Maraging C300 is the most widely used high-strength grade in the 18% nickel maraging steel family, prized across aerospace and high-performance tooling for reaching very high strength while keeping fracture toughness that a conventional through-hardened steel of similar strength simply can't match. The name comes from its two-step processing: cooling from solution-annealing temperature forms a soft, low-carbon nickel-cobalt-molybdenum-titanium martensite, and a subsequent aging treatment at moderate temperature precipitates fine intermetallic compounds through that martensite to develop the alloy's full strength.

The "300" in the designation reflects the approximate minimum yield strength once fully aged — roughly 300,000 psi, or about 2,070 MPa — achieved through a higher titanium content than the C200/C250 grades below it. That strength level, combined with good weldability, excellent dimensional stability through heat treatment, and reliable toughness even at high hardness, makes C300 a standard choice for rocket motor cases, aircraft landing gear, high-performance shafting, and precision die and mold components where both strength-to-weight ratio and predictable behavior under load are critical.

C300's machinability follows the same split personality as the rest of the maraging family, just with a wider gap between the two conditions. In the solution-annealed (unaged) state — typically around 32-34 HRC — the low-carbon martensite machines reasonably well with standard carbide tooling, similar to a moderately hard alloy steel. This is the condition in which most rough and semi-finish machining takes place, before the part is aged to reach its full 300 ksi capability.

Once aged, C300 climbs to roughly 54-56 HRC — among the hardest of the standard maraging grades — and machining becomes considerably more demanding. Cutting forces and tool wear both increase noticeably, and post-aging work is generally limited to grinding, EDM, or very light precision cuts rather than significant material removal. Because the strengthening mechanism is fine intermetallic precipitation rather than a coarse carbide network, aged C300 is less abrasive than an equally hard tool steel, but the combination of high hardness and high toughness still narrows the practical machining window considerably. The speeds below reflect the solution-annealed condition, which is how this material is normally machined before final aging.