Effect Of Heat Treatment On Structures And Properties Of K445 Superalloy

K445 superalloy, as a kind of as-cast Nickel-based superalloy, has excellent combined properties. It is mainly used for the third and the forth grade blades of gas turbine engines. In order to meet the demand of the products, K445 superalloy must have high strength, high toughness, excellent resistance to corrosion at elevated temperature and the stability of structures. The properties of the superalloy mainly depend on the chemical composition and structures. However for a given chemical composition of the superalloy, the effect of treatment on the structure is very sensitive. Heat treatment can change phase distribution, quantity and morphology as well as the state of grain boundary, thus improves the properties of the superalloy.The effect of heat treatment on structures and properties of K445 superalloy is systematically investigated by changing solid solution temperature and time, quenching rate, aging temperature and time, protecting gas. The microstructure is studied and hardness is measured before and after heat treatment. It can be drown the following conclusions from the above investigation:1. The microstructure of as-cast K445 superalloy mainly consists ofγmatrix, coarseγ′, petaline eutectic structures (γ+γ') and MC carbides inside grains and on grain boundaries. After heat treatment, micro segregation alleviates, and the coarseγ′changes into fine and dispersiveγ′; a certain amount of M23C6 and few MC distribute inside grains and on grain boundaries.2. The hardness of K445 superalloy firstly increases, then decreases after solid-solution treatment in the range of 1050℃~1300℃. When the solution temperature is 1250℃, the peak value of hardness can be obtained. With increase of the solution temperature, theγ′phase and secondary carbides in the K445 superalloy are fully dissolved, thus results in less retained carbides. The large amount of strengthening phases is dispersively precipitated after agingtreatment. If the solid-solution time is too much short, theγ′is still coarse and the hardness is low. After 1.5h solid-solution treatment, the size ofγ′become small and the hardness of the superalloy increase.3. A fewγ′phases are already precipitated in the superalloy after solid solution, followed by air cooling, and then they grow up during aging period. So the hardness of superalloy is low. However, cracks appeared in the superalloy quenched in water after solid solution, the better structure and hardness can be obtained after solid solution by oil cooling.4. In the range of 730℃~880℃aging temperature, the amount ofγ′strengthening phase, M23C6 and boride increase with increase of the aging temperature. The largest quantity and the highest hardness can be got at 880℃. Theγ′phases aggravate while aging at 920℃which result in decrease of hardness. The hardness is low due to insufficient precipitationγ′phases when the aging time is hold for 10h. While the aging time is extended to 16 h, more and fineγ′phases precipitate, and the hardness of the superalloy can reach the highest. However, while the aging time is above 20 h, the hardness of the alloy decreases, because of aggravation ofγ′phases.5. Compared with that of K445 alloy after first aging treatment, the hardness of K445 alloy after secondary aging treatment has not changed significantly. However, more M23C6 and MC are precipitated in the superalloy, and theγ′phases grow up slightly and distribute in a raft shape.6. The fineγ′phases and carbides in K445 superalloy are distributed dispersively and uniformly after heat treatment in atmosphere, and the hardness of the superalloy reaches the highest. However theγ′phases in superalloy are coarse and uneven, the hardness of the superalloy is almost same in nitrogen gas. While in argon and vacuum, theγ′phases are also fine and uniform, the hardness of the superalloy is higher as well, but the operating cost is high and the operating procedures are more complicated.