Microstructure Evolution, Growth Kinetics and Microhardness of Powder-Pack Borided Layers Produced on K190 Ledeburitic Chromium Steel Manufactured Using Powder Metallurgy

The unique powder-pack boriding technique using an open retort with boriding medium was applied for the first time in order to produce boride layers on K190 ledeburitic chromium steel manufactured using powder metallurgy. The processes were carried out using the commercial Durborid®G powder mixture at 1173 K, 1223 K, and 1273 K for 3 h, 6 h, and 9 h. As a result of the boriding of the high-carbon and high-chromium substrate, three zones were revealed in the produced surface layers: the outer FeB zone, the inner Fe2B zone, and the transition zone, with increased carbon content. The total thickness of the boride layers (FeB + Fe2B) ranged from 14.13 µm at the lowest temperature and shortest time to 65.49 µm at the highest temperature and longest duration. Increasing the temperature and extending the boriding time resulted in a deeper FeB zone as well as a thicker total layer (FeB + Fe2B). The growth kinetics of the produced layers on the surface of K190 steel were analyzed for the first time using the mean diffusion coefficient model. The thicknesses of the FeB zone and the total layer (FeB + Fe2B) were determined. The activation energies of boron for the FeB and Fe2B phases calculated in this work are comparable with other results for the powder-pack boriding of high-carbon tool steels. As a consequence of the high chromium content in K190 steel, chromium borides were observed in the boride zones, which increased the hardness of the surface layer. The highest temperature used resulted in the formation of vanadium borides. The presence of the transition zone with an increased carbon concentration and a high percentage of carbides resulted from the movement of carbon atoms toward the core by the advancing boron diffusion front. The parameters of boriding (temperature and time) as well as the presence of alloying elements in the substrate material influenced the microhardness of the boride layers.