Briefly explain why the hardness of tempered martensite diminishes with tempering time (at constant temperature) and with increasing temperature (at constant tempering time).

Answer: The decrease in hardness of tempered martensite is affected by both tempering time and temperature due to the progression of microstructural changes during the tempering process. With increased tempering time, more cementite particles precipitate and coarsen, and dislocation rearrangement becomes more advanced, leading to reduced hardness. At higher tempering temperatures, the mobility of atoms increases, resulting in a more rapid precipitation and coarsening of carbide particles and enhanced dislocation rearrangement. This causes a more pronounced decrease in hardness with increasing tempering temperature. These microstructural changes ultimately lead to a more ductile and less brittle material, which is the primary goal of tempering martensitic steels.