The seemingly perennial topic of the search for wrought steel properties by an economical powder route had yet another airing at the PowderMet 2007 conference in Denver, Colorado. In both regular technical sessions and special interest programmes, the usual suspects were seen pounding together powder compaction dynamics, alloy chemistry, sintering/hardening, surface densification and other “tricks” of the technology. The ambitious industry goal: “Single press to full density”, as detailed by Howard Sanderow of the Center for PM Technology, on the other hand, seems destined to remain on the slow road of collaborative/academic R & D.
Nevertheless, the ingenuity and entrepreneurship of industry individuals is likely, sooner or later to stumble on a solution or solutions and set PM off on a new round of expanded applications.
Leaving behind earlier attempts to achieve wrought low-alloy steel properties, Dennis Hammond, president of Apex Advanced Technologies, LLC, in concert with Richard Phillips, president of Engineered Pressed Materials, has pursued near-full density PM with a new approach. The new process uses activation chemistry in conjunction with Apex Superlube® powder lubrication, mixing and compacting technology. On the proprietary rights protection front, Apex has two issued patents on the lubricant and there are three patent applications in progress on the near-full density technology. In joint and separate presentations in Denver, Dennis Hammond and Richard Phillips showed that their advanced high density PM technology could be applied successfully to ferrous compositions beyond the low-alloy steel examples.
The emphasis was on the variety of near-full-density materials that could be manufactured by combining the Apex special lubricant mix with de-binding and sintering technology that fits the chemistry. In addition to extending the range of low-alloy steel compositions, results were shown for austenitic, ferritic, and precipitation-hardening stainless steels, iron-based soft magnetic alloys, and a synthetic “cast iron” that could be heat-treated to form a malleable iron microstructure. The technology can produce pressed-and-sintered materials close to full density (up to 99+ per cent) with some properties equal to or better than wrought. The process uses conventional minus-100 mesh grades of water-atomised PM powders, conventional blending, standard tooling and conventional pressing. Compacting pressures are at the high end (45-55 tsi) of the normal range to achieve green densities of around 7.2+ g/cm³, moulding to mass rather than volume.
According to Hammond, the secret is in the special lubricant and additives, the modified de-lube treatment and subsequent sintering and heat-treatment. The Apex proprietary lubricant is formulated to permit particle movement and re-arrangement by becoming liquid under pressure. It was emphasised that successful sintering to near-full density was not possible if there were density variations in the green part. Hence the choice of lubricant and its mobility during pressing was a key feature to facilitate the required movement of metal particles and additives. Likewise, during sintering it was important to ensure that parts were subjected to uniform heating. It was also important to allow for movement of the parts on the sintering substrate to prevent the “elephant’s foot” phenomenon. Zircar ZAL45, alumina, and BN-coated graphite or ceramic were mentioned as suitable substrate materials. A modified “de-binding” was done prior to sintering in a lean hydrogen atmosphere or in vacuum. Stainless steels were sintered in 100 per cent hydrogen. Sintering temperatures ranged from the conventional 2150°F (1175°C) up to the high temperature range around 1370°C. In general, heat-treatment of the sintered parts was required to optimize the properties, with the resulting mechanical and physical properties exceeding those of conventional PM due to the higher densities. Impact and toughness values of low-alloy compositions were said to approach and sometimes exceed those of wrought products. Compared with conventional PM, the new process required lower alloying content to reach equivalent properties. However, the process does not suit all PM alloy systems, or all types of iron powders.
In preparing the powder for compaction, a master-batch approach is employed. The master batch contains the special lubricant together with any additional additives. Calculations are required to ensure that the compacted mass fills 98.5 - 99.5 per cent of the volume at the target green density. This may require the inclusion of a space-filler material that will burn off with no residue in subsequent sintering. There are strict requirements in preparing the master batch and powder mix. The master batch contains all additives including proprietary additives, pre-mixed with no segregation, ready to mix with the base powder. Stainless steel powders are coated with additive before mixing with the master batch. A grounded anti-static feed tube should be used between the powder hopper and the press tooling. Apparent density and flow of the final mix may be different from that of conventional premixes.

