1. The problem statement, all variables and given/known data The lower yield point for an iron that has an average grain diameter of 1e-2mm is 230MPa. At a grain diameter of 6e-3mm, the yield point increases to 275MPa. At what grain diameter will the lower yield point be 310MPa? 2. Relevant equations Not sure 3. The attempt at a solution Not sure how to do it.