Titanium heating coils are commonly manufactured via cold bending to fit the limited internal space of reaction vessels, heat exchange tanks and closed circulating equipment. Improper cold bending operations such as excessive bending radius, local indentation, rapid forced stamping and uneven clamping pressure will leave micro-cracks, grain distortion, surface scratch defects and high residual tensile stress on the bending arc regions. Once these defective areas are immersed in chloride-containing, acidic or high-temperature industrial process media, tensile residual stress couples with corrosive environments to trigger stress corrosion cracking. Tiny surface micro-defects act as crack initiation points, and cracks gradually extend along grain boundaries under continuous thermal cycling and fluid vibration loads, eventually leading to coil leakage and equipment failure. Adhering to standardized controlled cold bending forming specifications can minimize surface damage and residual tensile stress during processing, eliminate hidden initiation points for stress corrosion cracking, and ensure the long-term structural reliability of bent titanium heating coil assemblies in corrosive service environments.
Setting a minimum allowable bending radius is the primary technical specification to prevent microstructural damage during titanium cold forming. Titanium alloys feature relatively low room-temperature ductility compared with conventional carbon steel; excessively small bending radii force severe plastic deformation on the outer arc of the bent section, resulting in grain elongation, intergranular micro-cracks and surface orange peel defects. These invisible micro-defects cannot be eliminated by subsequent conventional cleaning processes and become sensitive corrosion initiation sites in chloride-rich media. Formulating a unified minimum bending radius standard according to titanium tube outer diameter and wall thickness ensures the outer fiber deformation rate is controlled within the material safe ductility range. Auxiliary flexible mandrels are inserted inside thin-walled titanium tubes before bending to avoid inner wall collapse, wrinkling and local stress concentration during cold forming.
Surface protection and uniform clamping constraints during bending avoid artificial surface micro-scratches. Hard metal bending dies, rough clamping fixtures and unpolished contact surfaces easily leave linear indentations and abrasions on titanium tube outer walls. These scratches break the naturally formed continuous passive film and introduce local residual tensile stress along scratch edges. In corrosive conductive media, scratch locations preferentially develop pitting corrosion, which further evolves into stress corrosion cracks under the combined action of forming residual stress and operating cyclic thermal stress. All bending contact fixtures should be covered with high-temperature resistant PTFE protective liners, and clamping force must be evenly distributed to prevent point-type pressure indentation. Any accidental surface damage generated during forming requires fine abrasive polishing followed by chemical passivation to reconstruct a complete protective oxide layer before assembly.
Post-bending stress relief treatment paired with non-destructive defect inspection is a mandatory final process for bent titanium heating coils. Even if bending parameters strictly comply with design standards, cold plastic deformation inevitably generates uneven residual tensile stress at bending arcs. Low-frequency vibration stress relief or low-temperature thermal stress homogenization treatment redistributes residual stress to a low-level balanced state and greatly reduces the material sensitivity to stress corrosion. After stress relief, eddy current and fluorescent penetrant non-destructive testing are implemented on all bent sections to screen for hidden surface micro-cracks, wall thinning and folding defects. Coils failing defect inspection must be repaired or scrapped directly rather than put into service, to avoid delayed stress corrosion cracking failure after long-term industrial operation.
The following table presents classified cold bending forming protection schemes for different titanium heating coil service conditions:
表格
| Titanium Heating Coil Service Scenario | Recommended Controlled Cold Bending Specification & Supporting Process | Core Stress Corrosion Cracking Prevention Value |
|---|---|---|
| High-temperature high-chloride stirred reactor titanium heating coil | Minimum 3-times outer diameter bending radius + internal mandrel support + post-forming vibration stress relief | Avoids outer arc grain micro-cracks and releases tensile residual stress to suppress chloride-induced stress corrosion cracking |
| Medium-temperature fine chemical batch reaction bent heating assembly | PTFE die surface protection + uniform flexible clamping + post-bending penetrant inspection | Eliminates artificial scratch initiation points and filters hidden surface forming defects |
| Low-pressure softened water circulating heating coil with frequent thermal cycling | Standard bending radius constraint + local polishing & repassivation for minor surface abrasions | Repairs damaged passive film and reduces stress corrosion risk under alternating temperature loads |
| Long-service coastal wastewater buried titanium heating coil | Strict forming parameter control + full bent section eddy current scanning + low-temperature stress homogenization | Prevents delayed crack propagation under underground corrosive and vibration complex service conditions |
Controlled cold bending forming specifications eliminate stress corrosion hidden dangers at the source of titanium heating coil manufacturing. Titanium's excellent corrosion resistance cannot prevent crack propagation driven by the coupling effect of residual tensile stress and local micro-defects in aggressive industrial media. Standardized bending parameter control, surface protective processing, post-forming stress relief and non-destructive defect screening jointly guarantee the structural integrity of bent titanium components, cut off the initiation path of stress corrosion cracking, reduce post-operation maintenance and replacement costs, and realize the full-lifecycle safe operation of custom-shaped titanium heating coil equipment in various corrosive industrial scenarios.
