Alloy 625 Forgings: Thermo-Metallurgical Model of Solution-Annealing Treatment

Abstract

Metallurgical optimization of engineering alloys is traditionally addressed to improve the overall performance from a mechanical point of view. Grain size is one of the most influential and critical parameters to be controlled in nickel alloys, especially in the high-temperature shaping process and final solution treatment, since it can irremediably damage the alloy performance. For this reason, grain coarsening of alloy 625 was investigated in the temperature interval from 980 to 1150 °C ranging from 0.5 to 6 h. The grain-coarsening data were fitted via regression analysis as a function of time and temperature to develop a predictive model. Grain boundary strengthening was studied by hardness and tensile tests, and the relationships between the grain size and the mechanical properties were finally determined by regression analysis. Such equations were included in a thermo-metallurgical model able to predict the mechanical properties after annealing treatment. This predictive model was validated on a forged tube subjected to solution annealing at 1150 °C for 90 min. Then, it was finally used to compare different microstructural conditions in terms of the alloy impact on the environment.

Influence of Single- and Double-Aging Treatments on the Mechanical and Corrosion Resistance of Alloy 625

“…The possibility of estimating the tensile properties from the results of hardness tests represents a fast and useful tool in the analysis and comparison of different time-temperature aging conditions. The relationship between the hardness and the tensile strength was investigated by regression analysis of a dataset obtained combining the results reported in this work with those previously published in the literature [11,29]. The linear regression analysis of this larger set of experimental data is reported in Figure 8 together with the determination coefficient 𝑅𝑅 2 which was adopted to assess the goodness of fit.…”

“…The nickel-chromium-molybdenum-niobium Alloy 625, patented in 1964 by H. L. Eiselstein and J. Gadbut, provides excellent mechanical and corrosion properties thanks to the presence of high amounts of chromium, molybdenum and niobium [1][2][3][4][5][6][7][8][9][10][11]. The excellent potentialities of this material are mainly exploited for applications in the oil and gas and automotive fields [1,4,6,11]. The ASTM B446 standard [12] reports two possible heat treating conditions for this material: soft annealing and solution annealing.…”

“…The solution-annealing treatment is suggested for high creep strength, and it is performed at a minimum of 1093 °C for applications above 600 °C [12]. In the soft-annealed condition, this alloy shows a fully austenitic microstructure with a heterogeneous distribution of primary carbides and nitrides which are induced by the solidification process [8][9][10][11]13]. These compounds are mainly enriched in niobium and titanium, which are depleted from the surrounding regions.…”

“…Moreover, grain refinement via heat treatment cannot be obtained because of the absence of phase transformation temperatures. Furthermore, these detrimental characteristics are worsened by the fact that both the soft-and the solution-annealing treatments are inevitably associated with grain coarsening which degrades the overall mechanical performance [11,19,20]. Therefore, because the mechanical properties are directly related to the grain size [11], the heat treatment parameters should be tuned as to provide the best compromise between the solubilization level, which affects the corrosion resistance, and the grain size, which determines the mechanical strength.…”

“…Furthermore, these detrimental characteristics are worsened by the fact that both the soft-and the solution-annealing treatments are inevitably associated with grain coarsening which degrades the overall mechanical performance [11,19,20]. Therefore, because the mechanical properties are directly related to the grain size [11], the heat treatment parameters should be tuned as to provide the best compromise between the solubilization level, which affects the corrosion resistance, and the grain size, which determines the mechanical strength. The possibility to recover the strength loss induced by excessive grain coarsening by aging treatment is very useful, but this requires a careful optimization to find the best balance between the mechanical and corrosion properties and the heat treatment time, which directly affects costs and energy consumption.…”