Caracterização de juntas soldadas por TIG autógeno manual com arco pulsado e arco não pulsado do aço inoxidável hiperduplex SAF 2707 HD.

Abstract

Hiperduplex stainless steel, SAF 2707 HD, was developed due to the need of new alloys with greater corrosion resistance than that available in duplex / superduplex stainless steels. In addition to improved corrosion resistance, SAF 2707 HD also offers superior mechanical properties. Hiperduplex is a state-of-the-art steel that contains high levels of alloying elements, especially chromium, molybdenum and nitrogen. This type of steel is characterized by having a biphasic structure, comprising substantially equal proportions of ferrite and austenite due to the controlled distribution of the alfagenics and gamagenics elements. The interest in this type of steel increases gradually with the need of new materials for a variety of applications in the oil industry. However, when exposed to elevated temperatures and maintained between 600 °C and 1000 °C, some intermetallic phases can appear, principally consisting of the sigma phase (). The Sigma phase has a complex tetragonal structure rich of Cr and Mo, having a deleterious effect on the material affecting both the corrosion resistance and mechanical properties. Due to this, it is necessary to study welded joint of these materials to delineate the limits of these steels and improving their application. This study aimed to characterize the autogenous TIG welded joint by manual arc pulsed and non-pulsed hiperduplex stainless steel SAF 2707 HD. The techniques employed were metallography by electrolytic attack (NaOH reagent) and color etching (Behara reagent), micro-hardness measurements and microstructural quantification by Digital Image Processing. The microstructural aspects were observed by optical microscopy (OM) and scanning electron microscopy (SEM), and they have gone through stages of digital image processing (DIP) to quantify the percent of austenite phase. The chemical composition was verified by EDS. The results were analyzed statistically by testing hypotheses with Student's t distribution. The color etching technique made it evident that the austenite phase had a more homogeneous distribution for pulsed arc, than it did for non-pulsed. The electrolyte attack did not show a sigma phase () in the welded joint and the chemical composition verified by the EDS did not identified a significant change in these elements along the transition zone of the base metal to the fusion zone. Through the DIP the average percentages obtained for austenite were 36.38% (standard deviation 6.40%) for the pulsed method and 32.41% (standard deviation 6.67%) for the non-pulsed method. The Pulse method produced mean hardness values of HV 355.10 (SD = 28.60) for the base metal and HV 343.60 (standard deviation 20.51) for the fusion zone whereas, the non-pulsed method these values were HV 370.30 (standard deviation 34.51) for the base metal and HV 345.20 (standard deviation 41.33) for the fusion zone. Statistical analysis showed no significant variation in the percent of austenite phase in the welded joint for two tested conditions and no variation in hardness between the fusion zone and weld metal for samples tested by the two processes.