A comparison of the fracture behavior of two heats of the secondary hardening steel AF1410

The fracture behavior of two heats of AF1410 steel has been investigated for two aging temperatures, 425°C and 510°C. The first heat, modified by deliberate lanthanum additions, contains lanthanum-rich inclusions characterized by an inclusion spacing of 7.6 μm. The second heat, not modified by lanthanum additions, contains a comparable volume fraction of much smaller CrS inclusions characterized by an inclusion spacing of 2.3 μm. Of the mechanical properties measured, only the toughness appears to be influenced by inclusion type. For example, on aging at 510°C, δIC, the critical value of the crack tip opening displacement, for the lanthanum modified heat is 66 μm compared to 28 μm for the heat containing CrS inclusions. This influence of inclusion type is attributed to the larger spacing of the inclusions in the lanthanum modified heat. In addition, for both heats the toughness on aging at 510°C was higher than after aging at 425°C. Particle analysis from extraction replicas of polished cross-sections of as-quenched materials and of fracture surfaces of the 425°C and 510°C microstructures indicates that on aging at 510°C only undissolved particles inherited from the austenitizing temperature nucleate secondary voids, but that on aging at 425°C carbides precipitated on aging nucleate secondary voids as well.

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