PM Low-Alloy Steels
Results were shown for PM Ni-Mo low-alloy steels with compositions in the range 2 - 6.6 per cent nickel, 0.3 – 1.5 per cent molybdenum and 0.65 – 0.9per cent graphite, based on prealloyed Mo steel powders Ancorsteel A-30HP, A-85HP, A-150HP, and Astaloy 85Mo. When pressed to green densities in the range 7.22 - 7.34 g/cm³, these materials sintered to near full densities in the range 7.76 - 7.82 g/cm³. Mechanical properties after a martensitic heat-treatment were claimed to be far superior to comparable conventional PM compositions. Lower alloy compositions showed mechanical properties comparable to higher alloy samples, while modifications in heat-treatment could give improved tensile elongation and toughness.
In another series of prealloyed low-alloy Cr-Mo PM steels, compacted to 7.15 - 7.18 g/cm³, sintered densities rose to 7.69 - 7.79 g/cm³. A wide range of mechanical properties were shown for a low-alloy PM steel with 0.75 per cent chromium, 0.25 per cent molybdenum and 0.85 per cent carbon, according to the type of heat-treatment employed. And thanks to the elimination of inter-connected porosity, many surface treatments common to wrought steels are now possible that are more difficult and expensive with conventional PM parts. Finally, dimensional stability was illustrated by measurement data from sintered bushings of nickel - molybdenum steels 1.5” OD X 1” ID X 1” high (38 X 25 X 25 mm.), showing maximum out-of-roundness of 0.002” (0.051 mm.) and maximum taper of 0.003” (0.076 mm.). As already mentioned, dimensional control is maintained by eliminating green density gradients through the use of a highly effective mobile lubricant, reduction of the “elephant’s foot” effect by sintering on low-friction substrate plates, and by uniformity of temperature.

PM stainless steels
Several factors have so far limited the commercial application of PM stainless steels. These include relatively poor compressibility and the abrasive nature of stainless steel powder particles in compaction; and a high oxide content that is difficult to reduce during sintering without using very high temperatures, leading to distortion. In their joint presentation, Phillips and Hammond showed results for several PM stainless steel compositions based on AISI steel designations covering austenitic, ferritic and precipitation-hardening grades. The following types were sintered in 100 per cent hydrogen at 1175° - 1400°C for 60 minutes: 304L, 316L, 409Cb, 410L, 410 with graphite, 434L, and 17-4PH.
Despite lower overall densities compared with the low-alloy steel examples, some very interesting properties were shown for the various stainless steel grades that were processed with the new technology. For the popular 316L composition, the increase in density with sintering temperature provided significant increases in strength, elongation and toughness. With these materials, inter-connected porosity was eliminated after sintering at 2350°F (1290°C).
For similar densities, standard test bars showed much higher hardness and strength compared with the 316L material, but with lower ductility and toughness, as expected. Interesting results were also obtained for the other compositions tested. These mechanical properties are not exceptional when the sintered densities are taken into account. The benefit of activated sintering appears to be in facilitating densification.

Soft Magnetic Materials
High density is also a key factor in the search for improved soft magnetic properties in PM materials. In a series of tests reported by Phillips and Hammond, comparisons were made between standard and activated sintering of ferrous alloy compositions based on pure iron powder. Samples containing 0.45 per cent and 0.8 per cent phosphorus or 1.5 per cent and 3 per cent silicon were sintered at temperatures between 2150°F and 2534°F (1175° - 1390°C). Sintered density was increased by both the phosphorus additions as well as high sintering temperatures. The resulting magnetic properties for toroidal ring samples showed a similar trend, although here the activated sintering had a relatively small influence. Nevertheless, the achievement of densities upwards of 7.6 g/cm³ clearly provides attractive soft magnetic properties in terms of magnetisation, coercive force, and maximum permeability.

“Malleable Cast iron” by a PM Route
In a more novel development for PM, Hammond described how a synthetic form of “cast iron” was created by adding 0.7 per cent silicon and 2 per cent graphite to plain iron by mixing with the lubricant/additives as a master batch. This blended mix was compacted to 6.95 - 7.0 g/cm³ and then sintered at 2180°F (1190°C) in a 25/75 hydrogen/nitrogen atmosphere. The result was a material with a 7.67 g/cm³ density and a synthetic “cast iron” microstructure. After heat-treatment in a similar fashion to malleable cast iron, the microstructure changed to resemble that of malleable iron, with temper-graphite islands surrounded by ferrite in a pearlitic. Alternative heat-treatments are able to produce martensitic microstructures. This synthetic “ductile cast iron” had low porosity, was easy to machine, and in preliminary property evaluation was said to show “a fair degree of toughness” with high yield strength.
At the time of writing, a number of parts makers and several equipment manufacturers are signed up to exploit this technology, some with parts already in development. Lips remain sealed about any further details: time will tell whether this approach will prove to be the answer to the dream of full-density PM properties.