| Abstract: | Dual-phase structures are produced in the three experimental steels, namely A1, A2 and A3, a) by air-cooling from the austenitising temperature (910°C) and then intercritically annealing the ferrite-pearlite structure at 750°C and 810°C followed by water quenching, and b) by water-quenching from the same austenitising temperature and then intercritically annealing the martensitic structure again at 750°C and 810°C followed by water quenching. The ferrite phases present in the alloys A1 and A2 have formed in two different ways: i) before and/or during intercritical annealing (old ferrite) and ii) during cooling of the alloys from the intercritical annealing temperature (new ferrite). The amount of new ferrite has been found to be larger in alloy A1 as compared to alloy A2. Alloy A3 did not show any measurable amount of new ferrite. TEM studies did not reveal any significant difference in microstructure in any of the alloys as a result of the initial heat treatment. The volume percent of martensite is maximum in alloy A2 and minimum in alloy A1, with alloy A3 coming in between. Although the amount of martensite in alloy A1 is somewhat lower than that in alloy A3, the overall strength of alloy A1 is higher than that of alloy A3 due possibly to the significant solid solution hardening of the ferritic matrix caused by silicon. Alloy A2 has been found to have the highest strength amongst the three alloys. |
