Gear Material Selection: Steel Alloys and Heat Treatment

Gear performance starts with the material. The alloy and its heat treatment determine surface durability, core strength, fatigue life, dimensional stability, and how the gear behaves on the machine. Most precision steel gears are built from a short list of well-proven alloys, each chosen for a reason.

Common gear steels

• 4140: a versatile through-hardening chromium-molybdenum alloy steel; a workhorse for general-purpose gears.
• 4340: a nickel-chromium-molybdenum steel with higher strength and toughness for heavily loaded parts.
• 8620: a carburizing (case-hardening) grade that develops a hard, wear-resistant case over a tough core.
• 9310: a premium carburizing alloy used where high fatigue strength and cleanliness matter, common in demanding drive applications.
• 17-4 PH stainless: a precipitation-hardening stainless for gears needing corrosion resistance along with strength.

Heat treatment routes

Through hardening (quench and temper) raises strength uniformly through the section. Carburizing builds a hard, wear-resistant case over a tough core, which is ideal for contact-stressed teeth. Nitriding produces a very hard case with minimal distortion, useful for parts that cannot be ground afterward. Induction hardening selectively hardens the tooth surfaces.

Why grinding follows heat treatment

Heat treatment hardens the gear but also introduces distortion. For high-accuracy gears, the part is rough-cut soft, heat treated, and then finish ground, which removes the distortion and brings the tooth geometry back to tight AGMA tolerances. This sequence is exactly why AGMA 15 quality requires grinding after heat treat, not just precise cutting.

Material selection at INSCO

INSCO manufactures gears and components in 4140, 4340, 8620, and 9310 alloy steels, 17-4 PH stainless, and other specialty alloys, with case hardening, through hardening, and nitriding coordinated through qualified heat-treat partners. Ground gears are finished after heat treatment to hold up to AGMA Class 15, and material certifications and hardness results can accompany the shipment.