Thermal deformation and microstructure evolution of thick-walled welded steel pipe

Thick-walled welded pipe is a hard-to-deform precipitation-strengthened nickel-based superalloy, which is similar in composition to the ЭИ929 alloy of the former Soviet Union, and has a high level of solid solution strengthening of alloying elements and precipitation strengthening of γ' phase. It has excellent oxidation resistance, hot corrosion resistance, excellent yield strength, tensile strength, and creep strength at high temperatures. It is mainly used in environments with high temperatures, complex stress, and corrosive media, such as making engine turbine blades. Due to the relatively narrow range of thermal processing parameters of this alloy, when it is used as a turbine rotor blade for hot forging, the forging is prone to defects such as structural instability and cracks, resulting in a high scrap rate. Therefore, it is of great significance to study the hot deformation behavior of the alloy under different hot deformation conditions for obtaining qualified forgings. The researchers analyzed the rheological behavior characteristics of the alloy through the data obtained from the high-temperature compression experiment of the thick-walled welded pipe, established the constitutive equation of the thick-walled welded pipe within the range of thermal deformation parameters, and studied the influence of deformation temperature and strain rate on the microstructure of the alloy.

The raw material used in the experiment is a thick-walled welded pipe hot-rolled bar, and the original structure is mainly composed of equiaxed grains with a grain size of 10-30 μm. The bar is processed into a cylindrical sample of Φ8mm×12mm, and shallow grooves for storing high-temperature lubricant are processed at both ends of the sample. The isothermal compression test is carried out on the Gleeble-1500 testing machine. The deformation temperature is 1090, 1120, 1150, and 1180 ℃, the strain rate is 0.1, 1, 10, 50s-1, and the maximum deformation degree is about 60%. During the experiment, the testing machine automatically collects and calculates stroke, load, stress, and strain data. After the deformation is completed, the sample is water-cooled, and then the sample is cut longitudinally, ground, polished, and then corroded by CuSO4 (20g) + H2SO4 (5ml) + HCl (50ml) + H20 (100ml) solution, then observed under a metallographic microscope alloy microstructure. The results showed that:

1. When the thick-walled welded pipe is deformed under different conditions, as the strain increases, rheological softening occurs. The reason for rheological softening is that the alloy undergoes dynamic recrystallization during thermal deformation. As the strain rate decreases, the strain at which the flow stress reaches its peak value and the peak stress both decrease.

2. The high-temperature deformation constitutive equation of thick-walled welded pipe is established. The calculated value of the equation is in good agreement with the experimental value, and the relative error is below 8%, indicating that the equation accurately describes the rheological behavior of the alloy during thermal deformation.

3. The deformation temperature has a significant effect on the microstructure of the thick-walled welded pipe. With the increase in temperature, the dynamic recrystallization is fully enhanced, the grain size becomes larger, and the uniformity of the grain structure is improved; as the strain rate increases, the grain size first decreases and then increases. When the strain rate is 1s-1, the grain structure is finer.