G-CuZn40Fe is a cast copper-zinc alloy built on the classic 60/40 Muntz-metal ratio, with iron (0.2-1.2%) and manganese additions that raise its strength well above standard cast yellow brass — the reason it's often called a high-tensile brass or, informally, manganese bronze, even though it contains no tin and is technically a brass rather than a true bronze. The "G-" prefix is DIN shorthand for Guss (German for "casting"), and it's an important distinction: G-CuZn40Fe is formulated and processed as a sand, gravity, or centrifugal casting alloy, not a wrought bar or plate product, and it has the coarser, more variable grain structure that comes with cast copper alloys.

That combination of strength and castability made G-CuZn40Fe a traditional choice for marine propellers, worm gears, valve bodies, and other components needing more strength than a standard cast gunmetal or leaded brass casting can offer, while still being far more machinable and less costly than a cast bronze. It has largely been superseded by nickel-aluminum bronze in the most demanding marine applications, but remains common in gears, bushings, and structural brass castings where its strength-to-cost ratio still wins.

G-CuZn40Fe machines quite differently from a wrought, low-zinc brass like CuZn15. Its cast grain structure and iron-rich intermetallic particles make it noticeably more abrasive to a cutting edge than free-machining wrought brasses, even though the base copper-zinc chemistry is inherently softer than steel. The iron and manganese additions that give the alloy its strength also mean higher cutting forces and faster edge wear than a standard cast yellow brass, so tool life — not just chip control — becomes a real consideration here, unlike with dedicated free-cutting brasses.

Because there's no lead to promote chip fracture, chip control still requires a positive, sharp geometry, but the cast structure itself tends to produce shorter, more broken chips than an equivalent wrought alloy would, which helps offset the lack of a free-cutting additive. Running at moderate speeds — well below what a free-machining brass tolerates, but with a tougher edge than a wrought CuZn15 needs — gives the best balance between avoiding built-up edge and managing the abrasive wear from the iron-bearing microstructure.