4130 vs 4140 steel
Both are chromium-molybdenum steels with the same chromium and molybdenum. The difference is carbon and manganese, and it decides how deep each hardens and how easily each welds.
4130 has 0.28 to 0.33% carbon and 0.40 to 0.60% manganese; 4140 has 0.38 to 0.43% carbon and 0.75 to 1.00% manganese, with the same chromium and molybdenum. 4140 therefore hardens deeper and reaches higher strength, while 4130 is easier to weld. That is why welded high-pressure lines and fittings are usually 4130, and thick forged bodies that must be strong at the center are often 4140.
Chemistry side by side
| Element | 4130 | 4140 |
|---|---|---|
| Carbon | 0.28 to 0.33 | 0.38 to 0.43 |
| Manganese | 0.40 to 0.60 | 0.75 to 1.00 |
| Phosphorus, max | 0.035 | 0.035 |
| Sulfur, max | 0.040 | 0.040 |
| Silicon | 0.15 to 0.35 | 0.15 to 0.35 |
| Chromium | 0.80 to 1.10 | 0.80 to 1.10 |
| Molybdenum | 0.15 to 0.25 | 0.15 to 0.25 |
Hardenability and section size
Carbon and manganese both raise hardenability, the depth to which a section hardens when quenched. With more of each, 4140 reaches full hardness deeper into a thick wall. 4130 hardens moderate sections through, which covers most pipe, fittings and flowline iron, but in very thick bodies its center ends up softer than its surface. For a heavy forged body that must meet a high strength at mid-wall, 4140, or a modified 4130 with more alloy, is the usual answer.
Strength and hardness
Quenched and tempered, both steels cover the common oilfield levels. 4140 can be taken to higher hardness and strength than 4130. At equal hardness the difference narrows: at 22 HRC, the sour-service limit in NACE MR0175 / ISO 15156-2 for both, they give similar strength, so a sour-service part gains little from 4140 and gives up weldability.
Weldability
The carbon equivalent is a quick index of how prone a steel is to hardening and cracking beside a weld. Using the IIW formula and mid-range chemistry, 4130 comes out at about 0.62 and 4140 at about 0.78. Both need a qualified procedure with preheat and, on pressure parts, post-weld heat treatment, but 4140 needs more of both and is less forgiving. For a line that is welded at several joints, that difference matters.
| Steel | C | Mn | Cr + Mo | CE (IIW) |
|---|---|---|---|---|
| 4130 | 0.305 | 0.50 | 1.15 | 0.62 |
| 4140 | 0.405 | 0.875 | 1.15 | 0.78 |
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, with V, Ni and Cu taken as zero. Actual heats vary; the welding procedure uses the certified chemistry.
Which to choose
| Part | Usual choice | Why |
|---|---|---|
| Seamless pipe for choke, kill, mud and cement lines | 4130 | Welded in place; moderate walls |
| Butt-weld elbows, tees, reducers and long sweeps | 4130 | Welded to 4130 pipe; same weld procedure |
| Integral fittings and flowline iron | 4130 | Moderate sections, sour service at 22 HRC |
| Thick forged bodies, stems and bolting | 4140 or modified grades | Strength needed deep in the section |
Our pipe and fittings are made in 4130: see 4130 fittings and integral fittings.
Frequently asked questions
Is 4140 stronger than 4130?
It can reach higher strength and hardness, and holds its strength deeper into thick sections, because it has more carbon and manganese. At the same hardness, for example 22 HRC for sour service, the two give similar strength.
Can 4130 be welded to 4140?
Yes, with a qualified procedure. The preheat and post-weld heat treatment are set for the less weldable 4140 side, and for sour service the weld and both heat-affected zones must meet the ISO 15156-2 hardness limits.
Why are frac and choke line fittings usually 4130?
They are heavy-walled but welded or machined from moderate sections, so 4130 hardens through them while staying easier to weld and to keep at 22 HRC for sour service. 4140 is chosen where a thick body must reach higher strength at its center.
Sources
- ASTM A29 / SAE J404, chemical compositions of SAE alloy steels
- International Institute of Welding carbon equivalent formula
- NACE MR0175 / ISO 15156-2:2020, A.2.1.2