GGG-NiMn13.7 takes a different route to an austenitic matrix than the NiCr grades in this series. Instead of leaning on chromium as a secondary alloying element, it pairs a moderate 12–14% nickel addition with a substantial 6–7% manganese addition — manganese is itself an austenite stabilizer, so the combination reaches a stable austenitic structure at a lower nickel content than the NiCr20.3/NiCr30.1 grades need on their own. Chromium is deliberately kept minimal (0.2% max), which keeps the alloy free of the chromium carbides that would otherwise reduce ductility.

That composition gives GGG-NiMn13.7 the highest elongation of the austenitic ductile iron grades covered in this series — around 15% minimum — alongside good low-temperature toughness, non-magnetic behavior, and moderate corrosion resistance. It isn't as corrosion-resistant as the higher-nickel, higher-chromium NiCr grades, but its combination of toughness, ductility, and lower material cost (nickel is the most expensive alloying addition in this family) makes it an efficient choice where extreme corrosion resistance isn't the primary driver.

Typical applications include pump and compressor components, low-temperature service parts, and non-magnetic castings where cost-effective austenitic properties and high ductility matter more than the elevated corrosion resistance of the chromium-alloyed Ni-Resist grades.

GGG-NiMn13.7 is the most machinable of the austenitic grades covered in this series, though it's still noticeably tougher than any of the standard ferritic-pearlitic GGG grades. Its lower nickel content (12–14% versus 18–32% in the NiCr grades) means a somewhat less thermodynamically stable austenite, which translates to a modest reduction in work-hardening tendency and cutting forces compared with GGG-NiCr20.3 or GGG-NiCr30.1 — but the fundamental gummy, heat-concentrating behavior of an austenitic matrix is still present and still governs tool selection.

The absence of chromium carbides (chromium is capped at 0.2%) removes one abrasive wear mechanism that shows up in the NiCr grades, so flank wear tends to be somewhat more manageable here. The high manganese content doesn't introduce the same kind of hard second-phase particles, so the matrix itself — rather than embedded carbides — remains the dominant factor in tool wear and cutting force.

Sharp, positive-rake tooling and steady feed rates still outperform a cast-iron-style negative-rake edge on this alloy, and reduced speeds relative to standard ductile iron remain appropriate — just less aggressively reduced than for the higher-nickel NiCr grades. Good coolant and proactive tool indexing round out a sound approach.