GG40 sits at the top of the DIN 1691 flake graphite (gray) cast iron series, guaranteeing a minimum tensile strength of 400 N/mm². It's a niche, high-performance grade rather than a common one — reaching this strength in a flake graphite structure requires a very fine graphite network, a fully pearlitic (often lightly alloyed) matrix, and tightly controlled foundry practice, since flake graphite iron becomes progressively harder to cast reliably as strength climbs toward this ceiling.

Because gray iron's tensile strength is fundamentally limited by the stress-concentrating effect of its flake graphite, GG40 represents close to the practical upper limit for the flake graphite family; specifications calling for still higher strength typically move to nodular (ductile) iron instead, where spheroidal graphite doesn't concentrate stress the same way. Within its niche, GG40 is used for heavily loaded gray iron parts that still benefit from flake graphite's excellent damping and thermal conductivity — large diesel cylinder blocks and liners, high-performance brake rotors, and precision machine tool components subjected to significant wear and thermal cycling. As with every DIN 1691 grade, it remains brittle with essentially no ductility.

GG40 is the toughest-cutting member of the DIN 1691 gray iron family by a clear margin. Its very fine graphite structure still fractures the chip into short segments — gray iron never develops the long, continuous chips typical of steel — but the volume of free graphite is at its lowest point in the series, and the fully pearlitic, sometimes lightly alloyed matrix resists cutting forces far more than the ferrite-rich low grades. Expect meaningfully higher cutting forces, more heat concentrated at the edge, and faster abrasive wear than any other grade in this series.

The abrasive load comes from several directions at once on GG40: the graphite itself, iron carbide lamellae throughout the pearlitic matrix, any alloy carbides present, and the usual hard sand-embedded skin on as-cast surfaces. That combination pushes GG40 into territory where tool selection matters as much as it would on many alloy steels, even though the fundamental chip-formation advantage of gray iron — automatic, discontinuous chip breaking with no work hardening — still holds.

Coated carbide grades with high hot hardness and strong abrasion resistance are essential for economical tool life on GG40. Conservative speeds, rigid setups, and attention to edge condition matter more here than on any softer grade in the family, since a worn or chipped edge degrades quickly once it loses the sharpness needed to shear through the hard pearlitic matrix cleanly.