Iron ore, found lying around near the surface of the earth, can be heated and primitively refined (a cave man could do it) to become iron. Iron is soft and pliable, great for lots of things and better than a poke in the eye, but begging for improvement. With minimal additional elements and a bit more refining “mild steel” made its debut. Mild steel is the leanest form of carbon steel. It is generally considered non-hardenable and is fairly pliable; stranger than iron and still fairly ductile. From that point you can add and subtract elements (manganese, chromium, nickel, moly, etc.) add heat and then cool and you can pretty much tweak it to be particularly well suited for an infinite variety of specific uses (carbon, alloy, stainless, and tool steel). You could make steel that holds a sharp edge; slices and dices, and could even cut someone’s shoe in half. You can make steel that gets very hard but is also very brittle, steel that gets very hard on the surface but stays softer at the core, and steel that gets very strong and will stand up to impact, shock and gouging. Fortunately for us in the present day, most any type of carbon or alloy steel you would want already has a recipe and a full set of instructions that have been worked out since people have been throwing rocks.
For most of us in the industry who will encounter grades of steel in our work-a-day-lives, we can generally categorize them into two separate camps: steel that is made into parts sold commercially to the general public and manufacturing companies, “production-steel”. And, steel that requires special properties to fix the machines that make those parts, “maintenance-steel”. When working with steel to make things, the steel used for the tooling takes quite a beating. It may have to stand up to bending, or breaking, or abrasive wear. It may be subject to twist, or corrosion.
4140 is pretty much the “Defacto-Standard” for a good all-around general-purpose maintenance steel for replacement parts. It has been designed to possess high strength (sometimes referred to as ultra-high strength), along with noticeable resistance to gouging, bending, fatigue, wear, and to an extent even corrosion. 4140 may be supplied in several hardness ranges to deliver several levels of strength. In most cases it is considered to be fairly machinable and weldable. You can forge it and bend it with caution.
It is considered to be a “through hardening steel”, but it will also accept surface hardening to maintain a ductile core. While it may on occasion be used in production manufacturing, as the steel that becomes the parts being made, it is generally not used where speed of machining is the requirement. With 4140, you will give up a little bit of machining time to gain strength and toughness.
4140 is a great basic chemistry that is easily modified at the steel mill by use of additional elements to achieve additional, and/or, specific properties for specific industries or jobs. For instance, if you need high hardness that is deeper and more uniform, you could add Nickel to the 4140. The subsequent grade would be an alloy called 4340. Want to increase resistance to softening in higher service temperatures, like a bracket for an exhaust pipe, add Vanadium and increase the carbon a bit; there was a grade for that, 6150. Want easier welding, forging, and maybe a hardened chisel edge but soft core, lower the carbon a bit, and add a bit of Tungsten, and you have 4135 non-tempering steel. And so on. Need to make a high strength shaft with a high surface hardness for wear resistance but a very tough core, increase the carbon a bit. Remember, a rough calculation for potential surface hardness is ten points over carbon content i.e. 4150, potential surface, flame or induction hardness 60RC.
So, when do you use 4140 alloy steel?
Pretty much anywhere. It is a great all around general maintenance alloy chrome, moly, manganese steel. A great platform chemistry for many other application specific steel grades. It is fairly resilient in thermal treatment and may be quenched in water, oil, or other mediums. It is readily machinable and weldable (standard low-hydrogen process), forgeable, and with limitations; formable. It is suitable for many various hardening methods including; furnace, flame, induction, carburization, and cryogenic treatment. There are improvements to the basic grade, but, keep in mind that as steel grades get pushed toward higher and higher levels of performance, they become more and more temperamental.