a regular application of U.S. Provisional Application No. 63 / 614,154, filed on December 22, 2023, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] The present invention relates to a golf club head, a method for manufacturing the same, and a component for a golf club head. Background Art

[0004] From a technical perspective, golf clubs are subject to a series of unique and rigorous tests. For example, the ability of a golf club to meet performance thresholds (such as effectively transferring energy to a golf ball upon impact) is evaluated. The ability of a golf club to provide forgiveness upon off - center or mishit shots is also evaluated. In addition, the ability of a golf club to impart specific spin characteristics or other attributes to a golf ball upon impact in certain situations to shape the flight trajectory and / or ball roll characteristics is evaluated. In addition to performance, the club head should withstand repeated use, such as being wear - resistant, rust - resistant, and resistant to material fatigue. Some club heads should also allow for a certain degree of adjustment through plastic deformation, for example, bending the hosel to adjust the loft and / or lie angle.

[0005] Golf club manufacturers seek to succeed in all or as many of these aspects of use as possible. However, the diversity of these functional requirements and the limiting regulations (such as those regarding mass, physical dimensions, and cost) often lead to design conflicts and trade - offs. For example, increasing the forgiveness of a club head (such as by increasing its moment of inertia about a preferred axis) typically comes at the expense of the desired feel (hand - feel). Similarly, adjusting a club head to impart beneficial spin and wear resistance may involve material selection, which may have an adverse impact on other considerations. For example, traditional materials that can provide high hardness, high yield strength, and sufficient machinability often lack in other critical areas (such as ductility or softness), which are preferred for providing adjustability or bendability.

[0006] To minimize the severity of such design trade - offs, manufacturers consider selectively varying the club head material in different parts of the club head structure to better match the material properties with the structural functions. For example, the material of the face insert used in an iron - type club head (such as a wedge) is different from the material of the (club head) body into which the face insert is inserted. Thus, the face insert can be selected to exhibit properties suitable for striking the ball, such as relatively high hardness, relatively low density, sufficient wear resistance, and sufficient machinability. However, the material of the body can deviate appropriately from these properties and materials can be selected that exhibit, for example, relatively high density and greater ductility.

[0007] Selectively providing different materials in different parts of the club head structure, while having obvious benefits, is not without drawbacks. First, increasing the number of components that make up the club head increases the manufacturing cost and complexity. Thus, the margin of error in manufacturing may increase with the increase in the number of failure sites, such as in the case of improper use of adhesives, mechanical fasteners, and heat - affected zones caused by welding or brazing. In addition, the transition of the club head from a solid structure to a component - based structure may result in a harmful loss of feel and poor acoustic or vibration characteristics. SUMMARY OF THE INVENTION

[0008] Accordingly, an object of the present invention is to provide material compositions and their implementations that are themselves suitable for the various aspects of use expected of a golf club head. Thus, the benefits associated with material characteristics selectively corresponding to a particular club head structure can be obtained, while minimizing or avoiding the detriments associated with an over - componentized structure.

[0009] In one aspect, a golf club head has a single (unitary) component formed of stainless - steel material. The single component has variable hardness. A first portion of the single component has a first hardness H1 of not less than 50 HRC. A second portion of the single component has a second hardness H2 of not greater than 85 HRB.

[0010] In another aspect of the present invention, a method includes: forming a component of a golf club head. The component comprises stainless - steel material. The method includes: selectively case - hardening the component such that a first portion of the component has a first hardness H1 of not less than 50 HRC and a second portion of the component has a second hardness H2 of not greater than 85 HRB.

[0011] In yet another aspect of the present invention, a component for a golf club head comprises stainless - steel material. The nickel content of the stainless - steel material is not greater than 0.25 mass%.

[0012] In yet another aspect of the present invention, the golf club head component includes a stainless steel material. The carbon content of the stainless steel material is not less than 0.25% by mass.

[0013] In yet another aspect of the present invention, the golf club head component comprises a stainless steel material. The austenitizing temperature of the stainless steel material is not less than 800.

[0014] These and other features and advantages of the golf club head, its composition, and its manufacturing method in various aspects of the present invention will become more apparent upon consideration of the following description, drawings, and appended claims. The description and drawings given below are for illustrative purposes only and do not limit the scope of the present invention in any way. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A front view of an exemplary golf club head showing one or more aspects of the present invention.

