ASTM B874-96

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: B 874 – 96
Standard Specification for
Chromium Diffusion Coating Applied by Pack Cementation
Process
This standard is issued under the fixed designation B 874; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope E 766 Practice for Calibrating Magnification of a Scanning
Electron Microscope
1.1 This specification covers the requirements for chromium
E 1077 Test Method for Estimating Depth of Decarburiza-
diffusion of metals by the pack cementation method. Pack
tion of Steel Specimens
diffusion employs the chemical vapor deposition of a metal
F 1330 Guide for Metallic Abrasive Blasting to Descale the
which is subsequently diffused into the surface of a substrate at
Interior of Pipe
high temperature. The material to be coated (substrate) is
immersed or suspended in a powder containing chromium
3. Terminology
(source), a halide salt (activator), and an inert diluent such as
3.1 Definitions used in this specification are in accordance
alumina (filler). When the mixture is heated, the activator
with Terminology B 374.
reacts to produce an atmosphere of chromium halides which
3.2 Definitions of Terms Specific to This Standard:
transfers chromium to the substrate for subsequent diffusion.
3.2.1 diffusion coating—a diffusion coating is one produced
The chromium-rich surface enhances corrosion, thermal stabil-
by causing an element to react with or diffuse into, or both, the
ity, and wear-resistant properties.
surface of a metallic substrate, thus, chemically altering the
1.2 This standard does not purport to address all of the
surface of the substrate.
safety concerns, if any, associated with its use. It is the
3.2.2 retorts—containers in which powder and parts are
responsibility of the user of this standard to establish appro-
packed for processing. They can be constructed of carbon,
priate safety and health practices and determine the applica-
stainless, or high alloys and fabricated in all shapes and sizes
bility of regulatory limitations prior to use.
to accommodate parts being processed.
2. Referenced Documents 3.2.3 significant surface—areas that are essential to the
serviceability or function of the article. These surfaces must be
2.1 ASTM Standards:
identified on a drawing or marked-up sample of product. Areas
B 374 Terminology Relating to Electroplating
can fall into one of three categories as follows:
B 487 Test Method for Measurement of Metal and Oxide
3.2.4 coating required—these surfaces must be in accor-
Coating Thickness by Microscopical Examination of a
dance with all quality requirements of this specification.
Cross Section
3.2.5 no coating required—these surfaces are areas where
B 602 Test Method for Attribute Sampling of Metallic and
2 no coating is allowed due to a number of reasons including
Inorganic Coatings
dimensional, fabrication, and welding, as well as others.
B 697 Guide for Selection of Sampling Plans for Inspection
2 Materials used for masking are commercially available.
of Metallic and Inorganic Coatings
3.2.6 optional—these surfaces do not require coating, but at
B 762 Method of Variables Sampling for Metallic and
2 the same time do not require masking.
Inorganic Coatings
C 664 Test Methods for Thickness of Diffusion Coatings
4. Classification
D 3951 Practice for Commercial Packaging
4 4.1 There are four classes of chromium diffusion defined by
E 3 Methods of Preparation of Metallographic Specimens
base (basis) metal category.
4.1.1 Class I—Carbon steel.
This specification is under the jurisdiction of ASTM Committee B-8 on 4.1.2 Class II—Low-alloy steels.
Metallic and Inorganic Coatingsand is the direct responsibility of Subcommittee
4.1.3 Class III—Stainless steels.
B08.15on Diffusion Coatings.
4.1.4 Class IV—Nickel-based alloys.
Current edition approved June 10, 1996. Published August 1996.
Annual Book of ASTM Standards, Vol 02.05.
Annual Book of ASTM Standards, Vol 15.09.
4 5
Annual Book of ASTM Standards, Vol 03.01. Annual Book of ASTM Standards, Vol 01.07.
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NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
B874–96
5. Ordering Information 6.7 Visual Inspection—Inspect in accordance with 7.3.
6.8 Marking and Packaging—To be defined by the pur-
5.1 In order to make the application of this specification
chaser in the purchase order. Parts processed for the U.S.
complete, the purchaser shall supply the following information
government and military, including subcontract, shall be pack-
to the vendor through a purchase order and drawings:
aged in accordance with Practice D 3951.
5.1.1 Title, ASTM designation number, and date of issue of
this specification.
7. Coating Requirements
5.1.2 Deposit by classification (see Section 4).
7.1 Diffusion Thickness—The following are minimums for
5.1.3 Composition and metallurgical condition of substrate
the different classes of materials based on standard process
to be coated.
parameters. Thicker coatings require special processing and
5.1.4 Location of significant surfaces (see 3.2.3).
must be called out for in the purchase order if required.
5.1.5 Samples for destructive metallurgical test (see 8.1).
5.1.6 Any post heat treatment required. Class Minimum, in.
Class I (carbon) 0.003
5.1.7 Acceptance inspection procedure to be used (see
Class II (low alloys) 0.003
Section 9).
Class III (stainless steels) 0.002
Class IV (nickel base alloys) 0.001
5.1.8 Any requirement for certification (see Section 11).
7.2 Decarburization—For certain Class I and Class II ma-
6. Processing Requirements
terials, the maximum depth of decarburization to be defined by
6.1 Substrate Preparation—The metal to be chromized
the purchaser.
shall be free of flaws and defects that will be detrimental to the
7.3 Chromium Content—The outer 15 % of coating shall
coating. Thorough cleaning is essential to ensure satisfactory
contain a minimum of 20 % (by weight) chromium.
diffusion. Materials used for cleaning should not damage the
7.4 Appearance—The diffusion zone shall be nonporous
base metal. Oils, dirt, grease, and stains must be removed.
and adherent to the base metal and shall have a uniform surface
When blasting is also required, use aluminum oxide (90 to 220
free from objectionable imperfections. Minor variations in
mesh) at 60 to 80 psi from 4 to 6 in. standoff. (Refer to Guide
color and surface appearance shall be considered acceptable,
F 1330.)
providing the requirements of 7.1 and 7.2 are met (see 8.7).
6.2 Materials:
6.2.1 Masteralloy with 50 to 100 % Pure Chromium—Sold
8. Test Methods
under a number of trade names, in various mesh sizes.
8.1 Special Test Specimens:
Percentage in mix depends on material being processed.
8.1.1 The permission or the requirements to use special test
6.2.2 Activator—Most commonly used is ammonium chlo-
specimens, the number to be used, the material from which
ride, but many others are available. Percentage in mix depends
they are to be made, and their shape and size shall be stated by
on material and type of activity.
the purchaser.
6.2.3 Inert Filler—Aluminum oxide or calcined alumina is
NOTE 4—Test specimens are often used to represent coated articles in a
used and commercially available in numerous mesh sizes.
test if these articles are of a size,
...