1010 steel was welded under controlled atmosphere of argon and then treated by intermediate and intercritical quenching techniques to produce a duplex microstructure. Resulting microstructures and complementary hardness and tensile properties were studied. It is clear that the microstructure after intermediate quenching treatment is particularly more homogeneous, where irregularly distributed fine martensite islands are dense and mostly surround small equi-axed ferrite grains in the matrix. With increasing the annealing temperature in the two-phase region, martensite islands increase in number apparently and consequently beneficial mechanical properties can be achieved even the heat effected zone (HAZ) has the weakest strength. Thus, fracturing almost always took place along HAZ, showing an inter-granular mode for intermediate quenching treatment and trans-granular mode of fracturing for intercritical quenching treatment at the end of tensile tests. © 2005 Elsevier Ltd. All rights reserved.