Glossary

Chromoly—short for chromium-molybdenum steel—is a high-strength alloy steel that combines iron with small amounts of chromium (Cr) and molybdenum (Mo). These alloying elements dramatically improve the material’s tensile strength, toughness, and resistance to wear, heat, and corrosion compared to standard carbon steel.

The most common grades are AISI 4130 and AISI 4140, which differ slightly in their carbon content (and thus hardness and machinability). 4130 is easier to weld and form, while 4140 is stronger and used in applications that require higher fatigue resistance.

Chromoly’s key characteristics include:

High tensile strength: It can handle heavy loads and torque without stretching or breaking.

Excellent toughness and ductility: It can deform slightly under stress without cracking, making it ideal for structural and automotive applications.

Good machinability and weldability: Especially in the normalized or annealed condition.

Resistance to fatigue and impact: It holds up under repeated cyclic stresses (important for rotating fasteners and suspension components).

Moderate corrosion resistance: Better than plain carbon steel but not as good as stainless steel—often used with protective coatings.

Because of these properties, chromoly is widely used in aircraft parts, roll cages, bicycle frames, racing car chassis, shafts, and high-performance fasteners such as bolts, studs, and connecting rods.

In the fastener world, chromoly bolts are prized for their high yield and tensile strength, typically exceeding 150,000 psi, allowing them to provide reliable clamping force under extreme loads without failure.

AKA: Chromium-Molybdenum Steel