How does the duplex steel seamless pipe forming process solve the problem of heat exchanger corrosion?

In the field of heat exchangers, traditional welded duplex steel pipes have long been troubled by intergranular corrosion caused by the heat affected zone (HAZ). The essence of this phenomenon is that the local high temperature (1000-1350℃) during welding causes the diffusion of carbon and nitrogen elements in the duplex steel, forming a chromium-poor zone (Cr content <12%) at the interface between the austenite phase and the ferrite phase, which becomes a breakthrough for the corrosive medium. The duplex steel heat exchanger seamless pipe eliminates this hidden danger from the source of material forming through the innovation of hot extrusion and centrifugal casting processes, providing a new paradigm for the long-term operation of equipment under corrosive conditions.

The core of the manufacturing of duplex steel seamless pipes lies in the precise control of temperature and stress fields. In the hot extrusion process, the billet passes through a special die (deformation rate 0.1-10mm/s) in the range of 850-1150℃, and forms uniform equiaxed crystals (grain size 8-15μm) under the action of dynamic recrystallization (DRX). During this process, the internal dislocation density of the material is as high as 10¹²/m², which provides a driving force for the migration of the austenite/ferrite phase boundary and stabilizes the dual-phase ratio at 45:55±3%. Compared with the welding process, there is no local overheating zone in the hot extrusion process, and the diffusion coefficient of chromium is reduced by two orders of magnitude.

Centrifugal casting technology realizes directional solidification of molten metal through a centrifugal force field (100-200G). At a casting temperature of 1450℃, the dual-phase steel melt forms a columnar crystal structure in a rotating copper mold (speed 800-1200rpm), and its primary dendrite spacing (PDAS) is controlled within 30μm. The key process parameters include supercooling control (ΔT=15-25K) and mold cooling rate (>100℃/s), ensuring that the ferrite phase preferentially nucleates on the mold wall and the austenite phase precipitates uniformly at the end of solidification.

The lamellar dual-phase structure (lamellar spacing 0.5-2μm) formed during the seamless pipe forming process has a unique corrosion protection mechanism. In a Cl⁻-containing medium, austenite (γ phase) constitutes the skeleton of the passivation film as an electrochemically inert phase, and ferrite (α phase) dissolves preferentially as an anode, but the Cr element concentration gradient (Δ[Cr]=3-5wt%) at the interface between the two phases promotes the self-repair of the passivation film. XPS analysis shows that this dynamic balance maintains the thickness of the surface Cr₂O₃ film at 4-6nm, effectively blocking the penetration of corrosive media.

During the thermal cycle, the dual-phase structure of the seamless pipe exhibits excellent phase transformation toughness. When the temperature rises above the Ms point (about -40℃), part of the austenite undergoes a martensitic phase transformation (ε→α'), and the volume expands by about 3%. This reversible phase transformation (ΔV=0.02) can absorb thermal stress and inhibit the initiation of fatigue cracks. Experiments show that after 2000 times of -40℃→350℃ thermal shock, the surface roughness Ra of the seamless pipe only increases by 0.12μm, while the welded pipe has obvious microcracks due to HAZ embrittlement.

Through electrochemical impedance spectroscopy (EIS) analysis, the polarization resistance (Rp) of seamless pipes in 3.5wt% NaCl solution reached 1.2×10⁶Ω·cm², which is 40% higher than that of welded pipes. In the critical pitting temperature (CPT) test, the seamless pipe remained passive in 4mol/L FeCl₃ solution to 85°C, while the welded pipe showed stable pitting at 65°C. This is due to the elimination of the sensitization zone of the HAZ by the seamless structure (the width of the carbide precipitation zone is reduced from 20-50μm of the welded pipe to 0).

In the stress corrosion cracking (SCC) experiment, the four-point bending method was used to apply a tensile stress of 80% of the yield strength. After immersion in boiling MgCl₂ solution for 3000h, the crack growth rate of the seamless pipe was da/dt=5×10⁻¹¹mm/s, which was two orders of magnitude lower than that of the welded pipe. The microscopic mechanism is that the uniform dual-phase structure of the seamless pipe increases the hydrogen trap density (dislocation, phase boundary) by 3 times, effectively capturing the diffused hydrogen atoms.

Current research focuses on nano-scale phase boundary engineering: by adding trace amounts of Nb and Ti elements (0.1-0.3wt%), MC-type carbides (size 5-20nm) are formed at the dual-phase interface to further enhance the hydrogen trap effect. Develop a gradient structure seamless pipe (austenite-rich outer wall for erosion resistance, ferrite-rich inner wall for corrosion resistance), and achieve a composition gradient by controlling the solidification process through electromagnetic stirring.