Utilização da técnica de emissão acústica no estudo da trinca de hidrogênio na soldagem de um aço de alta resistência

Abstract

This work presents an investigation of the initial stage of formation and propagation of in hydrogen crack in welded steel API 5L X80, using the Implant and Tekken testing coupled to the acoustic emission (AE) technique. To this end, a equipment for acoustic emission called \"System for Acoustic Emission - SEA\" was developed and a computer program to record the acoustic signals generated by the events of the crack. In the development of the work two welding processes (FCAW-G and MIG/MAG) were used with different chemical compositions and mechanical properties consumables of the base metal. The shielding gases were mixtures Ar+25%CO2 and Ar+25%CO2 +3%H2, the latter to induce high levels of diffusible hydrogen in the joint and two levels of heat input, 1,058 Kj/mm and 1,500 Kj/mm. Microstructural analysis of the material showed that the MB consists primarily of polygonal ferrite and bainite; ZF grain boundary ferrite (PF (G)), ferrite with aligned second phase (FS (A)) and acicular ferrite (AF) and ZTA with bainite (B), FS (A), ferrite with non aligned second phase (FS (NA)) and martensite (M). The tests showed that the microhardness found in RGGZTA was higher than in other regions being greater than 300Hv and less than 350 Hv. In MB hardness varied but did not exceed the average 250 Hv and ZF showed valuesbetween 250 Hv and 320 Hv and the variation of heat input not interfere significantly in the values of microhardness of the weld region (ZF and ZTA). The acoustic emission was used to correlate the parameters of the signals with the modes of fracture or crack without the onset of fracture, which proves that the test with EA is fully suitable and can be used in determining the moment of formation and propagation of hydrogen crack. Has been observed the existence, in the fracture surfaces, the coalescing of microvoids (CMV), Quasi-cleavage (QC) and Intergranular (IG) modes. The material under study showed susceptibility to hydrogen cracking, even in the welded condition without hydrogen in the shielding gas.