During the welding process, photovoltaic steel structures are prone to residual stresses in the welds and heat-affected zones due to uneven localized heating and cooling. If these stresses are not promptly eliminated, they can lead to structural deformation, fatigue cracks, and even overall instability, directly impacting the safety and service life of the photovoltaic system. To address the issue of excessive residual stress in photovoltaic steel structures, heat treatment is a key method for effectively reducing residual stress. This method utilizes stress relaxation to induce plastic deformation or structural transformation in the material through creep within a temperature range below the phase transition point.
Universal high-temperature tempering is a primary method for heat treatment of photovoltaic steel structures. The entire photovoltaic steel structure is placed in a heating furnace and slowly heated to below the material's phase transition point (e.g., approximately 600-650°C for carbon steel). After holding at this temperature for a specified period, the structure is then air-cooled or furnace-cooled. During this process, tensile stress zones within the material soften at high temperatures and undergo plastic expansion, while compressive stress zones contract and compress, ultimately significantly reducing residual stress. This method can eliminate approximately 70%-90% of residual stress while improving material toughness. It is particularly suitable for steel structures in large photovoltaic racks or centralized photovoltaic power plants. However, it is important to note that steels containing alloying elements such as molybdenum and vanadium should be heated outside the temper brittleness temperature range (e.g., around 300°C) to prevent material performance degradation.
Local high-temperature tempering is used for photovoltaic steel structures that cannot be heated throughout, such as installed distributed photovoltaic racks or complex nodes. Using a gas flame, induction heating, or resistance heating, the weld and heat-affected zone are locally heated to the target temperature. After holding the temperature, insulation is applied to slow cooling. While this method cannot completely eliminate residual stress, it can significantly reduce peak stress and mitigate the risk of stress concentration. For example, local tempering at the rotating joints of photovoltaic tracking racks can effectively prevent stress-induced stalling or seal failure.
Post-weld dehydrogenation is a complementary method for heat treatment of photovoltaic steel structures. For welding materials with high hydrogen content (such as low-alloy high-strength steel), the structure should be heated to 250-350°C immediately after welding and held at this temperature for at least two hours to promote hydrogen atomic diffusion and escape, thereby preventing hydrogen-induced cracking. This method is particularly suitable for photovoltaic projects in humid environments or coastal areas, effectively reducing the risk of premature failure caused by hydrogen embrittlement.
Heat treatment process parameters must be strictly tailored to the material properties, thickness, and welding method of the photovoltaic steel structure. The heating rate should be controlled at 5-10°C per minute to avoid new cracks caused by thermal stress; the holding time should be calculated at 4-5 minutes per millimeter of thickness to ensure sufficient stress relief; and the cooling rate should be slow to prevent structural transformation stress caused by rapid cooling. For example, a 20mm thick photovoltaic bracket beam should be held at 620°C for 1.5-2 hours during overall tempering, followed by cooling to below 200°C before removal from the furnace.
Combining heat treatment with other stress relief methods can further enhance the effectiveness of the process. For thin-walled photovoltaic steel structures (such as color-coated steel roof supports), vibration aging can be combined with high-frequency mechanical vibration to induce micro-dislocation motion, eliminating 30%-60% of residual stress. This can then be supplemented with local tempering at key joints. For thicker plate structures (such as the main beams of ground-based power stations), explosive treatment or ultrasonic impact strengthening can be used to form a compressive stress layer on the weld surface, inhibiting crack propagation. Overall tempering can then be used to achieve stress uniformity.
Heat treatment to eliminate residual stress in photovoltaic steel structures requires a balance between efficiency and cost. Overall tempering is suitable for standardized components in mass production, while local tempering is more flexible and suitable for on-site repairs or special-shaped structures. By selecting a sound heat treatment method, strictly controlling process parameters, and coordinating with other stress relief techniques, the long-term stability of photovoltaic steel structures can be significantly improved, providing a reliable guarantee for the efficient operation of photovoltaic systems.
