Casting v. Forging

First, welding began to replace riveting in ship-and plane-building (TIME. Dec. 15). Now cast steel is replacing forged steel, and in tanks it is replacing both riveted and welded armor plate. Engineers call this technological shift "little short of a revolution."

Said Ford's chief metallurgist, Russell H. McCarroll, last week: "By making an airplane cylinder barrel as a centrifugal casting instead of a forging, we helped break all four of the main bottlenecks in production: materials, time, skilled manpower and machine tools."

Pound or Pour? Sample military gadget is a machine-gun trunnion block, on which the gun swivels. When it is forged, i.e.., pounded, into rough shape out of a hot chunk of steel, it weighs 20 lb. In machining the block into shape, 14 lb. of steel is drilled, planed and ground away before the crude lump becomes a finished 6-lb. trunnion block.

If the same part is cast instead of forged, it weighs only 11 lb. before machining, because molten metal can be poured into nooks & crannies where no trip hammer can force it. So only 5 lb.-about one-third as much-of steel remain to be tooled away. Result: a 35% saving in skilled man-hours (according to Metallurgist Carl F. Joseph of General Motors), plus a corresponding 35% increase in the capacity of machine tools.

Castings began to replace forgings on a big scale in 1932 when Ford first poured V-8 crankshafts instead of hammering them. Since then 10,000,000 Ford crankshafts have been cast with such success that many another part is made in the same way. But the trend toward casting did not get up real speed until many foundrymen last year began to fear that war contracts, escorted by priorities, would pass them by without a nod.

Abolition of the automobile industry might well have closed down a midwestern auto specialties plant had not this foundry begun casting 60-mm. U.S. and 3-in. British mortar shells, a practice unthinkable in World War I. Mortar shells must be of precise dimensions and exacting metallic analysis so they will burst into fragments not too large & not too small to insure maximum bloodshed. Hence they had always been forged, and many an ordnance officer swore they couldn't be cast: cast steel is full of pores and bubbles, it shrinks in its molds to nonuniform sizes, its metallic structure is unreliable.

All this was true-in 1918. But the metallurgist's laboratory, now equipped with such things as powerful X-ray machines to study flaws in castings, has replaced the foundry foreman's rule-of-thumb. Says Detroit Metallurgist E. C. Troy, "The foundry engineer has been able to improve his product almost beyond recognition."

Spinning Guns. "The U.S. today leads the world in the science of gun-making," says Brigadier General Holland W. Case, Commandant of the Watertown (Mass.) Arsenal, and he gives most of the credit to replacement since 1918 of forging by centrifugal steel casting.* Steel for cannon is poured into horizontal molds which whirl rapidly until the metal hardens. Impurities are forced to the hollow center of the barrel, where they are easily bored away, and blowholes and shrinkages are avoided.

"A single casting machine will turn out several gun tubes in the time it takes to forge one," says Case, and machining time is cut 25 to 40%. "Reduction in weight is considerable," he adds, "and is used in many guns to regain mobility."

Centrifugally cast cylinder barrels will be used in all of Ford's Pratt & Whitney engines by this month's end. Both Army and Pratt & Whitney engineers-mindful of the old cast-iron stove lid, which was almost as brittle as glass-were leary of steel castings until Ford testers showed that, while forged barrels burst at 5,000 to 7,000 lb. per sq. in., centrifugally cast barrels burst at 9,000 to 10,000 lb. "Moreover," said a Ford engineer last week, "$10,000 worth of centrifugal dies will turn out as many cylinder barrel blanks as a $110,000 forging hammer." Landing gear for bombers is also centrifugally cast by Ford.

Armor. Casting U.S. tank turrets and hulls, heretofore riveted or welded together from steel plate, gets rid of another traditional prejudice against cast steel: that it couldn't successfully be toughened by heating and quenching. About one-third of U.S. tanks are now built of cast armor, and all of them would be, if foundry and machining facilities were adequate (see p. 53).

The advantages of cast armor are that

1) there are no rivets, which a direct hit may drive into the interior of a tank, where they ricochet, as deadly as bullets,

2) it allows a goodly weight saving by eliminating angle-pieces and overlapping necessary for riveting plates together. Unlike welded armor, cast armor has no seams of uncertain toughness. And, unlike both riveted and all-welded armor, cast armor can have rounded corners to deflect glancing shells.

*First used in the U.S. in 1902 in making railroad wheels at the American Steel Foundries plant in East St. Louis.

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