Characterization of Microstructure and Fracture Performance of Boronized H11 Grade Hot-Work Tool Steel

Abstract H11 grade hot-work tool steel was subjected to powder boronizing at 1,030°C for different processing times. Subsequently, the steel was austenitized, quenched, and tempered to a bulk hardness of 47–48 HRC. The microstructure, phase constitution, and hardness of boronized layers were examined using scanning electron microscopy and microanalysis, glow discharge optical emission spectroscopy, X-ray diffraction, and microhardness measurements. The effect of the boronized region on the material fracture performance was examined using the Charpy impact test, according to the corresponding standard NADCA 202-97, Recommended Procedures for H13 Tool Steel. It was determined that the growth of the boronized layer begins with a boron enriched α-phase. Then, the (Fe,Cr)2B boride was developed, and in the case of longer processing time, the (Fe,Cr)B boride was grown. During boronizing, carbon atoms were transported from the surface toward the bulk, forming extra carbides beneath the compound regions. Their amount increased with increasing boronizing duration, which was reflected in the enhanced hardness of the intermediate region beneath the compound boronized layer. The microhardness of (Fe,Cr)2B boride layers was around 1,600 HV 0.1, and the microhardness values of (Fe,Cr)B considerably exceeded 2,000 HV 0.1. However, boronizing led to a substantial reduction of the Charpy impact toughness of the material, and this reduction was more pronounced when a thicker compound layer was formed on the steel surface. The reduction in toughness was clearly reflected on the fractured surfaces as they manifested clear cleavage fracture propagation.