Effect of TiN Inclusion on Fracture Toughness in Ultrahigh Strength Steel

抄録

The investigation was conducted on 6 specimens of an identical composition except for N and Ti contents, the specimens were remarkably free of nonmetallic inclusions other than TiN inclusion.
The quasi-dynamic method has been used to investigate the condition of nucleation, growth and propagation of micro-crack from TiN inclusion.
In summarizing the present study the following conclusion can be made:
(1) Fracture toughness K_IC is decreased rapidly with increasing volume fraction of inclusion (f_V) for f_V<0/1% and becomes insensitive when f_V>0.1% where the percentage of cracked-TiN reaches a limiting value.
(2) The percentage of cracked-TiN is determined by the true fracture strain ε_f and its propagation depends mainly on the critical true fracture strain ε_f^*. The ε_f^* can be computed by using both McClintock's model and Wall-Hill method as follows:ε_f^*=0.0345 ln (d_t/a)Thus the ε_f^* can be correlated with the inclusion spacing (d_t) and size (a).
(3) Based on the testing results, we get a similar relation to the ductile fracture model proposed by Rice, that is:K_IC=(E · σ_y · d_t)^1/2