By James M. Dahl, Ph.D., Product Application Specialist
Paul M. Novotny, Specialist - Tool & Alloy R & D
Carpenter Technology Corp., Reading, PA, U.S.A.

March 2002 - A new alloy that has been used in the landing gear of U. S. Navy carrier-based F-18 E/F jet fighters has brought a high level of excitement to golfers throughout Asia. That's because the new AerMet® TourAlloy™ high strength steel, invented by Carpenter Technology Corp., Reading, PA, U.S.A., just happens to offer a combination of properties that has been critical to improved golf club design and play.

AerMet TourAlloy, which has been used for face inserts in golf club woods and irons, has permitted the design of heads that are lighter, larger and offer greater energy transfer at impact with the golf ball. Players who have used clubs with AerMet alloy faces report driving the ball farther and with more accuracy off both the tee and fairway.

The new alloy is a premium-melted, ultra-high strength steel. In commercial as well as military applications, it has offered a unique combination of high strength and hardness, fracture toughness, exceptional ductility and resistance to both fatigue and stress-corrosion cracking.

AerMet alloy compares favorably with two other high-tech materials that have been commonly used in contemporary lines of clubs-titanium alloy Ti-6Al-4V and stainless steel alloy 17Cr-4Ni. Data has been developed (Charts 1 and 2) to compare key characteristics of the three materials such as strength, hardness, density, elasticity and strength-to-density ratio (specific strength).





Test Results
Tests show that AerMet alloy has a hardness of 54 HRC and tensile strength of 2083 MPa (302 ksi). Both values are significantly higher than those for conventional 17Cr-4Ni stainless steel and the Ti-6Al-4V titanium alloy. The 17Cr-4Ni stainless has a hardness of 44 HRC and tensile strength of 1365 MPa (198 ksi), while Ti-6Al-4V titanium alloy shows a maximum aged hardness of 40 HRC and maximum tensile strength when aged of 1140 MPa (165 ksi).

With their superior hardness and strength, golf club face plates that have been made of AerMet alloy give the ball, when struck, greater spring and velocity in flight. These two key properties, in combination, allow more of the swing energy to be transferred to the ball. Many golfers, in addition, say they enjoy the crisp sound made by the hard club face when it strikes the ball.

When aged at 468°C (875°F), AerMet alloy can reach a specific strength (strength-to-density ratio) of 26.9 km. To the surprise of many, this is slightly higher than that of Ti-6Al-4V titanium alloy, and at a much higher hardness (54 HRC vs. 40 HRC).

Ti-6Al-4V alloy can achieve a specific strength of 26.2 km when it is aged at 482°C (900°F) Without aging, the titanium alloy has a specific strength of only 20.7 km. The specific strength of 17Cr-4Ni stainless steel is 17.8 km, well below that of both the AerMet alloy and titanium. At that strength level, it has only slightly more hardness (44 HRC) than Ti-6Al-4V alloy.

Both AerMet alloy and the 17Cr-4Ni stainless alloy possess approximately 70% higher stiffness or modulus of elasticity than the Ti-6Al-4V alloy. AerMet alloy has a modulus of 192.5 Gpa, and 17Cr-4Ni stainless alloy a modulus of 196.5 Gpa, compared with a maximum modulus of 113.8 Gpa for Ti-6Al-4V titanium alloy when aged at 482°C (900°F). Thus, face plates or inserts of the two steels are stiffer, capable of supporting a greater load. That means they will absorb less energy at the point of impact than titanium. Instead, they transfer more energy to the ball put in flight.

AerMet alloy and the 17Cr-4Ni stainless steel have substantially higher elongation properties than the Ti-6Al-4V titanium alloy. This difference means that when either of the two steels are used, the face plate will be more ductile and tougher when striking the ball.

Design Flexibility
Face plates that have been made from AerMet alloy, with its exceptional combination of high specific strength and high modulus of elasticity, give the club designer wide latitude in designing the club head. He can use a thinner face than possible with the titanium alloy, without concern for its long term durability. In the process, he can make the head lighter, redistributing weight to enhance club efficiency in the hands of the average golfer, while providing excellent ball launch characteristics at the club face.

Tests have shown that the impact between the club head and golf ball can generate stress of up to 20 MPa (3000 psi). Thus it is essential that the face plate be made of a material that is strong enough to resist cracking or permanent deformation.

Golf clubs incorporating the thin AerMet alloy face plates already have been designed and manufactured that offer a larger "sweet spot" and more energy transfer. This improvement has helped the average weekend golfer in Asia to drive the ball farther and straighter without swinging harder.

Finally, it should be noted that the AerMet alloy contains martensite, the same martensite that has been the secret behind the sharp cutting edge of the Japanese Samurai sword for centuries. The martensitic grain structure gives the alloy long lasting high strength and hardness, just as those characteristics helped to perpetuate the performance and priceless value of the revered sword.