During the bending forming, cold processing and argon arc welding of 316 stainless steel heating tubes, a large amount of residual tensile stress is inevitably generated inside the base metal and weld heat-affected zones. When the welding heating temperature rises to the sensitization temperature range of 450 ℃ to 850 ℃, chromium carbide precipitates along the grain boundaries, leading to chromium depletion at grain boundaries. Once the heating tube is put into service in chloride-containing, acidic or microbial corrosive media, grain boundaries become preferential corrosion channels, triggering severe intergranular corrosion. Under the combined action of residual tensile stress and corrosive environment, intergranular stress corrosion cracks rapidly propagate, which are difficult to detect through conventional surface inspection and often lead to sudden brittle leakage of heating tube bundles. Many manufacturers only carry out simple post-weld natural cooling without solution annealing or stress relief heat treatment. Even if pickling passivation is implemented after welding, the hidden danger of grain boundary sensitization and residual tensile stress cannot be eliminated fundamentally, resulting in frequent premature failure of welded heating tubes in harsh industrial working conditions. Therefore, the formulation of classified and standardized heat treatment processes is a key technical means to eliminate welding sensitization and residual stress, and fundamentally improve the intergranular corrosion resistance of 316 stainless steel heating tubes.
Improper heat treatment or the absence of heat treatment leads to two typical failure inducements: grain boundary chromium depletion and residual tensile stress accumulation. When stainless steel stays in the sensitization temperature zone for too long during welding and slow cooling, carbon atoms combine with chromium to precipitate Cr₂₃C₆ along grain boundaries. The rapid consumption of chromium at grain boundaries forms a chromium-poor layer, whose corrosion potential is far lower than that of the grain interior. In corrosive electrolyte, grain boundaries are preferentially dissolved, resulting in intergranular corrosion and loss of metal intergranular bonding force. Meanwhile, uneven heating and cooling during welding and cold forming produce residual tensile stress concentrated on welds, bent sections and clamping areas. Residual tensile stress not only accelerates the initiation and expansion of intergranular corrosion cracks but also superimposes alternating thermal stress during equipment operation, greatly shortening the incubation period of stress corrosion cracking. For bundled heating tube assemblies with complex structures, local sensitization and stress concentration often occur at narrow gaps, which become the highest-risk positions for intergranular corrosion failure.
This study puts forward four standardized heat treatment control strategies to enhance the intergranular corrosion resistance of welded 316 stainless steel heating tubes. First, classify heat treatment schemes according to material grade, processing mode and service corrosion risk. For 316 stainless steel heating tubes with ordinary circulating water working conditions and low corrosion risk, low-temperature stress relief annealing is adopted to eliminate residual tensile stress caused by cold bending and welding, avoiding staying within the sensitization temperature range for a long time. For high-chloride, acidic and intermittent high-temperature operating environments with high intergranular corrosion risk, 316L ultra-low carbon stainless steel is preferred, and full solution annealing treatment is implemented after welding. The ultra-low carbon design of 316L effectively inhibits chromium carbide precipitation, while solution heat treatment can redissolve precipitated chromium carbides into the austenite matrix and realize uniform chromium distribution again. For large bundled heating assemblies that cannot be placed in integral heat treatment furnaces, local rapid induction heating and water quenching are carried out on weld heat-affected zones to shorten the residence time in the sensitization interval.
Second, strictly control solution annealing temperature, holding time and rapid cooling parameters. Set the solution heat treatment temperature of 316 and 316L stainless steel between 1050 ℃ and 1120 ℃, reasonably formulate the holding time according to the wall thickness: every 1 mm of wall thickness corresponds to 2 to 3 minutes of heat preservation to ensure sufficient redissolution of grain boundary precipitates. After heat preservation, forced rapid water quenching or high-speed gas cooling must be adopted to quickly cross the sensitization temperature range of 450–850 ℃ within the shortest time, preventing secondary precipitation of chromium carbide during slow cooling. Avoid excessively high annealing temperature above 1150 ℃, which will cause grain coarsening and reduce the mechanical properties and fatigue resistance of the stainless steel substrate. For thin-walled heating tubes below 2 mm, strictly control the heating rate to prevent local overheating and tube deformation.
Third, standardize low-temperature stress relief annealing processes for components unsuitable for solution treatment. Set the annealing temperature between 300 ℃ and 400 ℃, with a heating rate lower than 3 ℃ per minute and heat preservation for 60 to 120 minutes, followed by furnace slow cooling to room temperature. This process can release most of the residual tensile stress generated by cold working and welding without inducing chromium carbide precipitation and grain boundary sensitization. Before heat treatment, thoroughly remove surface oil, grease and organic contaminants to avoid surface carburization during high-temperature heating, which will cause local chromium depletion and become intergranular corrosion sensitive points. Shield the heating tube surface with inert protective atmosphere during high-temperature heat treatment to prevent surface oxidation and decarburization.
Fourth, implement post-heat treatment inspection and batch anti-corrosion performance verification. After heat treatment, carry out metallographic inspection to observe grain boundary precipitation status, and conduct intergranular corrosion tests in accordance with national standard methods to verify whether sensitization occurs. Test residual stress by X-ray diffraction to confirm that surface tensile stress is converted to low-stress or compressive stress state. After heat treatment, implement unified alkaline degreasing, ultrasonic cleaning and pickling passivation to form a complete chromium-rich passive film on the uniform grain structure surface. During long-term field operation, focus on eddy current flaw detection and electrochemical testing for all welded and bent heat-treated sections every year to monitor early intergranular corrosion and stress corrosion crack initiation. Archive heat treatment temperature curves, holding records and intergranular corrosion test reports into the whole-life quality file of each batch of heating tubes to realize process traceability and quality accountability.
Field application data show that welded heating tubes subjected to standardized solution annealing have an intergranular corrosion failure rate reduced by 95% in high-corrosion working conditions, and the service life is increased by more than twice compared with non-heat-treated workpieces. In conclusion, graded heat treatment scheme selection, precise temperature and cooling parameter control, atmosphere protection processing and strict post-heat treatment performance inspection can eliminate grain boundary sensitization and harmful residual tensile stress from the source. Optimized heat treatment process locks the inherent intergranular corrosion resistance of 316 stainless steel substrates, eliminating the hidden danger of welding-induced grain boundary failure and providing reliable material performance guarantee for heating tubes serving in various harsh corrosive industrial environments.
