Study of Precipation and Boron Distribution in 20Cr 25Ni Base Stainless Steels

摘要

The effect of alloying additions and heat treatment on the microstructural distribution of boron was studied. Eight alloys were investigated, low boron ( 2 wppm) and high boron (containing nominally 300 wppm) versions of a ternary; unstabilized; niobium stabilized and titanium stabilized stainless steels containing 25wt%Cr, 25wt%Ni. These alloys were investigated in the solution treated and in the aged condition by boron autoradiography, optical microscopy, and by transmission electron microscopy techniques. It was found that the distribution of boron within the microstructure could be affected by both alloying addition and heat treatment. The alloys with boron additions had boron segregation to and precipitation at the grain boundaries following solution treatment, whilst those that were boron free generally did not. These observations are related to the kinetics of nonequilibrium boron segregation and to the association of boron with certain precipitate types. The precipitation in aged specimens was also affected by boron: in the unstabilized alloy and stabilized alloys it was heavier. Where M23C6 was precipitated boron caused a marked increase in matrix precipitation.