*3.5. Impact Toughness and the Microstructure Effect*

The impact toughness of simulated CGHAZ and FGHAZ was present in Figure 11. At −60 ◦C, the mean absorbed energy of simulated CGHAZ was 42, 39 and 31 J with t8/5 of 10 s, 20 s and 30 s, while it was 36, 45 and 56 J in the FGHAZ at the identical cooling rate. The impact toughness seemed to be independent of the cooling time t8/5 at the different peak temperatures. In addition, the FGHAZ did not show more superior impact toughness than CGHAZ, which differentiates from conventional high strength low alloy (HSLA) steel. Even much finer effective grain and more 10~11% retained austenite, the impact toughness was not much better in FGHAZ than in CGHAZ. The harder martensite was the main factor in impact toughness. Moreover, the retained austenite may transform into martensite at −60 ◦C during the impact test, which is referred to as the transformation-induced plasticity (TRIP) effect [19]. This is due to the relatively lower enrichment factor of 1.8 in retained austenite, which means lower stability of retained austenite. The martensite due to the TRIP effect led to the decrease in impact toughness.

**Figure 11.** Absorbed energy of simulated (**a**) FGHAZ and (**b**) CGHAZ at the variation of t8/5.
