IPMD 14th Edition 2010-2011, 8 pages, 6037 words

Author: Dr Joseph M Capus, 520 Beaconsfield Blvd, Beaconsfield, Quebec, H9W 4C7, Canada

Dr 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. The recent economic situation has resulted in production consolidations in Europe and North America, while the surge in base metal prices has also 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]:

1. Chemical decomposition of an iron compound, e.g. solid-state reduction of iron oxide, or decomposition of gaseous iron pentacarbonyl.
2. Atomisation of liquid steel by high-pressure water jets or inert gas streams.
3. “Hybrid” process involving granulation or shotting of liquid cast iron, followed by ball-milling to powder and decarburisation in a high-temperature furnace.

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). More recently, JFE Steel in Japan has developed a new ultrafine powder (average particle size 1 µm) from iron oxide obtained by spray roasting of the waste hydrochloric acid used in washing cold rolled steel [2].

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. The liquid metal stream is broken up into particle-size droplets by very high pressure water jets that also provide rapid quenching (Fig.2) [3]. 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. The cake produced in the annealing step is processed into finished powder by crushing, screening and blending.........

Further sections of this article include:

• Current production routes
• Trends in production facilities
  - Stainless and high alloy steel powders
• Trends in ferrous powder products
  - Quality
  - Alloy developments
  - Sinter-hardening grades
  - Master alloys
  - Machinability improvement
  - Bonded powders
• EU REACH legislation
• Conclusions

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 of Höganäs AB, Sweden)

Fig. 3 Typical morphology of Iron powder particle (ASC100.29) produced by water atomisation (Courtesy of Höganäs AB, Sweden)

Fig. 4 Sponge iron particle (NC 100.24) showing (left) typical morphology and (right) cross-section with porous structure (Courtesy Höganäs 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 Höganäs 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 SEM of iron powder particle with graphite particles bonded to it (Courtesy Höganäs AB, Sweden)

Fig. 13 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. 14 Comparison of compressibility of AncorMax D^TM and regular EBS premixes [27] 

Fig. 15 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 200ºC for 1 hour [14]

Table 2 Mix compositions of powders used to compare properties after sinter-hardening. After Engström et al. [13]

Table 3 Mechanical properties of FLN2-4400 compared with wrought AISI 8620 steel in the quenched and tempered condition