Boride layers on high-carbon high-chromium tool steels: Microstructure-mechanical properties relationship

High-carbon high-alloyed steels (Sleipner, K 190, Sverker 21, Sverker 3) were boronised at 1323 K for 10 h. The layers that developed were composed of FeB and Fe 2 B phases. Beneath the borides, transient regions were found with the presence of ‘extra’ carbides, where the number of carbides was doubled compared with the substrates of steels. The extra carbides were chromium-rich, and were identified as M 23 C 6 for all the steels examined here. The borides mainly contained iron, although chromium was also present in FeB and Fe 2 B, in amounts of 7.8–10.1 and 10.3–11.9 wt.%, respectively. High-resolution transmission electron microscopy revealed only the Fe 2 B phase at a depth of 30 μm below the surface, in the close vicinity of insoluble high-chromium carbide in Sverker 3 steel. No borides of alloying elements were detected. The FeB had higher hardness and brittleness but lower fracture toughness than Fe 2 B. The content of alloying elements in borides had only a very limited impact on the mechanical properties. The amounts of undissolved carbides embedded in the borides increased their hardness and brittleness but reduced the fracture toughness.