Desenvolvimento e avaliação de calorímetros por nitrogênio líquido e fluxo contínuo para medição de aprote térmice

Abstract

One of the most influent parameter on the welding process at both industrial and research levels is the heat delivered to the workpiece (heat input) due to its direct connection with changes in metallurgical characteristics and mechanical properties of the weld joint. In order to quantify the heat input, different methods have been developed, both theoretical (analytical and numerical ones) and experimental (calorimetry). These techniques present large dispersion of results for thermal efficiency of welding processes. Therefore, this work aims to present the project, construction and assessment of a liquid nitrogen calorimeter and other one based on constant flow (water) measurement. Both calorimeters were evaluated for measuring heat input and thermal efficiency of GTAW and GMAW processes. For GTAW process, bead on plate welding was carried out with Ar as shielding gas over ASTM A36 plate, varying current, arc length, travel speed and bead length. For GMAW processes, both short-circuit and spray transfers were used with AWS ER70S-6 wire with 1.2 mm of diameter and Ar+25%C02 (short-circuit) and Ar+5%,02 (spray) as shielding gases. For derivative processes (STT - Surface Tension Transfer, RMD - Regulated Metal Deposition, CMT - Cold Metal Transfer, GMAW-P - Pulsed and GMAW-VP - Variable Polarity), the welding parameters were adopted from previous researches from Laprosolda (Center for Research and Development of Welding Processes). It can be concluded that the liquid nitrogen calorimeter presents good repeatability and stability (maximum data scattering of 3%). Also, the obtained results are coherent for different welding conditions applied through the work with global thermal efficiencies of 70,3%, for GTAW, 78,6% for conventional short-circuit GMAW, 73,6%, for GMAW with spray transfer and 7 6,1%, for GMAW derivative processes. The results measured by the continuous flow (water) calorimeter followed the trend of the liquid nitrogen one, but with average results 12% lower.