Coupled effects of nitrogen content and austenitizing temperature on the hardness-toughness balance of Cr–Mo–V hot-work tool steels

Nitrogen is widely regarded as beneficial for the hardness–toughness balance of Cr–Mo–V hot-work tool steels, yet retaining high nitrogen during steelmaking is costly and not always feasible. Whether a lower-nitrogen steel can still reach an adequate balance depends on how nitrogen interacts with the austenitizing condition—a coupling rarely examined directly. Here, two commercial Cr–Mo–V steels containing 200 ppm (N-200) and 50 ppm (N-50) nitrogen were austenitized at 1010, 1020, 1030, and 1040 °C and double tempered. The two properties responded oppositely to nitrogen. Counterintuitively, N-50 was harder at every condition: its less stable MC carbides dissolved more during austenitizing, releasing V and C that formed a denser population of nanosized V(C,N) precipitates during tempering. In contrast, N-200 retained better toughness, and this advantage was strongly condition-dependent—the N-200/N-50 toughness ratio stayed near 1.2 below 1020 °C but rose to 1.8 at 1040 °C, as the more stable carbides in N-200 continued to pin the prior austenite grain boundaries while those in N-50 dissolved. The toughness response thus reflects a dual role of the coarse undissolved carbides, which refine the martensitic structure but also provide crack-initiation interfaces. The benefit of nitrogen is therefore not absolute but tied to the austenitizing window: with proper temperature control, a lower-nitrogen steel from conventional melting can match a high-nitrogen steel that would otherwise need costly specialized routes.