Cold pilgering is widely utilized in high-end applications for the precise shaping of seamless tubes due to its capacity for large deformation, which reduces the number of deformation processes and shortens production cycles. However, there is a gap in the research on the cold pilgering of small-diameter, thick-walled seamless tubes, specifically those with an outer diameter–wall thickness ratio of ≤3. In this study, cold pilgering tests were performed on Cr-Mo-V hot-working die steel small-diameter thick-walled tubes. It was discovered that increasing the feed rate results in greater deviations in both inner diameter and wall thickness, although it has little effect on inner wall roughness. In contrast, increasing wall thickness reduction leads to higher wall thickness deviation but reduces inner surface roughness without significantly affecting inner diameter deviation. The study also found that a decrease in the initial inner wall roughness before pilgering results in improved final roughness. Under optimal conditions, the average inner surface roughness S_a can reach 0.177 μm, and small-diameter thick-walled seamless tubes with deviations in the inner diameter and wall thickness of 0.05 mm and 0.03 mm, respectively, are obtained. After tempering at 600 °C, the tensile strength (R_m) and yield strength (R_p0.₂) of the cold-pilgered tube reach 1092 MPa and 947 MPa, respectively, and the elongation (δ₅%) and impact energy (A_kU) increase to 20.4% and 61.5 J, respectively.

A hot-working die steel thick-walled tube Pilger rolling test was carried out using an LG40 Pilger mill, and the morphology and roughness evolution of the inner surface were examined using a white-light interferometer. The experimental results showed that micro-wrinkles formed on the basis of the original inner surface morphology, the altitude difference (Sz) between the peaks and valleys of the inner surface profile increased from 3.18 to 3.686 μm, and Sa increased from 0.722 to 0.892 μm in the diameter reduction zone. As the tube continued to feed into the wall thickness reduction zone, the micro-wrinkles gradually flattened, Sz and Sa were decreased to 1.625 and 0.174 respectively, and Sa maintained a slight fluctuation of 0.174~0.2 μm in the final sizing zone. From the diameters of the roller groove and taper of the mandrel, the three-dimensional strain of the tube in the wall thickness reduction zone was calculated, and the strain state of the tube in the complete deformation zone could be analyzed by finite element simulations. We found that in the diameter reduction zone, the inner surface was not supported by the mandrel and was free, while micro-wrinkles formed under circumferential compressive strain. In the wall thickness reduction zone, the deformation of the inner surface was controlled by the mandrel, and the micro-wrinkles were gradually flattened by radial compressive strain. The ratio of radial to circumferential strain was the key to flattening the micro-wrinkles, and when the ratio increased, the inner surface roughness (Sa) was reduced to 0.174 μm. In the sizing zone, the radial and circumferential strains were small, and the inner surface roughness showed no obvious fluctuation.

In this study, the cold rolling test on the quenched-tempered hot working die steel pipe with an outer diameter/thickness ratio of no greater than 3 was performed. The evolutionary trend of microstructure was examined by a combination of the microscope, SEM, and EBSD tests. The effect of feed rate on the inner wall roughness of rolled pipe was analyzed by means of white light interference. According to the experimental results, the maximum normal pressure per unit area increases from 1046.7 MPa to 1113.2 MPa with the rise in feed rate from 1 mm/stroke to 6 mm/stroke. Meanwhile, the inner wall roughness of the pipe declines from 0.285 μm to 0.146 μm after rolling. When the feed rate reaches 2 mm/stroke, the maximum normal pressure per unit area is 1058.4 MPa, which causes a significant plastic deformation to the inner wall of the pipe, and the average roughness below 0.2 μm. The microstructure of the pipe is dominated by tempered sorbite whether before or after rolling, and the grain size before rolling is 16.22 μm on average. After cold rolling, the longitudinal structure is deformed along the direction of rolling, in which the average grain size is 24.31 μm. With the increase in deformation work-hardening behavior in the rolling process, the tensile strength improves from 1134 MPa to 1178 MPa, the yield strength increases from 985 MPa to 1125 MPa, and the room temperature impact energy diminishes from 58 J to 52.5 J. After vacuum tempering at 600 °C, it is difficult to eliminate the deformed band microstructure along the rolling direction completely. However, the grain size is reduced after cold rolling, no coarsening occurs, and the impact toughness increases from 52.5 J to 60.5 J. With the recovery of the original microstructure, the mechanical properties are restored to the before rolling level.