(2) heat resistant steel notch sensitivity: gap caused by stress concentration to the persistent strength decided to temperature, the influence of geometric shape of the gap, durable plastic steel, heat treatment process and factors such as the composition of the steel Fen hot steel under a certain temperature to the view out of the notch sensitivity, such as for 25 cr2mo1v steel, when temperature is 550 ℃ show a larger notch sensitivity;A significant reduction in the heat resistant steel tooling lasting strength.For different steel, the persistent stretched at notch sensitivity that has not completely the same.Gap geometry has distinct effect on the rupture strength of steel is well known.But it must be pointed out that the gap depth of plastic material and the influence of the strength of a brittle material is vastly different.This factor must be given enough attention in the design.