Heat treatment of high temperature alloy
The properties of the high-temperature alloy are closely related to its microstructure. The microstructure of the high-temperature alloy can be adjusted by heat treatment, for example, the grain size, carbide shape and distribution, and the size and distribution of intermetallic compounds (r') are all controlled by heat treatment process. Heat treatment is especially important for deformation alloys. Heat treatment of high temperature alloy generally consists of solid solution treatment, intermediate treatment and aging treatment.
Vertical solid solution treatment
Solid solution treatment is used to dissolve the carbides in the matrix. When r' is equal to get uniform supersaturated and solid solution, it is convenient for fine precipitated particles to be re-precipitated after aging, uniform carbide and r' are distributed to strengthen the phase, and at the same time, stress caused by cold and hot processing is eliminated, so that the alloy recrystallizes. Secondly, solid solution treatment is to obtain suitable grain size to guarantee the creep resistance of alloy at high temperature. Solid solution treatment temperature range between 980 ~ 1250 ℃, mainly according to the law of precipitation and dissolution in each phase in the alloy to choose and use requirement, in order to make sure the main strengthening phase precipitation of necessary conditions and certain grain size. For alloys that are used in long-term high temperature, high temperature durability and creep properties are required. Higher solid solution temperature should be selected to obtain larger grain size. Tensile strength. The alloy of impact toughness and fatigue strength can adopt the lower solid solution temperature to guarantee the smaller grain size. High temperature solid solution. And there may be some precipitations. For low oversaturated alloys: low temperature solid solution treatment not only has the main enhancement of the dissolution of the phase, but also may result in the precipitation of some phases. For alloys with low oversaturation, faster cooling speed (e.g. oil, water cooling) is usually chosen: for alloys with high oversaturation, it is usually air cooling.
The main effect of intermediate treatment, i.e. secondary solid solution treatment or intermediate aging treatment, is to change the amount, shape and distribution of carbide precipitated on grain boundary, and then to cause a reasonable distribution of the size of two r' s in the alloy, so as to significantly improve the durability and plasticity of the alloy. Intermediate processing about 1000 ~ 1500 ℃, the temperature in the process of thermal insulation and cooling, grain boundary carbides precipitate chain-like, strengthen the grain boundary effect. For alloys with low oversaturated content, the precipitation of cell M23C6 in grain boundary can be avoided after intermediate treatment, and Cr rich bulk carbides can be produced at grain boundary. As Cr solubility in grain boundary decreases, the solubility of Al and Ti is improved, which is dissolved in matrix, resulting in poor r' area of grain boundary. The poor r 'zone with appropriate width has certain plasticity, which can relax under the action of high temperature stress, relieve stress concentration, delay the generation of cracks and improve the long life. Poor r' s' area is too narrow and long lasting plasticity is poor r' area is too wide, then the creep speed is high, which will lead to early break. For the alloy with high supersaturation, after intermediate treatment, the chain carbide M23C6 is precipitated out at the grain boundary, which leads to the dilution of Cr,Mo and so on near the grain boundary, while the solubility of Al and Ti is relatively increased, and the r' envelop coated with grain boundary carbide is often formed, which is beneficial to the long-term performance. The large size r' phase was precipitated during the intermediate treatment, and the two sizes r' phase were obtained after the final aging of the alloy, so as to improve the comprehensive performance and long-term tissue stability of the alloy. For carbide reinforced iron-based alloys, intermediate treatment is generally not used.
The aging treatment can make the alloy fully and uniformly separate out the intensified phase. The dissolution and polymerization of the enhanced phase should not be induced at the aging temperature to ensure the appropriate size of the enhanced phase. Aging temperature is in commonly 700 ~ 950 ℃, aging temperature depends on the number of strengthening phase and r 'in the alloy phase has a large amount of precipitation, so the final aging treatment only produce smaller organizational change.
Many cast alloys can be used without heat treatment or simple heat treatment, such as solid or aging treatment, which takes only a few hours. As the alloy becomes more and more complex, heat treatment similar to deformation alloys can be used to improve some comprehensive properties. After solid solution treatment, the casting structure can be locally homogenized. However, dendritic segregation of cast alloys will not be completely eliminated.
In conclusion, heat treatment is closely related to the microstructure and properties of alloy.