Relationship between microstructure and fracture types in a UNS S32205 duplex stainless steel

Abstract

Duplex stainless steels are susceptible to the formation of sigma phase at high temperature which could potentially be responsible for catastrophic service failure of components. Thermal treatments were applied to duplex stainless steels in order to promote the precipitation of different fractions of sigma phase into a ferrite-austenite microstructure. Quantitative image analysis was employed to characterize the microstructure and Charpy impact tests were used in order to evaluate the mechanical degradation caused by sigma phase presence. The fracture morphology of the Charpy test specimens were thoroughly observed in SEM, looking for a correlation between the microstructure and the fracture types in UNS S32205 duplex stainless steel. The main conclusion is the strong embrittlement effect of sigma phase since it is possible to observe a transition from transgranular fracture to intergranular fracture as increases the percentage of sigma phase. Thus, the mixed modes of fracture are predominant in the present study with high dependence on sigma phase percentages obtained by different thermal treatments.

Effect of the Temperature in the Mechanical Properties of Austenite, Ferrite and Sigma Phases of Duplex Stainless Steels Using Hardness, Microhardness and Nanoindentation Techniques

“…This σ phase shows a tetragonal crystallographic structure with 32 atoms per unit cell [22] making possible a considerable increase in the resultant hardness, whereas a decrease in the toughness as well as elongation is also observed [23][24][25][26][27]. In addition, a change in the fracture type from transgranular to intergranular is obtained as a function of higher amounts of σ phase [28].…”

Microscopic analysis of the stainless steel weld metal after long-term thermal exposition in the primary circuit of the nuclear power plant

“…• C [61,70]. σ-phase formation takes place by consumption of chromium and other alloying elements present in the δ-ferrite with very little or no contribution of these elements from adjacent austenite.…”

“…Therefore, considerable attention has been paid to the precipitation of secondary phases in the corrosion resistant steels with ferrite/austenite duplex microstructure in the conditions of high-temperature isothermal aging [61][62][63][64][65][66][67][68][69][70][71][72]. Less information is available concerning the formation of secondary phases during the welding and continuous cooling after high-temperature treatment [73][74][75][76][77][78][79].…”

Análise do ciclo térmico da soldagem por atrito com pino consumível do aço inoxidável superduplex ASTM A890 grade 6A.

“…É uma fase não-magnética na temperatura ambiente, porém apresenta ferromagnetismo em temperaturas muito baixas. Biezma et al (2013) afirmam que, para os aços inoxidáveis duplex, a fase sigma (σ) é a mais importante fase secundária, pois sua larga fração em volume produz perda de tenacidade, ductilidade e resistência à corrosão. A precipitação da fase σ em 4 Um composto intermetálico é formado por dois ou mais elementos metálicos que produzem uma fase com composição, estrutura cristalina e propriedades próprias.…”

“…Como comprovação da influência que a fase σ provoca nas propriedades dos aços inoxidáveis duplex, Biezma et al (2013) avaliaram o efeito do percentual de fase σ na energia de impacto em um aço inoxidável duplex UNS S32205 A Tabela 1.5 mostra o decréscimo da energia de impacto em relação ao crescimento da fase σ para amostras que permaneceram por diferentes períodos de tempo na faixa de temperatura de 850ºC e, em seguida, temperadas em água. …”