4130 Steel
Also: AISI 4130, SAE 4130, UNS G41300, chromoly, chromoly tube, chrome-moly, Cr-Mo steel, 4130 N, 4130 normalized, 25CrMo4 / 1.7218 (near), SCM430 (JIS, near)
4130 chromoly is the weldable Cr-Mo steel for tube frames, roll cages and aircraft structure: strong when normalized, TIG welds cleanly. Values normalized.
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At a glance
| Density | 7.85 g/cm³ (0.284 lb/in³) |
|---|---|
| Tensile strength | 670 MPa (97 ksi) normalized; about 560 MPa (81 ksi) annealed; Q&T commonly to 860-1,240 MPa (125-180 ksi) |
| Yield strength | 435-520 MPa (63-75 ksi) normalized, with thin sheet and tube at the upper end; about 360 MPa (52 ksi) annealed |
| Hardness | About 197 HB (low 90s HRB) normalized; about 156 HB annealed; about 25-40 HRC Q&T; tops out near 50 HRC as quenched |
| Elongation | About 25% normalized bar (thin tube and sheet specs set lower minimums); about 28% annealed; 12-18% Q&T |
| Modulus of elasticity | 205 GPa (29.7 Msi) |
| Thermal | Melts at about 1,430-1,510 °C (2,610-2,750 °F); thermal conductivity ~43 W/m·K |
| Composition | Fe, 0.28-0.33% C, 0.40-0.60% Mn, 0.80-1.10% Cr, 0.15-0.25% Mo, 0.15-0.35% Si, up to 0.035% P, up to 0.040% S |
Typical values, Normalized (Condition N, as stocked for tube, sheet and plate); annealed and quenched-and-tempered figures noted alongside.. Confirm against the mill certificate and the purchase spec.
- MachinabilityGood
- About 70% of the 1212 benchmark annealed, a little less normalized. Dead-soft annealed stock is gummy; normalized cuts cleaner. Tube cuts well on tube lasers and saws.
- WeldabilityGood
- TIG welds cleanly in thin-wall tube with no preheat beyond warming a cold part. Heavier sections, roughly 6 mm (1/4 in) and up, get a 150-260 °C (300-500 °F) preheat and low-hydrogen practice; stress relieve thick, highly restrained or fatigue-critical joints.
- FormabilityGood
- Normalized sheet bends cleanly at modest radii; tube bends on rotary-draw and mandrel benders, typically at a centerline radius of about 2.5-4x OD for cages and frames. Forges well hot.
- Corrosion resistancePoor
- Rusts like carbon steel, inside the tube as well as out. Powder coat or paint frames and oil or coat tube interiors.
4130 is a chromium-molybdenum steel at about 0.30% carbon, the lower-carbon sibling of 4140. That lower carbon is the point: it welds far more easily, it bends and forms well in the normalized condition, and it still offers roughly twice the yield strength of mild steel. It is sold mostly as normalized seamless and DOM tube and as normalized sheet and plate, and "chromoly tube" nearly always means 4130.
It built the steel-tube aircraft fuselage and engine mount, and it is still the standard for race-car chassis, roll cages, suspension arms, bicycle and motorcycle frames, and aerospace brackets. Normalized bar runs about 670 MPa (97 ksi) tensile and 435 MPa (63 ksi) yield, with thin normalized sheet and tube typically specified higher. It can also be quenched and tempered, commonly to 860-1,240 MPa (125-180 ksi) tensile, and it is widely used in that condition for oil and gas valve bodies, flanges and pressure-containing parts.
The limits: its hardenability is well below 4140, so thick sections will not harden through, and it will not reach the hardness of a 0.40% carbon steel. Stiffness is the same as any steel, so a thinner-wall 4130 frame is stronger but not stiffer than the mild-steel frame it replaces. Heavier welded sections want preheat and, for fatigue-critical or code work, a post-weld stress relief.
Heat treatment
Stocked normalized (870-900 °C / 1,600-1,650 °F, air cool), and most welded structures stay in that condition. Anneal at about 845 °C (1,550 °F) with a furnace cool for maximum formability. Harden by austenitizing at 855-870 °C (1,575-1,600 °F) and quenching in water, polymer or oil (water or polymer for heavier sections, oil for thin or crack-prone parts), then temper at about 480-650 °C (900-1,200 °F) to a strength band, usually stated as 860-1,240 MPa (125-180 ksi) tensile. Hardenability is well below 4140, so sections much over 25 mm (1 in) will not harden through. Stress relieve welded assemblies at about 600-650 °C (1,100-1,200 °F) when the spec or the service calls for it.
Tempers and conditions
- Normalized (Condition N, as stocked)
- Annealed
- Stress relieved (tube)
- Q&T to a tensile band (for example 125, 150 or 180 ksi)
- Cold drawn DOM tube, normalized
Stock forms
- Seamless and DOM round tube
- Square and rectangular tube
- Sheet and plate (normalized)
- Round and flat bar
- Forgings
Relative cost
Tier 2 of 5. Bar prices near 4140. Seamless or DOM 4130 tube costs several times mild-steel tube per foot, offset by thinner walls and lower weight.
