The steel-making practices followed for grain refinement can vary, particularly with respect to the grain refining element selected. Aluminum, niobium and vanadium are all used for grain refinement in medium carbon steels. AISI has examined this issue for SAE 1141 steel in both the normalized and quenched and tempered conditions.
Figure 1 shows strain-life fatigue data for three test iterations in the normalized condition. Iteration 11 was grain refined with aluminum, Iteration 13 with niobium, and Iteration 15 with vanadium. The hardness values fell in a fairly narrow range, 199-223 BHN, and the microstructures were ferrite-pearlite.
As can be seen, there is little variation in the strain-life curves for the three sets of data. This suggests that any of the three grain refining elements can be employed without worrying about differences in fatigue properties.
Figure 2 shows results obtained for SAE 1141 steel in the quenched and tempered condition. Iteration 12 was grain refined with aluminum, Iteration 14 with niobium, and Iteration 16 with vanadium.
In these cases there was more variation in hardness than in the normalized condition. Iteration 12 and 16 exhibited hardness values of 277 and 252 BHN and the microstructures were mostly martensite. Iteration 14 had a hardness value of 241 BHN, and the microstructure contained a significant amount of bainite and some ferrite.
There is somewhat more scatter in the strain-life fatigue data, however it can still be concluded that the various grain refining elements can be used interchangeably.
The data does indicate that even where fatigue performance is a consideration for a given application, grain refining elements can be selected based on other factors such as steel manufacturing preferences, economics, etc.