4140 Alloy Steel: Uses, Composition, Properties
4140 alloy steel is a highly versatile and popular low-alloy steel that is used across many industries. This steel is alloyed with chromium and molybdenum to give it specific properties. 4140 alloy steel is very tough, with particularly high torsional and fatigue strength. This makes it great for rotating components that experience significant stress, such as axles and shafts. 4140 steel is available in a variety of different forms but is most popular as bars.
Like all steels, 4140 alloy steel is composed primarily of iron. It is alloyed with carbon, chromium, molybdenum, and other minor components. It is therefore considered a “chromoly” steel. This alloy is tough and resists both wear and corrosion.
4140 alloy steel is typically used for rotating components that are placed under high stress such as axles, drive shafts, crankshafts, spindles, and gears. It is also used for static components that operate under high load such as bolts and chain links. The 4140 alloy excels in these situations thanks to its high tensile strength, toughness, and torsional strength.
4140 alloy steel is made using either an electric furnace or a basic oxygen furnace. The base alloy components are added and melted together at temperatures exceeding 1500 °C. The molten steel is then tapped off from the furnace and allowed to cool. It gets annealed through controlled heating and cooling. After annealing, the mill scale is removed by dipping the metal in hydrofluoric acid. Next, the steel is melted once more and cast into billets or slabs. These are then hot-worked or cold-worked — usually using rollers — to the desired finished dimensions.
The full composition of 4140 alloy steel is given in Table 1 below:
0.38 - 0.43
0.80 - 1.10
0.15 - 0.25
0.75 - 1.00
0.15 - 0.30
Carbon makes up 0.38-0.43% of 4140 steel’s composition. 4140 alloy steel can be considered medium-carbon steel since low-carbon steels generally have a carbon content of less than 0.3%.
4140 alloy steel is known for its strength and toughness. Some key properties are given in Table 2 below:
Yield Strength [MPa]
The machinability of 4140 alloy steel is approximately 65% in the annealed condition. If lead is added to the alloy (41L40), it will become more machinable but lead also makes it more difficult to weld and less ductile at high temperatures.
4140 alloy steel has a thermal conductivity of 42.6 W/m•K. This falls into the average range for steels but is low compared to most metals which are significantly more conductive. Its thermal coefficient of expansion is 12.2 µm/m•°C, which is also very typical of steels.
4140 alloy steel is available in multiple different forms, each with its own purpose:
4140 alloy steel is regularly cold rolled to create sheets. Sheets 1 to 10 mm thick are commonly available.
4140 alloy steel is manufactured in bar forms with flat, square, and round cross-sections. Due to their popularity and wide application, square and round bars are typically available in dimensions from 6 mm up to 120 mm.
4140 alloy steel is available in cold-rolled plate form. The most common plate thicknesses are in the range of 20-120 mm, although plates as thick as 300 mm are not unusual. Different finishes are possible, including annealed.
It is also possible to buy 4140 alloy steel as hot-rolled bars — flat, round, hex and square bars undergo this treatment. Hot-rolled bars have a different surface finish and tend to have lower strength than cold-rolled forms.
It is most common to purchase 4140 alloy steel in the annealed state. This is usually because the steel has been annealed to allow cold forming into bars or plates. Annealing improves the ductility of the steel in preparation for this cold forming.
4140 alloy steel can be cold drawn into flat, round, and square bars. Generally, the steel is treated to form the softer pearlite microstructure before cold drawing, as that improves the material’s ductility.
The composition of 4140 alloy steel goes by different names in certain parts of the world, depending on local standards. Table 3 below lists some of the common national standards and their equivalent 4140 alloy steel grade:
China GB/T 3077
Germany DIN 17200
Great Britain BS 970
Japan JIS G4105
USA ASTM A29
There are several advantages to using 4140 alloy steel:
- It has a very good fatigue strength, as well as a high tensile strength and high torsional strength.
- It is wear-resistant.
- It can be easily hardened by heat treatment.
- The chromium and molybdenum constituents give it very good corrosion resistance.
The clearest disadvantage of 4140 alloy steel is that it is difficult to weld. Its weldability depends on its thermal treatment, with welding only recommended on annealed material. The metal must be preheated before welding. Afterward, it usually needs to be slow-cooled and heat-treated again.
The difference between 4140 alloy steel and 8620 steel is that the latter has a slightly lower carbon content and also contains nickel. The proportions of other alloying elements such as chromium and molybdenum are similar. 8620 steel is ideal for case hardening by carburization.
The difference between 4140 alloy steel and 1045 steel lies in the alloying elements. 1045 steel is medium-carbon steel whereas 4140 alloy steel is a chrome-molybdenum alloy. Both have similar tensile strengths and are popular for industrial applications. However, 1045 is less resistant to wear and corrosion compared to 4140 alloy steel.
This article presented 4140 alloy steel, explained it, and discussed its various applications and its composition. To learn more about 4140 steel, contact a Xometry representative.
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