J. Unfried S., T. Hermenegildo, A. J. Ramirez, Brazilian Synchrotron Light Laboratory, Campinas-SP, Brazil; M. T. Piza, Petrobras, Rio de Janiero-RJ, Brazil
The Friction Hydro-pillar process (FHPP) is a solid state welding process developed by The Welding Institute (TWI), which has been successfully applied in underwater maintenance operations by the petrochemical industry. In this work it was evaluated the influence of the preheat temperature (PT) and axial force (AF) on the size, microstructural evolution, and mechanical properties of the thermo-mechanical affected zone (TMAZ) of FHPP welds in a low carbon C-Mn steel. The microstructure evolution and mechanical properties were studied through characterization techniques, and numerical analysis of equilibrium and continuous cooling transformations using Calphad method and a continuous cooling prediction software, respectively.
The used levels of AF were (a) low (25kN and 50kN), (b) intermediate (200kN), and (c) high (400kN). They were evaluated welded joints with and without PT application. When PT was used it was 200 ºC. The process time used was between 5-20s, 20-25s, and 50-70s for high, intermediate, and low AF levels, respectively. The characterization of TMAZ was made through optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Vickers microhardness profiles. The results show that TMAZ size decreased when the AF was increased. However, the joints obtained with higher AF are more susceptible to defect formation. As AF increases the TMAZ cooling rate also raises due to the shorter process time involved. Consequently, the microhardness obtained in joints produced with high AF is higher. The reduction in the cooling rate due to the application of preheats resulted in the reduction of TMAZ microhardness. A good correlation was obtained between the mechanical properties and the microstructures estimated using numerical analyses and the observed microstructures and measured microhardness.
Key words: Solid state welding, friction hydro-pillar process, thermo-mechanical affected zone, C-Mn steel.
Summary: The effect of welding parameters, preheat and axial force, during Friction Hydro-Pillar Process were evaluated on C-Mn steels. The microstructure evolution and mechanical properties of the thermo-mechanical affected zone (TMAZ) were studied through characterization techniques and numerical analysis of equilibrium and continuous cooling transformations using Calphad method and a continuous cooling prediction software, respectively.
