GG15 sits one step up from GG10 in the DIN 1691 gray iron family, guaranteeing a minimum tensile strength of 150 N/mm² on a separately-cast test bar. It's still a coarse-graphite, predominantly ferritic gray iron, but with slightly less carbon and silicon than GG10, which tightens the graphite network a little and nudges strength and hardness upward without giving up much of the machinability that makes the low grades attractive.

Like every grade in this series, GG15's mechanical behavior is governed almost entirely by its flake graphite morphology rather than by alloy content — there's no meaningful alloying here, just careful control of carbon equivalent and cooling rate during casting. That keeps costs low and machinability high, which is why GG15 shows up constantly in general-purpose castings: pump and valve bodies, machine bases, brackets, and housings where moderate strength is enough and the part benefits from gray iron's excellent damping and thermal stability. As with all the GG grades, it has essentially no ductility and should never be specified for parts that see shock loading or significant tensile stress in service.

GG15 machines on the same principle as every gray iron in the DIN 1691 family: free graphite flakes embedded through the matrix act as pre-built stress risers, so the material fractures into short, discontinuous chips instead of shearing into long continuous ones. That gives gray iron dramatically lower cutting forces and better chip control than steel of comparable hardness, with no risk of the stringy, tangled chips that plague many ductile or wrought materials.

The slightly finer graphite structure and marginally higher matrix strength in GG15 compared with GG10 mean marginally higher cutting forces and a touch more heat at the edge, but the difference is modest — GG15 still machines easily by any standard. The dominant wear mechanism remains abrasion: free graphite, silicate inclusions, and especially the hard, sand-embedded skin on as-cast surfaces all wear away at the flank face faster than the bulk metal underneath would suggest. First passes that break through the casting skin see the fastest wear; once past it, tool life on the cleaner subsurface material is considerably better.

Because there's no work hardening and chip evacuation is rarely an issue, tool selection for GG15 comes down mostly to picking a grade with enough abrasion resistance to survive the scale without sacrificing the edge sharpness that keeps cutting forces low on this naturally free-machining material.