American Iron and Steel Institute 4140 is a member of the low alloy tool steel family characterised by small quantities of trace elements added to an otherwise simple steel. These trace elements, when compared with more simple steel such as 1040, improves the characteristics of 4140 . However, because 4140 remains relatively simple, it is used in a wider range of applications than more specialised tool steels like the D or S series of steel.
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In terms of chemistry, AISI 4140 contains measurable amounts of carbon, Molybdenum, silicon, phosphorous, sulphur, chromium and manganese. Each of these elements are added for specific properties and in specific amounts. Carbon of between .38 and .43 per cent is essential to turn iron into steel and make the steel able to be heat treated. Chromium (.8 to 1.1 per cent), magnanese (.75 to 1 per cent), silicon (.15 to .35 per cent) and molybdenum (.15 to .25 per cent) give the steel added toughness and torsion strength. Phosphorous (less than.035 per cent), sulphur (less than .04 per cent) are present in all steel and are considered to be impurities.
AISI 4140 has enough carbon present to allow for adequate heat treatment. Recommended hardening procedures are to bring the steel up to 913 degrees Celsius for normalisation (removal of stress within the steel). After it cools properly, heat the steel to 843 degrees Celsius before quenching it in oil. The toughness of the steel makes machining in its hardened form difficult. However, after annealing the steel at 871 degrees C followed by a slow cooling, the steel is much softer and far easier to machine. It can then be heat-treated after machining to produce a hard, precise item.
Since AISI 4140 is a moderately simple steel with a high degree of toughness and tensile strength, it is useful for machined applications such as gears, bolts and fasteners. The lower cost due to simple chemistry coupled with the physical properties also makes it a good choice for axles, shafts and structural components. However, since the steel has a lower carbon content than other steels, it doesn't harden to the same degree. This makes hard, sharp edges, such as those found in knives, difficult.
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