Plasma paste boronizing of AISI 8620 steel by using different paste mixtures and investigation of their surface properties

In recent years, the surface modification of materials used as a result of expecting superior properties from metarials with the development of technology has been noted. For this purpose, boronizing is the first thing that comes to mind as an effective method of surface hardening.Above all, boronizing surface treatment within the mechanical and metallurgical industry in the world is in demand in tribological perspective. Plasma paste boronizing process is easily carried out by using boron products (borax, B2O3, B4C). The gases (B2H6, BCl3, BF3) used in plasma boronizing which are expensive and corrosive characteristics and purchasing boron resources used in pack boronizing from abroad makes plasma paste boronizing attractive. Using boron sources such as borax and boric acid that is common in our country in this process shall provide the opportunity in the consumption of boron and in the use of boron resources.In this study, AISI 8620 steel was plasma paste boronized with different paste mixtures which consist of borax, B2O3, SiC and B4C. Plasma paste boronizing process was carried out at temperature of 700, 750 and 800 °C for 2, 5 and 7 hours at gas mixture of 30% Ar + 70% H2 and 10 mbar working pressure. After boronizing, the samples were cut on cross section and prepared using standart metallographic techniques. The microstructure and thickness of plasma paste boronized samples were examined by an optical and SEM microscopy. The surface borides were identified by using XRD technique. The hardness of boride layers were measured by Vickers indenter. Moreover, growth kinetics and wear behaviours of boride layers were examined.Experimental results showed that boride layer thickness increased with the increase in the temperature and duration of the plasma paste boring process It has been found that Fe2B phase in all boron paste mixtures at 700 ° C is more dominant than FeB phase. There has been an increase in the Fe2B volume with the increase in SiC additive ratio. No boride layer could be obtained for pack boriding with the Ekabor II powder for 2 and 5 hours at 700°C. The SEM micrographs of plasma paste boronized samples showed that boride layers have saw tooth morphology.The thickness of boride layer increased with the increase in the temperature and time of the plasma paste boriding process. Generally, it has been found that increase occurs in the surface hardness of the samples with increase of the amount of B4C in both borax and B2O3 borax pastes. It has been observed that SiC added borax and B2O3 mixtures have less hardness values when compared to B4C added ones. Growth kinetics studies showed that using plasma paste process caused lower activation energy for the formation of the boride layer than that for the conventional boronizing processes.