[0016] Figure 2 Showing Figure 1 A rear view of an exemplary golf club head.

[0017] Figure 3 A flowchart showing a method of manufacturing a golf club head showing one or more aspects of the present invention.

[0018] Figure 4 A graph showing the material properties of an exemplary steel composition and known steel compositions providing one or more aspects of the present invention.

[0019] Figure 5 Showing Figure 4 A line graph showing relationship information between the tempering temperature and hardness of an exemplary steel composition and another known steel composition.

[0020] Figure 6 Showing Figure 4 A graph showing relationship information between the carbon content, tempering temperature, and "as-quenched" hardness of an exemplary steel composition and other known steel compositions. DETAILED DESCRIPTION

[0021] In one aspect of the present invention, referring to Figure 1 and Figure 2, which shows a club head 100. The golf club head 100 has: a front portion 122 (the front portion includes a hitting face 102), a top portion 106, and a bottom portion 108 opposite the top portion 106. In a reference position, the bottom portion 108 is configured to be supported on a virtual ground surface (e.g., the ground surface 114). The golf club head 100 also has a heel 112 and a toe 110 opposite the heel 112. A hosel 104 extends from the heel 112. The hosel 104 has a hosel bore (not shown in the figure), and the hosel bore is configured to receive a golf club shaft (not shown in the figure). When the golf club head 100 is combined with a golf club shaft, a golf club can be formed. The hosel 104 gives a virtual hosel axis 124, which is a central axis given by the hosel bore. The relative position of the hosel axis 124 with respect to the rest of the structure of the golf club head 100 gives the loft and lie of the golf club head.

[0022] Preferably, the golf club head 100 is an iron-type golf club head, for example, having a loft between 20° and 66°. More preferably, the golf club head 100 is a wedge-type golf club head, for example, having a loft between 40° and 66°. Additionally or alternatively, the golf club head 100 has a loft between about 62° and about 66°, and more preferably, has a loft between 61° and 63°. However, the structures and material compositions described in this specification can be easily applied to other types of golf club heads, such as woods including drivers, fairway woods, and hybrids, as well as putters, rescue clubs, etc.

[0023] The golf club head preferably comprises a steel material, and the steel material is preferably a stainless steel material. Generally, the construction of a golf club head, especially an iron-type golf club head, including a wedge-type golf club head, typically uses readily available grades of stainless steel. For example, AISI 431 alloy steel is commonly used for the components of a golf club head. However, considering a series of unique strict requirements and limitations of the golf club head (especially a wedge-type golf club head) and the desire to minimize the severity of design trade-offs, it may be advantageous to use a steel composition different from AISI 431. In the present invention, if the chromium content of the steel alloy is at least 10.5 mass%, the steel alloy is considered a stainless steel.

[0024] Preferably, most (i.e., greater than 50%) of the club head 100 by mass is composed of this steel, more preferably at least 85% of the club head 100 is composed of this steel, and particularly preferably substantially all of the club head 100 is composed of this steel (taking into account the presence of a small amount of auxiliary components / parts such as paint, thin coatings, adhesives (binders), ferrules, etc.). Alternatively or additionally, this steel preferably constitutes a single component of the club head 100, more preferably a single component including a first part (forming at least a part of the hitting face 102), particularly preferably further including a second part (forming at least a part of the hosel 104), and especially preferably further including a third part (forming at least a part of the rear portion 116 of the club head 100). The club head 100 preferably has a solid shape with an upper blade portion 118 and a lower muscle portion 120 near the bottom 108. In some embodiments, substantially the entire club head 100 is integrally formed of this steel. As described above, reducing the number of components required in the construction of the club head 100 can provide the following benefits: reducing manufacturing costs, reducing manufacturing tolerances, reducing the locations prone to failure, while maintaining the desired feel, acoustic, and vibration characteristics.

[0025] However, preferably, the composition of the above steel is selected to provide material properties that are particularly advantageous for use in a golf club head. Conventionally available grades of steel provide some suitable properties for the club head, but may also bring about performance that is not considered particularly suitable for the club head. At least for these reasons, modulating the steel composition according to the use of the club head can bring benefits with little or no harm. In addition, preferably, modulating the steel composition according to the use of the club head allows for greater flexibility in altering the club head characteristics through structure.

