Abstract
The H11 hot work tool steel was short-time boronized, austenitized, quenched and tempered to a standard core hardness of 47 - 48 HRC. Microstructure, phase constitution and microhardness of boronized layers were investigated. It was found that the boronizing begins with the formation of α-solid solution, enriched with boron, and continues with the development of firstly the Fe₂B-boride compound and finally with the FeB-phase. During the boronizing, carbon atoms are transported from the surface towards the parent material. They are accumulated in a narrow intermediate region beneath the borides, where they develop new carbides. These carbides were identified as M₂₃C₆. Their portion probably increases with prolonged boronizing time, which is reflected in higher hardness of intermediate region. Also the hardness of boron compounds was found to be increased slightly with longer processing time.

Key words
powder boronizing, H11 hot work tool steel, microstructure, phase constitution, borides, carbides

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