Novel ultrahigh strength crack free tool steel by laser powder bed fusion: revisiting carbon bearing Ni–Co–Cr–Mo system

Carbide-strengthened Ni–Co–Cr–Mo steels were developed to combine the high strength of Fe–C martensite with the weldability of Fe–Ni maraging alloys. Designed for laser powder bed fusion, Osprey® MAR55 achieves a balanced combination of printability, toughness, and strength through refined lath martensite and controlled carbide precipitation, addressing the typical weldability limitations of medium-carbon tool steels. After direct ageing at 480 °C, MAR55 shows excellent properties, with toughness above 30 J, hardness around 550 HV10, and tensile strength close to 1900 MPa. Thanks to its leaner alloy chemistry, particularly reduced Ni content, and solute depletion (C, Mo, Cr) during carbide precipitation, aged MAR55 exhibits ∼20% higher thermal conductivity than 18Ni300, reaching or even surpassing tempered H13 steels at elevated temperatures. In addition, carbon addition and the formation of stable Mo2C carbides significantly enhance wear and abrasion resistance. In-situ synchrotron measurements confirm high thermal stability and slower austenite reversion compared to 18Ni300, consistent with the lower Ni content. Microstructural analysis revealed ∼15 vol.% retained austenite (RA) in the as-built state. Continuous-heating dilatometry and Differential Scanning Calorimetry (DSC) further revealed six transformation mechanisms during heating, involving carbide clustering, RA decomposition, alloyed cementite and carbides precipitation, Ni-driven austenite reversion, and high-temperature shear transformation. Direct tempering at 480 °C reduces RA to ∼3 vol.% through decomposition of Mo- and C-rich austenite, while temperatures above 600 °C promote Ni-driven reversion. The combination of high strength, toughness, wear resistance, thermal conductivity, and thermal stability makes MAR55 a promising material for advanced additive manufacturing and demanding engineering applications.