During cold bending, stamping, welding and machining, 316 stainless steel heating tubes inevitably retain residual tensile stress inside the material. When such components are immersed in chloride-containing high-temperature industrial media, residual tensile stress works together with corrosive ions to trigger stress corrosion cracking (SCC). This failure mode features hidden initiation, rapid crack propagation and no obvious plastic deformation warning. Micro-cracks germinate from surface pitting defects, extend along grain boundaries or transgranularly, and eventually lead to sudden tube rupture and medium leakage. Many manufacturers only pay attention to dimensional precision and appearance quality after forming, ignoring residual stress detection and elimination. In high-temperature salt-containing environments, stress concentration areas rapidly develop into SCC failure points, causing frequent safety accidents and production shutdown losses. Therefore, effective residual stress control is essential to prevent stress corrosion cracking and guarantee the long-term structural safety of heating tubes.
The mechanism of chloride-induced stress corrosion cracking lies in the synergistic effect of tensile stress and electrochemical corrosion. Chloride ions break the integrity of the surface chromium-rich passive film, forming local anodic dissolution pits. Residual tensile stress concentrates at the pit tip, continuously opening tiny crack gaps and hindering passive film self-repair. Fresh metal inside cracks keeps dissolving anodically, pushing cracks to expand gradually. Cold-processed heating tubes have higher residual tensile stress, which significantly reduces the critical chloride concentration and temperature threshold for SCC. Weld heat-affected zones are also typical high-risk regions, where uneven thermal cycles lead to severe stress accumulation and material sensitization, further accelerating crack initiation.
This research puts forward four systematic technical measures to restrain SCC: residual stress relief annealing, optimized cold forming parameters, surface compressive stress strengthening and localized corrosion risk isolation.
First, low-temperature stress relief annealing is adopted for cold-bent, welded and thin-wall heating tubes. Workpieces are heated to 250–350 ℃ and held for 60–120 minutes before slow furnace cooling. This process can release most machining and welding residual tensile stress without changing the mechanical properties and corrosion resistance of 316 stainless steel. For components with complex forming deformation, solution annealing treatment at 1050 ℃ can thoroughly eliminate stress and welding sensitization, which is suitable for harsh high-temperature chloride working conditions.
Second, cold forming processes are optimized to reduce initial stress levels. The minimum bending radius shall not be less than three times the pipe outer diameter to avoid excessive tensile strain on the outer arc surface. Multi-pass gradual bending replaces one-time sharp forming to disperse strain distribution and prevent local stress concentration. Welding adopts low heat input segmented welding with forced rapid cooling, which narrows the high-stress heat-affected zone and avoids superposition of thermal stress and forming residual stress.
Third, surface shot peening treatment is applied to convert surface tensile stress into uniform compressive stress. High-speed tiny steel shots bombard the tube outer wall, forming a dense plastic deformation layer. Surface compressive stress can effectively restrain the opening and propagation of micro-crack tips, fundamentally cutting off the necessary tensile condition for SCC. This technology is especially suitable for bending sections, weld seams and other high-risk stress concentration positions.
Fourth, environmental parameter control reduces the driving force of SCC. Appropriately lower the operating temperature of the medium and avoid long-term service above 60 ℃ in high-chloride environments. Regular purification treatment reduces chloride ion concentration in circulating fluid, preventing local ion enrichment under sediment deposits. Enhanced surface passivation thickens the protective film to delay pitting initiation at stress concentration areas.
Accelerated SCC simulation tests show that heating tubes after stress relief and shot peening treatment have no crack propagation after thousands of hours of high-temperature chloride immersion testing. In contrast, untreated cold-formed samples produce penetrating stress cracks within a short test period. Industrial application data verify that standardized stress control measures reduce the SCC failure rate of heating tubes by more than 77%. In conclusion, eliminating residual tensile stress, optimizing forming and welding processes, introducing surface compressive stress and improving service medium conditions can effectively prevent chloride-induced stress corrosion cracking. These whole-process control measures block the synergistic failure path of stress and corrosion, greatly improving the safety service life of 316 stainless steel heating tubes in harsh industrial environments.
