By Tony Oldhand | Updated Mar 24, 2022
When 4140 steel exits the furnace it is a soft metal. Heating it to near melting and then rapidly cooling—typically by quenching in cold water—followed by tempering, transforms it into a very hard alloy. Because its carbon content (0.38–0.43 %) is intermediate, 4140 strikes a balance between the softness of low‑carbon steels and the hardness of tool steels. Experts at Interalloy Materials Company note that 4140 offers excellent hardening performance for many engineering applications.
Prior to hardening, 4140 is easy to cut: a simple hacksaw blade can shape it. It also forgings well—heat it close to melting and hammer it to densify the material. After hardening, 4140 remains machinable in sawing, milling, and grinding operations. Fox Valley Technical College instructors recommend milling speeds of 60–100 ft/min (fpm) with standard tool‑steel bits and 275–450 fpm when using very hard carbide tools. Using a cooling fluid on the cutting tools greatly improves surface finish and tool life.
Both Speedy Metals Supply Company and Interalloy warn that 4140 does not weld well. Its composition tends to develop stress cracks in the weld zone. Consequently, designers typically avoid welding 4140 or pair it with a different alloy in welded assemblies. If welding is unavoidable, Interalloy recommends performing the weld before hardening to reduce the likelihood of cracking.
4140 steel is available in round bar, square bar, flat stock, and hollow tube, in a wide range of sizes and quantities. Its versatility makes it a popular choice for gears, bearings, machine shafts, rollers, and bolts. Because it is inexpensive and widely available, 4140 is a cost‑effective solution for prototyping and production alike.
