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In-depth analysis of the relationship between post-welding temperature and color in stainless steel: from oxidation mechanisms to quality assessment. During the welding of stainless steel, the color of the weld is one of the key indicators reflecting the quality of the weld. The change in its color is essentially a reflection of changes in the thickness and composition of the surface oxide film at high temperatures, and it is closely related to the welding temperature, heat input, and the effectiveness of the shielding. By integrating principles of materials science with engineering practice, this paper systematically analyzes the relationship between the temperature and color of stainless steel after welding, and establishes a quality evaluation system. I. The mechanism linking temperature, color, and the oxide film: The formation of the oxide film is related to temperature gradients. During welding, the molten pool and the heat-affected zone (HAZ) are exposed to air at high temperatures, causing elements such as iron and chromium to oxidize and form oxides like FeO and Cr₂O₃. As the temperature increases (200–800°C), the thickness of the oxide film increases from the nanometer scale to the micrometer scale, resulting in different interference colors. For example: At 290°C, a thin layer of FeO is formed, appearing light wheat-yellow (interference wavelength λ=500nm) ； 600℃: Cr₂O₃ enrichment occurs, the oxide film thickens to the sub-micron level, and interference of reflected light produces a deep blue color. Effect of temperature on corrosion resistance: Silver-white (＜300℃): The oxide film is extremely thin (＜10nm), Cr₂O₃ is dense and defect-free, resulting in the best corrosion resistance ； Golden yellow (300~450°C): FeO/Cr₂O₃ mixed film; locally chromium-deficient regions appear, resulting in a 10%~20% decrease in corrosion resistance ； Blue (450–600°C): Cr₂O₃ grains become coarser; Cr-deficient zones form at the grain boundaries, significantly increasing the risk of intergranular corrosion ； Gray-black (＞600℃): The oxide film is loose and porous, with FeO accounting for >50%, resulting in a loss of corrosion resistance. II. Color Grading and Quality Assessment Criteria: Based on AWS D18.2 and ISO 15607 standards, along with experimental data, a correlation matrix linking the color, temperature, and quality of stainless steel welds was established: Color grading, Temperature range (°C), Oxide film thickness (nm), Quality assessment, Repair recommendations – Silver-white: <300, <10; Grade 1 (qualified), No treatment required. Golden-yellow: 300–400, 10–30; Grade 2 (qualified), Polishing or passivation required. Colored (yellow-blue): 400–450, 30–50; Grade 3 (critical), Chemical passivation required. Dark blue: 450–600, 50–100; Grade 4 (unqualified), Rework or re-welding required. Gray-black: >600, >100; Grade 5 (scrap), Entire component replacement required. Key criteria for qualification: Qualified levels are silver-white (AWS Grade 1) and golden-yellow (Grade 2); these can be used in high-demand applications such as the food and chemical industries ； Fail threshold: Deep blue and darker colors require rework, as the destruction of the Cr₂O₃ film causes the pitting potential to drop to -0.3V (compared to silver-white at -0.5V). III. Control of color by welding process parameters – Heat input control: The use of pulsed arc welding (PAW) can reduce the heat input per unit length by 40%, keeping the interpass temperature below 150°C and thus preventing re-oxidation. Formula verification: Q=ηI2t/v, where η is the thermal efficiency, I is the current, t is the pulse width, and v is the velocity. By optimizing the pulse frequency (80–120 Hz), the peak temperature of the thermal cycle can be reduced from 800°C to 550°C. Protective gas efficiency: When the purity of argon is increased from 99.99% to 99.999%, the oxygen partial pressure is reduced by two orders of magnitude, which can reduce the thickness of the oxide film by 60%. A Ar+2%He gas mixture is recommended, as it increases arc stiffness by 30% and improves the uniformity of pool mixing. Interlayer temperature control: During multi-layer welding, an infrared thermometer is used for real-time monitoring to ensure that the interlayer temperature remains ≤60°C. The cases show that by controlling the interlayer temperature, the area of the colored region in 4 mm thick 304L welds decreased from 35% to 8%. IV. Quality assurance strategies in engineering practice: Control of pre-welding cleanliness. Using **ultrasonic cleaning (28kHz) to remove oil residues, combined with mechanical grinding (Ra≤3.2μm) to eliminate oxide scale, can reduce the incidence of blue/grey spots in welds by 70%. Dynamic protection technology employs a drag shield for protection during pipeline welding (see Figure 1); by extending the delay time before shutting off the gas to 5 seconds, it is possible to raise the oxidation grade at the weld ends from level 3 to level 2. Post-welding treatment technique: Mechanical polishing – Grinding with a diamond wheel (grit size 80#) can remove an oxide layer of 5μm thickness and restore the silver-white color ； Electrolytic passivation: Electrolysis in a 10% nitric acid solution (2V, 5 min) restores the thickness of the Cr₂O₃ layer on the surface to 20 nm, doubling its salt spray resistance. V. Conclusion: Color is an indicator of quality. Best quality: silver-white welds (temperature < 300°C), offering the best overall performance ； Acceptable range: golden yellow (process variations allowed) and silver-white ； Failure warning: Blue and higher grades require rework, as their corrosion resistance is below 50% of that of the base material.