The lower yield point for a certain plain carbon steel bar is found to be 135 mpa, while a second bar of the same composition yields at 260 mpa. metallographic analysis shows that the average grain diameter is 50 µm in the first bar and 8 µm in the second bar. (a) predict the grain diameter needed to cause a lower yield point of 205 mpa. (b) if the steel could be fabricated to form a stable grain structure of 500 nm grains, what strength would be predicted? (c) why might you expect the upper yield point to be more alike in the first two bars than the lower yield point?