Distribution of chromium and nickel between the phases present in the borided layer of alloy steels
Abstract
The preferential distribution of transition elements in the borided layer consisting of phases derived from the borides FeB (rhombic) and Fe2B (tetragonal) has been verified using commercial steels containing chromium and nickel (UNI UX 100 Cr 13, UNI X8Cr 17 and UNI X8 CrNi 1808). Bonding treatment was carried out at 900° C with powders enriched in KBF4 and CB4. The average content of the alloying elements was determined using the microanalysis of areas of dimensions comparable with those of the single phase layers.
It was found that chromium diffuses from the parent metal into the borided layer and is distributed preferentially in the phase which is more rich in boron, as we have already discovered in investigation of the two phase field [(Fe,Cr)2B + (Fe,Cr)B] of the system Fe-Cr-B; nickel on the other hand tends to diffuse from the external layer into the bulk of the metal and the rhombic phase becomes depleted in this element in comparison with the tetragonal phase. These conclusions are in agreement with the generally valid observation which we have made in ternary systems between boron and pairs of transition metals as a result of which the element having the high atomic number is preferentially inserted into the phase which is less rich in boron in the distribution of the above mentioned metals between borides in equilibrium in the two phase zone. An intermediate zone between the borided layers and the bulk of the metal which is particularly rich in carbon, and, in the case of UNIX 8 CrNi 1808 steel only, nickel also, has been observed in all the steels examined
