Dehydrogenation Temperature and Time: How to Set Them Without Guessing
Most plants inherit a bake recipe: "200 °C, four hours". It is rarely wrong and rarely verified. The two failure modes are a bake that is too cool to drive hydrogen out, and a bake that is hot enough to temper the steel. Both look identical on paper.
Step 1: establish the temperature ceiling from the steel
The upper limit is set by tempering resistance, not by the coating. A rule used in practice:
| Property class / hardness | Typical bake temperature |
|---|---|
| Class 8.8, below 320 HV | 200 ± 10 °C, usually 2–4 h |
| Class 10.9, 320–380 HV | 200 ± 10 °C, 4–8 h |
| Class 12.9, above 380 HV | 190–210 °C, 8–24 h; confirm with the customer specification |
| Spring steel / case-hardened | Lower end of the range; check tempering curve before changing anything |
Where a customer specification exists, it wins. These figures describe common practice, not a universal standard.
Step 2: decide the time by thickness, not by habit
Hydrogen diffusion distance scales with the square root of time, so doubling the part's diffusion distance needs roughly four times the bake. In practice, plants use coating thickness and part section as proxies:
- Thin coatings (<10 µm), small parts: 2–4 h.
- Standard zinc 8–15 µm, class 10.9: 4–8 h.
- Thick zinc-nickel (12–15 % Ni, 8–15 µm) on high-hardness parts: 8–24 h.
- Parts with blind holes, pressed-in inserts or overlapping surfaces: add time; the oven air cannot reach those volumes quickly.
Step 3: prove the oven, not the controller
- Choose three positions: front-top, centre, back-bottom.
- Drill a representative part and insert the probe into the metal, not into a hole open to air.
- Run a full production load and record the curve.
- Repeat with the oven at 70–80 % load — behaviour changes with loading density.
Accept the recipe only when the coldest part reaches the minimum temperature for the specified time. If it does not, either extend time or reduce load; raising the set-point risks the hottest parts.
Step 4: control the interval before baking
Hydrogen diffuses out slowly at room temperature, but the risk window is the first hours after plating. Common requirements:
- Bake within 1–4 hours of plating, and never allow parts to sit wet overnight.
- Do not interrupt the bake. A bake stopped and restarted does not deliver the same thermal dose.
- Record entry and exit times, load, and the operator. A bake without a record is, in a dispute, a bake that did not happen.
Step 5: if the sealer goes on before the bake
The sealer now sits inside the thermal cycle. Two consequences:
- The sealer must be rated for the bake temperature and time — see the note on high-temperature sealers. If it degrades, corrosion protection is lost even though the bake worked.
- The film may slow the release of water and hydrogen from the surface. Dry parts properly before baking, and do not use the film as a substitute for a dry pre-bake step.