1.What is the core reason for coating burnout?
During welding, a high-temperature arc, molten pool heat or flame (such as gas welding) is generated, and the local temperature can reach thousands of degrees Celsius. If the coating is directly exposed to the high-temperature area, or the heat is transferred to the coating through the substrate and exceeds its tolerance temperature, it may cause the coating to decompose, carbonize, discolor or fall off, which is called "burning".
2.Do differences in the heat resistance limits of different coatings affect whether the coating is burned?
Low-temperature coatings (such as most organic coatings, plastic coatings, and electrophoretic paints): The resistant temperature is usually below 200°C. Even if it is indirectly heated during welding, it is easy to discolor, bubble, or carbonize.
Medium-temperature coatings (such as some inorganic zinc-rich paints and heat-resistant primers): It can withstand 200-400°C. Short-term contact with welding residual heat may only cause slight damage, but direct contact with the arc will burn it.
High-temperature coatings (such as ceramic coatings, high-temperature resistant enamels, and metal spray coatings): The resistant temperature can reach above 500°C, even exceeding 1000°C, and can withstand welding environments (such as some laser welding or brazing) under proper protection.
3.What effect do welding methods and heat concentration have on coatings?
Arc welding (such as arc welding, argon arc welding): The arc temperature is extremely high (3000-6000℃), and the heat is concentrated. If the coating is close to the weld (such as within 10mm), it is very easy to be directly burned or damaged by heat conduction of the substrate.
Gas welding/gas cutting: The flame temperature is about 3000℃, the heat diffusion range is large, and the coating damage area may be wider.
Laser welding: The heat is concentrated and the action time is short. If the parameters are properly controlled (such as low power, fast scanning), the heat-affected zone of the coating is small, and only the edge may be slightly damaged.
Resistance welding (such as spot welding): The heat is concentrated at the welding point. If the surrounding coating is far away (such as more than 5mm), it may only change color due to conduction heat, rather than being completely burned.
4.How does the relative position of the coating to the weld affect whether the coating is burned?
Coating in the weld area: If the part to be welded has a coating, the coating will be directly burned by the arc or molten pool during welding (becoming impurities and even affecting the quality of the weld), so the coating in the weld area usually needs to be removed before welding.
Coating near the weld: The closer the distance (such as <5mm), the greater the impact of heat conduction, especially the thin substrate (such as sheet metal) with fast heat conduction, the coating is more easily damaged; when the distance is far (such as >10mm), there may be only slight discoloration.
5.What are the measures to avoid coating burnout?
Before welding, remove the coating on the weld and nearby areas (at least 5-10 mm).
Choose a welding method with concentrated heat and small heat-affected zone (such as laser welding, plasma arc welding).
Control welding parameters (reduce current, increase speed) to reduce heat input.
Take cooling (such as water cooling) or heat insulation (such as asbestos board shielding) measures for the coating area.
If the coating needs to be retained, welding can be done first and then coating (applicable to scenarios where post-coating is allowed).