Impact of microstructural evolutions during thermal aging of Alloy 625 on its monotonic mechanical properties

Abstract

Abstract. Alloy 625 is widely used for petrochemical, marine and aerospace applications owing to its outstanding corrosion and mechanical properties at high temperatures. However, this alloy is prone to complex microstructure evolutions above 500• C that may impact its mechanical properties. In this study, the impact of its microstructure evolutions occurring upon thermal aging on the monotonic mechanical properties has been studied. Thermal exposures of up to ∼2000 hours in the 550• C -900• C temperature range have been investigated. TTT diagrams of the δ and γ phases were established based on high resolution scanning electron microscopy observations. The evolutions of secondary carbides distributions were also followed. It has been observed a steep increase of the room temperature micro-hardness after overagings performed at 650• C and 700• C due to the precipitation of the γ phase. Moreover, it is clearly demonstrated a strengthening effect of the δ phase observed after long term thermal exposures at temperatures in excess of 700• C. Finally, the impact of a thermal aging in the γ precipitation domain on the tensile properties was evaluated from room temperature up to 800• C. It is shown that the loss of high temperature ductility is not correlated to the precipitation of grain boundary secondary carbides.

Effects of cold rolling on the precipitation kinetics and the morphology evolution of intermediate phases in Inconel 718 alloy

“…11), which shows the decrease in the strengthening γ″-phase content and the increase in the brittle δ-phase content. Suave et al[40,41] obtained a similar result in alloy 625 samples that the microhardness reached a peak value after a 1000 hours thermal exposure at 650 o C and a subsequent small decrease was observed when increasing the exposure up to 2000 hours, which was attributed to the overaging and the onset of γ″→δ transformation occurring at this temperature for very long exposures.For the cold-rolled samples aged by procedure B, it is unrealistic to employ an equation/function for describing the variations in the hardness, which is due to the freshly recrystallized grain microstructure replacing the initial grain microstructure. However, the variations in the hardness of different degrees cold-rolled samples aged by procedure B can be roughly attributed to two aspects.…”

Avaliação de Revestimentos de Liga de Níquel 625 Depositados pelo Processo Eletroescória

“…A este respeito, Cortial et al [1] estudando o efeito do TTAT na microestrutura e propriedades mecânicas de depósitos realizados com a liga de níquel 625, observaram um aumento da resistência mecânica e redução na ductilidade devido à precipitação da fase γ" (Ni 3 Nb) no espaço interdendrítico somente para temperaturas superiores a 750°C. No entanto, em trabalhos recentes, Suave et al [41,42] observaram o aparecimento desta fase para temperaturas de envelhecimento entre 550-750°C. Conforme observado Figura 8, a observação por MET confirma a precipitação de γ" ao longo da matriz, na forma de precipitação fina, o que indica que o aumento da dureza no metal de solda após o TTAT (Figura 3) está associado com a precipitação desta fase, conforme verificado em outros estudos disponíveis [1,19,37,41,42].…”

Effect of Nb on δ Phase Precipitation and the Tensile Properties in Cast Alloy IN625

“…Although there are many studies in the literature on alloy IN625, most of the reports are about wrought alloy IN625 [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. So, studies on cast alloy IN625, such as the one presented in this paper, is needed to investigate its potential application in A-USC power plants.…”

“…According the studies on wrought alloy IN625, the δ phase is observed to precipitate easily after long-term aging at 700-760 • C (the service temperature of A-USC power plants) [9][10][11][12][13], although it is initially designed as a solid solution hardening alloy [14]. The δ phase is an equilibrium phase corresponding to the γ phase with a composition of Ni 3 Nb and an orthorhombic (D0 a ) crystal structure [15][16][17][18].…”