1045 vs 4140 Steel: Plain Carbon vs Alloy Workhorse

## Composition Both are medium-carbon steels with approximately 0.40-0.45% carbon, but 4140 contains critical alloying additions. | Element | AISI 1045 | AISI 4140 | |---------|-----------|----------| | Carbon | 0.43-0.50% | 0.38-0.43% | | Manganese | 0.60-0.90% | 0.75-1.00% | | Chromium | None | 0.80-1.10% | | Molybdenum | None | 0.15-0.25% | | UNS | G10450 | G41400 | The chromium and molybdenum in 4140 dramatically improve hardenability: the ability to form martensite throughout the cross-section during quenching rather than only at the surface. ## Hardenability: The Key Difference 1045 has limited hardenability. In a Jominy end-quench test, the hardness drops rapidly from the quenched end, and only thin sections (under approximately 25 mm diameter) can be through-hardened by water quenching. 4140 has substantially greater hardenability. Sections up to 75 mm diameter can be through-hardened by oil quenching. This means that for any component thicker than about 25 mm, 4140 will deliver more uniform mechanical properties across the cross-section after heat treatment. ## Mechanical Properties (Quenched and Tempered at 205C) | Property | 1045 | 4140 | |----------|------|------| | Tensile Strength | 1010 MPa | 1250 MPa | | Yield Strength | 590 MPa | 1140 MPa | | Elongation | 10% | 11% | | Impact (Charpy) | 14 J | 27 J | At similar hardness levels, 4140 has significantly better toughness (impact energy) than 1045 because the chromium and molybdenum refine the tempered martensite microstructure. ## Weldability 1045 is difficult to weld due to its high carbon content. Preheating to 150-260 degrees C and slow cooling are necessary to prevent hydrogen-induced cracking. 4140 is also challenging to weld but is done regularly in industry with proper procedures: preheat to 260-315 degrees C, use low-hydrogen electrodes (E7018 or E11018), and apply PWHT at 595-650 degrees C. ## Cost and Availability 1045 costs approximately 0.80-1.20 USD/kg and is available from virtually every steel distributor. 4140 costs approximately 1.20-1.80 USD/kg, a modest premium for significantly improved performance. ## When to Choose Each **Choose 1045 when**: The component is thin (under 25 mm), moderate strength without extreme toughness is acceptable, cost is a primary concern, and heat treatment facilities are limited. Common uses include shafts, axles, bolts, and agricultural equipment. **Choose 4140 when**: Through-hardening of thicker sections is required, toughness matters (impact loading, fatigue), or NACE sour service compliance is needed (4140 QT to max 22 HRC). Common uses include gears, crankshafts, connecting rods, and hydraulic shafts.