[0026] As described above, the golf club head 100 is preferably an iron-type club head, more preferably a wedge-type club head. Therefore, adjusting the main material composition according to the specific use of such a club head may bring benefits beyond expectations. Particular attention should be paid to hardness characteristics, wear resistance, and density, as they are considered relevant to a wedge-type club head. However, other characteristics can also be considered.

[0027] Hardness is an example of such a property, and the ideal application of hardness is unique for a golf club head. On the one hand, the hitting face (e.g., the hitting face 102) preferably comprises a relatively hard surface. However, other portions of the club head (e.g., the club head 100) are preferably softer and more ductile, such as the hosel 104. This ductility supports adjustability by plastically deforming the club head (e.g., by bending the hosel to change the loft and / or the sole angle). This duality of hardness requirements is a unique aspect of the function of a golf club head and is considered a legitimate reason to alter traditional material compositions.

[0028] One problem with traditional steels is their limitations in providing hardness control. For example, the austenitizing temperature of 431 stainless steel is about 720 °C, which makes the tempering temperature not greater than about 700 °C; exceeding this temperature will unfavorably increase the likelihood of austenite transformation and re-hardening. Therefore, it is challenging to obtain a relatively hard hitting face while maintaining ductility.

[0029] Despite other material composition factors, these limitations on hardening are considered to be closely related to the nickel content in the steel. Nickel is considered a strong austenite promoter. It is thought to affect the austenitizing temperature (lowering it). Thus, using this steel in the quenched and tempered state is thought to increase the minimum hardness achievable. For example, 431 stainless steel is thought to only be softenable to about 85 HRB. Remedial processes can counteract this deficiency, such as softening processes that include holding this steel at a temperature near its austenitizing temperature and slowly cooling. However, these remedial processes are not without drawbacks (e.g., they can reduce wear resistance). Such remedial processes also complicate the manufacturing process and increase manufacturing costs. Therefore, a steel composition that can achieve the desired hardness change solely through quenching and tempering is preferred.

[0030] Therefore, the nickel content of the steel composition of the club head 100 is preferably not more than 0.5% by mass, more preferably not more than 0.35% by mass. However, it is worth noting that even when the nickel component is reduced to not more than 0.25% by mass, or more preferably, about 0.2% by mass, the club head can still exhibit excellent properties (such as hardness change and wear resistance) with little or no damage. In such a nickel content case, the steel can exhibit an austenitizing temperature of not less than 800 °C, preferably not less than 850 °C, and more preferably about 870 °C. Thereby, the steel is expected to be tempered at a temperature of at least 800 °C. Thus, the steel can be softened to below 90 HRB after quenching and tempering, significantly improving the bendability, for example, by bending the hosel to accommodate the adjustment of the loft angle and / or the sole angle. Preferably, the steel exhibits material properties that allow for an angle adjustment of up to about 4° in the hosel neck, whether during loft angle adjustment or sole angle adjustment. Generally, in some other embodiments, the nickel content can be reduced to less than 0.2% without significant damage, especially in the case of wedge-type club heads. However, the lower limit of the nickel content may correspond to unacceptable impact energy and should be evaluated on this basis.

[0031] The hardness characteristics of the steel are also considered to be significantly affected by the carbon content. It is generally believed that increasing carbon increases the maximum hardness that can be achieved in the quenched state of the steel. For example, although there are factors of other material components, the carbon content of 431 stainless steel is considered to be about 0.1% by mass, and the maximum hardness in the quenched state is about 40 HRC. The carbon content of 8620 stainless steel is considered to be about 0.2% by mass, and the maximum hardness in the quenched state is about 45 HRC. Preferably, the carbon content of the steel of the club head 100 is not less than 0.13% by mass, more preferably not less than 0.25% by mass. Despite the factors of other components, the steel of the club head 100 can exhibit a hardness of not less than 50 HRC in the quenched state, more preferably not less than 55 HRC, particularly preferably not less than 60 HRC, and especially preferably between 60 HRC and 65 HRC.

[0032] In addition to its direct benefits itself, carbon is considered to be an effective substitute for nickel to a certain extent. Therefore, carbon can achieve the favorable reduction of the above nickel content. In addition to these hardness benefits, the carbon content limitation is considered to contribute to improving wear resistance and reducing material density.

