4130 vs 4140 Steel
4130 and 4140 are both chromium-molybdenum steels with the same alloying; the difference is carbon, 0.30% against 0.40%. That 0.10% is what separates the weldable roll-cage and aircraft tubing alloy from the pre-hardened shaft and gear alloy. 4130 welds with modest preheat and normalizes to a tough, moderately strong structure. 4140 hardens much higher but needs preheat, interpass control and post-weld tempering to weld without cracking.
Buy 4130 as tube, sheet and plate for welded structures and thin-wall parts. Buy 4140 as bar and plate, usually pre-hardened to 28-32 HRC, for shafts, gears, fixtures and anything that is machined rather than welded.
| Property | 4130 | 4140 |
|---|---|---|
| Type | Cr-Mo alloy steel | Cr-Mo alloy steel |
| Carbon | 0.28-0.33% | 0.38-0.43% |
| Cr / Mo | 0.8-1.1% / 0.15-0.25% | 0.8-1.1% / 0.15-0.25% |
| Yield (normalized) | ~435 MPa (63 ksi) | ~655 MPa (95 ksi) |
| Tensile (normalized) | ~670 MPa (97 ksi) | ~1020 MPa (148 ksi) |
| Max practical Q&T hardness | ~40-45 HRC | ~50-55 HRC |
| Density | 7.85 g/cm³ | 7.85 g/cm³ |
| Weldability | Good; the weldable chromoly | Fair; preheat and post-heat required |
| Machinability (annealed) | ~70% | ~65% |
| Product forms | Tube, sheet, plate, some bar | Bar, plate, forgings |
| Typical uses | Roll cages, aircraft tubing, bicycle frames, weldments | Shafts, gears, bolts, tooling, hydraulic parts |
Typical values for the stated conditions. Confirm against the mill certificate and your spec.
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Decision matrix
A check marks the side with the edge on that dimension.
| Dimension | 4130 | 4140 |
|---|---|---|
| Machining | Good; similar to 4140 annealed, softer | Good; widely machined pre-hard at 28-32 HRC |
| Welding | ✓ Good; TIG with ER70S-2 or ER80S-D2, light preheat on thick sections | Fair; preheat 150-300 °C, post-weld temper, crack risk |
| Forming | ✓ Good; tube bends and sheet forms annealed or normalized | Fair; annealed only |
| Strength | Up to ~1200 MPa tensile Q&T | ✓ Up to ~1500 MPa tensile Q&T, deeper hardening |
| Corrosion resistance | Poor; rusts | Poor; rusts |
| Finishing | Paints, powder coats, plates | Nitrides, blackens, plates |
| Material cost | Similar per pound; tube commands a premium | Similar per pound; pre-hard adds cost |
| Availability | Tube and sheet everywhere; bar less common | Bar and plate everywhere; tube rare |
Choose 4130 when
- Welded tubular structures: roll cages, chassis, aircraft fuselage, bicycle frames
- Sheet metal and thin-wall parts in chromoly
- A part that must be welded after machining without a furnace cycle
- Toughness and fatigue in the normalized condition are enough
Choose 4140 when
- Shafts, gears, spindles, pins and bolts machined from bar
- Hardness above about 40 HRC or deep hardening in thick sections
- Pre-hardened bar machined to size
- Tooling, dies and fixtures that see wear
Frequently asked questions
- What is the difference between 4130 and 4140?
- Carbon content: about 0.30% in 4130 and 0.40% in 4140, with the same chromium and molybdenum. The extra carbon lets 4140 harden higher and deeper, and makes it harder to weld. 4130 is the weldable, tubing grade; 4140 is the bar and shaft grade.
- Is 4140 stronger than 4130?
- Yes, in any equivalent condition. Normalized 4140 is about 655 MPa yield against about 435 MPa for 4130, and quenched and tempered 4140 reaches higher strength and hardness. 4130 is chosen for weldability and toughness, not peak strength.
- Can 4130 be welded without heat treatment?
- Yes, in thin sections, which is why it is the roll-cage and aircraft tubing steel. TIG with a mild-steel or low-alloy filler, clean joints and light preheat on anything over about 3 mm are standard. Heavy sections and critical structures are normalized after welding.
- Can I substitute 4140 for 4130?
- For machined parts, usually yes and it is stronger. For welded tubing, no: 4140 welds are far more crack-prone and the tube is hard to source. Match the product form to the grade.
