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The high temperature of annealing may result in oxidation of the metal's surface, resulting in scale. If scale must be avoided, annealing is carried out in a special atmosphere, such as with endothermic gas (a mixture of carbon monoxide, hydrogen gas, and nitrogen gas). Annealing is also done in forming gas, a mixture of hydrogen and nitrogen.
This produces steel that is much stronger than full-annealed steel, and much tougher than tempered quenched steel. However, added toughness is sometimes needed at a reduction in strength. Tempering provides a way to carefully decrease the hardness of the steel, thereby increasing the toughness to a more desirable point.
However, quenching certain steel too fast can result in cracking, which is why high-tensile steels such as AISI 4140 should be quenched in oil, tool steels such as ISO 1.2767 or H13 hot work tool steel should be quenched in forced air, and low alloy or medium-tensile steels such as XK1320 or AISI 1040 should be quenched in brine.
The earliest examples of quenched steel may come from ancient Mesopotamia, with a relatively secure example of a fourth-century BC quench-hardened chisel from Al Mina in Turkey. [6] Book 9, lines 389-94 of Homer's Odyssey is widely cited as an early, possibly the first, written reference to quenching: [ 3 ] [ 7 ]
This is the most common state for finished articles such as tools and machine parts. In contrast, the same steel composition in annealed state is softer, as required for forming and machining. Depending on the temperature and composition of the steel, it can be hardened or softened. To make steel harder, it must be heated to very high temperatures.
While pipe sizes in Australia are inch-based, they are classified by outside rather than inside diameter (e.g., a nominal 3 ⁄ 4 inch copper pipe in Australia has measured diameters of 0.750 inches outside and 0.638 inches inside, whereas a nominal 3 ⁄ 4 inch copper pipe in the U.S. and Canada has measured diameters of 0.875 inch outside and ...
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