Borides Effect on the Workability and Microstructure Evolution of As-Cast Borated Stainless Steel under Hot Compression

Hot compression tests at temperatures from 900 to 1100 degrees C and strain rates from 0.01 to 10 s(-1) are performed on as-cast borated stainless steel in order to investigate the effect of boride particles on the workability and microstructure evolution during hot deformation. Microstructure examinations are carried out by applying optical microscope, scanning electron microscope, transmission electron microscope, and electron backscatter diffraction. True stress-true strain curves indicate that borides increase the stress level at low temperature (<1100 degrees C) while sacrificing the strength at 1100 degrees C. Processing map shows that flow instability mainly concentrated at temperatures lower than 990 degrees C and strain rates higher than 2 s(-1). Microstructure study results reveal that boride particles can be the particle-stimulated nucleation (PSN) sites, but only small fraction of new grains formed by PSN. The nucleation of cavities will occur by: (i) boride cracking; (ii) vacancy condensation at boride-matrix interface; and (iii) debonding of boride-matrix interface. Moreover, cracks almost originate from cavities around borides and propagate mainly via debonding at boride-matrix interface and original grain boundary.