[0033] However, if the carbon content is too high, it may have harmful effects. For example, manufacturing problems may occur, such as problems related to weldability and curing. In addition, the carbon content, together with nickel and other components, is considered to contribute to the austenitizing temperature of the steel. Specifically, a relatively high carbon content will lower the austenitizing temperature because carbon is considered a strong austenite promoter. Therefore, the steel of the club head 100 may be adversely limited in its ability to soften to a hardness of not more than 90 HRB by tempering as described above. Therefore, the carbon content of the steel of the club head 100 is preferably between 0.13 mass% and 0.50 mass%, more preferably between 0.25 mass% and 0.50 mass%. However, in cases where the ability to achieve higher hardness takes precedence over lower minimum hardness (for example, for the above-mentioned adjustability purposes), a higher carbon content (for example, in the range of 0.45% to 0.50%) may be particularly preferred.

[0034] Although there are factors of other material components related to wear resistance, the increase in hardness itself is considered to be related to the improvement of wear resistance to a certain extent. Therefore, the ability to quench the above-mentioned steel to obtain higher hardness will in turn improve wear resistance and strength. This may be especially true in the case of combining a specific heat treatment process (such as laser etching or laser peening) with the above carbon content.

[0035] The chromium content is also considered to contribute to the material properties of the steel that are uniquely associated with the golf club head. This is especially true in terms of hardness and wear resistance (including rust resistance). Preferably, the chromium content present in the steel of the club head 100 is selected mainly based on these properties as described in further detail below.

[0036] As described above, the club head 100 preferably exhibits a relatively high hardness at the hitting face location and a relatively low hardness at other locations (e.g., the hosel 104 and / or the rear 116). In addition to nickel and carbon as described above, chromium can contribute to achieving these desired club head characteristics. For example, in addition to its basic hardness characteristics, the steel composition can potentially determine which surface treatment options (e.g., surface hardening or case hardening) may be effective and their degree of success. For example, in some embodiments, the hitting face 102 undergoes a surface hardening process, preferably a nitriding process. In some embodiments, other surface hardening processes can be applied, instead of or in addition to nitriding, such as carburizing, carbonitriding, normalizing, case hardening, induction hardening, cyaniding, flame hardening, and laser hardening. However, nitriding is preferred because it is considered cost-effective and can achieve the most satisfactory results. Due to the presence of chromium, the nitriding process enables chromium nitride to form on the hitting face 102. This causes the hitting face 102 to exhibit a surface hardness of no less than 1200 HV (0.05). For conventional carbon steel, for example, the maximum achievable hardness would be significantly lower, such as approximately 800 HV (0.05).

[0037] Nitriding may also have drawbacks. For example, nitriding stainless steel has been shown to reduce corrosion resistance. However, considering the overall usage characteristics of the club head (especially Figure 1 the wedge-type club head 100), the hardness benefits achieved through the steel composition and surface hardening (e.g., nitriding) are considered to offset this detriment.

[0038] Regarding rust, preferably, a steel composition suitable for reducing or minimizing the spread of rust or natural oxidation is employed. In other embodiments, rust or oxidation of the hitting face 102 of the club head 100 can be considered a positive development. For example, there is a niche market for golf club heads that seek a hitting face that exhibits rust or has features specifically selected to promote rust. This type of golfer market favors specific texture and / or surface roughness characteristics associated with a rusted face. However, generally, preferably, the club head 100 is configured to reduce the occurrence or spread of rust. It is believed that club heads susceptible to rust wear faster than those not susceptible to rust. This may be because rust on the hitting face is considered to wear faster than areas that do not exhibit rust, resulting in a greater volumetric loss rate for the rusted club head. This is of particular concern in terms of the scoring line structure. The presence of chromium in the steel reduces the occurrence and spread of rust and thus can reduce the wear rate.

[0039] Based on the above considerations, the chromium content of the steel used in the club head 100 is preferably not less than 13% by mass, more preferably not less than 16% by mass. Additionally or alternatively, the chromium content of the steel is not more than 21% by mass, more preferably not more than 18% by mass. An excessively high chromium content may hinder the transformation of the steel into a martensitic crystal structure, which may cause the steel to be too brittle and may disadvantageously result in the formation of the σ phase during the tempering process.

