Extraordinary toughness in AISI 1008 steel via controlling intercritical annealing time

Abstract

In the present work, extraordinary toughness in AISI 1008 steel was achieved by controlling the holding time of intercritical annealing. The as-received steel was heat treated at 1050 °C for 10 min, followed by quenching in water. Then, the quenched steel was intercritical annealed at 860 °C for four different times of 1, 5, 10, and 20 min, followed by water-quenching. The microstructural evolution and mechanical behavior of produced ferritic-martensitic dual-phase (DP) steel samples were examined. As the holding time increased from 1 min to 20 min, the martensite fraction of the sample increased from 0.31 to 0.79, and the carbon content of the α´ reduced from 0.225 wt% to 0.066 wt%. The formation of hard martensite from austenite during final water-quenching was limited in the 860–20 steel. The hardness increased from 176.5 to 209.0 HV with an increase in the holding time from 1 min to 10 min. However, after the holding time of 20 min, the hardness value decreased to 172.4 HV. All dual-phase steels (except the 860–20 sample) exhibited continuous yield behavior. With an increment of the holding time from 1 min to 10 min, the tensile strength was enhanced from 588.1 MPa to 648.2 MPa, the ductility was improved from 27.6 % to 36.4 %, and the energy absorption was increased from 146.0 J/cm3 to 224.8 J/cm3. The hardness, strength, ductility, and toughness were increased linearly with the increase of the martensite fraction. All DP steel samples revealed many dimples indicating a typical ductile fracture. By increasing the holding time from 1 min to 10 min, the size of the dimples increased.