Steam Treating
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team 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 1 7 or being whipped w ith wet eucalyptus bra nches 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 stea m as it gets. Yet steam is one of t he simplest and most basic of heat-treating atmospheres. Let’s learn more. Steam treating (a.k.a. bluing or blacken- ing) is a time-temperature-atmosphere-de- pendent process where the performance and quality of the surface and subsurface layers depends to a great extent on surface cleanli- ness (prior to stea m treatment) and the over - all integrity (i.e. gas tightness) of the equip- ment. 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 for- mation of undesirable Fe 2 O 3 (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 (H 2 O) into a tightly sealed heat- treating fur nace in the temperature ran ge of 315 -540˚C (600-1000˚F) so as to ensure that only Fe 3 O 4 (magnetite) is formed on the surface (Eq. 1). The following oxidizing reaction takes place at the surface of the parts: 3Fe + 4H 2 O (g) Fe 3 O 4 + 4H 2 (g) (1) A typical cycle begins by thoroughly c leaning 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 stea m introduction, while for nonfer rous 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%. Steam Treatment of P/M Parts Many P/M parts are processed in f urnaces exposed to superh eated (e.g. , 160˚C/320˚F steam) at a temperat ure around 550 ˚C (102 0˚F). The oxide layer formed i s typica lly 5- 7µm (0.00020-0. 00028 inch) thick on the surface with interconnected subsurface porosity (Fig. 1). Fe 3 O 4 is stable and tenacious, forming a bluish or bluish- black surface layer that does not easily break down. The Fe 3 O 4 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 var y 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 i s important, a retention time of only about 30 minutes may be required to achieve the desired results. 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 Steam Treating Daniel H. Herring | 630-834-3017 | [email protected] The Heat Treat Doctor S 16 September 2011 - IndustrialHeating.com 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) 25.0 µm
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team has been involved in some way, shape or formwith a number of memorable events in the Doctor’slife – whether doing calisthenics in a Turkish saunaat 17 or being whipped with wet eucalyptus branches
in a Moscow steam bath some 20 years later (stories best left foranother time and place). For many of us, however, staring down ata dinner plate of steamed broccoli or cauliower is as up close andpersonal 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 blacken-ing) is a time-temperature-atmosphere-de-pendent process where the performance andquality of the surface and subsurface layersdepends to a great extent on surface cleanli-ness (prior to steam treatment) and the over-all integrity (i.e. gas tightness) of the equip-ment. 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 necessaryfor all components, steam treatment benets include: reducing thesusceptibility to rusting on steel parts – that is, avoidance of the for-mation of undesirable Fe 2O3 (hematite), sealing porosity, providinga base material for additional (powder or paint) coatings, extendingshelf life, improving mechanical properties (e.g., apparent hardness,compressive strength, wear characteristics), and as a decorativecoating producing a blue-gray to blue-black surface appearance.
How it WorksSteam treating is performed on ferrous parts through a deliberateaddition of steam (H 2O) into a tightly sealed heat-treating furnacein the temperature range of 315-540˚C (600-1000˚F) so as to ensurethat only Fe 3O4 (magnetite) is formed on the surface (Eq. 1).The following oxidizing reaction takes place at the surface ofthe parts:
3Fe + 4H 2O (g) Fe3O4 + 4H 2 (g) (1)
A typical cycle begins by thoroughly cleaning the individualparts of oils and other contaminants. Heating then usually takesplace in air (or another oxidizing but non-carburizing furnaceatmosphere). Ferrous parts are typically heated in the 315-375˚C(600-700˚F) range before steam introduction, while for nonferrousparts this value is around 150˚C (300˚F). Purging of the furnacewith 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 toan intermediate temperature before removal or rapid quenchingnishes the cycle. Steam treatment is reportedly most effective onparts with a maximum carbon content of 0.5-0.8%.
Steam Treatment of P/M PartsMany P/M parts are processed in furnaces exposed to superheated(e.g., 160˚C/320˚F steam) at a temperature around 550̊ 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). Fe 3O4 is stable and tenacious, forming a bluish or bluish-black surface layer that does not easily break down.
The Fe 3O4 oxide has a hardness of approximately 50 HRC andis highly corrosion-resistant. The thickness of the coating growswith the square root of treating time and can vary from just over 1µm (0.00005 inch) to just over 7 µm (0.00030 inch). Applicationssuch as sealing require that the part be in the dry steam at 540˚C(1000˚F) for about 60 minutes. For applications where corrosionresistance or hardness is important, a retention time of only about30 minutes may be required to achieve the desired results.