[0040] In addition to the above considerations regarding rust prevention and wear, the above chromium content, together with the carbon and nickel contents, can desirably reduce the overall steel density. The density of a metallic material is mainly affected by two properties: (1) the composition of the alloy; (2) the form of its atomic arrangement. A simple way to estimate the alloy density is to obtain the weight percentages of the constituent elements and then divide them by the density of the elements to obtain the total volume of each element. Then, assuming a 100 g sample, divide it by the sum of the volumes, as shown in Equation 1 below:

[0041]

[0042] Another key factor affecting the steel density is the structure of the atomic arrangement or the crystal structure of the structure. Based on its above-mentioned constituent components, the steel of the club head 100, once tempered and / or quenched, is preferably a mixture of ferrite and martensite (or substantially completely or completely martensite) having a body-centered cubic structure (BCC). Martensite exhibits a body-centered tetragonal (BCT) structure in the quenched state and a body-centered cubic (BCC) structure in the tempered state. Therefore, the relative proportion of the crystal structure of martensite depends on the amount of tempering after quenching. The atomic packing factor (i.e., the amount of atomic volume per unit cell) of the BCC and BCT structures is 0.68. Since the packing factors of the BCC and BCT structures are similar, the overall material density is considered to be mainly determined by the alloy composition.

[0043] Another common metallic crystal structure is the face-centered cubic (FCC), whose atomic packing factor is greater than that of BCC or BCT, being 0.74, which indicates that it is a close-packed structure. Austenite has an FCC structure, so its density is generally higher than the density of the blended composition of the steel of the club head 100, although its density calculated according to Equation 1 above will be lower. For the above reasons, the density of the steel is preferably not more than 7.85 g / cm 2 and more preferably not more than 7.65 g / cm 2 and particularly preferably not more than 7.60 g / cm 2 . Reducing the density increases the discretionary mass (mass that can be freely traded off) of the club head 100, that is, the discretionary mass is not the mass required for the structural integrity of the club head and can thus be consciously placed at positions for enhancing various mass characteristics of the club head 100, such as the position of the center of gravity and the moment of inertia about the relevant axis passing through the center of gravity.

[0044] The nitrogen composition in the steel of the golf club head 100 is also important. Nitrogen can affect steel in a similar way to carbon. This is because of their similar sizes, i.e., both nitrogen and carbon can be regarded as interstitial elements. For example, nitrogen is a strong austenite promoter. Therefore, increasing the nitrogen component beyond a certain point may harmfully cause an increase in the minimum quenched and tempered hardness. In addition, if the nitrogen content is too high, then due to the small size and relatively high diffusion rate of nitrogen, nitrogen may affect the distribution of steel during welding and solidification. Further, if the nitrogen content is too high, nitrogen will also affect the loss of ductility, undesired toughness, and corrosion resistance of the steel due to the formation of CrN. Nitrogen may affect the maximum strength of stainless steel, although its influence is considered to be less than that of carbon.

[0045] Based on the above considerations, the nitrogen content of the steel of the golf club head 100 is preferably not more than 0.035 mass%, more preferably not more than 0.15 mass%, and particularly preferably in the range of about 0 mass% to 0.06 mass%. Since nitrogen and carbon have common characteristics, the total content of carbon and nitrogen is also significant. Preferably, the total content in the steel of the golf club head 100 is in the range of 0.13 mass% to 0.75 mass%, more preferably in the range of 0.20 mass% to 0.35 mass%. In this case, if the content is too low, the steel may not be hardened to the desired degree, may exhibit reduced wear resistance, and may exhibit an undesirable high density.

[0046] The above description details the embodiments of the preferred steel compositions for the golf club head 100. Table 1 below summarizes several exemplary steel compositions corresponding to the above description. Exemplary steel A corresponds to the first general example of the steel used in the golf club head 100. Exemplary steel B corresponds to the above steel composition but is specially tailored for the embodiment of the golf club head 100 corresponding to a hollow body type golf club head. Exemplary steel C corresponds to the above steel composition but is specially tailored for the embodiment of the golf club head 100 corresponding to a forged iron golf club head. 410SS, 440SS, 8620SS, and 431 are stainless steel alloys believed to be known in the golf club industry.

[0047] Table 1

[0048] Steel Carbon (mass%) Chromium (mass%) Nickel (mass%) Exemplary Steel A 0.26 16.6 0.15 410 Stainless Steel 0.135 11.7 0.244 440 Stainless Steel 0.6﹣1.2 16﹣18 0.75 8620 Stainless Steel 0.18﹣0.23 0.4﹣0.6 0.4﹣0.7 431 Stainless Steel 0.073 16.1 1.66

[0049] Table 2 below shows a more detailed evaluation of the chemical composition of exemplary steel A. Figure 4 The chart of... summarizes some of the material properties exhibited by exemplary steel A compared to known steel alloys.

