ASTM B935-05

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Designation:B935–05
Standard Guide for
Steam Treatment of Ferrous Powder Metallurgy (P/M)
Materials
This standard is issued under the fixed designation B935; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope hardness of magnetite (Fe O ) formed during steam treatment
3 4
is typically equivalent to 50 HRC, and when present in sintered
1.1 This guide is intended as an aid in establishing and
materials, their wear resistance can be improved significantly.
maintaining a procedure for the steam treatment, also referred
Steam treatment is also used to seal parts or provide a base
to as steam blackening, of sintered ferrous P/M materials and
material for additional coatings. Steam treated ferrous P/M
the appropriate use and evaluation of these materials. Addi-
materials are used in many industries, including automotive,
tional information concerning the effect of this process on
marine, home appliances and lawn and garden applications.
ferrous P/M material properties is contained in Appendix X1.
1.2 This standard does not purport to address all of the
4. Apparatus
safety concerns, if any, associated with its use. It is the
4.1 The material can be processed in either a batch-type
responsibility of the user of this standard to establish appro-
furnace or a continuous belt-type furnace. The furnace must be
priate safety and health practices and determine the applica-
capable of heating the load, maintaining it at the processing
bility of regulatory limitations prior to use.
temperature, and maintaining a steam atmosphere free of air
2. Summary of Guide leaks. Both batch type and continuous furnaces must meet the
same criteria described in the following procedure section.
2.1 A normal sequence of steps is: (1) preheat the load; (2)
When both batch and continuous furnaces are operated to meet
introduce a superheated steam-rich atmosphere; (3) heat the
the same processing times, temperatures, and atmosphere, the
load to the processing temperature and maintain the tempera-
results should be similar.
ture for the duration of the processing cycle; and (4) cool the
load to a temperature suitable for handling. This process will
5. Procedure
produce a layer of black iron oxide (magnetite) on the surface
5.1 Place the load in the furnace and heat in air until the
of the parts and on the surfaces of the interconnected porosity
temperature of the entire load is above 100°C (212°F) without
by the reaction.
exceeding 430°C (800°F).
3Fe 1 4H O ~gas!↔ Fe O 1 4H ~gas!
2 3 4 2
5.2 Once the load is preheated, introduce superheated steam
into the furnace until all of the air is purged from the furnace
3. Significance and Use
or processing zone.This steam atmosphere must be maintained
3.1 The performance and quality of steam-treated materials
until the processing cycle is complete.
depends upon the surface cleanliness of the material prior to
NOTE 1—The temperature of the parts must be in excess of 100°C
steam treatment and the adequacy of the processing. Steam
(212°F) before the superheated steam is introduced. If it is not, water will
treatment can be used as a decorative coating, producing a
condense on the parts and rust spots will form. The temperature must not
blue-gray to a blue-black appearance. It can reduce the
exceed 430°C (800°F) before the superheated steam is introduced or the
susceptibility of ferrous P/M materials to further oxidation and
parts will be covered with a smooth even coating of rust caused by air in
corrosion, thus providing better shelf life. More significantly,
the furnace atmosphere.
improvements in apparent hardness, compressive strength,
5.3 Raise the temperature of the furnace to the processing
wear characteristics, and some mechanical properties (see
temperature of between 430°C (800°F) and 590°C (1100°F)
Appendix X1) can be observed due to steam treatment. The
while maintaining the atmosphere of superheated steam.
NOTE 2—The processing temperature depends on the desired proper-
This guide is under the jurisdiction ofASTM Committee B09 on Metal Powder
ties. Lower temperatures result in deeper oxide penetration while higher
and Metal Powder Products and is the direct responsibility of Subcommittee B09.05
temperatures produce a thicker surface oxide layer.
on Structural Parts.
NOTE 3—Processes that decrease the amount or the size of the
Current edition approved Oct. 1, 2005. Published October 2005. DOI: 10.1520/
interconnected porosity such as copper infiltration, grinding, vibratory
B0935-05.
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B935–05
finishing, sizing, machining, burnishing, shot peening, or polishing will
batch-type furnace, the furnace should be flushed with nitrogen
reduce the effectiveness of the steam treatment by limiting the ability of
or an inert gas unless the parts and furnace are cooled to below
steam to penetrate the part. Increasing the density reduces the pore size,
the auto ignition point of hydrogen while still in the steam
which reduces penetration unless sufficiently large porosity is available to
atmosphere.
maintain a pathway for the steam. Copper infiltration also coats the
7.2 Foreign material can interfere with steam treatment and
surface of iron particles with copper, which prevents steam from reacting
possibly pose a serious health threat. Some chemical com-
with the iron and decreases the effectiveness of steam treatment as a
surface treatment.
pounds, for example, chlorinated hydrocarbons, decompose
into products that cause materials to rust during processing.All
5.4 Hold the load at the pr
...