When discussing AISI 4140, it is important to understand what the grade number means:

AISI 4140 is made by placing iron, carbon, and other alloying elements into an electric furnace or oxygen furnace. The major alloying elements added to AISI 4140 are:

• Chromium
• Manganese
• Molybdenum

Once the iron, carbon, and other alloying elements have been mixed together in liquid form, it is allowed to cool. The steel may then be annealed; possibly several times.

After the annealing is complete, the steel is heated to a molten phase again so that it can be poured into the desired form and can either be hot worked or cold worked through rollers or other tools to reach the desired thickness. Of course, there are other special operations that can be added to this to reduce mill scale or improve mechanical properties.

AISI 4140 is a low alloy steel. Low alloy steels rely on elements other than just iron and carbon to enhance their mechanical properties. In AISI 4140, additions of chromium, molybdenum, and manganese are used to increase the strength and hardenability of the steel. The additions of chromium and molybdenum are why AISI 4140 is considered a “chromoly” steel.

There are several important mechanical properties of AISI 4140, including:

• Tensile Strength: AISI 4140 steel typically has a target ultimate tensile strength of around 95,000 psi.
• Toughness
• Ductility
• Hardenability

The table below highlights the chemical composition of AISI 4140:

The addition of chromium and molybdenum promotes corrosion resistance. The molybdenum can be particularly useful when trying to resist corrosion due to chlorides. The manganese in AISI 4140 is used to increase hardenability and as a deoxidizer. In alloy steels, manganese can also combine with sulfur to improve machinability and make the carburizing process more effective.