GTW-35-04 is a whiteheart malleable cast iron under German standard DIN 1692, known as "Temperguss weiss." Like the blackheart GTS grades, it starts as white (chilled) cast iron with all of its carbon locked up as hard, brittle cementite rather than graphite. Where GTS castings are annealed in a neutral atmosphere to convert that cementite into graphite nodules throughout the section, whiteheart processing anneals the casting in an oxidizing atmosphere, which decarburizes the material from the surface inward. In thin sections this can burn out most of the carbon near the surface entirely, leaving a low-carbon, almost steel-like skin over a core that still contains some graphite and pearlite — the reverse of the blackheart approach, and the reason the fracture surface of a whiteheart casting looks pale ("white") rather than dark.

The designation numbers are guaranteed minimums: 340 N/mm² tensile strength and roughly 4% elongation. That decarburized surface layer gives whiteheart iron good weldability and a tough, wear-resistant skin, which is why GTW-35-04 is a traditional choice for small fittings, pipe hardware, hand tool parts, and thin-walled castings where the through-hardened, part-steel surface is an advantage over a fully graphitic blackheart or ductile iron part.

Whiteheart malleable iron machines differently depending on where in the section the cut is taken, which is the main practical difference from blackheart (GTS) grades. Near the surface, the decarburized layer behaves much like a low-carbon steel — little to no free graphite, higher toughness, and a tendency to produce longer, more continuous chips than a graphitic cast iron. Deeper into the section, where some pearlite and graphite remain, the material cuts more like a conventional malleable iron with somewhat better chip breaking.

Because the outer skin is essentially decarburized steel, tool wear on that layer is driven more by adhesion and built-up edge than by the abrasive graphite-and-carbide wear typical of cast iron, so a grade suited to low-carbon steel turning often outperforms a pure cast-iron grade on thin-walled or small GTW parts where most of the machined volume is near-surface material. Chip control benefits from a sharp, positive edge that shears the tougher decarburized layer cleanly rather than tearing it.

On heavier sections where more core material (with retained graphite) is being removed, treat the cut more like a standard malleable iron: moderate cutting speeds, good chip-breaker geometry, and a coated carbide grade with balanced toughness and wear resistance.