Stainless steel heat exchangers are widely used in chemical, energy, and other fields. The welding quality of their core components directly affects the corrosion resistance and service life of the equipment. Intergranular corrosion, one of the main failure modes of stainless steel welded joints, originates from chromium depletion at grain boundaries caused by carbide precipitation in the weld and heat-affected zone. Optimizing welding process control can effectively reduce intergranular corrosion susceptibility. Specific measures cover key aspects such as material selection, heat input management, protective measures, and post-weld treatment.
Material selection for stainless steel heat exchangers is fundamental to preventing intergranular corrosion. Low-carbon or ultra-low-carbon stainless steels (such as 304L and 316L) with a carbon content below 0.03% are preferred, significantly reducing chromium carbide precipitation. For scenarios where post-weld heat treatment is not feasible (such as large pipeline networks), 321 or 347 stainless steel containing stabilizing elements (such as titanium and niobium) can be used. These elements have a stronger affinity for carbon than chromium, preferentially forming stable carbides and avoiding chromium depletion at grain boundaries. Duplex stainless steel (such as 2205) utilizes its ferrite-austenite dual-phase structure, leveraging the carbon-dissolving ability of ferrite to reduce the tendency for grain boundary carbide precipitation, thus further enhancing its resistance to intergranular corrosion.
Controlling the welding heat input is crucial for suppressing carbide precipitation. Excessive high-temperature dwell time accelerates chromium carbide precipitation at grain boundaries; therefore, low-heat-input welding methods (such as pulsed TIG welding or plasma welding) must be employed to reduce arc energy input and shorten exposure time in the high-temperature range. Welding parameters need to be dynamically adjusted according to wall thickness: thin-walled tubes use low current and high welding speed, while thick-walled tubes require controlling the interpass temperature ≤150℃ to avoid localized overheating. Maintaining continuous operation during welding and reducing the number of reheating cycles can effectively shorten the dwell time of the material in the sensitization temperature range (450-850℃).
Welding protection measures for stainless steel heat exchangers are essential to prevent oxidation and impurity intrusion. Using argon gas with a purity ≥99.99% as the shielding gas, with a flow rate controlled at 8-15L/min, can isolate oxygen and nitrogen from the air. For thin-walled tubes (≤3mm), argon purging is required simultaneously on the back side to prevent oxidation at the weld root. Before welding, clean the surface of the weldment to remove oil, rust, and other impurities to prevent decomposition of impurities at high temperatures, which would generate corrosive elements and further reduce the risk of intergranular corrosion.
Post-weld heat treatment is a crucial step in eliminating chromium-depleted zones. Solution treatment involves heating the weld joint to 1050-1100℃ and holding it for a certain time, allowing chromium carbide to redissolve in the austenitic matrix, followed by rapid water cooling (water temperature ≤30℃) to prevent carbon re-precipitation. For stabilized stainless steels (such as 321 and 347), stabilization treatment at 850-900℃ can be used to promote the full bonding of titanium, niobium, and carbon, forming stable carbides. The solution treatment temperature for duplex stainless steel needs to be slightly lower (e.g., 1020-1080℃ for 2205) to avoid excessive ferrite dissolution affecting the duplex equilibrium.
Welding process optimization requires targeted design based on structural characteristics. For thick-walled or complex structures, multi-pass rapid welding and refined weld bead layout can avoid localized overheating caused by stress concentration. Numerical simulation technology can predict heat transfer and microstructure evolution, optimize bevel shape and weld bead sequence, and reduce repeated thermal cycles. Post-weld pickling and passivation treatment can remove surface oxide scale and form a dense passivation film to help resist the intrusion of corrosive media.
Quality inspection is an essential step in verifying corrosion protection effectiveness. Intergranular corrosion testing is performed according to ASTM A262 standards, observing grain boundary corrosion traces through oxalic acid etching or nitric acid testing. Non-destructive testing (such as penetrant testing and ultrasonic testing) can detect defects such as cracks and porosity on the weld surface, preventing defects from becoming entry points for corrosive media. Visual inspection must ensure that the weld surface is smooth and uniform, with the weld reinforcement controlled within 0.5-2mm to avoid stress concentration that accelerates corrosion.
Through comprehensive management of the entire process, including material selection, heat input control, protective measures, post-weld treatment, and quality inspection, the intergranular corrosion susceptibility of stainless steel heat exchanger welded joints can be systematically reduced. These measures not only extend the service life of equipment, but also reduce unplanned downtime and maintenance costs caused by corrosion, providing an important guarantee for the safe and stable operation of industries such as chemical and energy.