[0050] Table 2

[0051]

[0052] Based on the exemplary embodiments of the steel described above, after quenching, tempering, and case hardening, the steel of the golf club head 100 preferably exhibits a maximum hardness of not less than 50 HRC, more preferably not less than 55 HRC, and particularly preferably in the range of 60 HRC to 65 HRC. The minimum hardness of the same steel component of the golf club head 100 is preferably not greater than 90 HRB, more preferably not greater than 85 HRB. Preferably, the components of the golf club head 100 composed of this steel include a first position on the hitting face 102 of the golf club head 100, preferably include a second position at the hosel 104, and preferably include a third position at the rear portion 116 of the golf club head. In these embodiments, preferably, the hardness at the first position is not less than 50 HRC, more preferably not less than 55 HRC, and particularly preferably in the range of 60 HRC to 65 HRC. Alternatively or additionally, the maximum hardness of the steel component preferably coincides with the first position, for example, located on the hitting face 102. Preferably, the hardness at the second position (and optionally, the third position) is not greater than 90 HRB, more preferably not greater than 85 HRB. Alternatively or additionally, the minimum thickness of the steel component is preferably located on a portion other than the hitting face 102, preferably at the hosel 104. However, in some embodiments, the position of the minimum hardness of the steel component is located at the rear portion 116 of the golf club head 100 or another portion.

[0053] Additionally or alternatively, the ratio of the maximum hardness to the minimum hardness of the steel of the golf club head 100 (both the maximum and minimum values are expressed in terms of quantities related to the Rockwell C hardness (HRV) scale) is preferably not less than 4.5, more preferably not less than 6, particularly preferably not less than 9, and especially preferably not less than 12. In some specific embodiments, preferably, this ratio is in the range of 12 to 16.25.

[0054] Figure 3 A process flow diagram 200 is shown, which describes the preferred steps taken when forming the golf club head 100 using the exemplary steel composition embodiments described herein. Although the steps of process 200 are organized in sequence and are preferably intended to occur in the order shown, it is contemplated that one or more steps may occur in a different order or be omitted. Additionally, in some embodiments, other steps or processes may be performed before, after, or between any of the process steps shown and described.

[0055] In step 202, an intermediate (state) club head is formed by casting (e.g., investment casting or lost-wax casting). Next, optionally, in step 204, welding is applied to add material and / or repair any areas of the intermediate (state) cast club head body due to imperfections or defects in the casting process. For example, the welding material can be used as a filler in areas with porosity problems. The welding material is preferably a stainless steel material. However, other materials can also be used, but if so, it is preferably combined with additional post-treatment.

[0056] Next, optionally, in step 206, the intermediate (state) club head body is preferably polished to remove the residue of the sprue or other components generated by the casting process 202.

[0057] Next, in step 208, the intermediate (state) club head body undergoes a heat treatment 208. Preferably, the heat treatment process 208 at least includes a quenching process 208A and a tempering process 208B. First, in step 208A, preferably, the intermediate club head body is maintained at a temperature of about 1040 °C for about 90 minutes to about 120 minutes (preferably about 90 minutes). Subsequently, preferably, the intermediate (state) club head body is quenched by immersion in an N2 solution. As a result, most (i.e., greater than 50 mass%) of the intermediate (state) club head body, more preferably not less than 60 mass%, and particularly preferably substantially the entire intermediate (state) club head body is transformed into a harder martensitic structure.

[0058] Next, in step 208B, the intermediate club head body undergoes tempering. In this step, the intermediate club head body is maintained at a temperature not less than 800 °C (more preferably not less than 850 °C, particularly preferably in the range of 865 °C to 870 °C, and especially preferably at a temperature of about 870 °C) for about 2 hours. Preferably, subsequently, the intermediate (state) club head body is cooled in an N2 solution. By this step, the martensite is tempered, thereby softening the intermediate (state) club head body.

