GG25 and GG40 are two of the most common grey iron grades in industrial use. Both are DIN 1691 flake-graphite irons, but a 60% jump in minimum tensile strength between them makes the selection anything but interchangeable. Pick the wrong one and you'll either over-pay for capability you don't need or under-spec a part that fails in service.
Mechanical envelope
GG25 is the workhorse grade for machine bases, moulds, sliding surfaces and general engineering. GG40 is the choice when the same part must also carry higher loads or resist wear — heavy-duty gears, hydraulic bodies and rolls.
- GG25 — minimum tensile 250 MPa, hardness 180–220 HB, excellent damping and machinability.
- GG40 — minimum tensile 400 MPa, hardness 220–280 HB, stiffer and stronger but harder to machine.
Machinability and cost
GG25 machines faster, produces shorter chips and gives a better surface finish at the same tool cost. GG40 requires slightly slower feed and speed, and dulls tools faster because of its higher pearlite content — expect roughly 20% more machining time per kilogram.
When to switch grades
- Choose GG25 when the drawing does not require > 250 MPa tensile strength.
- Choose GG40 when the part sees bending, wear, or stress concentrations above 250 MPa.
- For heavy vibration duty (machine beds), GG25 damps better — do not upgrade to GG40 unnecessarily.
- GG25 = general-purpose grey iron; GG40 = heavy-duty grey iron.
- GG40 offers ~60% more tensile strength at the cost of ~20% more machining time.
- GG25 damps vibration better — don't upgrade for machine bases without a reason.
- Both ship from stock as continuous cast bar in round, square and hollow profiles.