Microstructural changes in SAF 2507 superduplex stainless steel produced by thermal cycle

Abstract

This research work studies the microstructural changes in SAF 2507 superduplex stainless steel after thermal cycle in the range from 200 to 900°C for 20 minutes. The results showed there is no microstructural change in the temperature range of 200-700°C. However, over 800°C the sigma phase precipitates; the percentage of this phase increases with temperature. Sigma phase precipitates at the boundaries of δ/δ and δ/γ. The experimental results also showed precipitation of small particles of sigma phase at 900°C all over the microstructure showing the initial stages of formation.Keywords: SAF 2507 superduplex stainless steel, sigma phase, thermal cycle. INTRODUCTIONDuplex stainless steel consists of approximately 50 per cent ferrite and 50 per cent austenite allowing a combination of excellent mechanical properties and high corrosion resistance. These alloys have a corrosion resistance similar to the ferritic stainless steels, although its toughness is inferior to that of austenitic steels but superior to ferritic stainless steels, while their mechanical strength is greater than that of austenitic stainless steels. These properties mean than duplex stainless steels are excellent materials for industrial applications and their use has increased in the oil, chemical, petroleum and electric power industries. As an example, these alloys find its application in the pumps for fuel gas and desulfurization plants, which have high corrosion and corrosion-erosion resistance requirements [1].The high mechanical properties and corrosion resistance of duplex stainless steel depends on adding Cr, Mo, Ni and N. However, this alloy elements, also promotes the precipitation of secondary phases in duplex stainless steels such as sigma phase, chi phase, chromium nitrides or secondary austenite. Since secondary phases are dependent on time and temperature, the mechanical properties and corrosion resistance of duplex stainless steel decrease significantly in the temperature range of 600-900 o C [2] where sigma phase and other secondary phases precipitates. The sigma phase is not only the phase that most precipitates in duplex stainless steel, but also it is the most detrimental for corrosion resistance and therefore mechanical properties.Several researchers have studied secondary phase precipitation in duplex stainless steel and its effect on corrosion resistance and mechanical properties. Kim and Kwon reported the degradation of pitting and stress corrosion resistance in ageing of 25% Cr duplex stainless steel at 850°C. They suggested that the decrease of resistance to localized corrosion of the alloy results from the sigma phase at δ/δ or δ/γ boundaries [3].Steigerwald in his excellent review of the effects of second phases in stainless steels addressed the complex relationship between sigma and other phases. Since the sigma phase contains more Cr than ferrite, its presence also could affect either the local or general corrosion resistance of duplex stainless steel [3].Despite the existence of studies about secondary phases, ...

Integranular corrosion susceptibility analysis in austeno‐ferritic (duplex) stainless steels

“…being the first reaction as the most effective in the investigated 2507 DSS. 28 2205 microstructural modification at 800°C is similar to 2507, with a carbides precipitation followed by the ferrite grains eutectoid reaction. The only difference that can be underlined with the super DSS concerns the shortest investigated tempering duration (1 hour), where the eutectoid reaction seems to be already initiated.…”

“…Considering a tempering heat treatment at 800°C (1–100 hours), the 2507 DDS shows the most evident microstructure evolution that is characterized, firstly, by carbides precipitation at α/γ grain boundaries after 1 hour at 800°C (Figure A; black spots) followed by the ferrite eutectoid reaction. According to the literature, these reactions can be described as follows, probably depending on the steel chemical composition: being the first reaction as the most effective in the investigated 2507 DSS …”

Reduction of Anisotropy in Cold-Rolled Duplex Stainless Steel Sheets by Using Sigma Phase Transformation

“…This is due to the element partitioning between austenite and sigma. Sigma changes its chemical composition gradually from the nominal chemical formula FeCr, via FeMo, and Fe(Cr,Mo) to (Fe,Ni) x (Cr,Mo) y [26,27] with decreasing ferrite volume content. Simultaneously, sigma's tetragonal lattice parameters a and c increase from a = 0.8799 to 0.9188 nm and from c = 0.4544 to 0.4599 nm.…”

Estudo Da Difusão E Fragilização Por Hidrogênio Em Aço Inoxidável Superduplex

“…As proporções exatas de ferrita e austenita são definidas através do controle da composição química e do tratamento térmico [1,3]. Os AID têm vantagens comparativas em relação aos aços inoxidáveis, entre as quais se incluem: maior resistência mecânica, maior resistência à corrosão e menor preço, principalmente devido ao menor teor de níquel na liga [2,4]. Em comparação com os aços inoxidáveis ferríticos, os AID têm resistência à corrosão semelhante e apresentam maior tenacidade [4].…”

“…Os AID têm vantagens comparativas em relação aos aços inoxidáveis, entre as quais se incluem: maior resistência mecânica, maior resistência à corrosão e menor preço, principalmente devido ao menor teor de níquel na liga [2,4]. Em comparação com os aços inoxidáveis ferríticos, os AID têm resistência à corrosão semelhante e apresentam maior tenacidade [4]. Em geral, os aços que tem a ferrita como fase predominante tem excelente resistência à corrosão, devido à elevada solubilidade do Cr nessa fase.…”