Steam has been involved in some way, shape or form with a number of memorable events in the Doctor’s life. For many of us, however, staring down at a dinner plate of steamed broccoli or cauliflower is as up close and personal with steam as it gets. Yet steam is one of the simplest and most basic of heat-treating atmospheres. Let’s learn more.
Steam has been involved in some way, shape or form with a number of memorable events in the Doctor’s life – whether doing calisthenics in a Turkish sauna at 17 or being whipped with wet eucalyptus branches in a Moscow steam bath some 20 years later (stories best left for another time and place). For many of us, however, staring down at a dinner plate of steamed broccoli or cauliflower is as up close and personal with steam as it gets. Yet steam is one of the simplest and most basic of heat-treating atmospheres. Let’s learn more.
Steam treating (a.k.a. bluing or blackening) is a time-temperature-atmosphere-dependent process where the performance and quality of the surface and subsurface layers depends to a great extent on surface cleanliness (prior to steam treatment) and the overall integrity (i.e. gas tightness) of the equipment. As an atmosphere, steam can be used for scale-free tempering and stress relief of wrought or powder-metal (P/M) parts of ferrous or nonferrous materials. While not necessary for all components, steam treatment benefits include: reducing the susceptibility to rusting on steel parts – that is, avoidance of the formation of undesirable Fe2O3 (hematite), sealing porosity, providing a base material for additional (powder or paint) coatings, extending shelf life, improving mechanical properties (e.g., apparent hardness, compressive strength, wear characteristics), and as a decorative coating producing a blue-gray to blue-black surface appearance.
How it Works
Steam treating is performed on ferrous parts through a deliberate addition of steam (H2O) into a tightly sealed heat-treating furnace in the temperature range of 315-540°C (600-1000°F) so as to ensure that only Fe3O4 (magnetite) is formed on the surface (Eq. 1). The following oxidizing reaction takes place at the surface of the parts:
3Fe + 4H2O (g) <> Fe3O4 + 4H2 (g) (1)
A typical cycle begins by thoroughly cleaning the individual parts of oils and other contaminants. Heating then usually takes place in air (or another oxidizing but non-carburizing furnace atmosphere). Ferrous parts are typically heated in the 315-375°C (600-700°F) range before steam introduction, while for nonferrous parts this value is around 150°C (300°F). Purging of the furnace with steam then takes place to an oxygen level less than 1% (10,000 ppm), typically around 0.1% (1,000 ppm). This step must be complete before the temperature exceeds 425°C (800°F) for ferrous parts. A dew point in the furnace of +15°C (+60°F) or higher is typical. After soaking, parts are cooled in steam to an intermediate temperature before removal or rapid quenching finishes the cycle. Steam treatment is reportedly most effective on parts with a maximum carbon content of 0.5-0.8%.
|Fig. 1. 4200 steel (0.60%Mo, 0.45%Ni, 0.28%Mn) with 2% Cu and 1% graphite admixed; steam treated for one hour at 575°C (1067°F)|
Steam Treatment of P/M Parts
Many P/M parts are processed in furnaces exposed to superheated (e.g., 160°C/320°F steam) at a temperature around 550°C (1020°F). The oxide layer formed is typically 5-7µm (0.00020-0.00028 inch) thick on the surface with interconnected subsurface porosity (Fig. 1). Fe3O4 is stable and tenacious, forming a bluish or bluish-black surface layer that does not easily break down.
The Fe3O4 oxide has a hardness of approximately 50 HRC and is highly corrosion-resistant. The thickness of the coating grows with the square root of treating time and can vary from just over 1 µm (0.00005 inch) to just over 7 µm (0.00030 inch). Applications such as sealing require that the part be in the dry steam at 540°C (1000°F) for about 60 minutes. For applications where corrosion resistance or hardness is important, a retention time of only about 30 minutes may be required to achieve the desired results.
|Fig. 2. Combination annealing and bluing cycle for motor laminations|
Steam Treatment of Motor Laminations
The performance of steel in an electro-magnetic circuit is measured by variations in eddy current, hysteresis losses and changes in (magnetic) permeability. The presence of carbon and to a lesser extent sulfur, oxygen and nitrogen in steel increases eddy current and hysteresis losses while lowering permeability. In-process annealing, in addition to removing stains induced from cold working, reduces carbon levels (from steels with up to 0.08%) to typically less than 0.01%C. Eddy current losses vary with lamination thickness and lamination coating. For most 60 Hz applications, 0.60 mm (0.024 inch) is reported to balance optimum stamping qualities with acceptable eddy current losses.
To reduce eddy current losses between laminations, electrical steels are normally coated to increase inter-laminar resistance. These coatings may be organic, inorganic or an oxide (Fe3O4) applied by exposing the lamination to either a high dew-point exothermic atmosphere or a super-heated steam atmosphere (Fig. 2) for a period of up to one hour at a temperature of around 510°C (950°F). This coating generally increases resistance between laminations and provides rust and corrosion protection.
Steam Treatment of High-Speed Steels
The presence of a tenacious oxide coating on some high-speed steel cutting tools is reported to help prevent chip buildup on cutting edges while enhancing grinding, drilling, cutting and endurance (feed rates). However, steam treating is not recommended in cutting applications for very soft or nonferrous materials.
Steam Treatment of Nonferrous Materials
For nonferrous materials, loads are purged at 150°C (300°F), heated to the required soak temperature, and cooled under a steam atmosphere back down to 150°C (300°F) after the final soak and before air or water quenching. The treatment performs both a stress relief and anneal for bronze, brass, copper and silver alloys. Post treatments such as bright dip, buffing or pickling are reportedly reduced.
|Fig. 3. Acceptable and unacceptable steam-treatment conditions for ferrous materials|
The most commonly reported problems with steam treating could be categorized as follows:
The hydrogen (H2) gas created by the steam-treating process is constantly diluting the steam (H2O). If the hydrogen concentration rises too high, the reaction is reversed and the oxide layer reduces. In order to prevent this, three measures are important: (1) maintain a sufficiently high turbulence in the steam; (2) create good circulation paths throughout the load; and (3) bleed controlled amounts of air (or oxygen) into the furnace chamber to keep the hydrogen concentration down to an acceptable level.
In most instances, neither the steam temperature nor the part temperature should exceed 550°C (1020°F) because, above this temperature, the reaction (Eq. 1) is more and more superseded by the reaction:
Fe + H2O (g) > FeO + H2 (2)
which forms a gray, flaky and loosely adhering layer of FeO (wüstite) on the surface of the parts and provides no corrosion protection at all.
Pinkish or inconsistent discoloration of the part surface may indicate the presence of undesirable Fe2O3, suggesting that the entire load was not above 315°C (600°F) prior to the introduction of the steam. Black spots on parts suggest improper cleaning and residues left on parts prior to processing, while white spots often indicate water contaminants or chemicals.
Reddish discoloration suggests that the steam was not dry – that is, liquid water was present and reacted with the iron on the surface of the iron part to form Fe2O3. A brown or brownish-black discoloration suggests air was in contact with the ferrous component while in the presence of the steam.
Steam treating is a versatile tool in the heat-treater’s arsenal and one that should not be forgotten when its benefits are needed. Typical steam-treating applications include automotive, hydraulics, agriculture, marine, home appliances, lawn and garden, and off-road construction components of wrought and powder metal. IH