Figure 1. Ferrous Steam Treated Components
The steam treatment process is the controlled oxidation of metals to produce a thin layer of oxide on the surface of a component. This process can be used to provide a component with increased corrosion resistance, better wear resistance, increased surface hardness, an attractive surface finish and, in the case of porous materials such as powder metal, seal the part porosity and increase the density.
Like any thermal process, the time, temperature and atmosphere relationship is critical to the success of the steam treatment process.
Figure 2. Thermal History of a Steam Treated Part
When processing a ferrous component, the first step in the process is to heat the component in air to a temperature above 300ºF. A typical “rule of thumb” is to heat the part above 700ºF before exposing it to steam to be sure that the entire load of parts is above 300ºF. If a ferrous component is exposed to steam below 300ºF, the water vapor will oxidize the iron to form Fe2O3on the surface of the part. The Fe2O3shows up as pink inconsistencies in the color of the part surface. Inconsistencies in the surface color may indicate a potential reduction in the performance of the oxide layer of the component.
Once at 700ºF, the component is brought into contact with dry steam, meaning that any condensate as a result of the steam cooling while going between the boiler and steam treating unit is allowed to flow to a drain and not into the steam treating unit.
Figure 3. Schematic of the Desired Oxide Formation
The water vapor in the steam will begin to react with the iron in the part to form Fe3O4. This is an oxide of iron that is blue in color.
If the steam is not dry, the liquid water will react with the iron on the surface of the part to form a very dense form of Fe2O3on the surface of the part, that appears as a red discoloration on the surface of the part. A brown discoloration may also result if air is allowed to contact the ferrous component while in the presence of the steam.
The ferrous parts continue to be heated to a temperature of ~ 1000ºF. It is at this temperature that the reaction and the reaction rate for the formation of Fe3O4by water vapor are optimized. The time that the components are held at 1000ºF in the steam is a function of the application. Applications such as sealing require that the part be in the dry steam at ~1000ºF for ~ 60 minutes; while other applications, like corrosion resistance or hardness, may only require a retention time of ~30 minutes to achieve the desired result.
The final step in the steam treatment process is to allow the component to exit the steam treating unit. Even though the component is still very hot, exposure to open air is generally not a problem. Many producers collect the part in a container and allow them to cool naturally.