4140 Pre-Hardened Alloy Steel Bar Blanks

4140 pre-hardened bar stock is a chromium-molybdenum alloy steel delivered in the quenched and tempered condition at 28-32 HRC, ready for machining directly into high-strength components without additional heat treatment. The alloy's nominal composition of 0.40% carbon, 0.95% chromium, and 0.20% molybdenum provides an exceptional combination of through-hardenability, fatigue resistance, and toughness that has made 4140 the most widely specified alloy bar steel in North American industry. The pre-hardened condition eliminates the heat treatment step from the production process, which saves time, reduces distortion risk, and removes the need for in-house heat treating equipment. At 28-32 HRC, the material machines predictably with carbide tooling at moderate speeds, producing a clean surface finish suitable for hydraulic cylinder rods, mold bases, and precision mechanical components. Fatigue strength is a defining advantage of 4140 over plain carbon steels. The chromium and molybdenum additions refine the grain structure during heat treatment, producing superior resistance to cyclic loading. This makes pre-hardened 4140 the standard material for rotating shafts, gears, and other components subjected to repeated stress reversals. The alloy also responds well to nitriding, which can produce a surface hardness exceeding 60 HRC while maintaining the tough 28-32 HRC core. 4140 pre-hardened bar is weldable with precautions. Preheating to 400-600°F and using low-hydrogen electrodes or ER80S-D2 MIG wire minimizes the risk of hydrogen-induced cracking in the heat-affected zone. Post-weld stress relief at 1,050-1,100°F is recommended for structural applications.