Cr-Mo alloy steel · UNS G41300

4130 Chromoly Steel

The weldable Cr-Mo alloy steel. Lower carbon than 4140 means it welds without preheat, machines easily, and still hardens to 50+ HRC for strength-critical parts. Standard for aircraft tubing, race car frames, firearms barrels, and lightweight weldments.

UTS (normalized)

670 MPa

Density

7.85 g/cc

Modulus

200 GPa

Mach.

70 /100

When to use

Pick 4130 when you need to weld it later.

4130 is essentially 4140 with less carbon (~0.30% vs 0.40%). That single chemistry difference is why it dominates aerospace tubing and weldable structural applications: it welds without preheat or post-weld heat treatment, and the heat-affected zone retains useful properties without becoming brittle.

The trade-off is a lower strength ceiling than 4140 — fully hardened 4130 reaches ~50 HRC vs ~58 HRC for 4140. For most weldments and tube structures the strength is plenty; for non-weld applications where higher strength matters, look at 4140 or 4340. "Chromoly" in the bicycle and motorcycle communities almost always means 4130.

Strengths

Why aerospace builds with it.

Weldable as-is

TIG welds with ER70S-2 filler — no preheat, no post-weld stress relief required for most applications.
Machinability 70/100

Cuts faster and cleaner than 4140 in the annealed condition. Sharp tools, normal feeds.
Wide stock availability

Round bar, hex, plate, sheet, AND seamless tube in every aerospace size — uniquely common in tube form.
Heat-treatable

Q&T to 50+ HRC for non-welded applications. Normalized to ~95 HRB is standard for welded structures.
Excellent fatigue

Well-understood fatigue behavior — used in aircraft cyclically loaded tube structures and motorsport frames.
Good toughness

Better Charpy than 4140 at the same hardness due to lower carbon content.

Trade-offs

Where 4140/4340 win.

Lower max hardness

Tops out around 50 HRC vs 58 HRC for 4140. For wear surfaces, 4140 or A2 are better.
Will rust

No corrosion resistance. Plate, paint, or oil for any wet environment.
Lower strength than 4140

Same alloy chemistry but less carbon = lower max strength. ~80% of 4140 at peak hardness.
Section size matters

Hardenability is shallow vs 4340. Use 4340 for through-hardening of large sections.
Hydrogen embrittlement

Same as 4140 — bake-out after acid pickling/plating at hardness ≥35 HRC.
Annealed is common — but spec it

Chromoly bar can ship in normalized OR annealed condition. Specify which condition matters.

Specs

Mechanical, physical & chemical data.

Properties shown for normalized condition (the most common purchase form for tube and weldable structural use). Q&T values follow.

Mechanical

Properties (normalized)

Ultimate tensile strength	670 MPa (97 ksi)
Yield strength (0.2% offset)	435 MPa (63 ksi)
Elongation at break	25%
Reduction of area	60%
Modulus of elasticity	200 GPa (29.0 Msi)
Hardness (normalized)	95 HRB (~209 HB)
Hardness (Q&T max)	~50 HRC
Charpy V-notch impact (norm.) at 21 °C	85 J

Q&T to ~38 HRC: UTS 1130 MPa, YS 970 MPa. Annealed: UTS 560 MPa, YS 360 MPa, hardness 90 HRB.

Physical & thermal

At room temperature

Density	7.85 g/cc (0.284 lb/in³)
Melting range	1416 – 1455 °C
Specific heat	0.477 J/g·°C
Thermal conductivity	42.7 W/m·K
CTE (20–100 °C)	12.2 µm/m·°C
Electrical resistivity	0.223 µΩ·m
Magnetic	Ferromagnetic
Critical temp (Ac1)	732 °C

Physical properties essentially identical to 4140 — the difference is metallurgical (carbon content) not thermal.

Composition

Chemistry, weight %

Carbon (C)	0.28 – 0.33
Manganese (Mn)	0.40 – 0.60
Chromium (Cr)	0.80 – 1.10
Molybdenum (Mo)	0.15 – 0.25
Silicon (Si)	0.15 – 0.35
Phosphorus (P)	0.035 max
Sulfur (S)	0.040 max
Iron (Fe)	balance

The 0.30% nominal carbon (vs 0.40% in 4140) is what gives 4130 its weldability — and limits its hardness ceiling.

Specifications

Common purchase specs.

AMS 6346

Bar, hardened & tempered, 125 ksi (862 MPa) min tensile — aerospace

AMS 6350

Sheet, plate

AMS 6371

Tubing, mechanical

AMS 6360

Seamless tubing — normalized or stress relieved

ASTM A322

Hot-finished alloy steel bar

MIL-T-6736

Tubing — round mechanical

AISI 4130 / SAE 4130

Standard designation

DIN 25CrMo4 / 1.7218

European equivalent

Machining

How we cut it.

4130 in normalized condition is one of the easier alloy steels to machine. Sharp tools, normal feeds, no surprises. Q&T condition is similar to 4140 prehardened — predictable but slower.

Cutting speed (norm.)

200–350 SFM with carbide. Better than 4140 due to lower carbon.

Feed rate

Aggressive constant feed. 4130 doesn’t work-harden meaningfully.

Tooling

Coated carbide standard. HSS works fine for prototype work.

Coolant

Flood is standard. Some shops run with MQL for chip control.

Threading

Standard taps work cleanly. Cut taps preferred over form taps in normalized condition.

Q&T after machining

For high-strength applications, rough machine + Q&T + finish-grind. Allow ~0.001″/inch dimensional growth from quench.

Tube machining

4130 tubing is common. Slow turning speeds for thin-wall to avoid chatter; sharp tools.

Welding considerations

If machined parts will be welded later, leave material distribution symmetric to control HAZ distortion.

Applications

Where 4130 shows up.

Aircraft tubing structures

Light aircraft fuselages, engine mounts, structural tubing in Cessna, Cub, kit aircraft. The aerospace welded-tube standard.

Race car / motorsport

Roll cages, tube frames, suspension links. SCCA, NHRA, FIA roll cage specs are typically 4130. 'Chromoly' = 4130 in this world.

Firearms & ordnance

Rifle barrels, action receivers, machine gun components. Q&T to ~38 HRC for the strength + machinability balance.

Compare

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