GTS-35-10 is a blackheart malleable cast iron under German standard DIN 1692, known in German as "Temperguss schwarz." Malleable iron starts life as white (chilled) cast iron, cast with all of its carbon locked up as cementite rather than graphite, then goes through a long, carefully controlled anneal. For the GTS blackheart process, that anneal takes place in a neutral or protective atmosphere at high temperature, giving the cementite time to decompose into iron and irregular, rosette-shaped graphite nodules distributed through a ferritic matrix, rather than burning the carbon out near the surface the way the whiteheart (GTW) process does. The result is a casting with graphite nodules throughout its cross-section, not just a decarburized skin.

The two numbers in the GTS-35-10 designation are its guaranteed minimums: 350 N/mm² tensile strength and 10% elongation. That combination of moderate strength with genuine ductility is what sets malleable iron apart from as-cast gray or white iron — it can absorb shock and deform somewhat before fracturing rather than failing in a brittle manner. GTS-35-10 is a common choice for small- to medium-size fittings, pipe hardware, wheel hubs, shift forks, axle housings, and other parts that need to survive impact loading and vibration in service without the added cost of a nodular ductile iron.

Blackheart malleable iron machines noticeably differently from either gray or ductile cast iron because its graphite is dispersed as irregular, compact rosettes rather than flakes or true spheroids, set in a soft ferritic matrix. That combination gives GTS-35-10 good machinability at a low hardness, but the material's toughness — the same property that makes it shock-resistant in service — also means it doesn't shear as cleanly as gray iron. Chips tend to be tougher and less prone to breaking on their own than in gray cast iron, so chipbreaker geometry and insert selection matter more here than the raw hardness number would suggest.

Because the ferritic matrix is soft and free of hard pearlitic carbides, abrasive tool wear is minimal, and tool life is generally strong with a properly matched grade. The practical machining challenge is chip control and finish rather than wear resistance: a low cutting speed or a dull edge can leave a torn or dragged surface instead of a clean shear, since the ductile matrix wants to deform before it separates.

A sharp, positive-rake carbide grade with good chip-breaker geometry generally outperforms a heavily honed edge suited to harder, more brittle cast irons. Adequate cutting speed and feed keep the chip breaking cleanly and prevent the smeared finish that under-speed cutting tends to produce on this ductile, low-hardness material.