Influence of Ni and Mn on Microstructure and Mechanical Properties of Coarse Grain Heat Affected Zone in Low Carbon Microalloyed Steel

Abstract

A Gleeble simulation of the coarse grain heat affected zone (CGHAZ) with two different heat inputs (20 and 30 kJ/cm) welding was performed on low carbon steels with different Ni and Mn contents to investigate the effect of Ni and Mn addition on toughness and microstructure evolution. The results indicated that Ni and Mn have considerable effect on variation of the microstructure and mechanical properties, but this effect is associated with heat input. With increasing Ni content the amount of granular bainite decreased and acicular ferrite or lower bainite increased under the condition of 30 kJ/cm heat input, and this effect was not obvious with the heat input of 20 kJ/cm though the prior austenite grain size was increased by the Ni addition. The effect of Mn (with 2.0% Ni content) was mainly to promote the formation of martensite structures. The impact toughness at −20 °C was shown a good and stable value unrelated to Ni content with the heat input of 20 kJ/cm, while it had a significant rise from 15 J to 39 J on increasing Ni content from 2.0% to 3.0% with the heat input of 30 kJ/cm. The lowest impact energy was obtained at a very high level of Mn (i.e. 2.0% Ni-2.5% Mn). Thorough investigation of fracture surface of the simulated specimens, after impact test, and combined with the microstructure observation, elucidated that the formation of martensite even coalesced/massive martensite, deteriorated toughness. Based on hardness and impact resistance, the optimum levels of Mn and Ni were suggested to be 1.5% and 2.0–3.0%, respectively.