Investigation On Microstructure And Properties Of Inconel 625 Alloy During Heat Treatment

Inconel 625 alloy is typical Nickel-based corrosion-resistant alloy, which has high tensile, creep and rupture strength. The alloy is, therefore, being widely used in aerospace, marine, nuclear industries and so on. By employing optical metallography, scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), transmission electron microscopy (TEM), Physicochemical phase analysis (PPA) and the double loop electrochemical potentiokinetic reactivation test (DL-EPR), microstructure and other related properties of Inconel 625 alloy was investigated with respect to variation of grain size, the types and contents of precipitates, mechanical properties and mechanism of intergranular corrosion.Grain size of Inconel 625 alloy increased obviously with either increasing temperature or extending holding time at temperatures above 1050℃. However, at temperatures lower than 1050℃, GG was sluggish due to the pinning effect of precipitation of carbide particles on GB. Critical temperature for transition from duplex grain structure to homogeneous one is considered to be around 1100℃. Once the temperature exceeded 1200℃, an instant increase in the grain size occurred showing no dependence on holding time. Basing on the experimental results, mathematical model for grain growth of Inconel 625 alloy at temperatures ranging from 1100 to 1250℃ was established.Work-hardening exponent of Inconel 625 alloy increased with increasing temperature between 930 and 1230℃, especially at temperature from 1130 to 1230℃, the exponent was increased above 0.54. While the strength of yield and tensile under room temperature decreased with increasing temperature, the plasticity and toughness at room temperature was improved obviously. The results of EPR tests show that the susceptibility to intergranular corrosion was reduced with the increasing of temperature. Solution treating above 1130℃ can impart excellent intergranular corrosion resistance to Inconel 625 alloy. Heat-treatment of alloy 625 is recommended to be performed in the temperature range of 1130 to 1230℃.Physicochemical phase analysis was carried out on the Inconel 625 alloy following aging at temperatures between 650 and 1000℃. The results showed that the dominant second phase formed was M23C6 from 650 to 800℃ and its peak temperature was about 750 ℃. It was found that M23C6 precipitatedat initial stage of aging treatment at 900 ℃, transformation of which into M6C, however, occurred by prolonging aging time. In addition, the largest amounts of M6C and MC precipitates can be obtained at 900℃. Basing on the EPR results, it was concluded that the temperature at which the value of R was largest varied with holding time, and the value was relatively high at 750 and 800℃.