AISI 4140 vs 4340 Steel: Equivalent Grades Compared

4140 and 4340 are both chromium-molybdenum low-alloy steels supplied in the quenched and tempered condition, and the honest answer to “what is the 4140 steel equivalent” is that it depends on the standard your drawing calls up. In Chinese production 4140 is ordered as 42CrMo to GB/T 3077 and 4340 as 40CrNiMoA. Both map cleanly onto ASTM/AISI, EN, JIS, DIN and ISO designations — but they are not interchangeable. 4340 carries a deliberate nickel addition that 4140 does not, and that one alloying element is the whole reason the two behave differently in heavy sections.

Short answer

For a like-for-like substitution, 4140 = 42CrMo = EN 42CrMo4 = JIS SCM440, and 4340 = 40CrNiMoA = EN 40NiCrMo6 = JIS SNCM439. The two are different steels, not two names for one steel. 4140 is the general-purpose Cr-Mo grade: widely stocked, easy to machine, and the default choice for shafts, gears, bolts and tooling that are not pushed to the limit. 4340 adds nickel for deeper hardenability and better toughness through the section, which is why it is called up for heavily loaded shafts, crankshafts and aerospace parts. Pick the grade from the section size and the toughness requirement, not from the price list.

Equivalent grades across standards

Both grades sit in the alloy structural steel family (GB/T 3077), which is a different family from the structural carbon grades such as Q235, Q355 and S355JR used for buildings and general fabrication. When a buyer asks for an “equivalent”, the useful question is which property has to match — chemistry, hardenability, strength level or toughness — because an equivalent grade number does not always mean the same behaviour in a large section.

Chemical composition compared

The single meaningful difference is nickel: 40CrNiMoA (4340) specifies it and 42CrMo (4140) does not. Chemically both are medium-carbon Cr-Mo steels, so both harden and temper in the same way; the nickel in 4340 is what keeps the core tough when the section is thick. This is also why a mill will quote 4140 and 4340 at different prices — nickel-bearing heats cost more to make.

Mechanical properties after quenching and tempering

These are the specification levels the two grades are capable of after quenching and tempering, not the result of any particular heat. In practice the achievable figure depends on section size: the larger the bar or forging, the harder it is to develop the full property through the core, and 4340 holds up better as the section grows. Always state the condition on the enquiry — annealed, normalised or quenched and tempered — because a supplier cannot guess it.

Where each grade earns its place

• 42CrMo / 4140 — general machine parts, axles, bolts and studs, connecting rods, gear blanks, mould and die bases, and any application where a good strength-to-cost ratio matters more than extreme toughness
• 40CrNiMoA / 4340 — heavily loaded shafts and gears, crankshafts, high-stress fasteners, landing gear and other aerospace or defence parts, and thick sections that must stay tough to the core
• Neither — is a structural carbon steel. For frames, towers and general fabrication the Q235 / Q355 / S355JR family applies, and calling up 4140 there wastes money

How to choose between them

What to put on the enquiry

1. The grade and the standard — for example 42CrMo to GB/T 3077, or AISI 4140
2. The supply condition — annealed, normalised, or quenched and tempered, and the target hardness or strength level if one applies
3. Size and form — bar diameter or thickness, and whether black, peeled, ground or cut to length
4. Quantity and the end use, so the mill can check that the grade is right for the duty
5. Whether a mill test certificate is required, and in which format, so the paperwork is arranged before production rather than after

Related reading

• Alloy steel round bar
• Q355B vs S355JR vs Q235B
• Chinese steel grades explained
• Deformed bar vs round bar
• Quality and certification

Frequently asked questions

What is the 4140 steel equivalent?

AISI 4140 is equivalent to 42CrMo under GB/T 3077, 42CrMo4 under EN 10083, SCM440 under JIS and 42CrMo4 under ISO 683-1. These designations describe the same nominal grade, so a drawing calling up any of them can be produced to the same specification.

Is 4340 the same as 4140?

No. Both are chromium-molybdenum alloy steels, but 4340 adds nickel, which 4140 does not carry. The nickel gives 4340 deeper hardenability and better toughness in thick sections, so it suits heavier duty and larger sections while 4140 remains the general-purpose, lower-cost choice.

Can 4140 replace 4340?

Only if the section is modest and the toughness requirement is not severe. In a large section, or under high fatigue or impact loading, 4140 will not develop the same core properties, and the substitution should be agreed against the drawing rather than assumed from the grade number.

What is 42CrMo steel?

42CrMo is the Chinese designation for the medium-carbon chromium-molybdenum alloy steel that matches AISI 4140. It is produced to GB/T 3077 and supplied in the quenched and tempered condition for shafts, gears, fasteners and general machine components.

What is 40CrNiMoA?

40CrNiMoA is the Chinese nickel-bearing chromium-molybdenum alloy steel that matches AISI 4340. It is produced to GB/T 3077 and is specified where maximum toughness and deep hardenability are required, such as heavily loaded shafts and crankshafts.

Do the mechanical values apply to any size?

No. The values are the levels the grade can reach after quenching and tempering; what a given bar actually achieves depends on its section size and on the heat treatment applied. The size, the condition and the required property level all belong on the enquiry.

Send the grade, size, quantity and destination port you need and we will come back with a worked offer. See the steel product catalogue, how we inspect and certify each order, or request a quotation.