Number names, used in sports cars, fighter jets and golf clubs, hint at calculated awesomeness, but what do they mean? In a set of irons, the metal name is typically its component makeup: 17-4 stainless steel is 17 percent chromium and 4 percent nickel; 6-4 titanium is 6 percent aluminum and 4 percent vanadium; 1040 carbon steel is 0.4 percent carbon, and 1020 is 0.2 percent.
"Composition affects atomic grain structure, which determines stiffness, strength, density and ductility," says David Lee, associate professor of physics at Gordon College and a Golf Digest Hot List Advisor. "It's a trade-off. The stronger the material, the more difficult it is to work into shapes."
"The first decision is to forge or cast," says John Rae of Cleveland. "Next is choosing between metals that are nearly identical, which is when the science becomes art." Strong, light and easy to work with, 17-4 is popular for casting large cavity-back designs, like the Callaway X-20 and Cleveland CG irons, because it allows a thin face and discretionary weight for the perimeter. The 431 stainless steel in irons like Cobra's S9 and Nike's CCi is slightly softer. Conversely, in a forged iron more metal must fit into a smaller space. So forgers use highly formable, dense steels that are soft, like the 1025E of Mizuno's MX-25 irons.
Like any commodity, the price and availability of metals fluctuate with supply and demand. Saving dollars can be as much a consideration as saving grams. The goal, though, remains that elusive element of feel, an equation designers pursue with metallurgy and economics. Says Cleveland's Rae, "More so than material, what a player feels is the frequency at which a head design vibrates."