An uniform nitrogen content in the surface and interior was
obtained by solution annealing some steels in an atmosphere
blowing 0.3 l/min argon gas at 1100 ◦C for 2 h after the HTGN
heat treatment for 10 h (HTGN-SA).
Subsequently,the steels
were tempered in an atmosphere blowing 0.3 l/min argon gas
from 200◦ to 750◦ for 1 h. Optical microscopy (OM), scanning
electron microscopy (SEM-EDS) technique (JSM-6700f,
Japan) were used to observe the microstructure and morphology of the samples after being etched by the solution of 15 ml
hydrochloric acid,