GTW-40-05 is a whiteheart malleable cast iron under German standard DIN 1692 — "Temperguss weiss" — and the higher-strength counterpart to GTW-35-04 within the same whiteheart family. Like all GTW grades, it starts as white (chilled) cast iron with its carbon locked up as hard, brittle cementite, then goes through a long anneal in an oxidizing atmosphere that decarburizes the casting from the surface inward. In thinner sections this can remove most of the carbon near the surface, leaving a low-carbon, near-steel skin over a core that retains some pearlite and graphite — the opposite of the blackheart (GTS) process, which anneals in a neutral atmosphere to graphitize carbon throughout the section rather than remove it.

The designation numbers are guaranteed minimums: 400 N/mm² tensile strength and roughly 5% elongation, giving it a useful step up in strength over GTW-35-04 while retaining the decarburized surface layer's weldability and toughness. That combination makes GTW-40-05 a common choice for exhaust manifolds, agricultural machinery parts, pipe and hydraulic fittings, clutch components, transmission cases, and steering housings — applications that need more strength than the lightest whiteheart grade but still benefit from a tough, part-steel surface over a fully graphitic core.

GTW-40-05 shares the same section-dependent machining behavior as GTW-35-04, but its slightly higher strength means the decarburized surface layer and pearlitic core both run a notch tougher. Near the surface, where carbon has been largely removed, the material cuts much like a low-carbon steel — minimal free graphite, higher toughness, and longer, more continuous chips than a typical cast iron produces. Deeper into thicker sections, where more of the original pearlite and graphite survive the anneal, the material behaves closer to a conventional malleable iron with somewhat better natural chip breaking.

Because the outer skin is essentially decarburized steel, adhesion and built-up edge are bigger factors in tool wear here than the abrasive wear typical of graphitic cast iron, so a grade with good toughness and edge stability — similar to what works well on low-carbon steel — often outperforms a pure cast-iron grade on thin-walled GTW-40-05 parts. A sharp, positive edge shears the tougher surface layer cleanly rather than tearing or work-hardening it.

On heavier sections where more core material is being removed, treat the cut like a moderate-strength malleable iron: a coated carbide grade with balanced toughness and wear resistance, adequate cutting speed to keep the chip breaking, and stable clamping to control the tougher, less brittle chip this material produces compared with gray cast iron.