In-Depth Interpretation of the Relationship Between Post-Welding Temperature and Color in Stainless Steel: From Oxidation Mechanism to Quality Assessment
I. Correlation Mechanism of Temperature-Color-Oxide Film
Formation of Oxide Film and Temperature Gradient
During welding, the molten pool and Heat-Affected Zone (HAZ) are exposed to air at high temperatures. Elements like iron (Fe) and chromium (Cr) oxidize, forming oxides such as FeO and Cr₂O₃. As the temperature increases (200–800°C), the oxide film thickness grows from nanometers to micrometers, exhibiting different interference colors. For example:
290°C: Thin FeO layer forms, appearing light straw (interference wavelength λ = 500 nm).
600°C: Cr₂O₃ enrichment occurs, oxide film thickens to the sub-micron level, and light reflection interference produces dark blue.
Effect of Temperature on Corrosion Resistance
Silver-white (<300°C): Oxide film is extremely thin (<10 nm), Cr₂O₃ is dense and defect-free, optimal corrosion resistance.
Golden yellow (300–450°C): Mixed FeO/Cr₂O₃ film, localized chromium-depleted zones appear, corrosion resistance decreases by 10%–20%.
Blue (450–600°C): Cr₂O₃ grains coarsen, Cr-deficient bands form at grain boundaries, significantly increased risk of intergranular corrosion.
Gray-black (>600°C): Oxide film is loose and porous, FeO content >50%, corrosion resistance is lost.
II. Color Grading and Quality Assessment Standards
Based on AWS D18.2 and ISO 15607 standards, combined with experimental data, establish the stainless steel weld color-temperature-quality correlation matrix:
| Color Grade | Temperature Range (°C) | Oxide Film Thickness (nm) | Quality Assessment | Remedial Action |
| Silver-white | <300 | <10 | Grade 1 (Acceptable) | No treatment required |
| Golden yellow | 300~400 | 10~30 | Grade 2 (Acceptable) | Polishing or passivation |
| Chromatic (yellow-blue) | 400~450 | 30~50 | Grade 3 (Marginal) | Chemical passivation required |
| Dark blue | 450~600 | 50~100 | Grade 4 (Reject) | Rework or repair welding |
| Gray-black | >600 | >100 | Grade 5 (Scrap) | Component replacement |
Key Criteria:
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Acceptance threshold: Silver-white (AWS Grade 1) and golden yellow (Grade 2) permitted for high-demand applications (food, chemical, etc.).
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Rejection threshold: Dark blue and above require rework, as Cr₂O₃ film failure reduces pitting potential to -0.3V (vs. -0.5V for silver-white).
III. Welding Parameter Control for Color Management
Heat Input Control
Pulsed argon arc welding (PAW) reduces heat input per unit length by 40%, stabilizing interpass temperature below 150°C to prevent repeated oxidation. Formula verification:
Q = ηI²t/v
Where η = thermal efficiency, I = current, t = pulse width, v = speed. Optimizing pulse frequency (80–120Hz) lowers peak thermal cycle temperature from 800°C to 550°C.
Shielding Gas Efficacy
Increasing argon purity from 99.99% to 99.999% reduces oxygen partial pressure by two orders of magnitude, decreasing oxide film thickness by 60%. Recommended: Ar+2%He mixture improves arc stiffness by 30% and enhances molten pool stirring uniformity.
Interpass Temperature Management
For multi-layer welding, use infrared thermometers for real-time monitoring to ensure interpass temperature ≤60°C. Case study: Controlling interpass temperature reduced chromatic zones in 4mm thick 304L welds from 35% to 8%.
IV. Quality Assurance Strategies in Engineering Practice
Pre-weld Cleanliness Control
Acetone + ultrasonic cleaning (28kHz) removes oil contamination, while mechanical grinding (Ra≤3.2μm) eliminates scale, reducing blue/gray spot occurrence by 70%.
Dynamic Shielding Technology
Applying trailing shields (Fig.1) in pipe welding with extended post-flow time (≥5s) improves weld end oxidation color from Grade 3 to Grade 2.
Post-weld Treatment Techniques
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Mechanical polishing: Diamond grinding wheel (80# grit) removes 5μm oxide layer, restoring silver-white appearance.
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Electrolytic passivation: 10% nitric acid solution (2V, 5min) recovers Cr₂O₃ thickness to 20nm, doubling salt spray resistance.
V. Conclusion: Color as a Quality Barometer
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Optimal quality: Silver-white welds (<300°C) with peak comprehensive performance.
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Acceptable range: Golden yellow (tolerable process fluctuation) and silver-white.
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Reject alert: Blue and above require rework, exhibiting <50% of base metal’s corrosion resistance.
Technical Insight: Welders should integrate color observation into process control as a "visual infrared thermometer" for real-time quality feedback. Future intelligent welding systems may incorporate spectral analysis modules to achieve closed-loop color-temperature-quality control.