Effect Of Cu And Nb On Microstructure And Tensile Strength Of Multi-Layer Weld HASL Steel

In this paper, weld metals with Cu of different levels ranged from 0 to 3.2wt% and 0.03wt% Nb or not were prepared by deposition on plate of a commercial Q345-grade steel with fine pure copper wire and Nb-Fe power pre-depositing in a V-shape groove using inner gas shielded metal arc welding and a commercial C-Mn welding wire. The different zones of multi-layer weld metal characterized by peak temperature of thermal cycles ranged from 650 to 1300?C and energy input of 10-75kJ/cm, respectively were successfully simulated in a Gleeble-3500 thermal-mechanical simulator. The hardness of simulated samples was measured and the microstructure was observed. The possible precipitated particles embedded in the matrix of simulated weld metal were examined using transmission electron microscope (TEM). The role of Cu and Nb in improving the uniformity of microstructure and mechanical properties of reheated weld metal was emphasized for the present study.It was shown that 1.0wt% or less Cu contained in single-pass weld metal resulted in a limited strengthening effect until 3.0wt% Cu level reached, at which the hardness of weld metal as high as 280 HVN measured or tensile strength of approximate 900Mpa estimated was achieved. The addition of Nb in single-pass weld metal did not enhance significantly the strengthening effect of copper. For the simulated multi-layer weld deposit, the addition of single Cu of any level produced poor distribution uniformity of hardness. However, dual addition of 0.5-3.0wt% Cu and 0.03%Nb resulted in a uniform distribution of hardness and microstructure of dominantly acicular ferrite and lower bainite in reheated weld deposit. Moreover, in comparison to that of reheated weld metal with single copper added, the hardness of reheated weld metal with both Cu and Nb addition decreased less as the heat input increased. The obvious dual precipitation of Cu and Nb were observed in the simulated multi-layer weld metal with 3.0wt% Cu and 0.03wt% Nb added. The uniform precipitation ofε-Cu was therefore enhanced and hardness of more than 280 (HVN) was achieved. The multi-layer weld metal with tensile strength of more than 900MPa and hardness distributed uniformly could be accordingly obtained.