I. What does decarburization of the galvanized steel base look like (key distinction: decarburization refers to the loss of carbon from the surface layer of the steel base, not the peeling off of the zinc layer)?
❗The temperature of the hot-dip zinc bath is only about 450°C, and the zinc tank itself does not cause decarburization; decarburization must occur in the high-temperature process before galvanizing (≥700°C).
1. Visual appearance (it is difficult to directly determine with the naked eye, it is a hidden defect)
- Normal zinc layer: uniform silver luster, consistent luster, stable alloy layer thickness;
- Zinc layer at decarburization sites: grayish, dark, matte gray spots, uneven luster; the zinc-iron alloy layer is prone to abnormal thickening, roughness, and coarse grains; severely, the adhesion of the zinc layer deteriorates, peels off, and detaches from the base (many sites directly consider it as "poor zinc layer", the root cause is the decarburization of the base material)
- Knocking off/scrapping off the zinc layer: the base material at the decarburization area is softer, the grinding feels more "meaty", the hardness is significantly lower than the core of the pipe;
2. Metallography (GB/T 224-2019 determination standard, the most accurate)
- Complete decarburization layer: only white ferrite on the surface, no pearlite; carbon content is below the solubility limit of ferrite;
- Partial decarburization layer (transition layer): the pearlite gradually increases from the outside to the inside, the carbon content gradient rises, until it is consistent with the normal structure of the base;
- The cross-section is a layer of low-carbon gradient from the surface to the inside, not a thin layer of oxide skin
3. Hardness characteristics
From outside to inside, the hardness gradually increases; the surface hardness is lower, the yield strength decreases, and the fatigue performance deteriorates; it is prone to cracking when subjected to flattening or expansion.
❗Easy to confuse: zinc layer detachment/leakage of zinc coating, acid washing corrosion, sand holes, not decarburization; decarburization is a change in the steel base structure, the coating is only a secondary abnormality.
II. Fundamental principle & specific reasons
Essence: At high temperatures, the carbon on the surface of the steel diffuses outward and reacts with the decarburizing medium in the furnace gas to turn into gas and escape; the carbon potential of the furnace gas is lower than that of the steel → decarburization; if the oxidation rate is greater than the carbon diffusion rate, only iron oxide scale is generated, and no obvious decarburization is visible; a weakly oxidizing atmosphere is most likely to produce a deep decarburization layer
Reaction equation:
Fe3C + H2O &→ 3Fe + CO + H2
Fe3C + CO2 &→ 3Fe + 2CO
Fe3C + O2 &→ 3Fe + CO2
Fe3C + 2H2 &→ 3Fe + CH4
Decarbonization activity strength: Water vapor H₂O > CO₂ > O₂ > H₂; CO and CH₄ are carbon-increasing atmospheres, which can inhibit decarburization
Direct causes (sorted by frequency in the field)
1. Insufficient carbon potential in the annealing furnace atmosphere, and high dew point (the top reason for continuous hot-dip galvanizing lines)
Enclosure annealing/continuous annealing furnace (800~900℃) protective gas nitrogen purity is insufficient, with air introduced; hydrogen contains high moisture and exceeds dew point; poor internal sealing, moisture absorption, and air leakage into the furnace
2. High temperature, too long time of insulation/transport stay
The higher the temperature, the longer the high-temperature stay, the faster the carbon diffusion, and the deeper the decarburization layer; equipment failure shutdown, steel strip staying in the high-temperature zone for a long time, prone to batch decarburization
3. Pre-process of raw steel strip/black pipe has already undergone decarburization
During hot-rolling heating, post-welding stress relief annealing, and cold drawing intermediate annealing, the furnace gas is not protected; the black pipe itself already has decarburization, and galvanizing only retains and enlarges the defect
4. Residual oil, rust scale, iron oxide scale, and lubrication residues on the steel tube surface
After entering the furnace, they decompose and release water vapor and CO₂, forming a strong decarburization micro-atmosphere, with local strip-like/banded decarburization
5. The steel type itself is sensitive
The higher the carbon content, the easier it is to decarburize; Si and Al increase the tendency of decarburization; Cr and a small amount of Mn can inhibit decarburization; Ordinary Q235 and Q355 welded pipes are prone to occur when the dew point is out of control
