Chromium–Molybdenum steels (Cr–Mo steels) are a family of low-alloy, heat-treatable steels that derive their superior mechanical and thermal performance from the combined effects of chromium (Cr) and molybdenum (Mo).
These alloying elements significantly enhance strength, hardenability, toughness, creep resistance, oxidation resistance, and fatigue life. Cr–Mo steels are engineered for high-stress, high-temperature, and pressure-bearing applications across automotive, oil & gas, petrochemical, power generation, aerospace, and heavy engineering industries.
Typical Chemical Composition
| Element | Typical Range (%) | Contribution |
|---|---|---|
| Carbon (C) | 0.25 – 0.50 | Strength & hardenability |
| Chromium (Cr) | 0.80 – 1.20 | Wear & oxidation resistance |
| Molybdenum (Mo) | 0.15 – 0.40 | Creep resistance, toughness |
| Manganese (Mn) | 0.60 – 1.00 | Strength, deoxidation |
| Silicon (Si) | 0.15 – 0.35 | Improves strength |
| Phosphorus (P) | ≤ 0.035 | Controlled impurity |
| Sulfur (S) | ≤ 0.040 | Controlled impurity |
| Iron (Fe) | Balance | Base metal |
Microstructure & Metallurgical Behavior
• Annealed: Ferrite + pearlite (good machinability)
• Normalized: Fine pearlite + ferrite (uniform strength)
• Quenched & tempered: Tempered martensite (high strength & toughness)
• Chromium carbides improve wear resistance
• Molybdenum suppresses grain growth and enhances fatigue life
Mechanical Properties (Typical)
| Property | Typical Range |
|---|---|
| Tensile Strength | 650 – 1,100 MPa |
| Yield Strength | 415 – 900 MPa |
| Hardness | 180 – 320 HB (Q&T) |
| Elongation | 12 – 25% |
| Impact Toughness | Excellent |
| Modulus of Elasticity | ~200 GPa |
| Density | ~7.85 g/cm³ |
Heat Treatment & Refining
Annealing: 830–870°C – machinability and stress relief
Normalizing: 870–920°C – grain refinement and uniformity
Quenching: Oil or polymer – martensitic structure
Tempering: 200–650°C – toughness and fatigue resistance
Stress Relieving: Prevents distortion in welded/machined parts
High-Temperature & Pressure Performance
• Excellent creep resistance up to 500–600°C
• Maintains strength under continuous thermal load
• Low hydrogen embrittlement susceptibility
• Preferred for pressure vessels, boilers, and steam piping
Machinability & Weldability
Machinability: Good in annealed or normalized condition
Weldability: Good with controlled procedures
• Preheating required
• Post-weld stress relief recommended
Corrosion & Wear Resistance
• Better corrosion resistance than carbon steel
• Chromium improves oxidation resistance
• Molybdenum enhances pitting and fatigue resistance
• Not stainless – coatings recommended for aggressive environments
Available Forms
✔ Bars (round, flat, square, hex)
✔ Plates & sheets
✔ Pipes & tubes (seamless / welded)
✔ Forgings & shafts
✔ Billets & blooms
✔ Castings & rolled sections
✔ Custom-machined components
Common Cr–Mo Steel Grades
| Grade | Key Feature |
|---|---|
| AISI 4130 | High strength & weldability |
| AISI 4140 | Excellent toughness & wear resistance |
| AISI 4145 | Heavy-section strength |
| AISI 4340 | Ultra-high strength & fatigue resistance |
| ASTM A387 | Pressure vessel steel |
| ASTM A335 (P11, P22) | High-temperature piping |
Industrial Applications
Automotive: Axles, gears, crankshafts, drivetrain components
Oil & Gas: Pressure vessels, drill collars, boiler tubes, piping
Power Generation: Turbine casings, high-temperature bolting
Industrial Machinery: Shafts, couplings, gear assemblies
Aerospace: Landing gear parts, high-fatigue fasteners
Advantages
✔ High strength and deep hardenability
✔ Excellent fatigue and creep resistance
✔ Reliable high-temperature performance
✔ Good machinability and weldability
✔ Long service life under extreme conditions
Limitations
⚠ Higher cost than carbon steel
⚠ Requires controlled heat treatment
⚠ Not corrosion-proof (not stainless)
Chromium–Molybdenum steels are the industry benchmark for strength-critical and temperature-resistant applications. Their balanced alloy design ensures reliability, durability, and long-term performance where failure is not an option.