Quick Answer
Quick Answer
Alloy steels 4130 and 4140 are chromium-molybdenum ("chromoly") grades with excellent hardenability and high-temperature strength. 4130 contains ~1.0 % Cr and 0.20 % Mo; 4140 adds more carbon and manganese for higher as-quenched hardness. Both are widely used in pressure vessels, valves, drilling equipment, and structural components requiring high strength.
Overview
Alloy steels are distinguished from carbon steels by deliberate additions of alloying elements — principally chromium, molybdenum, nickel, vanadium, or combinations thereof — that improve hardenability, high-temperature strength, toughness, or corrosion resistance beyond what is achievable with carbon alone.
The 4xxx series (AISI/SAE designation system) are chromium-molybdenum steels. The first digit "4" indicates a molybdenum base; the second "1" indicates the addition of chromium. The last two digits represent the nominal carbon content in hundredths of a percent:
- 4130: ~0.30 % C
- 4140: ~0.40 % C
These grades can be hardened and tempered to a wide range of strength levels, making them among the most versatile engineering steels.
Chemical Composition — AISI 4130
Per ASTM A29 / SAE J1397 (heat analysis):
| Element | Range (wt%) |
|---|---|
| Carbon (C) | 0.28 – 0.33 |
| Manganese (Mn) | 0.40 – 0.60 |
| Silicon (Si) | 0.15 – 0.35 |
| Phosphorus (P) | ≤ 0.035 |
| Sulfur (S) | ≤ 0.040 |
| Chromium (Cr) | 0.80 – 1.10 |
| Molybdenum (Mo) | 0.15 – 0.25 |
Chemical Composition — AISI 4140
Per ASTM A29 / SAE J1397 (heat analysis):
| Element | Range (wt%) |
|---|---|
| Carbon (C) | 0.38 – 0.43 |
| Manganese (Mn) | 0.75 – 1.00 |
| Silicon (Si) | 0.15 – 0.35 |
| Phosphorus (P) | ≤ 0.035 |
| Sulfur (S) | ≤ 0.040 |
| Chromium (Cr) | 0.80 – 1.10 |
| Molybdenum (Mo) | 0.15 – 0.25 |
The main difference between 4130 and 4140 is carbon and manganese content. Higher carbon in 4140 means higher as-quenched hardness and tensile strength, but reduced toughness and weldability compared to 4130.
Mechanical Properties
Properties are highly dependent on heat treatment condition. The table below gives representative values for common conditions.
AISI 4130
| Condition | UTS | YS (0.2%) | Elongation | Hardness |
|---|---|---|---|---|
| Annealed | 560 MPa | 360 MPa | 28 % | 156 HBW |
| Normalized | 670 MPa | 435 MPa | 25 % | 197 HBW |
| Q&T (870/480 °C) | 1040 MPa | 900 MPa | 16 % | 302 HBW |
AISI 4140
| Condition | UTS | YS (0.2%) | Elongation | Hardness |
|---|---|---|---|---|
| Annealed | 655 MPa | 415 MPa | 26 % | 197 HBW |
| Normalized | 1020 MPa | 655 MPa | 18 % | 302 HBW |
| Q&T (870/480 °C) | 1090 MPa | 960 MPa | 14 % | 315 HBW |
Q&T = quenched and tempered. Tempering temperature strongly influences the strength-toughness balance. Higher tempering temperatures produce lower strength but better impact toughness.
Hardenability
The Jominy end-quench hardenability bands for 4130 and 4140 show that both grades through-harden effectively in section sizes typical of pressure equipment and valve bodies. 4140's higher carbon and manganese give marginally better hardenability at equivalent section sizes.
| Grade | Critical diameter (full hardening in oil) |
|---|---|
| 4130 | ~25–50 mm (1–2 in) |
| 4140 | ~38–75 mm (1.5–3 in) |
Standards Coverage
| Grade | Standard | Application |
|---|---|---|
| 4130 / 4140 | ASTM A29 | Bar |
| 4130 / 4140 | ASTM A519 | Seamless tube |
| 4130 | ASTM A322 | Bar, hot-rolled |
| F22 (~2.25Cr-1Mo) | ASTM A182 | Forgings for pressure service |
| P22 (~2.25Cr-1Mo) | ASTM A335 | Seamless pipe, elevated temperature |
| 42CrMo4 | EN 10083-3 | European bar, comparable to 4140 |
Note: ASTM A182 Grade F22 and A335 Grade P22 are 2.25Cr-1Mo alloy steels — a higher-alloy relative of 4130/4140 specifically qualified for elevated-temperature pressure service under ASME code.
Applications
4130 Typical Uses
- Aircraft structural tubing and airframe components
- Pressure vessels and autoclaves (moderate strength)
- Bicycle frames and motorsport roll cages (weldable, good toughness)
- Valve bodies and fittings in oil and gas wellhead equipment
4140 Typical Uses
- Drill collars, drill pipe tool joints, and BHA components (API 7-1)
- High-strength bolting and fasteners
- Heavy-duty gears, shafts, and spindles
- Mold bases and tooling
Weldability Considerations
Both grades are weldable but require attention to preheat and post-weld heat treatment:
| Grade | CE (approx.) | Minimum Preheat | PWHT Required? |
|---|---|---|---|
| 4130 (annealed) | ~0.60 | 175–230 °C | Yes (stress relief) |
| 4140 (annealed) | ~0.75 | 260–315 °C | Yes (stress relief or full Q&T) |
High carbon equivalent values mean that cold cracking risk is significant. Hydrogen-controlled (low-hydrogen) electrodes or filler metals are mandatory.
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What does '4130' mean in the AISI designation system?
In the AISI four-digit system: the first digit (4) indicates molybdenum as a primary alloying element; the second digit (1) indicates the addition of chromium; and the last two digits (30) represent approximately 0.30 % nominal carbon. So 4130 is a Cr-Mo steel with ~0.30 % C and 4140 has ~0.40 % C.
Is 4130 or 4140 better for pressure vessel applications?
4130 is generally preferred for welded pressure vessels due to its lower carbon content and better weldability. 4140's higher strength is advantageous for non-welded components such as bolting, valve stems, and machined fittings. For elevated-temperature pressure service under ASME Section VIII, 2.25Cr-1Mo (A182 F22) is typically preferred over 4130/4140 because it has established code allowable stresses and better creep resistance.
Can 4130 and 4140 be used in sour service (H₂S)?
With restrictions. NACE MR0175 / ISO 15156-2 permits low-alloy steels including 4130/4140 in sour service, subject to maximum hardness limits (typically 22 HRC / 248 HBW in the base metal and HAZ). Tempered condition with controlled hardness and heat treatment documentation is required. Over-tempered 4140 with hardness well below the maximum is commonly used for wellhead components.
What should an MTC for 4140 bar include?
A conforming 4140 MTC per ASTM A29 should include: heat number, heat analysis (all specified elements), product analysis if required by the purchase order, heat treatment condition, mechanical test results (UTS, YS, elongation, reduction of area, hardness), and certification of conformance to the applicable standard and purchase order. If sour service per NACE MR0175 is specified, hardness test results and heat treatment records must also be included.