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

• Published in: Metals (Basel ) Vol. 14; no. 7; p. 823
• Main Authors: Rivolta, Barbara, Gerosa, Riccardo, Panzeri, Davide
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01-07-2024
• Subjects: Aging; Aging (metallurgy); aging treatment; Alloy 625; Annealing; Chemical composition; Corrosion rate; Corrosion resistance; Corrosion resistant alloys; Corrosion tests; Energy consumption; Grain size; High temperature; Intergranular corrosion; Intergranular precipitation; Investigations; Mechanical properties; mechanical strength; Molybdenum; Molybdenum base alloys; Nickel base alloys; Phase transitions; Plastic deformation; Precipitation hardening; Precipitation heat treatment; Superalloys; Temperature; Transformation temperature; Italy
• DOI: 10.3390/met14070823
• EISSN: 2075-4701

Nickel–chromium–molybdenum Alloy 625 exhibits an excellent combination of mechanical properties and corrosion resistance. However, the high-temperature plastic deformation process and the heat treatment represent critical aspects for the loss in mechanical strength by grain coarsening. This detrimental behavior is worsened by the absence of phase transformation temperatures. However, the chemical composition permits slow precipitation-hardening response upon single aging. Therefore, when the soft- or solution-annealed condition is associated with insufficient mechanical properties, this potentiality can be exploited to improve the mechanical strength. Since the γ″ precipitation can be accelerated by double-aging treatment, different time–temperature combinations of double aging at 732 °C and 621 °C are investigated. The simultaneous precipitation of intergranular carbides can dramatically affect the corrosion resistance. Such an undesired phenomenon occurs very quickly at 732 °C, but it is obtained only after very long exposure times at 621 °C. For this reason, a performance chart is developed to compare all the tested conditions. In particular, single aging at 621 °C for 72 h and 130 h are associated with an acceptable combination of mechanical and corrosion properties. Double aging permits a conspicuous acceleration of the aging response. For instance, with double aging at 732 °C 3 h and 621 °C 72 h, it is possible to obtain the same mechanical properties of single aging at 621 °C for 260 h. Such acceleration is accompanied by a more critical corrosion behavior, especially because of the primary step. However, even after its optimization, none of the tested conditions were acceptable.