Effects of Ni, Ti, Si and Mn on the Microstructure and Mechanical Properties of Multi-pass Flux-cored Arc Weld of SS400 Base Steel

In this study, effects of Ni, Ti, Si and Mn on the microstructure and mechanical properties of multi-pass welded SS400 base steel have been investigated. Flux-cored arc welding (FCAW) was performed at the electrical current of 220- 240 A and the voltage of 21- 24 V using carbon dioxide shielding gas and the welding rate of 800 mm/ min. Four flux- core welding wires with different chemical compositions were used in the present investigation leading to different chemical compositions of the weld metals in the range of 0. 012- 0. 024 wt% Ti, 0. 187- 0. 474 wt% Si, 0. 832- 1. 383 wt% Mn and 0. 024- 1. 472 wt% Ni as analysed by spark optical emission spectroscopy. The microstructure of welded specimens was investigated by optical microscopy ( OM) , scanning electron microscopy (SEM) and energy dispersive x-ray spectrometry (EDS) in SEM. The results indicated that low Ni content about 0. 02 wt% gave the microstructure containing mainly polygonal ferrite ( PF) and possibly bainite, whereas high Ni content about 1.3 wt% promoted acicular ferrite (AF). Ni was not involved in inclusions within the welded metals, while Ti, Si and Mn accompanied in inclusions, some of which contains other elements such as Fe, Zr and Mg. These inclusions are mainly aluminosilicates ( Ti, Mn, Fe, Al, Si) Ox or oxysulfides ( Ti, Mn, Al, Mg, Si, Zr) OxSy. The formation of uniform AF in the microstructure of the welded metals requires the presence of Ni higher than approximately 1.3 wt% Ni and the uniform distribution of aluminosilicate or oxysulfide inclusions acting as nucleation sites for AF. The Charpy impact toughness at two different temperatures (-20 and -30 °C) and the strength of the steel joint, as well as the Vickers microhardness of the welded zone, were measured. The welded specimens with relatively higher Ni content and the AF microstructure tended to possess acceptable Charpy impact toughness and strength. The welded specimens with relatively lower Ni content, higher Si and Mn content, and comparable Ti content tended to possess lower strength, even though the microhardness of their welded zones was higher.