At the time that I started working on the background for this article, I thought, “I wonder what Wikipedia says about steam?” According to Wikipedia, steam is water in the gas phase commonly formed by boiling or evaporating water. Steam and the steam engine played a central role in the Industrial Revolution, with the modern steam engines generating more than 80% of the world’s electricity.
Steam is used for many things, including cleaning, sterilization, heating and concrete treatment. Wikipedia, however, does not mention metals treatment when talking about steam. In order to raise awareness of the use of steam in metals treatment, this article will discuss the steam treatment of metals: the process, the equipment and the benefits.
The steam-treat process is a thermal process requiring good controls for time, temperature and atmosphere. The thermal profile can be tailored to achieve specific oxide-layer thicknesses. The process can be completed in a batch-style furnace design or a continuous-belt furnace, depending on product volume.
The first step of the process is heating the product in a batch-type furnace in a chamber with air to or above 315°C (600°F). A soak time at that temperature is typically employed to give time for the removal of any residual oils or fluids left on the product surface from previous processing steps and to ensure the entire load is at a uniform temperature.
The next step raises the temperature to above 370°C (700°F), and the chamber is purged with dry steam at that point. The product load must be at 315°C (600°F) or higher when steam is introduced or there is danger that the produced oxide is red rust. The steam must be dry with any condensate being evacuated from the steam line through a trap so it does not enter into the chamber with the steam.
A typical final processing temperature of 538°C (1000°F) is reached, and the product is left to soak in steam at this temperature. The soak time can be tailored to the specific application. Typical soak times are between 30-60 minutes, where the length of time ensures porosity-sealing or magnetite thickness on the surface. During this time, the steam or gaseous H2O reacts with the iron to form the magnetite (Fe3O4) layer (Fig. 1), usually in the thickness range of 3-7 µm (0.0001-0.00028 inch). This is the chemical reaction that takes place:
4H2O (gas) + 3Fe Fe3O4 + 4H2 (gas)
The process ends with cooling and a purge of nitrogen to push out any free hydrogen left in the chamber from the chemical reaction prior to introducing the air back into the chamber. In most cases, the load is pulled from the chamber in the 315-425°C (600-800°F) range out into air and placed on a cooling table. The product is not negatively impacted by pulling the load of product into air at these temperatures (Fig. 2).
As mentioned earlier, the steam-treat process can be completed in a batch furnace or a continuous furnace. I’ll review each and provide advantages and disadvantages with some depictions.
The batch furnace is an electrically heated, top-loading, pit-type furnace (Fig. 3). The furnace can be installed in a pit to lower the loading height or installed at ground level with a mezzanine. The typical maximum temperature is 760°C (1400°F). The work envelope is typically cylindrical and surrounded on all sides by high-temperature resistance heating elements. The work chamber includes an alloy retort to protect the surrounding high-performance insulation package from the process steam. The workload is maintained in mesh alloy baskets that are lowered into the heat chamber from the top.
A counterbalanced cover lid is utilized to seal the work chamber. A shielded high-temperature circulation fan is located in the floor of the unit. The circulation fan is utilized to provide the best temperature and steam uniformity to the workload. An ambient air blower can be included to accelerate the cooling cycle.
The controls can be either a programmable step/soak sequence controller or a human-machine interface (HMI) that manages the thermal ramping, steam input, soak times and cooling cycle. The unit can be sized to handle just about any preferred load size.
The batch system is best suited for low- to mid-size production volumes or a high mix of products where cycles may differ substantially. The footprint required can be less than the continuous furnace. Anecdotally, many users claim they achieve higher magnetite thicknesses and better sealing properties from the batch-style furnace. However, large load sizes can take time to reach temperature and can require costly soak times to achieve temperature uniformity throughout the load.
High-volume production requirements may lend themselves to a continuous-belt steam-treat furnace. The humpback (Fig. 4)
or straight-through belt furnace are two types currently in operation. The designs are both electrically heated units where the product is loaded directly onto the continuous mesh belt. The typical maximum temperature is 760°C (1400°F).
The humpback design utilizes an elevated hearth and alloy muffle chamber that takes advantage of the fact that steam wants to rise. The design aids in reducing the ability of air entering and mixing with the steam. The straight-through design eliminates the raised hearth.
The heating is done by wire-wound heating elements above and below the alloy muffle for good temperature uniformity. The cycle times are controlled by the belt drive speed and the length of the heating chamber. Continuous designs typically have multiple heating zones individually controllable via universal digital control instruments or via HMI. Continuous furnaces can be customized to specific production load/volume requirements.
Continuous-belt designs are suited for high-volume production runs requiring ongoing, consistent 24-hour operation. The design is not suited for situations that require constant start-ups and shutdowns. Operating the furnace in this manner can put significant stress on components and reduce their life.
Footprint can be an issue because the units can get lengthy based on cycle requirements. The load is continuous and lighter than the batch, however, which allows the product to reach temperature much faster. This provides shorter soak times, potentially saving on energy costs.
Steam and iron typically should not be in close contact or detrimental oxidation will begin. Red rust (Fe2O3) is the result of this oxidation process, which is undesirable in nearly all applications. However, steam treating is a controlled-oxidation process that produces a thin layer of a beneficial oxide on the surface of a ferrous component.
The process yields a layer of magnetite (Fe3O4). The magnetite on the surface is a blue-gray to dark-blue color and is extremely hard. The benefits of the magnetite layer can include: improved magnetic response; improved surface wear resistance; sealing of open porosity in powder-metal components; inhibiting the formation of red rust; improving apparent hardness and mechanical properties in powder-metal components; providing a surface layer for better adherence of paint or other coatings; and providing a blue-gray to dark-blue decorative look.
The process is used on drills, taps and other machine tools where the oxide layer is able to counteract galling or loading of the surfaces. Due to the tightening of environmental regulations forcing companies to look for alternatives, steam treatment is replacing black-oxide coatings in some instances. Steam treatment is more environmentally friendly than other similar processes because it uses no caustic oxidizers, oils or other chemicals to develop its surface oxide.
Steam treatment is an effective thermal process for improving both mechanical properties and the decorative look of metal components. The process can be more energy-efficient and environmentally friendly than other treatment processes. The equipment and controls are not complicated, so the process is easily developed and understood.
Our engineers have a wide range of expertise with this process and helping customers understand which piece of equipment fits their requirements.