Release time：2020-01-16 18:44 Browse：
Direct Answer: Yes, there are significant differences in the galvanizing results for elongated screws that are heat-treated compared to those that are not.
These differences are mainly evident in the appearance, uniformity, thickness, adhesion of the zinc coating, and the mechanical properties of the screw after galvanizing.
Heat treatment fundamentally alters the internal crystal structure of the steel and its surface chemical composition (especially the decarburization layer). These changes directly affect the electrochemical reactions during the galvanizing process.
1. Impact on Hot-Dip Galvanizing
Untreated Screws: The surface is typically in a rolled or drawn state. With proper pre-treatment (cleaning, pickling, fluxing), they generally galvanize evenly, resulting in a bright, smooth coating.
Heat-Treated Screws: The heating process forms scale and a decarburization layer on the surface.
Decarburization Layer: This is key. The surface layer becomes soft, pure iron. During hot-dip galvanizing, this layer reacts excessively with the molten zinc, causing the zinc-iron alloy layers to grow too thick.
Result: A dull, rough, matte-gray coating that is thicker and less uniform. This coating is also more brittle and prone to flaking.
2. Impact on Electrogalvanizing
Untreated Screws: The uniform surface structure allows for even current distribution, facilitating a bright, smooth, and uniform zinc plating.
Heat-Treated Screws: The decarburized surface and varied microstructure lead to inconsistent electrochemical activity. This can cause non-uniform plating thickness, "staining," or dull spots.
3. Hydrogen Embrittlement Risk
This is a critical issue for high-strength heat-treated screws (e.g., Grade 8.8 and above). The acid pickling step before galvanizing introduces hydrogen atoms into the steel, which can cause sudden brittle failure.
Heat-treated, high-strength screws must undergo a post-galvanizing baking process to remove hydrogen. This is generally not required for low-strength, non-heat-treated screws.
| Characteristic | Not Heat-Treated (e.g., Grade 4.8, Low-Carbon Steel) | Heat-Treated (e.g., Grade 8.8 and above) |
|---|---|---|
| Suitable Process | Both Hot-Dip and Electrogalvanizing | Primarily Hot-Dip Galvanizing (Electroplating requires extreme caution) |
| Coating Appearance (Hot-Dip) | Bright, Smooth, Shiny | Dull, Rough, Matte |
| Coating Thickness | Relatively thin and uniform | Thicker and less uniform |
| Adhesion | Generally good | Can be poorer due to the decarburization layer |
| Hydrogen Embrittlement Risk | Low | Very High - Post-plate baking is mandatory |
Performance First: The need for heat treatment is determined by the required mechanical strength.
Trade-offs for Strength: If high-strength screws must be galvanized, the inferior coating appearance must be accepted, and strict process control (especially de-embrittlement baking) is essential for safety.
Appearance Priority: For applications where appearance and corrosion resistance are key, but strength requirements are low, using non-heat-treated low-carbon steel screws will yield a better-looking, more consistent finish.