[0059] In some embodiments, in step 208, the intermediate club head body undergoes multiple heat treatment cycles. In some cases, it is preferred to apply multiple heat treatment cycles in order to both maintain a relatively high surface hardness potential of the steel at the hitting face 102 and further reduce the final club head hardness at locations including, for example, the hosel 104 and / or the rear portion 116. Such multiple cycles can include multiple tempering cycles. For example, as Figure 6As shown, at the same temperature, using a double tempering process as opposed to a single tempering will result in a lower hardness. For example, a double tempering for about 2 hours at a temperature of about 863 °C will result in a hardness of 80 HRB, while a single tempering at the same temperature and duration will result in a hardness of about 96 HRB. The multi-cycle heat treatment process may, for example, include the sub-steps shown in Table 4 below:

[0060] Table 3

[0061] Step Process Temperature (°C) Duration (hours) Resulting Hardness 1 Heating in a Vacuum Furnace 1040 2 47HRC - 50HRC 2 <![CDATA[Quenching and cooling of N2]]> To below 100 3 Heating in a Vacuum or Conventional Furnace 870 2 87HRB 4 <![CDATA[N2 Quenching and Cooling]]> To below 100 5 Heating in a Vacuum or Conventional Furnace 830 2 80HRB 6 <![CDATA[Quenching and cooling of N2]]> To Room Temperature

[0062] Next, in step 210, the hitting face 102 is preferably surface milled. Next, in step 212, the hosel / neck region of the intermediate (condition) club head body is polished to harmonize the appearance between the hitting face 102 and the hosel portion 104. Next, in step 214, scoring lines are machined on the hitting face 102, preferably by milling.

[0063] Next, in step 216, the hitting face 102 undergoes surface hardening. Preferably, the surface hardening step 216 includes a nitriding process 216A and another hardening operation, such as laser hardening or laser peening in step 216B.

[0064] The nitriding step 216A is preferably carried out at a temperature of not less than about 550 °C, more preferably not less than about 575 °C, and particularly preferably about 580 °C, for a duration of about 50 minutes. However, additional or alternative surface hardening or other protective or decorative surface finishing processes may be contemplated, such as carburizing, carbonitriding, normalizing, case hardening, induction hardening, cyaniding, flame hardening, and coating with PVD, ceramics, and / or diamond. Due to the reasons of the various components described above, the steel used in the golf club head 100 is considered to exhibit sufficient wear / anti-rust properties by itself, and thus plating is not considered necessary for this purpose. On the other hand, for example, for 8620 steel, nickel plating and chrome plating are generally considered necessary for the purpose of improving the wear / anti-rust properties to an acceptable level.

[0065] Preferably, in step 216B, the hit face after nitriding is laser hardened. In this step, focused thermal energy is directed to the hit face 102 in a short period of time (e.g., a few seconds). After the laser surface hardening process, it is preferred to cool the hit face 102 using, for example, gas or water. This laser hardening process preferably affects the material extending from the hit face 102 to a depth of not less than 0.5 mm (more preferably not less than 0.7 mm, and particularly preferably about 0.8 mm). Such a laser hardening process further hardens the hit face 102 to a hardness of not less than 50 HRC, more preferably not less than 52 HRC. Based on the above steps, the hit face 102 of the club head 100 preferably achieves hardness and wear resistance values consistent with the embodiments of the club head 100 described above.

[0066] As a result of the surface hardening process, the hardness of the club head 100 (more specifically, the hardness of any component made of the steel of the present invention described herein) exhibits a hardness gradient throughout its thickness. Preferably, the club head 100 exhibits a martensitic structure at a depth of not less than 50% of the total depth of the club head 100 (more preferably not less than 90% of the total depth of the club head 100, and particularly preferably substantially all of the total depth of the club head 100) from the hit face (i.e., measured backward in a direction perpendicular to the virtual hitting face plane that is substantially coplanar with the hit face 102). In a specific embodiment where the hit face 102 undergoes nitriding, preferably, by the nitriding process, the thickness of the CrN layer on the hit face 102 is not less than 0.02 mm, more preferably not less than 0.03 mm, and particularly preferably about 0.04 mm.

