Compare AISI 4340 and 4140 alloy steel bar to choose the right grade for high-strength, high-stress parts and learn what heat treatment and tolerance to specify.
4140 and 4340 are two of the most requested quenched-and-tempered alloy steel bars, and buyers often quote them against each other for shafts, gears, and high-stress components. The names differ by one digit, but that digit adds nickel to the chemistry and changes the toughness ceiling. Choosing the cheaper 4140 where 4340 is needed can lead to brittle failure under shock, while over-specifying 4340 adds avoidable cost. This guide explains how to match the grade to the real loading.
What the Nickel in 4340 Actually Does
4140 is a chromium-molybdenum alloy with about 0.40 percent carbon, giving it excellent strength and good hardenability in moderate sections. 4340 keeps similar carbon and adds roughly 1.8 percent nickel along with chromium and molybdenum. Nickel is what raises through-hardening depth and impact toughness, so 4340 can reach higher strength while keeping enough ductility to absorb shock. In practice, 4340 is selected where a part must combine very high tensile strength with resistance to sudden impact or fatigue.
Where 4340 Earns Its Place
4340 is the safer choice for aircraft landing gear components, heavy-duty shafts, high-stress fasteners, gears under shock load, and any part that must hold high strength in a large cross-section. Its deeper hardenability means the core of a thick bar reaches the target hardness instead of staying soft, which is exactly the failure mode that catches buyers who substituted 4140 to save cost. Where the loading is cyclic or impact-based rather than steady, the toughness margin of 4340 usually pays back the price difference.
Where 4140 Is the Better Value
For pump shafts, axles, spindles, bolts, and machinery components under steady or moderately high load, 4140 delivers excellent strength at a lower price and is far more widely stocked. In moderate sections its hardenability is sufficient, and because it is produced in large volumes, both availability and lead time are usually better than 4340. Unless the design specifically calls for high toughness in a thick section or under impact, 4140 is the pragmatic choice.
What to Specify on the RFQ
State the grade by full AISI or DIN equivalent, the heat-treat condition such as normalized, annealed, or quenched and tempered to a target hardness, and the mechanical requirements including tensile and yield strength. Because both grades are sensitive to heat treatment, request the actual hardness on the mill test certificate rather than accepting the condition name alone. For machined parts, also specify the straightness tolerance, since cold-drawn or quenched bar can carry enough bow to affect machining setup.