Densification and Microstructure Formation of Systems Based on HSS M2-SiC-Cu at Low Temperature Vacuum Sintering
Year: 2026
Type: article
Abstract: Authors: A. Simchi and M. KhakbizAffiliation: Department of Materials Science and Engineering, Sharif University of Technology, Tehran, IranJournal: Powder Metallurgy Progress (2003), Vol. 3, No. 4, pp. 165–174. How to Cite Simchi, A.; Khakbiz, M. Densification and Microstructure Formation of Systems Based on HSS M2-SiC-Cu at Low Temperature Vacuum Sintering. Powder Metallurgy Progress 2003, 3(4), 165–174 Abstract The densification and microstructural features of M2 grade high speed steel powder compacts with SiC and Cu additions were studied. The test specimens were prepared using uniaxial cold compaction followed by vacuum sintering at relatively low temperature, i.e. 1100-1120°C. Microstructural analysis of sintered samples and identification of phases were made using optical microscopy, SEM and EDX-analysis. The density and the mechanical specifications of the specimens were evaluated in accordance to the standard methods. The results show that the compressibility of the steel powder enhances with addition of tough Cu powders, while introducing the SiC hard particles has detrimental influence on the consolidation. In contrast, it was found that during vacuum sintering at low temperature the influence of Cu on the densification is marginal; instead, SiC addition enhances the sintering kinetics considerably. Based on the results of metallographic study, this behavior is attributed to the reaction between M2 matrix and SiC particles at around 1100°C, resulting in the formation of low melting Fe-Si-C phase. Although high densification rate was obtained for the specimens containing SiC particles, grains- and carbides- coarsening were likely to occur. By varying the composition of the mixture and sintering condition, the structure, density, and properties such as wear resistance, resistance to hot crating and hot hardness may be optimized for a given application. Keywords: high speed steel, SiC, compaction, sintering, microstructure Introduction High speed steels (HSS) are used for applications that require strength and hardness combined with the ability to withstand high temperatures, e.g. cutting tools, and for demanding cold work applications such as fine blanking tools and dies [1]. Although these materials have been produced by conventional steel making methods for several years, the powder metallurgy route has contributed to dramatic cost reductions in mass production of HSS [2]. In fact, during the last couple of decades the process has been satisfactorily developed, for producing parts with improved properties and lower production costs than the conventional methods. For instance, manufacturing of wear resistant automotive and aerospace components such as valve seat inserts for internal engines and bearing materials for aerogas turbines has been done successfully [3-5]. The main advantage of using this technique refers to the ability of obtaining a fine and uniform microstructure in which segregation is reduced to insignificant levels [2,6]. Accordingly, better strength and grinding characteristics as well as higher hardness and resistance to wear can be achieved. Nowadays, cold compaction and vacuum sintering of prealloyed HSS powders to full density is a well established process [3]. The powders are pressed to a density of about 70 % theoretical, and are sintered in the temperature range 1240-1330°C to promote the supersolidus liquid phase sintering process. In spite of the advantages of achieving full density, it is well known that application of the supersolidus sintering process of HSS requires keeping a close interval of temperature of liquid phase formation [7]. This problem suggests an important field of study of densification by formation of a secondary liquid phase using several additives. For instance, introduction of copper phosphide is successful in lowering the sintering temperature and fully dense parts can be produced using continuous mesh belt furnaces [8-10]. The vacuum sintering of systems based on HSS M2 powder chemically coated with Cu and graphite additions was successfully implemented [11, 12]. The positive influence of FeB powder on the densification of HSS was also studied [10]. It is known that carbon and silicon additions to tool steels significantly affect the sintering temperature by lowering the solidus temperature. However, blending of more than 0.15 to 0.2 wt% C may cause several detrimental effects, such as nonuniform distribution of carbon, variable response to sintering, variable part-to part density, and part distortion [6]. On the other hand, admixed silicon does not homogenize rapidly because of its low diffusivity; consequently, it usually causes incipient melting. One possible way is to use SiC particles as sintering aid instead of elemental graphite and silicon powders. This work offers better blend homogeneity during mixing, which in fact may suppress the detrimental influences of using elemental powders. The aim of this article