Thermal deformation and microstructure evolution of thick-walled welded pipes

Thick-walled welded pipe is a kind of precipitation-strengthened nickel-based superalloy that is difficult to deform. It is similar in composition to the former Soviet Union's ЭИ929 alloy. The solid solution strengthening of alloy elements and the precipitation strengthening of γ' phase are very high. It has excellent oxidation resistance, hot corrosion resistance and 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 hot working parameters of the alloy, when it is used for hot forging of turbine working blades, the forgings are prone to defects such as structural instability and cracks, resulting in a high rejection 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 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 effect of deformation temperature and strain rate on the microstructure of the alloy.

The raw material used in the experiment is a hot-rolled bar for a thick-walled welded pipe, 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 the two ends of the sample are processed with shallow grooves for storing high-temperature lubricants, and the isothermal compression test is carried out on a 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, then the sample is cut longitudinally, ground and polished, and then corroded by CuSO4 (20g) + H2SO4 (5ml) + HCl (50ml) + H20 (100ml) solution, and 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 the rheological softening is that the alloy undergoes dynamic recrystallization during thermal deformation. As the strain rate decreases, both the strain at which the flow stress peaks and the peak stress decrease.

2. A constitutive equation for high-temperature deformation of thick-walled welded pipe is established. The calculated value of the equation is in good agreement with the experimental value, and the relative errors are all 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 sufficient, the grain size becomes larger, and the uniformity of the grain structure increases; with the increase of the strain rate, the grain size first decreases and then increases. When the strain rate is 1s-1, the grain structure is relatively fine.