Effects of cooling rate on the microstructure and mechanical properties of the Ni-base superalloy UDIMET 500

The Ni-base superalloy UDIMET 500 indicates good hot corrosion resistance, high stability and strength at high temperatures and for this reason the alloy is used in manufacturing of gas turbine hot components. UDIMET 500 is employed in manufacturing of the second stage blades of high-power gas turbines. The microstructure consists mainly, of austenite γ-matrix and ordered and coherent γ' precipitates and also carbides dispersed in the matrix. The alloy gains its appropriate microstructure and strength through precipitation hardening mechanism. The heat treatment cycle consists of three stages: homogenization, partial solution and aging. Heat treatment parameters such as: time and temperature of homogenization, partial solution and aging temperatures, and cooling rate from homogenization and solution temperatures affect the microstructure of the alloy. Among these parameters cooling rate from partial solution is the most effective. Therefore, in this research the effects of cooling rate on microstructure and mechanical properties such as: tensile, creep and hardness were investigated. For this purpose, six different cooling rates were applied on the cast UDIMET 500 specimens after partial solution at 1080 deg. C for 4 h. Microstructures of the specimens then were studied using optical and electron microscopy. Tension and creep tests were performed at different conditions. It was found out that with increasing cooling rate the volume percent of the γ' precipitates decreases. Also, it was shown that size, shape and volume fraction of primary γ'-particles are largely influenced by the cooling rate following homogenization and partial solution treatments