Efeito dos ciclos de tratamentos térmicos de alívio de tensão na microestrutura e nas propriedades mecânicas da ZTA do aço API 5l x80 após soldagem dissimilar com o aço AISI-4130

Abstract

This work aims to study the modifications in the microstructures and mechanical properties in the Heat Affected Zone - HAZ, of API 5L X80 steel, caused by the welding processes and Post-Weld Heat Treatment - PWHT. The motivation for this work is to understand the effects on HAZ of API 5L X80 steel, after welding processes and PWHT, in dissimilar welding with AISI 4130 steel, used in the manufacture of drilling risers. For this study, a tubular welded joint with a TIG root pass and filling passes with a coated electrode was submitted to 5 (five) cycles of PWHT, always with the same parameters, from which the coupons for the manufacture of samples, with 1 (one), 3 (three) and 5 (five) cycles of PWHT were extracted. The test bodies were used for metallographic analyzes (macrography, optical microscopy and scanning electron) and mechanical tests (traction, microhardness and CharpyV), characterization of microstructure and mechanical properties, in the three situations studied. The results obtained in the microstructures and in the mechanical properties of the HAZ of the API 5L X80 steel, after the welding process and the PWHT cycles, show that the decomposition of the MA constituent and the reduction of the ferrite-carbonate volumetric fraction initially did not cause losses in the mechanical properties, due to the hardening caused by the precipitation of carbides, but with the increase of the PWHT cycles, the microstructural changes continue to occur, without the precipitation of the carbides, promoting significant losses in the mechanical properties. The impact toughness for all conditions studied was above 200 J, higher than the minimum limit of 68 J, for the temperature of 0 ºC, as determined by the standard. The average values of the microhardness in the HAZ of the X80 steel showed reduction with the increase of the PWHT cycles. Some regions presented an average of less than 200 HV, for an expected value close to 250 HV, as determined by the standard.