is to report on the interaction that occurs during processing of M2 grade high speed steel with SiC particles. The influence of Cu addition was also studied. Experimental Procedure The starting powders used for manufacturing the sintered materials were water atomized M2 grade HSS powder from Hoganas, Belgium; SiC powders with mean powder particle size of 14, 18 and 24μm from Zamin Tavana Co., Iran; water atomized copper powder from MPM Co., Iran. The compositions, and some characteristics of HSS powder used, were summarized in Table 1. Table 1. Composition and properties of M2 grade high speed steel examined Element (wt%) C Cr W Mo V Mn Si O+N Composition 0.83 4.19 6.13 4.08 1.925 0.27 0.19 * M2 powder sieve analysis Sieve fraction <45 µm 45–53 µm 53–75 µm 75–106 µm Weight percentage 35.6 11.9 21.8 17.8 Apparent density: 2.55 g/cm³; Hall flow: 29.7 s/50 g. The composition of the investigated powder mixtures was 1-5 wt% SiC and the balance of water atomized based powder. The composite powders were blended in a Tumbling mixer for 30 min. A small batch of the HSS powder was also modified by coating particles with copper by a cementation reaction in an aqueous CuSO4 solution as cited in [8]. In this process, the steel powder was poured into stirred CuSO4 solution at ambient temperature until the blue color of the solution had completely disappeared, and then filtered off, rinsed with water and acetone, and dried at 70°C. The copper content was adjusted to 2 wt% by controlling the powder mass. Moreover, some runs were made by 2 wt% elemental Cu additions. The powder blends were uniaxially cold compacted to cylindrical (25 mm diameter and 6 mm high) and rectangular (90x10x10 mm) specimens at 550 MPa, die-wall lubrication being afforded. The compacts were then sintered in a laboratory tube furnace in a vacuum of the order of 10-2 torr at 1120°C for 20 and 60 min. It is pertinent to point out that an attempt to process the green compacts at higher temperatures resulted in compact slumping as well as melt oozing out of the specimens containing high amounts of SiC particles. After sintering, the samples were furnace cooled to ambient temperature. The mean cooling rate was around 10 K/min in a range from 800 to 580°C. The density of the sintered samples was measured by the water displacement method. A surface impregnation technique with a water stop spray was employed to prevent penetration of water into the pores open to the specimen surface. The hardness of the samples was evaluated by using the Vickers method under 0.05 and 30 kg force. Microstructural analysis of sintered microstructure and identification of phases were made using optical microscopy, SEM and EDX-analysis. Results and Discussion Density The green density of the investigated material systems is shown Fig.1. One can see that the copper coating decreases the green density. However, addition of elemental copper powder has a positive influence on the densification. The advantage of blending hard high speed steel particles with tough and highly compressible powders to enhance the compressibility has been already established [5]. On the other hand, Strobl and Danninger [8] have shown that Cu-coated M2 powders were more difficult to compact in comparison to the base powders, and the green bodies being comparatively fragile. Similar results were found in this study. It may be due to surface condition of the coated powder, which influences friction, interlocking, and cold welding between particles. Fig. 1. The green density of the investigated material systems compacted at 550 MPa; Cu-C: copper coated; Cu-E: elementally prepared by admixing with copper powder. From Figure 1, it is also visible that additions of SiC particles to the M2 base powder adversely affect the compressibility. When the M2/SiC composite is accompanied by Cu coating, the green density is reduced further. It seems that the adverse effect of adding SiC and Cu coating was superimposed. It is evident that the SiC particles are rigid bodies that do not deform to fill in the voids. On the other hand, the M2 particles are comparatively more ductile plastic bodies. It is therefore expected that additions of SiC particles suppress the compressibility of the steel powder. It is obvious that this effect depends on the size and the amount of the hard particles added. Figure 2 depicts the influence of SiC addition on the green density of M2/SiC composite compacts. Here, the relative density was used to omit the effect of the lower density of the carbide particles. On can notice that the adverse effect of SiC addition is more pronounced for the composite compacts containing >2 wt% SiC. On the base of the percolation theory [13,14], which analyzes the connectivity of a two-body system, it can be noticed that the 2 wt% SiC should be the percolation threshold. It means that at higher SiC additions, the ceramic particles are not distributed predominantly as isolated grains spanning M2 particle
Author: Khakbiz, Mehrdad
Language: English
Subfield: Mechanical Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7225591357