Chrome Moly 40CrMo4 Alloy Steel Bars

Product Description

40CrMo4 is a high-strength, high-hardenability Chromium-Molybdenum (Chrome Moly) alloy steel specified under the European EN 10083-3 standard. If you are sourcing or machining this grade, you are working with the direct metallurgical sister to DIN 1.7225 and AISI 4140.

The designation itself tells you the precise structural formula: 40 indicates an average carbon content of 0.40%, Cr highlights Chromium for surface wear resistance, and Mo signifies Molybdenum for deep core strength and high-temperature integrity.

1. Chemical Composition (EN 10083-3)

The material's reliable toughness under high torque and cyclic fatigue is achieved via this strict chemical window:

2. Mechanical Profile vs. Bar Cross-Section

A critical trap when engineering with 40CrMo4 bars is assuming its strength remains identical across all dimensions. Because it lacks a large dose of nickel, its through-hardening depth drops as the bar thickness increases. When ordered in the standard Quenched & Tempered (+QT) condition, its mechanical thresholds scale as follows:

• Small Bars ($\le$ 16 mm): Tensile strength reaches an intense 1100 – 1300 MPa with a minimum yield strength of 900 MPa.

• Medium Bars (40 mm – 100 mm): Tensile strength shifts to 900 – 1100 MPa with a minimum yield of 650 MPa.

• Massive Bars (160 mm – 250 mm): Tensile strength measures 750 – 900 MPa with a minimum yield strength of 500 MPa.

3. Core Delivery Conditions

When specifying 40CrMo4 bars from a service center, you will typically choose between three distinct structural states:

Soft Annealed (+A)

• Hardness: $\le$ 241 HBW (Brinell)

• Usage: Best for extensive material removal. If your process involves deep drilling, gear-tooth hobbing, or heavy milling, machining in this soft state preserves your cutting inserts and keeps residual internal stress low before final hardening.

Pre-Hardened / Quenched & Tempered (+QT)

• Hardness: Typically 280 to 340 HBW ($\sim$29 to 36 HRC).

• Usage: Ideal for direct component manufacturing. You can machine your parts straight to their final dimensions and place them directly into service, completely bypassing the risk of dimensional warping or scaling caused by post-machining heat treatment lines.

Shearable State (+S)

• Hardness: $\le$ 255 HBW

• Usage: Specifically softened to allow industrial cold-shearing or cold-heading machines to chop or form the bar stock without causing localized stress cracks.

4. Key Engineering Applications

Thanks to its premium torsional fatigue resistance, 40CrMo4 bar stock is widely machined into:

• Powertrain Elements: High-torque drive shafts, axles, high-stress crankshafts, connecting rods, and universal joint components.

• Heavy Industrial Gearing: Pinions, spur gears, machine tool spindles, and planetary gear systems.

• High-Stress Fastening: Grade 10.9 structural bolts, heavy anchor studs, and specialized clamping hardware for wind turbines or heavy machinery.

• Oil & Gas Hardware: High-pressure valve bodies, drill pipe couplings, and manifold connectors.

5. Direct Global Equivalents

If you are converting international blueprints or matching foreign material test certificates (MTCs), 40CrMo4 corresponds exactly to:

• Germany / Europe: DIN 1.7225 / 42CrMo4

• USA: AISI 4140 / SAE 4140

• Japan: JIS SCM440

• United Kingdom: BS 708M40 (Classic EN19)

• China: GB 42CrMo

• Russia: GOST 38KhM (38ХМ) / 42KhM (42ХМ)

6. Surface Hardening and Welding Guidelines

⚠️ Technical Consideration

• Nitriding / Induction Hardening: While the core of a +QT bar remains tough at $\sim$32 HRC, its skin responds beautifully to surface treatments. Induction hardening or nitriding can easily raise the surface layer to 50–55 HRC, creating a glass-hard wear sleeve perfect for needle bearings or high-friction gear faces.

• Welding Sensitivity: Because of its high carbon equivalent, 40CrMo4 is highly susceptible to cold cracking. It is not considered a weldable steel under normal shop conditions. If welding is required for assembly or salvage, the bar must be preheated to $200^\circ\text{C} - 300^\circ\text{C}$, welded with low-hydrogen filler metal, and followed by immediate Post-Weld Heat Treatment (PWHT) to temper the brittle heat-affected zone (HAZ).