Steam Treatment of Motor LaminationsThe performance of steel in an electro-magnetic circuit ismeasured by variations in eddy current, hysteresis losses andchanges in (magnetic) permeability. The presence of carbon andto a lesser extent sulfur, oxygen and nitrogen in steel increaseseddy current and hysteresis losses while lowering permeability.In-process annealing, in addition to removing stains inducedfrom cold working, reduces carbon levels (from steels with up to0.08%) to typically less than 0.01%C. Eddy current losses varywith lamination thickness and lamination coating. For most60 Hz applications, 0.60 mm (0.024 inch) is reported to balance
Steam Treating
Daniel H. Herring | 630-834-3017 | [email protected]
The Heat Treat Doctor
S
16 September 2011 - IndustrialHeating.com
Fig. 1. 4200 steel (0.60%Mo, 0.45%Ni, 0.28%Mn) with 2% Cu and1% graphite admixed; steam treated for one hour at 575°C (1067°F)
25.0 µm
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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 (Fe 3O4)applied by exposing the lamination to either a high dew-pointexothermic 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 betweenlaminations and provides rust and corrosion protection.
Steam Treatment of High-Speed SteelsThe presence of a tenacious oxide coating on some high-speed steelcutting tools is reported to help prevent chip buildup on cuttingedges while enhancing grinding, drilling, cutting and endurance(feed rates). However, steam treating is not recommended incutting applications for very soft or nonferrous materials.
Steam Treatment of Nonferrous MaterialsFor nonferrous materials, loads are purged at 150˚C (300˚F),heated to the required soak temperature, and cooled under asteam atmosphere back down to 150˚C (300˚F) after the nalsoak and before air or water quenching. The treatment performsboth a stress relief and anneal for bronze, brass, copper and silveralloys. Post treatments such as bright dip, bufng or pickling arereportedly reduced.
Potential ProblemsThe most commonly reported problems with steam treating couldbe categorized as follows:
Layer ReversalThe hydrogen (H 2) gas created by the steam-treating process isconstantly diluting the steam (H 2O). If the hydrogen concentrationrises too high, the reaction is reversed and the oxide layer reduces.
In order to prevent this, three measures are important: (1) maintaina sufciently high turbulence in the steam; (2) create good
circulation paths throughout the load; and (3) bleed controlledamounts of air (or oxygen) into the furnace chamber to keep thehydrogen concentration down to an acceptable level.
Flaky SurfacesIn most instances, neither the steam temperature nor the parttemperature should exceed 550˚C (1020˚F) because, above thistemperature, the reaction (Eq. 1) is more and more superseded bythe reaction:
Fe + H 2O (g) FeO + H 2 (2)
which forms a gray, aky and loosely adhering layer of FeO(wüstite) on the surface of the parts and provides no corrosionprotection at all.
Discolored SurfacesPinkish or inconsistent discoloration of the part surface mayindicate the presence of undesirable Fe 2O3, suggesting that theentire load was not above 315̊ C (600˚F) prior to the introductionof the steam. Black spots on parts suggest improper cleaning andresidues left on parts prior to processing, while white spots oftenindicate water contaminants or chemicals.
Reddish discoloration suggests that the steam was not dry –that is, liquid water was present and reacted with the iron onthe surface of the iron part to form Fe 2O3. A brown or brownish-black discoloration suggests air was in contact with the ferrouscomponent while in the presence of the steam.
In ConclusionSteam treating is a versatile tool in the heat-treater’s arsenal andone that should not be forgotten when its benets 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
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100 90 80 70 60 50 40 30 20 10 0
0 10 20 30 40 50 60 70 80 90 100
Fe
Fe
550˚C
Fe3O4
Minimum H 2O/H2
FeO
1200
1000
800
600
400
200
T e m p e r a t u r e
, ˚ C
T e m p e r a t u r e
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Cool in steam
Steam blue
Airblast
Controlledcool
Soak
Heat
Preheat
Time, hrs. % H2O
% H2
I n c r e a s
i n g r i s
k o
f
F e
O f o r m a t i o n
F e
3 O
4 f o r m a t i o n
s l o w
i n g
d o w n
Fig. 2. Combination annealing and bluing cycle for motor laminations [1]Fig. 3. Acceptable and unacceptable steam-treatment conditions for ferrous materials [3]
