AISI/SAE 4520 rounds out this group of low-alloy steels with chromium and molybdenum content in the same range as 4137, 4140, and 4419 (roughly 0.70 to 0.90% Cr and 0.20 to 0.30% Mo), plus a meaningful nickel addition — around 1.65 to 2.00% — that isn't present in the leaner members of the family. Nickel contributes toughness and ductility beyond what chromium and molybdenum alone provide, which pushes 4520 toward applications that combine wear resistance with a need to absorb shock loading without cracking.

That combination of alloying elements makes 4520 a common choice for piston pins, bearings, gears, machine tool arbors, die blocks, and cutting tool shanks — components that see both surface wear and impact. Like the rest of this alloy group, it responds well to heat treatment and can be supplied in a range of conditions depending on the target hardness and toughness balance a given part requires.

The nickel content in 4520 changes the cutting picture slightly compared with the plain chromium-molybdenum grades in this family. Nickel adds toughness to the microstructure, which generally means the material resists chipping at the tool edge a bit better, but it can also make chip separation less crisp — expect chips that hold together a little longer before breaking, so chipbreaker geometry matters more here than on a straight Cr-Mo grade.

Cutting forces and heat generation track closely with the alloy's heat-treated condition, similar to 4140 and its siblings. The combined Cr-Mo-Ni carbide and solid-solution strengthening raises abrasive wear on the tool compared with plain carbon steel, so a coated grade remains the practical choice once you move past light finishing cuts. Because this alloy is often specified for parts needing both wear resistance and shock resistance, it's frequently supplied in a hardness range where tool life becomes the primary limiting factor.

Rigid setups and steady feed rates matter as much here as anywhere else in this alloy family — a tool that's allowed to rub rather than cut will wear faster on 4520 than the raw hardness numbers alone would suggest. Matching grade and geometry to the operation is the most reliable way to keep tool life predictable.