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4140 Chromium-Molybdenum High-Tensile Steel CNC Machining

4140 (Chromoly) is an alloy steel offering exceptional toughness, high torsional fatigue strength, and versatile heat-treatability, making it the industry standard for shafts, gears, and hydraulic cylinders.

Engineering Specifications & Parameters

Density 7.85 g/cm³
Fatigue Limit 420 MPa
Hardness (Q&T) 28 - 32 Rockwell C (HRC)
Machinability Rating 65% (Annealed) / 55% (Q&T)
Yield Strength (Q&T) 720 MPa (105,000 psi)
Tensile Strength (Q&T) 950 - 1050 MPa

Annealed vs Pre-Hardened (Q&T) 4140

4140 is commonly machined in two standard conditions:

  1. Annealed (~200 HB): Optimized for fast roughing and heavy chip loads at cutting speeds of 120 – 180 m/min.
  2. Quenched & Tempered / Pre-Hard (28–32 HRC): Delivers excellent surface finish and eliminates secondary heat-treat distortion.

Tooling & Chip Control

  • Tooling: Coated carbide (TiCN / TiAlN) with chipbreakers engineered for medium-to-high feed rates.
  • Coolant: Water-soluble synthetic coolant at 7–9% concentration to dissipate cutting heat and prevent build-up.

Frequently Asked Questions

What is the recommended cutting speed for pre-hardened 4140 steel?

For solid carbide milling in pre-hard 4140 (30 HRC), maintain 100 – 150 m/min cutting speed with a rigid setup.

Can 4140 steel be induction hardened locally?

Yes, bearing surfaces and gear teeth can be induction hardened up to 55 HRC after CNC machining while maintaining a tough ductile core.