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Master's Dissertation
DOI
10.11606/D.3.2019.tde-21032019-095002
Document
Author
Full name
Henrique Boschetti Pereira
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 2018
Supervisor
Committee
Azevedo, Cesar Roberto de Farias (President)
Ferrari, Jean Vicente
Ito, Hamilton Lelis
Title in Portuguese
Corrosão sob tensão de junta soldada de aço inoxidável duplex: ensaio de flexão em quatro pontos sob gotejamento de solução de água do mar sintética.
Keywords in Portuguese
Aço inoxidável
Água do mar sintética
Corrosão
Soldagem
Abstract in Portuguese
O presente estudo objetivou caracterizar as juntas soldadas de uma chapa de aço inoxidável duplex UNS S32205 soldada utilizando diferentes energias (0,5 kJ/mm, 1,0 kJ/mm e 3,5 kJ/mm) e avaliar a susceptibilidade dessas juntas à corrosão sob tensão (CST) em ambiente contendo cloreto. O ensaio por gotejamento "drop evaporation test" (DET) de água do mar sintética foi utilizado para avaliar a susceptibilidade à CST das juntas soldadas e investigar os efeitos da temperatura (70 ºC, 90 ºC e 110 ºC) e da tensão de tração (70%, 90% e 100 % do limite de escoamento do metal base) na resistência à CST. Os resultados da caracterização mecânica, suscetibilidade à corrosão intergranular e caracterização microestrutural das juntas soldadas não evidenciaram a presença de fases deletérias, como as fases ? e ?. A proporção da fase ferrita na zona fundida e na zona afetada pelo calor (ZAC) foi inversamente proporcional à energia de soldagem. Por exemplo, a ferrita na ZAC variou de 68% para 0,5 kJ/mm a 54% para 3,5 kJ/mm. Os resultados dos ensaios de CST mostraram houve trincamento e fratura para todos os corpos de prova ensaiados a 110 ºC (mesmo com 70% do limite de escoamento), enquanto que nenhuma trinca foi observada nos corpos de prova ensaiados a 70 ºC e 90 ºC. Os ensaios de CST realizados a 90 ºC apresentaram corrosão localizada e preferencial da fase de ferrita longe do cordão de solda, enquanto os ensaios realizados a 70 ºC não apresentaram sinais significativos de corrosão. Todos os corpos de prova ensaiados a 110 ºC apresentaram fratura abaixo do deposito de sal com aspecto de vulcão formado pela evaporação da solução gotejada. As trincas propagaram pela interface ?/? ou clivagem transgranular nas fases ferrita e austenita. Para menores tensões ensaiadas, a proporção de clivagem transgranular da fase de ferrita foi mais significativa que a fase de austenita. O ensaio de DET não foi eficiente para investigar o efeito das alterações microestruturais na ZAC na susceptibilidade à CST. A análise de distribuição de temperatura na superfície dos corpos de prova indicou que o gotejamento da água do mar sintética teve um efeito de resfriamento e a temperatura na região de gotejamento foi pelo menos 10 ºC mais baixa quando comparada a outras regiões do corpo de prova. Além disso, o pH na região gotejante foi mais básico (pH = 11) do que a solução de água do mar sintética (pH = 8,2).
Title in English
Stress corrosion cracking of welded duplex stainless steel joints: four point bend and drop evaporation test of synthetic sea water.
Keywords in English
Duplex stainless steels
Stress corrosion cracking
Synthetic seawater
Welding
Abstract in English
The present investigation characterized the welded joints of a UNS S32205 duplex stainless steel plate using different welding energies (0.5 kJ/mm, 1.0 kJ/mm and 3.5 kJ/mm) in order to evaluate the susceptibility of these joints to stress corrosion cracking (SCC) in Cl- environments. Drop evaporation test (DET) of synthetic seawater was used to assess the SCC behavior of the welded joints and to investigate the effects of the temperature (70 ºC, 90 ºC and 110 ºC) and the tensile strength (70%, 90% and 100% of the base metal's yield strength) on their SCC resistance. The results of the mechanical, intergranular corrosion susceptibility and microstructural characterization of the welded joints did not show the presence of deleterious phases, such as ? and ? phases. Additionally, the proportion of ferrite phase in the molten zone and in the heat-affected zone was inversely proportional to the welding energy. For instance, the ferrite in the HAZ varied from 68% for 0.5 kJ/mm to 54% for 3.5 kJ/mm. The results of SCC testing showed the cracking and fracture for all specimens tested at 110 ºC (even at 70% of the yield strength), while no cracking was observed at 70 ºC and 90 ºC. DET performed at 90 ºC showed that there was a localized and preferential corrosion of the ferrite phase far away from the weld bead, while DET performed at 70 ºC did not show any significant sign of corrosion. All samples tested at 110 ºC presented a noteworthy salt deposition in the shape of a volcano and their cracks were formed underneath the salt layer by the pit corrosion of the ferrite phase. The cracks propagated by either ? / ? interfacial or transgranular cleavage at ferrite and austenite phases. For lower stresses, the proportion of transgranular cleavage of the ferrite phase was more significant than the austenite phase. DET was not an efficient test to investigate the effect of the microstructural changes in the HAZ on the SCC susceptibly of the welded joints as the crack took place below the salt deposit and away from the weld bead. The temperature distribution analysis on the surface of the DET samples indicated that the seawater drip has a significant cooling effect and the temperature of this region was at least 10 ºC lower when compared to other regions of the samples. Additionally, the pH on the drip region was more basic (pH = 11) than the synthetic seawater solution (pH = 8.2). Keywords: Duplex stainless steels; stress corrosion cracking; synthetic seawater; welding.
 
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Publishing Date
2019-04-01
 
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