Standard Alloys and Metallurgy
MKI produces Dynapore® diffusion-bonded porous
metals in a variety of metals and metal alloys. The most frequently used
materials are the AISI 300 series austenitic stainless steels*, such as
304, 304L, 316, 316L, and 347 stainless steels; and related alloys such
as type 430 stainless, Carpenter Alloy 20-Cb3 and 904L. We offer products
in nickel and its alloys, including Monel 400, Inconel 600, Hastelloy
C-276, Hastelloy C-22, Hastelloy X, and Nichrome 80-20 Cb. Further, MKI
has produced porous materials in copper and phosphor bronze.
here for an in-depth discussion of the metallurgy of 300 Series stainless
steels, including carbon content, stabilization, intergranular corrosion,
sensitization, and the "Strauss Test."
In some cases we have produced porous structures wherein dissimilar materials
are bonded together. Examples include 304L stainless and Monel 400, 316L
stainless and phosphor bronze or pure OFHC copper, and 430 stainless bonded
to 304 stainless. These structures present process challenges due to the
differences in the coefficient of thermal expansion between dissimilar
Due to metallurgical and process considerations, alloys containing certain
reactive or volatile elements are generally incompatible with our process.
In particular, alloys containing reactive elements such as aluminum or
titanium are difficult to sinter, particularly if the total content of
these elements exceeds 0.3%. Therefore, for example, in choosing between
the stabilized grades of austenitic stainless, 347 is a better choice
for sintering than 321, as the latter alloy contains titanium. Also, our
process does not permit the presence of volatile (high vapor pressure
or low boiling point) elements such as cadmium, zinc or lead.
The alloys we can offer depend not only on metallurgical considerations,
but also on availability in each product category, as follows:
In woven wire
meshes, 304, 304L, 316 and 316L are usually readily available, while
347 or Monel 400 are occasionally available. Most other alloys can be
woven on special order, as long as wires can be drawn to the appropriate
diameter with enough ductility and elongation to permit weaving. Typically,
custom woven meshes in exotic alloys may require lead times of 3-4 months
media are normally produced in 316 stainless, but may be ordered in
316L, 304, 304L or 347. Oxidation resistant alloys such as Inconel 600
or Hastelloy X may be produced on special order.
five-layer filter plate are normally produced in 316L stainless, but may
be special ordered in 304L, 904L, or Monel 400 for applications demanding
specific corrosion resistance. MKI also produces and stocks certain standard
grades of TWM
filter plate in Hastelloy C-22.
powder metals, availability depends on the commercial availability
of pre-alloyed powders. Typical alloys are 310, 316 or 316L stainless,
Monel 400, Inconel 600, Hastelloy X and Hastelloy C-276. Other alloys
may be available on special order. The production of SPM
also requires that the powder be classified or separated into "cuts"
having a predetermined particle size distribution in order to achieve
the correct final pore size.
fiber metal media, availability is typically restricted to 316L stainless
steel. A few other alloys may be available on special order.
note: Dynapore is a registered trademark of Martin Kurz & Co., Inc.
SWM, TWM, SPM, SFM, LFM and HFM are trademarks of Martin Kurz & Co.,
Inc. Hastelloy is a registered trademark of Haynes International. Alloy
C-22 is a trademark of Haynes International. Monel, Inconel, and Nichrome
are registered trademarks of the International Nickel Company.
300 Series Stainless Steels:
The standard 300 Series austenitic stainless
steels are the most important group of alloys used to produce Dynapore®.
It is therefore useful to provide the basic history and metallurgy of
About Stainless Steels
Steel is an alloy formed by the addition of carbon
to iron, which makes the iron hardenable by heat treatment into a high-strength
material. However, steel, like iron, can rust by the formation of iron
oxides. By adding at least 12% chromium to steel, a "passive"
layer of chromium sesquioxide (Cr2O3) can be formed and maintained at
the surface of the metal, which prevents the formation of rust (oxides
of iron). The material therefore remains bright and clean. This is the
most basic formulation of a "stainless" steel. ("Passivation"
of stainless steel refers to the deliberate inducement of a passive layer,
usually achieved by immersion of the metal in an oxidizing bath such as
Subsequently the famous "18-8" alloy of iron with 18% chromium
and 8% nickel was developed. The higher chromium content, along with the
addition of nickel, imparted even greater corrosion and oxidation resistance,
and superior ductility in the annealed condition. Unlike basic carbon
steel or 12% Cr stainless, this alloy is non-magnetic and is not hardenable
by heat treatment; although it does work harden readily.
The American Iron and Steel Institute ("AISI") assigned the
designation "type 300 stainless steel" to 18-8 stainless steel.
The AISI 300 Series stainless steels are all variations on the original
18-8 alloy. Like basic steel, this alloy contains carbon. Whereas carbon
was desirable in steel to impart strength and hardenability, in the 18-8
formulation the carbon actually causes problems. Excessive carbon may
produce brittleness, susceptibility to stress fracture, too much work
hardening, and a form of corrosion known as intergranular attack. Therefore
it has always been desirable to control, reduce or otherwise limit the
presence or effect of carbon in these alloys.
The first AISI specification for commercial 18-8 stainless steel, known
as "type 301 stainless steel," was written with a ceiling on
the allowable carbon content. As refining and alloying techniques improved,
it was possible to further reduce the maximum permissible carbon content,
first to 0.15% (302 stainless) and then to 0.08% (304 stainless). Type
304 stainless steel, the most widely used variant of 18-8, has a low enough
carbon content to avoid many problems, but may still become susceptible
to intergranular attack resulting in corrosion.
Susceptibility to Intergranular Attack
Intergranular attack ("IGA") may occur
when carbon in the alloy has combined with chromium to form chromium carbides.
