Trends in ferrous powders for use in powder metallurgy

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Trends in ferrous powders for use in powder metallurgy

IPMD 15th Edition 2010-2011, 9 pages, 5904 words

Author: Dr Joseph M Capus, Quebec, Canada


ferrous-powders_1Dr Joseph M Capus updates us on the progress and trends in the ferrous powder industry with specific reference to global capacity and advances in powder processing and material properties for high performance PM steel components. Advances include a combination of alloy developments and also premixes with proprietary binders and lubricants that are helping PM to close the properties gap with wrought steels. Recent economic developments, particularly in the North American automotive industry resulted in production consolidations in Europe and North America, while the price volatility in alloy ingredients has focused attention on achieving target properties with the lowest materials cost.

Commercial grades of iron and steel powders for use in PM part production, including metal injection moulding, are manufactured by three types of process [1]:
The popularity of the various processes has changed over time with the advance of technology and the demands of the powder-consuming industries. Due to cost factors and the economics of scale, the bulk of commercial iron and steel powders for the production of PM parts are now manufactured in large volumes on a semi-continuous basis by either water atomisation, oxide reduction, or the “hybrid” process. Stainless steel and tool steel powders for part fabrication are produced either by water- or gas atomisation, but on a smaller scale. For metal injection moulding (MIM) applications, very fine iron and stainless steel powders are produced on a considerably smaller scale, mostly by carbonyl decomposition (iron) or by gas or water atomisation processes (stainless steels).

Current production routes

The proportion of iron and steel powders made by atomisation has been steadily increasing for more than three decades. Over three-quarters of ferrous powders are now produced by the water atomisation of liquid steel in more than a dozen plants around the world, with the largest consumer being the structural PM part sector (see ‘Powder Metallurgy – A Global Market Review’ in this edition of the IPMD). Typically, selected steel scrap is melted in an electric arc furnace and refined to reduce impurities, before pouring through a tundish nozzle into a vertical atomisation chamber (Fig. 1). The liquid metal stream is broken up into particle-size droplets by very high pressure water jets that also provide rapid quenching (Fig. 2) [2]. The water/powder slurry is pumped out of the bottom of the atomisation tank to be dried. After screening off oversize material, the freshly atomised powder is annealed in a belt furnace under a hydrogen-rich atmosphere, such as dissociated ammonia, or now more likely pure hydrogen. The cake produced in the annealing step is processed into finished powder by crushing, screening and blending.

In a few instances, liquid steel is obtained directly from a neighbouring steel plant. The resulting minus 80 mesh (minus 180 microns) high-compressibility powder may be subjected to a second annealing step to further purify and increase compressibility (Fig. 3). The atomisation of liquid steel is versatile in that it permits economical manufacture of prealloyed powders as well as plain iron.

The production of sponge iron powder by the reduction of iron oxide played a very significant and historical role in the early development of the PM industry. The HöganĂ€s (Sweden) sponge iron process [3], in use for about a century, involves the reduction of high-purity magnetite ore (Fe3O4) with carbon. The ore is first pulverised and then reduced to metallic iron by heating to over 1200ÂșC with carbon. This is accomplished by packing the ore and a mixture of coke and limestone into large ceramic tubes and passing these through a gas-fired tunnel kiln on a series of kiln cars. The iron oxide is reduced to a spongy cake in the form of a hollow cylinder; the limestone decomposes and reacts with sulphur in the coke. HöganĂ€s sponge iron was originally used as a melting stock in the steel industry but eventually became the source of iron powder. To obtain PM grade powder, the sponge iron cakes are ground up and passed through magnetic separation and screening steps prior to purification and softening by annealing in a belt furnace..........

Further sections of this article include:

Figures and Tables:

Fig. 1 Molten steel being transferred into a ladle prior to atomisation (Courtesy Epson Atmix Corporation, Japan)
Fig. 2 Iron and steel powder production by water atomisation (Courtesy HöganÀs AB, Sweden)
Fig. 3 Typical morphology of Iron powder particle (ASC100.29) produced by water atomisation (Courtesy HöganÀs AB, Sweden)
Fig 4. Sponge iron particle (NC 100.24) showing (left) typical morphology and (right) cross-section with porous structure (Courtesy HöganÀs AB, Sweden)
Fig. 5 (Left) Iron powder particle produced by reduction of mill scale; (right) Cross-section of reduced mill scale iron powder particle (Courtesy JFE Steel Corp., Japan)
Fig. 6 Alternative methods for forming PM alloy materials: admixed, diffusion-alloyed, prealloyed and hybrid alloy powders (Schematic) [9]
Fig. 7 SEM showing fine particles of Cu, Ni and Mo agglomerated to the surface of a Distaloy iron powder particle (Courtesy HöganÀs AB, Sweden)
Fig. 8 Yield strength and UTS comparison of as-sintered Distaloy HP-1 and hybrid 1.5% Mo steel alloy powders [10]
Fig. 9 Comparison of Automotive powertrain sprockets made from Ancorsteel 4300 + 0.3% graphite SP/SS and FLN2-4405 DP/DS: wear patterns on sprocket teeth after 22 hours dynamometer test [15]
Fig. 10 Yield strength versus sintering temperature for Ancorsteel 4300, 4300L and FD-0405 (Cooling rate 0.7°C/sec) [16]
Fig. 11 Ultimate tensile strength values for low-alloy steel compositions in Table 2 after sinter-hardening [13]
Fig. 12 As-sintered UTS and hardness of Mn-containing FLM-4005 and FLM-4405 low-alloy PM steels with and without accelerated cooling (After Lindsley et al.)
Fig. 13 SEM of iron powder particle with graphite particles bonded to it (Courtesy HöganÀs AB, Sweden)
Fig. 14 Yield strength and tensile strength of cast iron compared with Ancordense-processed PM steels warm-compacted to 7.3+g/cm³ and sintered 30 minutes at 1260°C in 25/75 nitrogen/hydrogen. Material A: Ancorsteel 85HP + 4% Ni + 0.6% graphite; Material B: Ancorsteel 85HP + 3% Ni + 0.75% Cu + 0.6% graphite; Material C: Ancorsteel 85HP + 2% Ni + 1% Cu + 0.6% graphite
Fig. 15 Comparison of compressibility of AncorMax D™ and regular EBS premixes [27]
Fig. 16 Comparison of compaction modes: AncorMax 200 versus Ancordense and conventional wax lubricant (28)

Table 1 Effect of sintering temperature on properties of Ancorsteel 4300 cold-compacted at 690 MPa; sintered for 30 min. in 90/10 nitrogen/hydrogen; conventional cooling, tempered at 200ÂșC for 1 hour [14]
Table 2 Mix compositions of powders used to compare properties after sinter-hardening (After Engström et al.) [13]
Table 3 Nominal compositions of FLM-4005 and FLM-4405 powders containing Mn as proprietary master alloy and comparison of low-alloy steel powders (After Lindsley et al.)
Table 4 Mechanical properties of FLN2-4400 compared with wrought AISI 8620 steel in the quenched and tempered condition (After Hanejko) [29]
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