Futiguc limit and crack propagation behavior of TRIP aided dual-phase (TDP) steel associated with the TRIP of retained austenite were investigated under the stress ration R of 0.1 and at three testing temperatures 20, 100 and 200°C. For comparison, a conventional ferrite-marlensite dual phese (DP) sted was also tested. The fatigue limit of TDP steel was considerably higher than that of DP steel. Furthermore, the fatigue limit decreased with increasing testing temperature, differing from the case of DP steel. These results were ascribed to the solution strengthening of the ferrite matrix due to the large amount of added silicon and the "block effect"against crack initiation due to the strain-induced transformation of the retained austenite. In a low ΔK region the TDP steel crack propagation rate da/dN was higher than that of DP steel, and was accompanied by a smaller threshold value ΔKm. On the other hand, in a high ΔK region, da/dN was somewhat lower than that of DP stecl. Such crack propagation behavior was mainly explained on the basis of the crack closure effect induced by fracture surface roughness and fretting oxide films.
