Until World War I, forged iron chain was used on naval vessels, and the forge shop at the Charlestown yard was a leader in the industry. But it was a laborious process, and the demands of war spurred the development of cast steel chain, which could be produced more quickly. Charlestown was soon experimenting with detachable links to connect standard chain lengths. This led to the development in 1926 of a new chainmaking process, in which each link was made from half-links joined in a die under a drop-forge hammer—“die-lock” chain. It was clearly superior: more uniform, stronger, cheaper to make. By the early 1930s Charlestown was producing die-lock chain in several sizes, and by 1936 die-lock had superseded cast steel chain for all sizes. The shop made the chain used in most U.S. naval vessels built during World War II and was the only forge to make chain for the largest postwar aircraft carriers. [Illustration: Finished chain is loaded for shipping.] [Illustration: Decorative background] [Illustration: The Die-lock Chainmaking Process] 1 Rolled nickel-steel rods (from ¾-inch to 4¾-inch in diameter) are cut into shorter bars. 2 The cut bars are heated in a gas furnace to 2100°F. The now-malleable bars are bent by machine into U-shapes. 3 The U-bars are stamped to form stems, with tapered and ridged ends, or they have holes punched in the ends to form sockets. 4 Stem is hooked onto last completed link and placed in die; socket is heated, and the two are joined under a 10,000-pound hammer (_next page_ ). 5 Largest 4½-inch chain for supercarriers could withstand up to 2.5 million pounds. Each two-foot-long link weighed 360 pounds. Red undercoat and grey paint helped retard rusting. [Illustration: 10,000-pound hammer] [Illustration: _At a shift change in 1943, departing workers hurry past destroyer escorts being outfitted for war. At its peak during the war, the Charlestown yard and its annexes employed more than 50,000 men and women._] The Yard Transformed It was not a dramatic launch—no gathering speed down the shipways and plunging into Boston Harbor. Instead, the water flowing into Dry Dock 1 rose slowly around U.S.S. _MacDonough_ until the destroyer lifted off the keel blocks and was towed out of the dock. The 1934 “floating” was low-keyed but significant. _MacDonough_ was the first warship built by Charlestown since the wooden screw sloop _Vandalia_ slid down the ways in 1874.