Role of alloying elements

Carbon has a large effect on the range of temperature over which upper and lower bainite occur. The B_S temperature is depressed by many alloying elements but carbon has the greatest influence, as indicated by the following empirical equation:
B_S (° C) = 830-270C -90Mn-37Ni-70Cr-83Mo
where the concentrations are all in wt.%. Carbon has a much larger solubility in austenite than in ferrite, and is a very powerful austenite stabiliser which leads to a general retardation of reaction kinetics. The fraction of carbides to be found in the final microstructure increases in proportion to the carbon concentration, so that the concentration must be kept below about 0.4 wt.% to ensure reliable mechanical properties. We have already seen that an increase in carbon makes it easier for lower bainite to form because it becomes more difficult for plates of supersaturated bainitic ferrite to decarburise before the onset of cementite precipitation.

In plain carbon steels, the bainitic reaction is kinetically shielded by the ferrite and pearlite reactions which commence at higher temperatures and shorter times, so that in continuously cooled samples bainitic structures are difficult to obtain. Even using isothermal transformation, difficulties arise if, for example, the ferrite reaction is particularly rapid. The addition of metallic alloying elements usually results in the retardation of the ferrite and pearlite reactions. In addition, the bainite reaction is depressed to lower temperatures. This often leads to a greater separation of the reactions, and the TTT curves for many alloy steels show much more clearly separate C-curves for the pearlite and bainitic reactions. However, it is still difficult to obtain a fully bainitic microstructure because of its proximity to the martensite reaction.

A very effective means of isolating the bainite reaction in low carbon steels has been found by adding about 0.002 wt.% soluble boron to a 0.5 wt.% Mo steel. While the straight molybdenum steel encourages the bainite reaction, the boron markedly retards the ferrite reaction, probably by preferential segregation to the prior austenite boundaries. This permits the bainite reaction to occur at shorter times. At the same time, the bainite C-curve is hardly affected by the boron addition, so that martensite formation is not enhanced. Consequently, by the use of a range of cooling rates, fully bainitic steels can be obtained.