Magnetic detection of sigma phase in duplex stainless steel UNS S31803

Abstract

a b s t r a c tDuplex stainless steels are high strength and corrosion resistant steels extensively used in the chemical and petrochemical industry. The best mechanical properties and corrosion resistance are obtained with a microstructure composed by equal parts of ferrite and austenite and free from tertiary phases. Sigma phase is one of these deleterious tertiary phases. In the present work different amounts of sigma phase were precipitated by heat treatments in a UNS S31803 stainless steel. Some specimens were cold rolled before sigma phase precipitation in order to evaluate the effect of deformation on the magnetic measurements. The amount of sigma phase was precisely determined by microscopy and image analysis for each heat treatment condition. The effects of sigma phase on the steel properties were investigated, confirming the detrimental effects of very small percentages on corrosion resistance and toughness. Two magnetic methods were used to detect sigma phase: magnetization saturation measurements in a Vibrating Sample Magnetometer and ferritoscope testing. Both methods were found to be sensitive to small percentages of sigma phase in the microstructure.

Avaliação da Técnica de Voltametria Linear para Determinação Quantitativa de Fase Sigma no Aço Inoxidável Duplex UNS S31803

“…Conforme o esperado, as micrografias mostram que na medida em que se aumenta o tempo de tratamento térmico, aumenta-se consideravelmente a concentração de precipitados de sigma na superfície de material, estando em concordância com as referências [2,12].…”

“…Desta forma, estudos apontam para a necessidade da criação de um Ensaio Não Destrutivo (END) que possa ser utilizado para quantificar, preferencialmente em campo, a presença de fase sigma em aços duplex [10][11][12][13][14][15].…”

“…Além disso, outro fator limitante se dá pelo fato de que para longos períodos de exposição em temperaturas elevadas, a fase sigma também pode precipitar da fase austenita (não magnética), não sendo detectada pelo método [18]. Ademais, propriedades magnéticas utilizadas para avaliação, tal como a permeabilidade magnética, também são sensíveis a outros fatores microestruturais como aqueles causados pela deformação plástica e também pelo tamanho de grão da fase ferrítica [12].…”

Use of Magnetic Barkhausen Noise (MBN) to Follow Up the Formation of Sigma Phase in Saf2205 (UNS S31803) Duplex Stainless Steel

“…The correspondence between the microstructure and the magnetic response of stainless steels, including its variation during phase transformations, was the object of a series of studies by Tavares and co-workers [31][32][33] . Their work was done using measurements of saturation magnetization, remanence, coercive force and magnetic transition temperature, and included the magnetic response as a function of ferrite/austenite proportion 31 , high and low temperature embrittlement 32 and specific measurements aimed at sigma phase formation 33 .…”

“…Their work was done using measurements of saturation magnetization, remanence, coercive force and magnetic transition temperature, and included the magnetic response as a function of ferrite/austenite proportion 31 , high and low temperature embrittlement 32 and specific measurements aimed at sigma phase formation 33 . In the latter case, the Ferritscope and Magnetic saturation Use measurements were used, and the authors observed that both methods are sensitive to small fractions of sigma.…”