These carbides tend to form when the material is exposed to temperatures
within the range of 900° to 1500°F, such as may occur in welding.
This is known as the "sensitization range." Prolonged exposure
to temperatures within the sensitization range produces substantial formation
of chromium oxides. The oxide molecules precipitate or gather at the grain
boundaries, forming a "network" of carbides. These carbide chains
are hard, and may embrittle the metal. In addition, the supply of chromium
atoms at the metal surface becomes depleted, so that there is not enough
chromium to form an effective passive layer. Material in this condition
is considered "sensitized" and is susceptible to intergranular
corrosion. In the extreme case, the sensitized material may simply crumble
or disintegrate when exposed to mildly corrosive environments.
Stainless steel which has been sensitized may be restored or "desensitized"
by a solution annealing heat treatment. "Annealing" refers to
heat treatment processes which soften a metal, remove work hardening,
and render the material more ductile. "Solution annealing" is
an annealing process in which the critical alloy constituents (in this
case chromium and carbon) are redistributed uniformly throughout the alloy
matrix. The word "solution" does not refer to a liquid, but
rather to the dissolution of the chromium carbide molecules and networks.
A desensitizing heat treatment restores the resistance of the metal to
IGA, and remains effective as long as the metal is not reheated within
the sensitizing range.
Low Carbon and Stabilized Grades
One obvious solution to the problem of susceptibility to IGA would be
to reduce the carbon level in the alloy chemistry to the point where it
is no longer problematic. If the carbon level is less than .03%, there
is simply not enough carbon to create the carbide networks. The introduction
of vacuum melting technology enabled the production of alloy heats containing
these low levels of carbon. Accordingly, AISI type 304L stainless was
introduced, containing a maximum of .03% instead of .08% as found in 304.
All 304L stainless steel also qualifies as 304, but the converse is not
necessarily true. Only 304 with a carbon level less than or equal to .03%
Before the technology for producing low carbon (304L) heats was available,
another technique was employed to prevent sensitization. Certain "stabilizers"
were added to 304 to prevent chromium carbide formation. Type 347 stainless
steel is one of the stabilized grades, to which columbium and tantalum
are added. Another is type 321 stainless, in which the stabilizing element
is titanium. The stabilizing elements combine preferentially with the
carbon, forming carbides other than chromium carbide. These other carbides
do not cause the same problems as chromium carbide. From the standpoint
of producing diffusion-bonded porous metals, 347 is preferable to 321,
as the titanium in 321 is reactive at elevated process temperatures and
may interfere with diffusion-bonding.
Another variant of 304 stainless is 316 stainless steel, to which 2-3%
molybdenum has been added for improved resistance to salt pitting. All
of the same considerations with respect to sensitization apply to 316
stainless, and so an analogous low carbon grade, 316L, was developed to
provide resistance to sensitization.
Types 304L and 316L stainless steel are the most widely employed in the
production of Dynapore®, with 316L taking top honors for its superior
corrosion resistance in saline environments. Accordingly, almost all classes
of porous metals are available in 316L stainless steel.
The Strauss Test
Various corrosion tests may be used to determine whether or not material
is in a sensitized condition. The most commonly employed is the "Strauss
Test" as described in ASTM specification A262, Practice "E."
In this test, a sample of metals is boiled in an acidic solution for 24-72
hours. In the case of fine wire mesh or porous metals, failure is usually
fairly dramatic, with the material cracking or disintegrating. Materials
which are neither low carbon nor stabilized, and which have been subjected
to temperatures within the sensitization range, will most likely fail
the Strauss test. Such materials may be rehabilitated by solution annealing.
The efficacy of the annealing may be determined by performing another
Strauss test after annealing.
Materials which are either low carbon or stabilized should be immune
to sensitization. This may be tested by performing a deliberate "sensitizing
heat treatment" (i.e. prolonged exposure to temperatures within the
sensitizing range) followed by a Strauss test. Material meeting 304L,
316L, 321 or 347 should still pass the test after being subjected to this
treatment. Sometimes this method is employed to test these alloys, to
determine whether they are indeed low carbon or stabilized. Of course,
chemical analysis by a competent laboratory can also answer this question.
Carburization and Decarburization
One important point to note is that any thermal process above 1600°F
is capable of affecting the carbon content of steels, including 300 series
stainless steels. For example, if a diffusion-bonding, sintering, or solution
annealing process is not run carefully, it is possible for the metal to
absorb additional carbon during the cycle. This carburization may occur
due to contamination of a furnace chamber, excessive hydrocarbon lubricants
on the metal surfaces, or a furnace atmosphere with too high a "carbon
potential" (e.g. too much methane, carbon monoxide, etc.).
In the case of porous metals or fine wire mesh, the high ratio of surface
area to mass can spell disaster. The high surface area allows a much higher
degree of carbon infiltration per pound of metal than would be found in
machined parts such as nuts and bolts. Carburization can turn low carbon
grades into high carbon rejects, and can overcome the benefits of stabilizers.
Conversely, at even higher temperatures such as would be encountered
during diffusion-bonding, it is possible to reduce the carbon content
of the metal if all process variables are carefully controlled. In this
manner, materials meeting 304 and 316 stainless steel may be decarburized
or "reduced" to 304L and 316L respectively. This can be quite
useful when the low carbon grades are not available. For example, coarser
woven wire meshes are often commercially unavailable in low carbon grades.
As low carbon content is so important to
the corrosion resistance of 300 Series austenitic stainless steels, particularly
in the case of high surface area meshes and porous metals. The performance
of thermal processes such as solution annealing or sintering is best left
to the care of experienced specialists. At Martin Kurz & Co. we pride
ourselves in our thirty year track record of superior product performance.