Effect of long-term aging on the microstructure and mechanical properties of Haynes282 heat-resistant alloy
In order to improve the thermal efficiency of coal-fired power plants, reduce environmental pollution and improve resource utilization, the development of ultra-supercritical power plants has become one of the effective measures. The thermal efficiency of coal-fired power plants is improved mainly by increasing steam parameters, namely steam temperature and pressure. The improvement of steam parameters puts forward higher requirements for the high-temperature durability performance of materials. When the steam temperature reaches 700°C or higher, traditional ferritic steel and new austenitic steel can no longer meet the use requirements, and nickel-based heat-resistant alloys must be used.
Haynes282 alloy is an age-strengthened nickel-based heat-resistant alloy. This alloy has high high-temperature creep strength, strain-aged rupture strength and good thermal stability and has been selected as one of the candidate materials for 700℃ advanced ultra-supercritical units. one. At present, foreign reports are mostly focused on the research on its steam oxidation resistance, weldability and fatigue properties. There are few studies and reports on the changes in microstructure evolution and mechanical properties during the aging process of alloys. The researchers studied the changes in the structure and mechanical properties of 282 alloy during long-term aging at 700°C, and analyzed the impact of the changes in the structure on the mechanical properties.
The test material uses a 25kg vacuum smelting method. After homogenization and annealing, it is forged into a Φ16mm round bar. After standard heat treatment (1120℃×2h, air cooling + 1010℃×2h air cooling and 788℃×8h, air cooling), the test materials were aged at 700℃ for 100, 300, 1000 and 3000h respectively.
After aging, the metallographic samples were ground and polished, and then electrolyzed with 60% HNO3 aqueous solution. The microstructure was then observed and combined with energy spectrum analysis to determine the distribution of precipitated phase elements. Observe the morphology of the precipitated phase and determine the type of precipitated phase, measure the hardness value, and test the room temperature impact energy absorption of the alloy. Finally, fine phase analysis was performed on the aging samples, and the samples were subjected to electrolytic extraction to separate different precipitated phases one by one for XRD analysis and chemical quantitative analysis. The test results are as follows:
(1) As the aging time increases, the grain boundary M23C6 carbides and intragranular MC carbides of Haynes282 alloy grow. The γ' phase grows slowly with the extension of aging time. It is still spherical after 3000h aging and is finely and diffusely distributed in the matrix.
(2) The impact absorption energy of the alloy in the standard heat treatment state is 20.5J. As the aging time increases, the impact absorption energy decreases rapidly. Between 300 and 3000h, the impact absorption energy of the alloy remains at about 10J. The continuous distribution of M23C6 carbides at the grain boundaries is an important reason for the low impact absorption energy of 282 alloy. As the aging time increases, the grain boundary carbides coarsen and grow, causing the impact absorption energy of 282 alloy to gradually decrease.
(3) The hardness of 282 alloy increases with the aging time. After standard heat treatment, the hardness value of the alloy is low. After aging for 100 hours, the hardness value increases significantly from 272HBS in the standard heat treatment state to 342HBS. After aging for 300 to 3000h, the hardness value increases, but the trend is slow. Because the γ' phase content in the standard heat treatment state is low, the alloy hardness is low. After long-term aging, the γ' phase content increases and the particles grow significantly, so the alloy hardness increases.
