The SAE four-digit numbering system encodes the alloy family and approximate carbon content. For the 4xxx series: the first digit (4) indicates a molybdenum-bearing alloy; the second digit indicates the primary secondary alloying element (1 = chromium-molybdenum; 3 = nickel-chromium-molybdenum); and the last two digits give the nominal carbon content in hundredths of a percent.[1]
| Designation | What it encodes | Key alloying elements | Carbon content |
|---|---|---|---|
| 4140 | 4 = Mo alloy, 1 = Cr-Mo, 40 = 0.40% C | Cr 0.8–1.1%, Mo 0.15–0.25% | 0.38–0.43% |
| 4340 | 4 = Mo alloy, 3 = Ni-Cr-Mo, 40 = 0.40% C | Ni 1.65–2.0%, Cr 0.7–0.9%, Mo 0.2–0.3% | 0.38–0.43% |
| 4130 | 4 = Mo alloy, 1 = Cr-Mo, 30 = 0.30% C | Cr 0.8–1.1%, Mo 0.15–0.25% | 0.28–0.34% |
The most important distinction jumps out immediately: 4140 and 4130 are both chromium-molybdenum steels that differ only in carbon content. 4340 adds nickel to the mix, and that addition is responsible for 4340's dramatically higher hardenability and why it can achieve high strength in large cross-sections where 4140 cannot.
Values for normalized bar stock, the "as-received" condition from most distributors before heat treatment. Properties depend on cross-section and mill processing.[1][2]
4140 is the most commonly specified alloy steel for machined components. We machine 4140 in all conditions at C&W, including prehardened. It offers a good balance of strength, toughness, and wear resistance in the quenched and tempered condition. Prehardened 4140 (typically 28–32 HRC) is widely stocked and can be machined without additional heat treatment for many applications. When higher hardness is needed, 4140 responds well to Q&T cycles up to approximately 54 HRC.[1][2]
4340 adds nickel to the chromium-molybdenum chemistry, which gives it significantly better hardenability and toughness than 4140, especially in large cross-sections. When you need a shaft or structural component over approximately 3 inches in diameter that has to be through-hardened uniformly, 4340 is the correct choice. It is the standard for aerospace landing gear, high-strength fasteners, and critical structural members where both high strength and resistance to impact or fatigue are required.[1][2]
4130 has lower carbon content than 4140, which makes it significantly more weldable per AWS D1.1. It is the standard choice for welded structures that need to be heat treated after fabrication, including aircraft tube frames, roll cages, and welded structural components. It will not achieve the same peak hardness as 4140, but it provides good strength with much better weldability.[1][3]
The defining advantage of the 4xxx alloy steels over plain carbon steels is their response to heat treatment. Through a quench and temper cycle, properties can be adjusted across a wide range. The temper temperature is the primary control: higher tempering temperature produces lower strength but higher toughness.[2][4]
The chart below shows how properties change with tempering temperature for all three grades. Toggle between properties using the tabs. The dramatic drop in strength as temper temperature increases is the fundamental trade-off between strength and toughness.[2][4]
These are the conditions you'll see most frequently on drawings and purchase orders.[2][4]
| Condition | Hardness | UTS (typical) | Availability | Notes & Applications |
|---|---|---|---|---|
| Annealed | 4140: ≤197 HB 4340: ≤217 HB 4130: ≤156 HB | 90–110 ksi 621–758 MPa | Widely stocked | Softest condition. Best machinability. Order for parts to be rough machined, then heat treated to final condition. |
| Normalized | ~197–217 HB | 97–185 ksi 669–1276 MPa | Widely stocked | Air cooled from austenitizing temp. Common as-received condition from distributors. |
| Pre-hardened (4140 PH) | 28–34 HRC | ~130–150 ksi ~896–1034 MPa | Widely stocked (4140) | 4140 bar Q&T by the mill. Extremely convenient — significant strength without an additional heat treat step. Machines well with carbide. 4340 and 4130 PH less commonly stocked. |
| Q&T 400°F | ~54–56 HRC | ~237–280 ksi ~1634–1931 MPa | Q&T required | Near-peak hardness. Low-temper, minimum toughness. Tooling, cutting dies, wear surfaces. Caution: temper embrittlement risk — avoid 500–700°F range for 4140. |
| Q&T 1000°F | ~34–38 HRC | ~168–196 ksi ~1158–1351 MPa | Q&T required | Excellent balance of strength and toughness. Most common high-performance condition for structural applications. |
| Condition | 4140 | 4340 | 4130 |
|---|---|---|---|
| Normalized UTS | 148 ksi / 1020 MPa | 185 ksi / 1276 MPa | 97 ksi / 669 MPa |
| Q&T 400°F UTS | ~237 ksi / ~1634 MPa | ~280 ksi / ~1931 MPa | ~185 ksi / ~1276 MPa |
| Q&T 800°F UTS | ~196 ksi / ~1351 MPa | ~232 ksi / ~1600 MPa | ~160 ksi / ~1103 MPa |
| Q&T 1000°F UTS | ~168 ksi / ~1158 MPa | ~196 ksi / ~1351 MPa | ~140 ksi / ~965 MPa |
| Q&T 400°F Hardness | ~54 HRC | ~56 HRC | ~46 HRC |
| Q&T 1000°F Hardness | ~34 HRC | ~38 HRC | ~28 HRC |
| Max through-harden section | ~2–3 in (50–75 mm) | 6+ in (150+ mm) | ~1–2 in (25–50 mm) |
| Governing spec | ASTM A29 / AMS 6349 | ASTM A29 / AMS 6414 | ASTM A29 / AMS 6370 |
Our team at C&W can advise on heat treatment sequence, stock allowances for Q&T, and coordinate heat treat with certified vendors. Submit your print for a quote.
