GGG120B sits well above the conventional ductile iron strength range, at a level where the material is almost certainly being produced by austempering rather than a simple quench and temper. Following the number convention used throughout the "GGG" family from the older DIN 1693 system, the designation points to a roughly 1200 N/mm² minimum tensile strength — a figure that lines up closely with EN-GJS-1200-2, one of the austempered ductile iron (ADI) grades standardized under EN 1564. Austempering isothermally quenches the casting into a bainite-forming temperature range rather than all the way to room temperature, producing a distinctive "ausferrite" matrix of fine bainitic ferrite plus stabilized, carbon-enriched retained austenite around the graphite nodules — a structure that delivers an unusual combination of very high strength and genuine ductility that conventional quenched-and-tempered ductile iron struggles to match at the same hardness.

The "B" suffix on this designation most plausibly reflects that bainite-based structure, though we can't point to one specific, independently verifiable clause of a current standard that defines this exact letter code, so it's best treated as an indicator of a strengthened, specially heat-treated delivery condition rather than a citation to a single numbered standard. With a 2% minimum elongation paired with roughly 1200 N/mm² tensile strength and a proof strength in the 800-900 N/mm² range, GGG120B is suited to highly stressed components — gears, crankshafts, and wear-resistant machinery parts — where the strength-to-weight and fatigue performance of ADI genuinely competes with forged and case-hardened steel.

GGG120B is substantially harder to machine than the standard-grade ductile irons the rest of this family builds on, and the reason is more specific than "it's harder." Austempering leaves a meaningful fraction of stabilized retained austenite in the matrix alongside the bainitic ferrite, and that retained austenite work-hardens rapidly under cutting forces — much like the mechanism that makes austenitic stainless steels notoriously tough on tooling. Every pass that doesn't cut cleanly through the material risks strain-hardening the surface it leaves behind, which then resists the next pass even more.

Combined with a bulk hardness typically in the 340-420 HB range, this makes GGG120B one of the more demanding cast irons a shop will encounter — closer in difficulty to a hardened alloy steel than to any as-cast iron grade. Cutting forces and heat generation are high, and tool wear (both flank wear and edge chipping from the harder bainitic constituents) is the dominant concern rather than achievable finish.

Sharp, positive-leaning geometries that shear rather than rub help minimize the work-hardening tendency, and maintaining adequate feed and depth of cut to stay below the previous pass's hardened layer matters more here than on any other grade in this family. Coated carbide grades built for high-hardness ferrous machining, run at conservative speeds with rigid, vibration-free setups, give the most consistent results.