[0067] In the foregoing discussion, the present invention has been described in connection with specific exemplary aspects. However, it is obvious that various modifications and changes can be made to these exemplary aspects without departing from the broader substance and scope of the present invention. It is contemplated that the exemplary steel compositions described herein can be applied to the following scenarios, for example, in application examples where the known functions are similar to the functions of the golf club heads described herein. For example, such steel compositions can be used for figure skating or ice skating blades that require a certain strength and ductility due to typical cyclic loads and forging or cold working. In such cases, surface hardening may also be beneficial. Similarly, for the same reasons, such steel compositions can be used for the cutting edges used in alpine skis, snowboards, and related snow sliding equipment. Especially for snow sliding equipment, such cutting edges must exhibit sufficient ductility to form irregular or complex peripheral shapes. Additionally, considering their expected repeated interaction with snow and ice, such cutting edges must also exhibit sufficient surface hardness and wear resistance / rust resistance. Therefore, the foregoing discussion and drawings should be regarded as illustrative of the present invention only and should not limit its scope in any way.

Claims

1. A golf club head, comprising: A single component formed from stainless steel. The single component has a variable hardness, a first portion of the single component has a first hardness H1 not less than 50 HRC, and a second portion of the single component has a second hardness H2 not greater than 85 HRB.

2. The golf club head according to claim 1, wherein: The carbon content of the stainless steel material is not less than 0.25 mass %.

3. The golf club head according to claim 1, wherein: The nickel content of the stainless steel material is not more than 0.35 mass %.

4. The golf club head according to claim 3, wherein: The nickel content is not more than 0.25 mass %.

5. The golf club head according to claim 1, wherein: The chromium content of the stainless steel material is not less than 16% by mass.

6. The golf club head according to claim 1, wherein: The density of the stainless steel material is not more than 7.55 g / cm 3 .

7. The golf club head according to claim 1, wherein: The golf club head also has: Hitting surface, rear, and Rod neck, The first portion of the unitary component is located on the ball striking face.

8. The golf club head according to claim 7, wherein: The second portion of the unitary component is located at the hosel or rear of the club.

9. The golf club head according to claim 1, wherein: H1 is not less than 60HRC.

10. The golf club head according to claim 1, wherein: The hardness ratio H1 / H2 is not less than 4.5 in terms of a value corresponding to the Rockwell C hardness, ie, HRC scale.

11. The golf club head according to claim 1, wherein: The golf club head also has a club head mass Mh, and the single component also has a component mass Mc, and Mc / Mh is greater than 0.

5.

12. The golf club head according to claim 1, wherein: Substantially the entire club head is formed from the single component.

13. The golf club head according to claim 1, wherein: The golf club head also has a club head depth Dc, the single component has a martensitic structure throughout a depth Dm from the striking face, and Dm / Dc is greater than 0.

5.

14. The golf club head according to claim 1, wherein: The striking face has a nitrided surface.

15. A method comprising: a component forming a golf club head, the component comprising a stainless steel material; The component is selectively surface hardened so that the first portion of the component has a first hardness H1 of not less than 50 HRC and the second portion of the component has a second hardness H2 of not more than 85 HRB.

16. The method according to claim 15, wherein: The carbon content of the stainless steel material is not less than 0.25 mass %.

17. The method according to claim 15, wherein: The nickel content of the stainless steel material is not more than 0.35 mass %.

18. The method according to claim 16, wherein: The nickel content is not more than 0.25 mass %.

19. The method according to claim 15, wherein: The chromium content of the stainless steel material is not less than 16% by mass.

20. The method according to claim 15, wherein: The density of the stainless steel material is not more than 7.55 g / cm 3 .

21. The method according to claim 15, wherein: H1 is not less than 60HRC.

22. The method according to claim 15, wherein: The hardness ratio H1 / H2 is not less than 4.5 in terms of a value corresponding to the Rockwell C hardness, ie, HRC scale.

23. A golf club head component comprising a stainless steel material having a nickel content of not more than 0.25 mass %.

24. The component according to claim 22, wherein Furthermore, the density of the component is not greater than 7.6 g / cm 3 .

25. The component according to claim 23, wherein The density is not more than 7.55 g / cm 3 .

26. The component according to claim 22, wherein The nickel content is not more than 0.20 mass %.

27. A golf club head having the component according to claim 23, wherein: The golf club head also has: Hitting surface, rear, and Rod neck, The member has at least a first portion located at the ball striking face and a second portion located at the rear or hosel portion.

28. A golf club head component comprising a stainless steel material having a carbon content of not less than 0.25 mass %.

29. A golf club head component comprising a stainless steel material having an austenitizing temperature of not less than 800°C.

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