4140 steel is a high-alloy tool steel designed to have good strength and toughness characteristics, such as limited corrosion resistance. It is typically available in two forms; the annealed and most common form is designated as simply 4140, while the prehardened form is heat-treated at the foundry and designated as 4140 HT.
Chemically, 4140 HT steel is relatively simple compared to other tool steels. It contains .42 percent carbon, .30 percent silicon, .20 percent molybdenum, 1.00 percent manganese and 1.00 percent chromium by mass. It can also contain the impurities sulfur and phosphorous in trace amounts -- up to .025 percent.
4140 HT steel is prehardened during manufacture to a medium hardness of 28-32 HRC. The steel has a modulus of elasticity of 200 GPa and a specific gravity of 7.81. It also has a density of .282 pounds per cubic inch, or 7,810 kilograms per cubic meter.
Since 4140 HT is prehardened, additional heat treatment is often avoided. However, 4140 HT can be annealed at 50-60 degrees Fahrenheit to return the steel back to its softest state. It can then be hardened by preheating the steel at 1,250-1,300 degrees Fahrenheit followed by heating the steel to 1,550-1,600 degrees and holding it at that temperature for up to 30 minutes. Once heated, the steel should be quenched in oil and immediately tempered.
Since 4140 HT is prehardened, it has specific applications in which the added expense of prehardening is offset by increased performance. 4140 HT is often used in the manufacturing of punch holders, clamps, gauges, jigs, molds and dies.
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