SAE/AISI 9840 is a nickel-chromium-molybdenum alloy steel from the 98xx family, combining a medium carbon content with meaningful nickel, chromium, and molybdenum additions to deliver deep hardenability and a strong, tough microstructure through relatively large sections. Nickel builds core toughness and impact resistance, chromium adds hardenability and wear resistance, and molybdenum both deepens the hardening response in thick sections and helps guard against temper embrittlement during heat treatment — the same alloying logic behind 8740, but with a slightly different balance suited to heavier-duty, higher-strength parts.

Because 9840 through-hardens well even in substantial cross-sections, it's a common choice for large shafts, heavy-duty gears, crankshafts, and other highly stressed rotating components in industrial machinery and equipment where both strength and toughness need to hold up under sustained cyclic loading. It's typically supplied and machined in a normalized or quenched-and-tempered condition rather than fully annealed.

9840's full nickel-chromium-molybdenum alloy content and medium-carbon base make it a demanding grade to machine, even in the normalized or quenched-and-tempered condition most shops encounter. Cutting forces and edge temperatures run well above plain carbon steel, and the combined alloying elements resist the cutting edge more than any one element alone would, making flank wear the dominant limiting factor on tool life.

Chip formation is generally manageable with a properly matched chipbreaker, producing segmented rather than continuous chips, but the nickel content can add a degree of gumminess and a tendency toward built-up edge at low cutting speeds or with a dull tool. Staying within the recommended speed range and keeping the edge sharp both help maintain consistent chip control.

Given the strength levels this grade is typically supplied at, a rigid setup, sharp coated carbide with good hot hardness, and steady feeds that keep the tool cutting rather than rubbing are all important for controlling both tool wear and surface finish. This steel work-hardens readily under light or interrupted cuts, so consistent engagement helps avoid a hardened surface layer that complicates the next pass.