The Effect of Niobium and Tantalum on Grain Size of Steel

Abstract

The effects of niobium and tantalum on austenitic grain size and ferritic grain size of pure iron, Fe-C (C=0.1%) and Fe-N (N=0.02%) alloys have been studied and compared with each other. The austenitic grain size were revealed by thermal etching method. The effects of small addition of each element on the grain coarsening temperature of these alloys were also studied. The main results obtained are as follows;
(1) Small addition of niobium or tantalum to Fe-C alloys is effective for austenitic grain size refining and this may be due to the precipitation of δ-niobium-carbide or δ-tantalum-carbide. Tantalum is required two times as much amounts (in wt%) as niobium is, to obtain the same fine grain size number such as No 7·5.
(2) The addition of niobium or tantalum to pure iron and Fe-N alloys is also effective for austenitic grain size but the finer grain size than No 6 cannot be obtained in these cases.
(3) An approximately linear relationship is observed between ferritic and austentic grain size in the series of niobium-added steels as well as of tantalum added steels.
(4) Small addition of niobium or tantalum to Fe-C alloys is effective in retarding the austenitic grain coarsening.