Fourth, the persistent creep performance is not qualified
In power plants, chemical plants, boilers and aircraft engines, some parts need to operate at high temperatures for a long time, for these parts, excessive creep deformation and creep fracture is one of the common failure modes. Creep limit, lasting strength and lasting plasticity are the main high temperature mechanical properties of materials. The creep limit represents the resistance of steel material to creep deformation under the action of high temperature and long-term load. The durable strength is the resistance index to evaluate the resistance of a material to creep fracture, while the capacity of a material to withstand creep deformation is expressed by the durable plasticity. The deformation and fracture of materials at high temperature are not only affected by temperature and external force, but also closely related to the composition and microstructure of materials. The early failure of high-temperature components caused by improper heat treatment and poor organization that the high temperature mechanical properties of materials cannot meet the service requirements should be regarded as a heat treatment defect and be prevented.
4.1 Heat treatment and persistent creep properties of superalloys
Aviation superalloys include nickel based superalloys, iron based superalloys and cobalt based superalloys. Nickel-based superalloy refers to the alloy with a mass fraction of nickel > 50%; Iron based superalloys are actually iron-nickel based alloys, and the mass fraction of nickel can be roughly divided into 25%, 35%~40% and 45% grades; Cobalt-based superalloy is rarely used in China.
Superalloys are complex alloying systems, and most of them use solid solution strengthening, second phase strengthening, grain boundary strengthening and comprehensive strengthening to obtain the desired properties.
The main task of heat treatment of super alloy steel is to adjust the process parameters according to the service conditions of the workpiece, restrain the harmful phase precipitation and change the number, shape, size and distribution of the beneficial phase in order to obtain the desired performance.
The substandard persistent creep strength of super alloy steel is often caused by low solution treatment temperature and improper aging process, and too high solution treatment temperature will cause the decrease of room temperature strength and the decrease of durable plasticity. The properties of superalloys can be controlled within a wide range by adjusting the heat treatment process. Optimizing the heat treatment process according to the workpiece service conditions is particularly important for superalloys.
4.2 High temperature creep brittleness
Under the action of long-term stress at high temperature, the elongation and section shrinkage of heat-resistant steels and alloys are greatly reduced, which often leads to brittle fracture, which is called high temperature creep brittleness. This brittleness is measured by the plasticity at the lowest point of the curve of the relationship between δ and experimental time at the time of creep fracture. Creep embrittlement is caused by the change of the internal structure of the material under the action of high temperature and long-term load, which occurs in both the metal and austenitic steel of the body-centered cubic lattice.
The way to reduce the creep brittleness is to reduce the intragrystalline strength to balance the intragrystalline strength with the grain boundary strength. Strengthen grain boundary or reduce the influence of weakening factors of grain boundary. The results show that the addition of trace elements such as B, B+Ti and B-Nb to the low-alloy heat-resistant steel can increase the permanent plasticity of the steel by forming fine TiC near the grain boundary or changing the morphology of carbides on the grain boundary.
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