Effect of Torch Weaving on the Microstructure, Tensile and Impact Resistances, and Fracture of the HAZ and Weld Bead by Robotic GMAW Process on ASTM A36 Steel

Abstract

Abstract:The effect of torch weaving on the microstructure, tensile strength, impact resistance and fracturing in robotic welded joints using varing welding speeds, voltages and currents was evaluated through fractography using scanning electron microscopy, tension, impact and micro-hardness tests and optic microscopy. Results indicated that linear, sinusoidal and circular weavings favored an increase of the width of the HAZ as well as a slight increase in yield resistance accompanied by hardening in comparison with a triangular weave. The latter favored larger impact energy in the HAZ with less width, containing coarse-grained ferrite due to lower tensile strength and Vickers hardness. A circular weave generated the highest level of hardening and the lowest energy absorbed in the HAZ as a consequence of an increase in yield strength related to the fine needles of acicular ferrite. A linear weave favored the greatest width of the HAZ compared with other weld weavings due to heat accumulation along the fusion line of the welded joint. Hardening and loss of toughness were evaluated through fractographic analysis showing mixed fractures mainly composed of brittle fracturing made by transgranular cleavages with facets containing well defined river patterns.Key-words: HAZ; Cleavage fracture; Robotic welding; Torch weaving; ASTM A36 steel. Efeito do Movimento da Tocha na Microestrutura, Resistência à Tração e ao Impacto, e Fratura da ZAC e Cordão de Solda por Processo GMAW Robotizado em Aço ASTM A36Resumo: O efeito do movimento da tocha sobre a microestrutura, resistência à tração e impacto e fratura em juntas soldadas por processo robotizado, variando velocidade, voltagem e corrente de soldagem foi avaliado através de fratografia, usando microscopia eletrônica de varredura, provas de tração e impacto e testes de microdureza e microscopia óptica. Os resultados indicaram que os movimentos lineares, senoidais e circulares favorecem um aumento da largura da ZAC, assim como, um ligeiro aumento do limite elástico acompanhado de endurecimento em comparação com o movimento triangular. A energia de impacto é mais favorecida na ZAC com menor largura, contendo grãos grosseiros de ferrita devido à menor resistência à tração e dureza Vickers. Um movimento circular produziu o nível mais elevado de endurecimento e a menor energia absorvida na ZAC como consequência de um aumento na limite elástico relacionado com as agulhas finas de ferrita acicular. Um movimento linear favoreceu maior largura da ZAC em comparação com outros movimentos da tocha, devido à acumulação de calor ao longo da linha de fusão da junta soldada. Endurecimento e perda de tenacidade foram avaliados através de análise fratográfica mostrando fraturas mistas compostas principalmente de fratura frágil por clivagem transgranular com facetas contendo marcas de rio bem definidas.Palavras-chave: ZAC; Fratura por clivagem; Soldagem robotizada; Movimento de tocha; Aço ASTM A36.

Influencia del calor de entrada sobre el comportamiento a la fractura dúctil Charpy de la ZAC recalentada en uniones soldadas multipasos GMAW en acero HSLA utilizando análisis fractográfico digital

“…The semi-automatic welded joints of ASTM A-633 (2013) HSLA steel obtained at a greater heat input and a higher wire feed rate exhibited lower Charpy impact energy in the reheated HAZ of the weld metal, as well as greater flat area of mixedfracture surfaces showing mainly brittle fracture in transgranular cleavage with bigger facets, containing well-defined river patterns and a reduction of small micro-hollows distributed throughout the surface (Mendoza et al, 2013;Guzmán et al, 2017). This impact fracture behavior was linked to the microstructural features of a fine network of fine acicular ferrite (Thewlis, 2004;Wan et al, 2010;Guzmán et al, 2017) surrounded by allotriomorphic ferrite, which changed in size and amount as a consequence of the increase in wire rate.…”

“…The brilliance and granular appearance of the planar central zone, in differing sizes, could be associated with the fractographic features of the typical brittle fracture. From the observations made on the fractographs obtained by SEM from impact fractured surfaces, we observed that the fracture was mainly brittle with well-defined transgranular facets containing river patterns in the direction of propagation, as well as minimum ductile fracture representing by microvoid bands (Guzmán et al, 2017) within the surface and surrounding these facets. These fractographic features confirmed that the brittle fracture was made by cleavage.…”

“…According to the aforementioned fractographic features, and as part of the conventional fracture analysis, the number of cleavage facets and microvoid bands (Guzmán et al, 2017) was directly measured on the fractographs and was reported as the volume fraction. It was observed that this fraction of transgranular cleavage facets slightly increased as a consequence of the increase in the wire rate ( Fig.…”

“…There has been some research on the mechanical and microstructural behavior of the HAZ, for example in ASTM A36M (2014) carbon steel using robotic GMAW process with different torch weaving modes (Guzmán et al, 2017). Some works are related to the simulated coarse-grained HAZ for welded HSLA steel, which showed an evident decrease in fracture toughness with an increase in the size of the austenite grain and the volume fraction of martensite/austenite constituent distributed in a network (Shi and Han, 2008).…”

“…However, fewer contributions have been published on conventional or digital fractography (Guzmán et al, 2017) under impact loads in reheated HAZ on steels such as ASTM A633 (2013). The objective of this investigation is to produce a clearer understanding of the causes and effects of impact fracturing, such as the degradation of the absorbed energy and microstructural zones.…”