Automotive applications have so far dominated the industry growth. However, non-automotive applications are .... The microstructures of the sinterings are important, of course, because they ..... 105 College Road East, Princeton, NJ 08540. 6.
Paper No. 2007-01-0145
SINTERING OF POWDER PREMIXES – A BRIEF OVERVIEW Kalathur S.Narasimhan and Frederick J.Semel Hoeganaes Corporation 1001 Taylors lane Cinnaminson,NJ 08077
Abstract Advances in the understanding of the sintering of powder premixes have contributed significantly to the growth of the ferrous powder metallurgy industry. This includes sintering both in the solid state and in the presence of a liquid phase. In this article, the sintering of iron powder premixes containing: 1) graphite; 2) nickel and graphite; 3) copper and graphite; 4) Phosphorus as ferrophosphorus; and, 5) boron as ferroboron are discussed. The evolution of microstructure and mechanical properties are discussed as well.
Paper No. 2007-01-0145
INTRODUCTION The growth of ferrous powder metallurgy (P/M) over the past five decades has been outstanding as this technology is proving itself as an alternative lower cost process to machining, casting, stamping, forging, and other similar metal working technologies. The growth has been spurred by continuing technological advancements in powder making and processing, alloy development, and parts production methods. The markets for the powder metallurgical parts are varied. Automotive applications have so far dominated the industry growth. However, non-automotive applications are becoming increasingly important. The latter applications mainly include hand tools, household and lawn appliances and industrial motor controls and hydraulics The growth of P/M is dependent on the ability of technological advancements to deliver products with higher densities than are currently available. Figure 1 shows how parts distribution by density has progressed and the anticipated use of higher density parts in the future. The indicated density increases to date are mainly due to improvements in powder making and compaction processing. The future advances are likely to come mostly from our understanding of the sintering process. P/M parts are characterized by density, composition, and microstructure. For a given part, these three parameters are optimized. Sintering and additional heat treatments of powder premixes generate the microstructures that are needed to meet the required performance. Generally, there are four types of powder premixes that are used in the P/M industry, (2). These are: 1) iron powder mixed with alloying ingredients such as C, Cu, Ni and P; 2) iron pre-alloyed with Mo or with Mo and Ni and mixed with C; 3) partially alloyed iron with Mo, Ni and Cu and mixed with carbon; and, 4) hybrid systems that use various combinations of 1, 2, and 3. All of these premixes contain a lubricant and in many cases also contain a binder to prevent compositional variations due to demixing and dusting. Prior to sintering and any additional heat treatments, the premixes are compacted to near net shape. Typically, the compaction process determines the final density to within one or two percent and the sintering process provide the balance.
Paper No. 2007-01-0145
P/M Part Distribution by Density
2005 7.8
6.2 6.2-6.6
7.2-7.6 6.6-6.9
1979 7.2-7.6
1989
6.9-7.2 7.8
6.2
7.2-7.6
7.8
7
6.2
6.9-7.2 6.2-6.6
6.9-7.2
6.2-6.6
6.6-6.9 6.6-6.9
Figure 1: History projection of density advances with time. In this article, we will discuss the sintering, microstructure, and mechanical properties of the four most common alloy systems including: 1) iron-carbon; 2) iron-carbon-nickel; 3) iron-carbon-copper; and, 4) iron-phosphorus. The iron-boron system as potential future alloy will also be discussed. In general, when powder premixes are sintered, sintering is by solid-state processes or a combination thereof with liquid phase processes. In both cases, atom movements plays a key role. The basic mechanisms include vapor transport and surface, volume and grain boundary diffusion. The vapor transport and surface diffusion mechanisms effect metallurgical bonding and pore rounding but not densification. The volume and grain boundary diffusion mechanisms effect all three: bonding, pore rounding and densification. Typically, in ferrous PM, all four processes occur to some extent. Since vapor transport and diffusion are thermally activated processes and iron is subject to oxidation, sintering necessarily involves high temperatures and protective atmospheres. In the case of iron base premixes, this usually means temperatures in the range of 1100 to 1300 oC, (2000 to 2350 oF) and nitrogen/hydrogen atmospheres ranging anywhere from 5% to 100% hydrogen. Hold times are typically from 15 to 30 minutes but on occasion may be as long as an hour or more. Excellent reviews of the sintering process and of the various sintering mechanisms and their effects are available in the open literature, (3, 4). Fe-C and Fe-C-Ni Systems: The response of Fe-C and Fe-C-Ni premixes to sintering in terms of the microstructures and mechanical properties that are obtained are very similar. A typical Fe-C
Paper No. 2007-01-0145
microstructure based on an iron powder premix containing 0.5% graphite as the carbon source is shown below in Figure 2. Such a composition is normally designated as F-0005, (F = Iron, 5 = 0.5% C).
F- 0005
Figure 2: Microstructures of sintered iron with 0.5% carbon. Sinter bonding of the powder particles and two phases, pearlite and ferrite, are visible. The micrographs each show two phases. The light-etching phase is ferrite, essentially, pure iron or if nickel is present, a solid solution of nickel in iron. The dark etching phase is pearlite, a lamellar structure of the ferrite and iron carbide, Fe3C, known as cementite. The relative amounts of the two phases are largely dependent on the carbon content and to a lesser extent on the process conditions. Their morphologies, on the other hand, are almost entirely dependent on the process. Typically, higher carbons and faster cooling rates subsequent to sintering lead to higher pearlite contents, decreased lamellae spacing and finer grains of both phases. The micrographs also clearly show a good ‘degree of sinter’ in terms of pore rounding and sinter bonding of the iron particles. The porosity seen here is typical of most ferrous P/M materials. More generally, the numbers and shapes of the pores are a function of the as-compacted density and the sintering conditions, especially the temperature. The microstructures of the sinterings are important, of course, because they largely determine the resultant mechanical properties. In general, increases in the pearlite content and decreases in the lamellae spacing and grain size increase strength but decrease ductility while increases in the ferrite content increase ductility but decrease strength. Increases in the density and degree of sinter increase both strength and ductility. The mechanical properties of an iron powder premix with 0.8% graphite, (i.e. F0008), are shown in Table 1, (5).
Paper No. 2007-01-0145
Table 1: Mechanical Properties of F-0008 carbon steel sintered at 1120°C Density UTS YS Elongation Impact Hardness (g/cm³) (MPa) (MPa) In 25.4 mm Energy (J) (HRB) 6.2 240 210
