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「鋼鐵是怎麼煉成的?」- How It's Made: Steel Forgings


When does forging not involve fraud? When you're talking about metal.

Many industrial machines contain steel parts that have to withstand great stress. The metal working method that produces the strongest steel parts is forging: heating the metal, then forming it to the required size and shape.

Steel forgings are made from scrap iron, pieces of used iron recovered from demolished buildings and old cars, as well as bits of new iron leftover from the manufacturer of iron products. Forge workers maneuver a gigantic magnet to lift almost six tons of scrap at a time until they fill the twenty-eight-ton scrap bucket. The bucket empties two such loads into a fiery furnace, whose temperature peaks at three thousand degrees Fahrenheit. This is called an electric arc furnace, because the heat is created by a strong electric current running in an arc between three electrodes.

There are about twenty main grades and one hundred sub-grades of steel. They are produced by adding specific metals or non-metallic chemical elements, such as aluminum, nickel, chromium, vanadium, manganese, carbon, and molybdenum. They also add the mineral fluorite to help fuse the metals, some of which are in the form of blocks called ingots.

Throughout the three and a half hour meltdown, they test and adjust the chemical composition. They try to retain the lowest hydrogen, oxygen, nitrogen and sulfur levels: the key to producing strong, high quality steel. Then they pour the sizzling molten metal into a fifty-ton ladle that's lined with heat-resistant brick. They add aluminum in line to chemically counteract oxidation, rusting caused by air exposure.

Now they cast an ingot, a block of steel that fills shape into a forging. The molten metal flows out the bottom of the ladle into a mold below. This bottom pouring, as it's called, makes for a smoother casting because liquid flowing downward doesn't swirl around as much as liquid poured sideways. This forge can cast ingots weighing up to forty-six tons.

After several hours, the ingot solidifies. And they turn the mold upside down to extract it. Then over twelve hours, they reheat the ingot to twenty-two hundred degrees Fahrenheit. This makes it soft enough to hammer or press into shape. To make large forgings, they use a machine called the forging press, mounted inside a forming die or a pair of dies depending on the technique they're using. The press applies thousands of tons of pressure, pressing the ingot and forcing it against the die. The steel is quickly oxidizing with all this air exposure. They use a high-pressure water gun to remove the scales of rust.

An ingot often passes through a die several times or through a series of dies arranged in sequence. Each pressing forms the metal bit by bit into the final shape.

Metal is composed of microscopic crystals. Squeezing it in the press bends these crystals, destabilizing the metal structure. But reheating the metal creates new crystals to replace the deformed ones. This restablizes the structure, and because the new crystals are smaller, makes the metal stronger than before.

That's why finished forgings require heat treatment. It was called an annealing furnace. They heat for a day or two depending on the grade of steel. Then they soak in a water and chemical bath for about five hours. This strengthens the steel even more.

Finally, workers mount the steel forgings on lace and milling machines. They remove the rust scales that formed during heat treatment, then precision-machine the forgings into what the client ordered, usually industrial parts, such as rotors, spindles and shafts. Many factories prefer to buy forgings in the shape of blocks or bars, and machine the parts themselves.





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