Fe-Me-B diffusion layers for surface modification of carbon and alloy steels

Mishigdorzhiyn Undrakh - East Siberia State University of Technology and Management (Russia)

Multicomponent boron-based diffusion layers are capable to provide a wider variety of surface improvements compared to pure boriding. In this research, we consider a way to increase mechanical properties of carbon and alloy steels by using two-component thermal-chemical treatment (TCT) such as borochromizing (B-Cr), boroaluminizing (B-Al), and boronickelizing (B-Ni). Diffusion treatment of steel surface was carried out by pack method in powder mixtures, containing the above-mentioned elements and sodium fluoride as an activator. The exposure time was 2-4 hours, the treatment temperature was 950-1050 °C. Pure boriding was conducted additionally to compare with two-component methods. The metallographic analysis revealed diffusion layers with tooth-like structure after B-Cr and B-Ni diffusion TCT, which consists of iron boride FeB as an outer phase and Fe2B as an inner one. EDS analysis revealed that these compounds contain a small amount of Ni and Cr. Another structure was obtained after B-Al diffusion TCT – namely a complex heterogeneous structure, where hard iron borides were mixed with softer phases – Fe3Al and α-Fe(Al). The layer thickness after boroaluminizing increased from 110-190 μm to 550-570 μm, compared to pure boriding, borochromizing, and boronickelizing. The maximum microhardness values reached about 2000 HV after all three methods. Wear tests indicated that the samples with the tooth-like structure were slightly superior to the ones with heterogeneous structures.
Keywords: thermal-chemical treatment, borochromizing, boroaluminizing, boronickelizing, multicomponent diffusion treatment, metallography, microhardness, wear resistance.

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