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.

* Click 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 materials.

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 or more.

Dynapore® LFM™ and HFM™ fluidizing 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.

Dynapore® TWM™ and SWM™ 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.

In SPM™ 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.

For SFM™ fiber metal media, availability is typically restricted to 316L stainless steel. A few other alloys may be available on special order.

Please 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.

The 300 Series Stainless Steels:
Metallurgical Considerations

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 these alloys.

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 nitric acid.)

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% meets 304L.

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.