AISI 4140 and AISI 4340 are the two workhorse Cr-Mo alloy steels specified for power-transmission shafts, with 4140 used in roughly the majority of general-purpose industrial shafting and 4340 reserved for the most demanding duty cycles [S2][S4]. Both share a 0.38–0.43% carbon range, but 4340 carries 1.65–2.00% nickel, which is the single compositional change that drives the mechanical and cost gap between the two grades [S2][S4][S5].
Procurement engineers usually arrive at the same question: is the extra 1.8% of nickel worth the premium for the part in front of them, or is 4140 a defensible, cost-effective substitute [S5]. The answer lives in three numbers: shaft diameter, peak load profile, and minimum required through-thickness hardness.
Chemistry: Where the Two Grades Actually Differ
The AISI 4140 chemistry, expressed as Fe balance with 0.80–1.10% Cr, 0.75–1.00% Mn, 0.15–0.25% Mo, and residual Ni below 0.25%, places it in the Cr-Mo "chromoly" family used widely in alloy steel stock bar [S2][S4][S5].
The AISI 4340 chemistry is identical in carbon (0.38–0.43%) and broadly similar in Cr (0.70–0.90%) and Mn (0.60–0.80%), but adds 1.65–2.00% Ni and slightly more Mo (0.20–0.30%) [S2][S4][S5]. Sulfur, phosphorus and silicon ranges overlap and are not the deciding factors between the two grades [S1].
Independent composition databases put the shared alloy content at roughly 98% of the average formula, confirming that nickel is the only meaningful alloying addition that separates the two grades [S6]. Molybdenum in both grades stays below ~0.30%, the upper limit for the 4xxx series of low-alloy steels [S3].
Hardenability: The Decision That Actually Drives the Choice
4140 is classed as a medium-hardenability steel, and a 4140 bar above roughly 100 mm diameter will harden at the surface during oil quenching but leave a softer, lower-strength core, which limits its use in large transmission shafts [S4][S5].
4340 is classed as a high-hardenability steel, and a 200 mm diameter 4340 shaft can achieve consistent hardness and tensile strength from the surface to the center on oil quenching, because nickel suppresses the pearlite transformation and pushes the hardenability curve outward [S4][S5].
Hardness is a separate concept: it is the maximum value a grade can reach at the surface under a given heat treatment, while hardenability is the depth to which that hardness penetrates. A 4140 part can be just as hard as a 4340 part at the surface, but the core of a thick 4140 section will be softer and weaker than the core of an equivalent 4340 section [S4].
Mechanical Properties: Tensile, Yield, Hardness, Toughness
In the quenched-and-tempered (QT) condition, 4140 typically delivers 850–1,000 MPa tensile strength, around 655 MPa yield strength, and 28–32 HRC, while 4340 reaches 1,000–1,200+ MPa tensile, 740–1,000 MPa yield, and 30–40+ HRC [S2]. Elongation sits at roughly 25% for 4140 and 22% for 4340, so 4340 trades a small amount of ductility for a step-change in strength and toughness [S2].
Yield strength of quenched-and-tempered 4140 commonly falls in the 655–900 MPa band, and 4340 in the 860–1,050 MPa band, with 4340 capable of going higher under specialised heat-treatment cycles [S5]. Impact strength is graded "moderate" for 4140 and "high" for 4340 in side-by-side OEM comparison tables, reflecting nickel's well-known effect on low-temperature notch toughness [S2][S5].
Fatigue strength is rated "good" on 4140 and "excellent" on 4340, the practical consequence of 4340's higher cleanliness, higher tensile ceiling, and the nickel-stabilised tempered martensite that resists crack initiation under cyclic loading [S2][S4].
Heat Treatment, Machinability, and Weldability
Both 4140 and 4340 respond to annealing, normalizing, quenching, and tempering, and a tailored cycle (normalized + tempered, or quenched + tempered) is the standard route to the properties quoted above [S4]. 4140 is described as easier to heat treat, while 4340 needs tighter control of austenitising and tempering temperatures to hit its higher strength band without distortion or retained austenite [S2].
Machinability favours 4140: 4140 is rated "good" and 4340 "moderate" to "slightly more difficult," because the higher hardness and work-hardening tendency of 4340 reduces tool life at the same cutting parameters [S2][S4]. For tight-tolerance shaft machining, this is a real cost and lead-time factor, not a marketing line.
Weldability on both grades is "fair" rather than good; preheat, post-weld stress relief, and low-hydrogen consumables are required because the carbon content sits in the hardenable range, and the 0.4% carbon level (4140 is essentially the same as 4130 in weld behaviour, see this 4130 vs 4140 vs 4340 review for context) puts both grades in the "weld with procedure" category [S2][S3][S4]. 4340 is the harder of the two to weld successfully, so welded fabrications usually default to 4140 or to 4130.
Cost and Availability
4140 raw-material and processing cost sits below 4340, and 4140 is the more widely stocked grade in service-centre bar and tube inventory [S2]. The 4340 premium reflects the nickel alloy surcharge, the tighter heat-treatment window, and the lower production volume relative to 4140 [S2][S5].
For procurement, 4340 costs significantly more than 4140 in bar form, with the price premium driven by its nickel content, which 4140 lacks [S5]. Both grades are routinely available in hot-rolled bar, cold-finished bar, and forged billet, and both are covered by the same AISI/SAE designation system that engineers already use for shaft material selection [S1][S2].
Application Split: Where Each Grade Earns Its Keep
4140 fits general-machinery shafts under roughly 100 mm diameter operating under steady torque: axles, conveyor pins, small gears, hydraulic cylinder rods, pump shafts, sprockets, and standard shaft collars where the loading is predictable and the section size is moderate [S3][S4][S5].
4340 fits the heavy end of the duty spectrum: large-diameter drive shafts, aircraft landing-gear pins, rock-crusher shafts, forging dies, die blocks, pressure vessels, and machine-tool spindles, in other words the parts where a soft core would be a fatigue-initiation site [S2][S4][S5]. For dynamic-loaded splined interfaces, 4340 is the safer pick on a shaft key or shaft coupling hub, because through-hardening keeps the keyway teeth in the high-hardness zone.
For shock-loaded, high-stress rotating equipment, the standard 4340 recommendation is "use it when the part has to take shock loading and still hold strength and toughness," as one OEM metallurgist summarises in published guidance [S4].
Selection Rule of Thumb for Shaft Designers
Default to 4140 (or 4140 QT bar) when the shaft diameter is under ~100 mm, the load is steady or mildly cyclic, machinability matters, and weld fabrication is in scope [S5]. Choose 4340 when the diameter is over ~100 mm, the loading is severely shock or fatigue-dominated, low-temperature toughness is specified, or the through-thickness hardness requirement cannot be met by 4140's hardenability curve [S4][S5].
For most general industrial shafting, 4140 in the QT condition at 28–32 HRC and 655–900 MPa yield is the spec that gives the best cost-versus-performance balance, while 4340 is reserved for the small minority of parts where the nickel-driven hardenability and toughness step-up is genuinely required [S2][S4][S5].