Ductility of 18Cr-12Ni Steel Containing Coarse Irregular Carbide Precipitates along Grain Boundaries in Tensile Test at Room Temperature

Abstract

Creep-rupture strengths of austenitic steels are improved by coarse irregular carbide precipitates along grain boundaries. This structure is obtained by a two-step heat treatment, i. e., steels are solution-treated above the solvus of carbide and then directly cooled to and held at an intermediate temperature below the solvus to make a portion of carbide precipitate preferentially along grain boundaries. The improvement in creep-rupture strengths by this treatment, though it is quite remarkable and attractive, accompanies a reduction in ductility at lower temperatures.
The present study aims to find such conditions of the above two-step treatment as the creep-rupture strength is highly improved but yet the room-temperature tensile ductility does not seriously decrease.
In an 18Cr-12Ni-0.3C steel, a holding time of two to 5h at the second-step temperatures yielded substantial covering of grain boundaries by carbide precipitates which were appropriate in thickness.
This state of precipitation gave a maximum creep-rupture strength and still retained a sufficient ductility at room temperature. Longer holding times, which gave no further improvement in creeprupture, decreased room temperature ductility seriously.
The coarse irregular carbide precipitates were exposed by deep etching, observed in a scanning electron microscope, and were found to be “dendritic”.