SAE/AISI 8740 is a nickel-chromium-molybdenum alloy steel from the 87xx family, distinguished from lower-numbered grades like 8630 by a higher carbon target that pushes it toward through-hardened, high-strength applications rather than carburizing stock. The nickel content builds core toughness and impact resistance, chromium contributes hardenability and moderate wear resistance, and molybdenum both deepens hardenability in larger sections and helps resist the temper embrittlement that can affect nickel-chromium steels during slow cooling through certain temperature ranges.

This combination makes 8740 a step up from the general-purpose nickel-chromium-molybdenum grades, delivering higher strength and fatigue resistance at a given hardness level than plain carbon or simpler alloy steels. It's a common choice for highly stressed shafts, gears, bolts, and other rotating or load-bearing components in heavy equipment, oilfield tooling, and aerospace-adjacent applications where both strength and toughness need to hold up under repeated or shock loading.

8740's higher carbon and full nickel-chromium-molybdenum alloy content make it more demanding to machine than the lighter 86xx grades. Even in the annealed or normalized condition, cutting forces and edge temperatures run noticeably higher than on plain carbon steel, and flank wear accelerates as the combined alloy content resists the cutting edge more than any single element would on its own. Shops that run 8740 regularly typically see it in a normalized or quenched-and-tempered condition rather than fully annealed, which further increases the cutting forces involved.

Chip formation is generally manageable with the right chipbreaker geometry, producing segmented chips rather than long stringers, but the nickel content can make this grade a bit gummier than pure chromium-molybdenum alloys, with some tendency toward built-up edge if speeds are too low or the tool is allowed to dull.

A rigid setup, sharp coated carbide with good hot hardness, and steady feeds that keep the edge cutting rather than rubbing all matter more on 8740 than on lower-alloy grades. Because this steel work-hardens readily under light or interrupted cuts, consistent engagement and avoiding dwell time help prevent a hardened surface layer that makes the next pass more difficult.