Hydrogen embrittlement of fasteners is a result of electrochemical surface treatments whereby hydrogen (in atomic form) enters the lattice of the steel and causes microcracks in the structure, followed by mechanical failure.
The most common method to prevent hydrogen embrittlement is through a process called baking. Baking consists of heating the fasteners to a specified temperature and holding them at temperature generally between 175-220ºC (350-430°F) for a period of 8-24 hours. The process is done in a convection-style industrial oven, either continuously or on a batch basis. The specific time and temperature necessary are determined by the hardness of the steel and are also affected by the permeability of the coating. For zinc-electroplated fasteners above 39 HRC, for example, common practice is a baking time of 8-10 hours at 190-220ºC (375-430°F).
It is critical to perform the baking process as soon as possible after plating. Otherwise, the hydrogen will become fully absorbed into the steel lattice, preventing it from being baked out. Although some specifications call for baking to be performed within 24 hours of plating, it is generally accepted practice to bake within three hours of plating, and many manufacturers specify no more than 1 hour. For this reason many fastener manufacturers use a continuous flow (first in, first out) process so that the work in process is minimized and the chance of shipping improperly baked fasteners is reduced.
For example, a manufacturer of helicopters required processing of steel fasteners for the purpose of hydrogen embrittlement relief. Due to the long baking times required and the restricted floor space available, three electric-heated five-drawer ovens (Fig. 1) were used. The ovens each have a capacity of 90 kg (200 pounds) per drawer, for a total capacity of 455 kg (1,000 pounds) per oven. With an operating range of 175-260°C (350-500°F), the oven uses closed-loop temperature control to provide tight temperature tolerances. It maintains a temperature uniformity of ±5.5°C (±10°F) throughout the heating chamber. The air is delivered from ductwork located on both sides and discharges horizontally to each drawer before traveling vertically and returning to the heating/recirculation system at the top.
In another example, a high-production fastener manufacturer required a continuous hydrogen embrittlement relief oven (Fig. 2) to process miniature screws for eyeglasses and other applications. In order to handle the throughput of 1,815 kg (4,000 pounds) per hour, a double conveyor-belt arrangement was utilized. It used a compound-weave conveyor belt riding on top of a conventional flat wire belt. The upper belt provided a very tight weave to carry the tiny screws without getting caught in the mesh. The lower belt served as the carrier and was driven by the conveyor drive. The lower belt rides on a low-friction roller bed, which allows a loading of 366 kg/m3 (75 pounds/foot2) without overloading or excessive wear.
The 18.3-meter-long (60-foot-long) oven has three heating zones to provide a one-hour heat-up and a seven-hour soak time at temperature. A high-volume recirculated airflow system delivers air to both the top and bottom of the conveyor belt to provide a temperature uniformity of ±5.5°C at 205°C (±10°F at 400°F).
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