UNS N08810 is a Cr23Ni32-type corrosion-resistant high-carbon alloy with a carbon content of 0.05% to 0.1%. The structure of UNS N08810 under high-temperature solid solution conditions is austenitic, but when heated at medium temperatures of 500 to 900°C, this alloy may enter the γ+σ phase region. Long-term heating between 650°C and 750°C results in phase precipitation, which can easily cause over-acidification issues during material pickling. The production of this alloy faces challenges such as high deformation resistance, high deformation temperatures, uneven grains after cold expansion heat treatment, and a narrow processing temperature range. It is primarily used at temperatures above 600°C, with characteristics of coarse grains and high creep strength. It is used in superheaters, reheaters, high-temperature heating, and other applications in chemical plants, power plants, petrochemical industries, and polysilicon equipment manufacturing. The processing technology for UNS N08810 seamless large-diameter pipes with an outer diameter ≥ 300mm involves hot perforation, cold expansion, and cold rolling. However, this process is not as mature as the cold processing of other 300-series, leading to many quality issues and a lack of successful experience to draw from. The main specifications currently required by the market are 323.8mm × 10.31mm, 355.6mm × 11.3mm, and 406mm × 12.7mm. In actual production, the outer diameter of UNS N08810 increases by 15 to 30mm during cold expansion, and transverse cracks appear on the inner surface after multiple expansions. Research and analysis of the transverse cracking in UNS N08810 seamless large-diameter pipes during cold expansion are needed to optimize the cold processing process, ensuring safe production with guaranteed quality and quantity.
 

The specifications of the perforated rough pipe of UNS N08810 are 250mm × 20mm or 290mm × 25mm. The entire cold processing sequence includes rough pipe lubrication, cold expansion, degreasing, heat treatment, straightening, intermediate product lubrication, cold expansion, degreasing, heat treatment, straightening, intermediate product lubrication, welding the head, wall reduction drawing, degreasing, heat treatment, straightening, flat head, pickling, and whitening. The cold expansion unit uses a 700-ton hydraulic drawing machine. The equipment operates stably during the expansion process, with the cold expansion machine running at a speed of 1.5 m/min. The single-pass cold expansion is controlled at 15 to 25 mm, increasing the expansion per pass and reducing the total number of expansion and heat treatment passes. The "Standard Specifications for Iron-Chromium Alloy Seamless Steel Pipes" is referenced, and the actual solution temperature on-site is set between 1050 and 1180°C. The holding time is adjusted according to the wall thickness changes during the process pass, and the heat treatment is followed by rapid water cooling and solid solution treatment. HF and HNO3 are used for mixed acid pickling, with the HF concentration around 30 to 35 g/cm³ and the HNO3 concentration around 180 to 200 g/cm³. Calcium grease and lime are used for lubrication during cold expansion, with a ratio of calcium grease to lime between 1:6 and 1:7. In actual production, serious quality defects such as transverse cracks in the inner surface are generated.
 
Table 1 Chemical composition of UNS N08810 (wt/%)

 

Samples were taken from the 406mm × 12.7mm stainless steel seamless pipe that failed cold expansion, and the composition, physical, chemical, and process analyses were performed. The chemical composition of the sample was analyzed in accordance with GB/T 11170-2016, "Determination of Multi-element Content of Stainless Steel by Spark Discharge Atomic Emission Spectrometry (Conventional Method)," and the results are shown in Table 1. Upon comparison, its composition meets the requirements of ASTM B407-08a (2014), "Standard Specifications for Nickel-Iron-Chromium Alloy Seamless Pipes" for UNS N08810, and there is no obvious abnormality in the main components of the matrix.
 

According to GB/T 228.1-2010, "Tensile Test of Metallic Materials Part 1: Room Temperature Test Method," the room temperature tensile test was conducted. The results are shown in Figure 1. The strength and plasticity are good, exceeding ASTM B407 standard requirements.
 

As shown in Figure 2, the macromorphology of the inner surface of the stainless steel pipe after cold expansion primarily features transverse cracks along the circumferential direction, with a crack width of ≤4 mm and a length of ≤100 mm. There are several very small cracks near the larger cracks, and an over-acid is consistently observed at the transverse cracks.
 
 
Figure 1 Mechanical properties of UNS N08810 steel pipes
 
 
Figure 2 Schematic diagram of macroscopic and over-acid of transverse cracks on the inner surface of cold-expanded stainless steel pipes
 
The seamless steel pipe was sampled after pickling, grinding, and cold expansion, and the sampling size was 10 mm × 10 mm. The metallographic structure of the inner surface is shown in Figure 3. It can be seen from the figure that the cracks propagated along the crystals, with the grains being coarse and fine, and the grains around the cracks being finer, leading to the appearance of mixed crystals.
 

Figure 3 Metallographic structure of the inner wall transverse crack
 

As shown in Figure 2, the crack is transgranular. To further analyze the crack, it was observed using a field emission scanning electron microscope. Figure 4 shows the micromorphology of the microcrack. A composition analysis was conducted on both the defective and non-defective parts of the crack. As shown in Table 2, the Cr and O element contents at the crack are significantly increased, indicating the presence of chromium oxides, primarily generated during hot perforation and hole expansion. During hole expansion, the inner wall stress is concentrated, leading to the tearing of the metal structure. The stress concentration may be caused by lubrication failure or severe polishing marks, as shown in Figure 5.
 
 
Figure 4 Micromorphology of transverse pockets
 

Table 2 Micro-region composition percentage (mass fraction %)

 

Figure 5 Macroscopic morphology of serious polishing marks