Structural Reasons of the Trough-Thickness Variation of Mechanical Properties in Large Forgings of Nuclear Reactor Pressure Vessel Steel

The ductile-brittle transition temperature is the usual reference parameter for the safety assessment of the reactor pressure vessel. In large forgings this parameter can change with the distance from the surface. Thus, the safety assessment must consider the effect. To evaluate the significance for the VVER 440-type pressure vessel the relationship among mechanical properties, through-the-thickness position and microstructure was investigated at a forging of 15Kh2MFA steel. The change of the mechanical properties (hardness, strength, transition temperature) are relatively low. The surface-near position is harder and tougher than the deeper range. Both ranges differ in the content of preeutectoid ferrite and in the arrangement, but not in the content of carbides. The Orowan model of the mechanism of precipitation hardening can describe quantitatively the dependence of the yield strength and the number of M23C7 carbides per unit area of section plane found experimentally on the trough-the-thickness position.