Role of Microstructure Evolution on Hardness, Machinability, and Corrosion Resistance of a Recycled IN738 Superalloy
摘要
The effect of the microstructure of recycled IN738 (RIN738) on the hardness, machinability, and corrosion resistance was investigated. Three conditions were employed: one in the as-cast state, and two subjected to heat treatment involving a double solution treatment (DST) and aging. The solution treatment consisted of heating to 1180 °C for 120 min, followed by cooling, and then reheating to 1220 °C for 90 min, followed by aging. After treatments, samples that were cooled in air were named as AAC, while the ones quenched in water were labeled AWQ. The microstructure was analyzed using scanning electron microscopy combined with energy-dispersive x-ray (SEM & EDX) and x-ray diffraction (XRD). Machining tests were performed using a conventional drill. The surface roughness, tool wear, and sub-machined surface features were investigated. The corrosion resistance at 40 °C in 10% NaCl solution was also evaluated. It was observed that DST lowered the segregation and homogenized the microstructure. After aging, the AAC sample exhibited coarser γ′ precipitates compared to the AWQ. The hardness of the as-cast and AAC samples was around 399 HV, whereas that of the AWQ sample increased to 465 HV. The changes in the microstructure and hardness following the heat treatment consequently influenced the machinability. The machining results showed that the thrust force increased from 1454 to 1555 N for the as-cast and the AAC samples, respectively, to 1581 N for the AWQ, which is in accordance with the hardness values. The AAC sample recorded the lowest average surface roughness of 0.77 and a minimum average tool wear of 33.8 µm, while the AWQ showed the highest roughness of 1.1 and a tool wear of 43.2 µm. Clear tool wear was observed for the as-cast samples, averaging 63.6 µm, which can be attributed to the inhomogeneity of the microstructure, where segregation and agglomeration of carbides can heavily affect the drilling tool. Regarding the corrosion behavior of the investigated samples, the AAC sample exhibited the best corrosion resistance among the samples, with a corrosion rate of 1.795 × 10−3 mm/year and a polarization resistance of 3061 kΩ cm2. The findings demonstrate that the heat treatment using DST and air cooling, followed by aging, provides the optimal balance among machinability, surface integrity, and corrosion performance of RIN738.
Graphical Abstract