In the production process of deformed superalloy, ingot opening is the key process step to obtain the forged isaaxial structure of the material, improve the cold and hot deformation ability and obtain the uniform property. This process mainly involves the thermal deformation of the as-cast material. Because the as-cast structure has the characteristics of serious segregation, large initial austenite grain size, and confused dendrite morphology and orientation, the preforming has always been one of the bottleneck processes restricting the production of deformed superalloys. At present, there are few research reports about the hot deformation of nickel-based superalloy and high alloying stainless steel ingot in China, and they mainly focus on the process exploration and finite element simulation, and the research on the microstructure evolution process and mechanism is not in-depth.
The wear-resistant plate with relatively simple structure composition was selected and samples were taken from different areas of the ingot to obtain samples with initial structure of fine columnar crystals, coarse columnar crystals and equiaaxial dendrites. The effects of dendrite morphology, size and orientation on rheological behavior and microstructure evolution were studied through unidirectional compression experiment, and the deformation microstructure orientation was analyzed by backscattered electron diffraction (EBSD) technique. The deformation mechanism is determined to provide the theoretical and experimental basis for the formulation of the preforming process.
The wear-resistant plate ingot was made by vacuum induction smelting. The ingot size was Φ150mm×500mm, the mass fraction of each element was 0.13%C-30%Cr-9%Fe, and Ni was the matrix. In order to obtain the macroscopic structure of different regions of the ingot, the alloy was corroded with CuSO4+HCl+C2H5OH mixed solution stipulated in the national standard. The hot compression samples were obtained from the equiaxed dendrite transition region (A region) at 1/2 radius, the fine columnar crystal region (B region) at the edge of the ingot, and the coarse columnar crystal region (C region) at the center of the ingot. The hot compressed sample is a Φ14mm×20mm cylinder. In order to uniform the influence of dendrite direction on thermal deformation behavior, the height direction (loading direction) of the cylindrical sample in B and C region is perpendicular to the direction of the columnar crystal. The hot deformation experiments were carried out on the MTS test machine with compression temperatures of 1100, 1150 and 1200℃, strain rates of 0.01, 0.1 and 1s-1, and deformation amount of 50% to simulate the free forging process of the ingot. Load-displacement data is automatically output during the compression process, and the specimen is water-cooled at the end of the deformation to retain the compressed structure. The longitudinal section of the compressed sample was treated by standard metallographic sampling method and boiled in a mixture of 2.5gKMnO4+10mLH2SO4+90mLH2O for 30min for thermal deformation observation. After some samples were electropolished, the orientation relationship of the deformed structures was analyzed by the backscatter electron diffraction (EBSD) module of scanning electron microscope (SEM).
The test results show that:
(1) Under the as-cast condition, the rheological resistance at high temperature of the wear-resistant plate increases with the decrease of deformation temperature, the increase of strain rate and the increase of deformation amount. At 50% compression, high temperature (1200℃) and high speed (1s-1) are conducive to the occurrence of dynamic recrystallization.
(2) When the compression direction is perpendicular to the columnar direction, the secondary dendrite sliding, as a deformation mechanism, leads to the increase of strain rate sensitivity factor. In this case, the fine columnar sample has the smallest deformation resistance and the lowest recrystallization ratio, while the sample with the initial structure of coarse columnar and equiaaxial dendrites has the greatest deformation resistance and the most favorable dynamic recrystallization conditions, respectively.
