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发表于 2019-6-6 14:27:18
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本帖最后由 茉莉素馨 于 2019-6-6 14:31 编辑
300M Steel
Alloy 300M is basically a silicon-modified (1.6% Si) 4340 steel, but it has slightly higher carbon and molybdenum
contents and also contains vanadium. This steel exhibits deep hardenability and has ductility and toughness at tensile
strengths of 1860 to 2070 MPa (270 to 300 ksi). Many of the properties of this steel are similar to those of 4340 steel,
except that the increased silicon content provides deeper hardenability, increased solid-solution strengthening, and better
resistance to softening at elevated temperatures. Compared to 4340 of similar strength, 300M can be tempered at a higher
temperature, which provides greater relief of quenching stresses. The so-called 260 °C (500 °F) embrittlement is
displaced to higher temperatures. Because of the high silicon and molybdenum contents, 300M is particularly prone to
decarburization. During thermal processing, care should be exercised to avoid decarburization, or the decarburized layer
should be removed after processing. When heat treated to strength levels higher than 1380 MPa (200 ksi), 300M is
susceptible to hydrogen embrittlement. If the steel is properly baked after plating, the resulting improvement in properties
is better than that for 4340 or D-6ac steel of equal strength.
The steel 300M is forged at 1065 to 1095 °C (1950 to 2000 °F). Forging should not be continued below 925 °C (1700 °F).
After forging, it is preferred that parts be slowly cooled in a furnace, but they may be allowed to cool in air in a dry place.
Although 300M can be readily gas or arc welded, welding is generally not recommended; welding rod of the same
composition should be used. Because 300M is an air-hardening steel, parts should be either annealed or normalized and
tempered after welding. The machinability rating of annealed 300M is about 45% (B1112, 100%). A partially
spheroidized structure, obtained by normalizing and then tempering at 650 to 675 °C (1200 to 1250 °F), gives optimum
machinability.
Typical applications of 300M, which is available as bar, sheet, plate, wire, tubing, forgings, and castings, are aircraft
landing gear, airframe parts, fasteners, and pressure vessels. |
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