4140 vs 4340 Steel Hardening: Differences & HRC

If you specify shafts, gears, axles or crankshafts in Ludhiana or Punjab, choosing between 4140 and 4340 is one of the most common decisions in heat treatment. Both are chromium-molybdenum-based alloy steels that respond beautifully to hardening, but a small chemistry difference changes how deep they harden, how tough they stay, and how much they cost. This guide compares the two head to head.

Quick comparison table

Here is an at-a-glance summary of 4140 vs 4340 steel hardening across composition, hardenability, achievable HRC, cost, typical uses and EN equivalents.

The headline takeaway: 4140 and 4340 share almost the same carbon level, so their peak surface hardness is similar. What separates them is nickel, which transforms 4340 into a deeper-hardening, tougher grade that commands a higher price.

Composition comparison

Both grades are medium-carbon low-alloy steels with around 0.40% carbon. That carbon content is what makes either steel capable of reaching high hardness, because carbon is the element that forms hard martensite during quenching. Below that shared foundation, the alloying tells two different stories.

4140 — chromium-molybdenum steel

4140 is a clean chromium-molybdenum (Cr-Mo) alloy. Roughly 0.80–1.10% chromium and 0.15–0.25% molybdenum give it good hardenability, strength and resistance to softening when tempered. With no significant nickel, its chemistry is lean and its cost is lower. This is the workhorse grade for shafts, gears, spindles and pins of small to medium section, and it responds extremely well to localized induction hardening.

4340 — nickel-chromium-molybdenum steel

4340 adds about 1.65–2.0% nickel to a similar chromium-molybdenum base, making it a nickel-chromium- molybdenum (Ni-Cr-Mo) steel. Nickel does two things: it deepens hardenability so the part hardens fully through thick sections, and it raises toughness and impact resistance at a given strength. That makes 4340 the premium choice for large, heavily stressed components — but the nickel also pushes the price up.

In short, you can think of 4340 as 4140 with nickel added. The carbon does the hardening; the nickel does the deep, tough hardening. It is worth noting that both grades are normally supplied in the hardened and tempered (Q&T) condition for machining, and then selectively induction hardened at the wear surfaces. That two-stage approach gives a strong, tough core with a very hard skin, and it works equally well on either grade — only the achievable depth and the price differ.

Hardenability & HRC

Hardenability is not the same as hardness. Hardness is how hard the surface gets; hardenability is how deep that hardness reaches into the cross-section before the cooling rate falls too low to form martensite. This distinction is the heart of the 4140 vs 4340 decision.

Achievable hardness (HRC)

On the surface, both grades land in a similar range. After induction hardening and a low temper, 4140 typically reaches about 54–58 HRC, and 4340 about 54–60 HRC. For a thin part or a shallow hardened case, you would struggle to tell them apart by surface HRC alone. Our 4140 steel hardening service routinely delivers these values with controlled quenching.

Depth of hardening

The difference appears as parts get bigger. Because 4340 has higher hardenability, it keeps high hardness much deeper into a large bar, so a thick 4340 axle can be hardened nearly through-and-through. A comparable 4140 part of the same large diameter will harden well at the surface but soften toward the core, where the quench could not cool it fast enough. For small and medium sections, 4140 hardens fully and the extra hardenability of 4340 is simply unnecessary.

This is why ruling section matters more than the grade name. A 25 mm 4140 shaft and a 25 mm 4340 shaft will behave almost identically after hardening, so paying for nickel buys you nothing. Push the diameter to 100 mm or more, add an interrupted oil quench, and the 4340 will hold its strength and toughness right through the centre while the 4140 cannot. Matching the grade to the section size is the single most cost-effective decision you can make when specifying either steel.

Both grades pair well with induction hardening, where the surface is heated and quenched to form a hard martensitic case over a tough core. If you are weighing surface versus through approaches, our overview of induction hardening vs case hardening explains how the case is produced and controlled.

4140 = EN19, 4340 = EN24 (equivalents)

Indian and European drawings often call out EN grades rather than the AISI/SAE numbers, so knowing the cross-reference avoids costly material mix-ups on the shop floor.

• 4140 is the close equivalent of EN19 (BS 970 709M40) and the European 42CrMo4.
• 4340 is the close equivalent of EN24 (BS 970 817M40) and the European 40NiCrMo family.

The chemistries are nearly identical within each pair, so a 4140 callout can normally be satisfied with EN19 stock, and a 4340 callout with EN24. We treat both naming systems every day — see our dedicated EN19 steel induction hardening and EN24 steel hardening pages for grade-specific parameters and hardness ranges.

For authoritative chemistry limits and heat-treating data on these alloy steels, the ASM International handbooks are the recognised reference.

When to choose which

The right grade depends on the size of the part, the loads it carries, and your budget. Because their surface hardness is similar, the decision usually comes down to cross-section and toughness, not peak HRC.

Choose 4140 (EN19) if:

• The part is a small or medium section — shafts, gears, spindles, pins.
• You want strong, wear-resistant components at a lower material cost.
• Surface or localized induction hardening meets the duty requirement.

Choose 4340 (EN24) if:

• The part is large or thick and must harden deep into the core.
• It carries shock or fatigue loads where extra toughness matters.
• You are making heavy axles, large crankshafts, big shafts or rams.

In practice, many Ludhiana and Punjab manufacturers standardise on 4140/EN19 for the bulk of their shafts and gears, then step up to 4340/EN24 only where section size or load demands it. That keeps cost in check while reserving the premium nickel grade for the parts that truly need deep, tough hardening. If you are unsure which grade your drawing calls for, send it over and we will confirm the equivalent and recommend a hardening route.

Frequently asked questions