Typical applications
- Race-car chassis, roll cages and suspension arms
- Light-aircraft fuselage trusses, engine mounts and landing gear legs
- Bicycle and motorcycle frames
- Aerospace brackets and fittings in normalized sheet
- Oil and gas valve bodies, flanges and wellhead parts (Q&T)
- Seamless gas cylinders and pressure-containing parts
- Machine parts that must be welded and still be stronger than mild steel
Process compatibility
How 4130 takes each process, and what to specify. Each process links to its own guide and to the shops that do it.
| Process | Rating | Notes |
|---|---|---|
| TIG Welding | ExcellentExcellent | The standard for thin-wall chromoly frames and cages; ER70S-2 filler, clean fit-up, back purge not required. |
| Welding | GoodGood | Welds far more easily than 4140; preheat heavier sections and stress relieve fatigue-critical or code joints. Can 4130 chromoly be welded? |
| MIG Welding | GoodGood | Fine for heavier sections and production frames; TIG gives more control on thin-wall tube. |
| Tube Bending | GoodGood | Normalized tube bends well on rotary-draw benders; watch wall thinning and ovality on tight radii. |
| Laser Cutting | GoodGood | Sheet, plate and tube cut cleanly; tube lasers cope and notch joints for tight TIG fit-up. |
| Press Brake Forming | GoodGood | Normalized sheet brackets and gussets bend at modest radii; anneal for tight bends. |
| CNC Machining | GoodGood | Machines like a slightly tougher 4140; normalized stock cuts cleaner than dead-soft annealed. |
| Heat Treating | GoodGood | Quenches and tempers to 125-180 ksi tensile in modest sections; hardenability limits thick parts. |
| Forging | GoodGood | Forged fittings, flanges and aircraft parts; normalize after forging. |
| Nitriding | FairFair | Takes a nitrided case, but a softer one than 4140 or nitriding steels give; use it for mild wear, not heavy loads. |
| Powder Coating | GoodGood | The usual finish on frames and cages; blast and phosphate first, and treat tube interiors separately. |
| Induction Hardening | FairFair | At 0.30% carbon the surface tops out in the mid to high 40s HRC; 4140 or 1045 harden better. |
| Carburizing | PoorPoor | Possible but pointless: 8620 is the carburizing grade and gives a better core and case for the same money. |
| Anodizing | Not recommendedNot recommended | Anodizing is for aluminum and titanium; steel frames get powder coat, paint, zinc or electroless nickel. |
Compare 4130
Design and sourcing tips
- Call out tube by OD x wall, "4130 normalized, seamless" or "DOM", and the tube spec (AMS 6360 for aircraft, ASTM A519 for mechanical tube).
- For as-welded frames specify ER70S-2 (or ER70S-6) filler; the softer weld is more ductile and the joint design carries the load. Use a filler that responds to heat treatment only when the assembly will be heat treated after welding.
- Coped and notched joints cut on a tube laser give tight TIG fit-up; ask for it in the RFQ on production frames.
- Check the governing rulebook (SCCA, NHRA, FIA, FAA) for minimum OD, wall, material certification and welding process before designing a cage or airframe.
- Do not thin the wall just because 4130 is stronger: stiffness depends on the section, not the grade, and thin walls dent and buckle.
- Protect tube interiors from rust with an internal oil or coating, and drill drain or vent holes where water can collect.
Frequently asked questions
- Can 4130 be welded?
- Yes; it is the most weldable of the chromoly steels and is TIG welded every day in race cars, bicycles and aircraft. Thin-wall tube needs no preheat beyond warming a cold part. Heavier sections, roughly 6 mm (1/4 in) and up, should be preheated to 150-260 °C (300-500 °F) with low-hydrogen practice. ER70S-2 is the common filler for as-welded frames.
- Does 4130 need to be heat treated after welding?
- Most thin-wall tube frames are left as welded and perform well. Post-weld stress relief, typically around 600-650 °C (1,100-1,200 °F), is worth it for thick sections, highly restrained joints, fatigue-critical parts and whenever a code, customer or sanctioning body requires it. Assemblies that will be quenched and tempered after welding need a filler that responds to the heat treatment.
- What is the difference between 4130 and 4140?
- Carbon. 4130 has about 0.30% and 4140 about 0.40%, with the same chromium and molybdenum. 4130 welds and forms more easily and is the usual choice for tube, sheet and welded structures. 4140 hardens deeper and reaches higher strength, and it is the usual choice for machined bar parts such as shafts and tooling.
- Is 4130 chromoly stronger than mild steel?
- Yes. Normalized 4130 has a yield strength of roughly 435-520 MPa (63-75 ksi), against about 250 MPa (36 ksi) for A36 structural steel, so a 4130 part can carry the same load with less material. Its stiffness is the same as any steel, though, so a thinner-wall 4130 tube is stronger but flexes more than a thicker mild-steel tube.
Processes that commonly use it
Related materials
Where to verify these values
- ASTM A519 (mechanical tube) and ASTM A29/A29M (bar)
- AMS 6360, AMS 6350 and AMS 6370 (aerospace tube, sheet and bar)
- MMPDS (design allowables for normalized and heat-treated 4130)
- ASM Handbook Vol. 1 (properties) and Vol. 4 (heat treating)
