GTS-55-04 is a blackheart malleable cast iron from the older German DIN 1692 system, carried forward today as EN-GJMB-550-4 under EN 1562. Like all malleable irons, it starts life as a fully carbidic "white" iron casting with no free graphite, and only takes on its working properties through a long post-casting anneal. The blackheart route used here differs from the whiteheart process in one key way: instead of packing castings in an oxidizing medium to decarburize the surface, blackheart castings are annealed in a neutral or protective atmosphere. No carbon is removed — it simply precipitates in place as fine, rounded "temper carbon" particles distributed evenly through the section, leaving a uniform structure from surface to core rather than the two-zone skin-and-core structure of whiteheart grades. That uniformity is why blackheart malleable iron can be used in thicker sections than whiteheart material.

Within the blackheart family, GTS-55-04 sits toward the higher-strength, pearlitic-matrix end rather than the fully ferritic grades. By controlling the later stages of the anneal cycle, foundries leave a portion of the matrix as pearlite instead of fully graphitizing it to ferrite, which trades away some elongation in exchange for meaningfully higher tensile strength and hardness. With a 550 N/mm² minimum tensile strength and 4% minimum elongation, GTS-55-04 lands between the tough, low-strength ferritic blackheart grades and the harder, lower-ductility pearlitic grades further up the series (GTS-65-02, GTS-70-02) — a practical middle ground for crankshafts, camshafts, levers, and hubs that need more strength than ferritic malleable iron but still benefit from cast iron's damping and machinability.

GTS-55-04's pearlitic-plus-ferrite matrix makes it noticeably harder and more abrasive to cut than a fully ferritic blackheart grade, running around 205 HB against roughly 150 HB or less for the softer ferritic grades in the same family. The pearlite constituent raises cutting forces and puts more abrasive wear on the tool, so this grade sits closer to a mid-carbon steel in cutting difficulty than to soft ductile iron. That said, the temper-carbon particles distributed through the matrix still act as internal chip-breakers and provide a measure of self-lubrication, so chip control stays reasonable and built-up edge is less of a concern here than on the softer, more ductile grades in the malleable iron family.

Because the matrix is uniform from surface to core — unlike whiteheart iron's decarburized skin — machining behavior stays consistent regardless of how deep the cut goes, which simplifies process planning on thicker sections. Tool wear is driven mainly by abrasion from the pearlite rather than by heat, so a coated carbide grade with good abrasion resistance generally outperforms an uncoated grade, especially at the cutting speeds and feeds typical of production turning and milling on crankshaft and lever-type parts.

A moderately honed edge with a positive-leaning geometry balances edge strength against the cleaner shear this grade rewards; too negative a rake tends to push cutting forces and heat higher than necessary given the material's hardness level.