Influência do teor de CO2 na soldagem MIG/MAG do aço inoxidável superduplex UNS 32750 (SAF 2570) sobre a geometria e o balanço de fases

Abstract

Super duplex stainless steel (SDSS) is gaining more employability every day. It has properties that are ideal for certain applications, good mechanical properties and high resistance to corrosion. These properties are due to the biphasic constitution of these alloys that combine ferrite (α) and austenite (γ) and the balance between those two phases is extremely important to keep them. These steels impose difficulties in the welding process. The protective gas is already known to have influence on the welding process. The use of CO2 is recommended to minimize the difficulties that these alloys impose, therefore it is necessary to investigate the effects of its use in the process, on the phase balance and its geometric properties. Simple deposition gas metal arc welding (GMAW) was carried out in pulsed transfer mode of AWS E2594 electrode on base metals SAE 1010 and S32750 (SAF 2507), altering the composition of the ternary protection gas (He + Ar + CO2) through addition of CO2, altering the Ar content in the mixture at different energy levels. Afterwards, surface analysis of the strands, of geometric properties (penetration, reinforcement, width and dilution) and of phase balance of the weld metal was performed to evaluate effects of the energy variation and of the change of protection gas in these properties. During welding, it was noticed an increase of average tension, caused by increase of the CO2 content in the mixture. In the surface analysis of the cord, it was possible to notice that the enhance of CO2 content increased the amount of spatter around the weld bead. In geometric properties, there was a great variation of behaviour. In the penetration, the highest values occurred in the energy of 1.5 kJ mm-1 and in the contents of 4% of CO2 in the mixture. In the reinforcement, the fall occurred more significantly from CO2 content at 4%, while higher energies produced higher reinforcement values. In terms of width, high energy caused greater widths by increasing the volume of material deposited by the energy increase technique, since increase in CO2 content contributed to the decrease in cord width, which can be attributed to presence of the He in the mixture and its preferential movement of the welding well. Dilution had lower values for higher energies, which is believed to be the effect of the barrier generated by the greater amount of deposited material that prevented the melting of the base metal. In the quantification of the ferrite content, a marked decrease in dissimilar welding samples was observed due to the introduction of iron into the weld metal from the base metal. In addttion for the dissimilar weldings, the increase of CO2 content, promote a decrease of ferrite contente, excepting 0,5 kJ/mm weldings, where increasing CO2 content led to an increase of the ferrite contente, this could be explained due increase of the colling speed.For the similar weldings,energy had a tendency effect of the decrease of ferrite content, followed by increase of CO2 content, as both promoted reduction of the cooling rate. For energy levels of 0.5 kJ/mm, increase of CO2 content, promote increase in ferrite content. In summary, the results indicate that CO2 content has effects on geometric properties, generates an increase of arc voltage, increase the number of spatter around the weld bead and promotes a decrease in the ferrite content, according to the SDSS alloy used.