ASC with Cu and 0.5C consists of ferrite and perlite, whereas in the case of ... distinguish between lower bainite and martensite, but the differences in .... [4] Houdremont, E.: Handbuch der Sonderstahlkunde. ... Binary Alloy Phase Diagrams.
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FORMATION OF STRUCTURES WITH A MICROGRADIENT IN SINTERED IRON-BASED POWDERS BY COPPER-COATING S. Strobl, C. Gierl, T. Konegger, M. Kupková, M. Kabátová Abstract Iron and Cr-Mo-steel powders were coated with different amounts of Cu (3 or 8 wt%) by cementation. For studying the influence of carbon on these systems 0.5 wt% graphite was added partially. After sintering at 1120°C, 60 min in H2 or N2 and metallographic examinations, microgradient structures were found. The samples were tested in regard to bending and tension. They were investigated by LOM (light optical microscopy) and SEM (scanning electron microscopy) to get detailed informations about microstructures and fracture surfaces. Keywords: PM steel, microgradient, copper-coating, fracture INTRODUCTION To generate microgradient structures in sintered parts with the ambition to increase the mechanical properties, powder metallurgy can be used. During the sintering process contacts between the powder particles are developed, which are the most important structural elements. The formation of microgradients is of interest, since the regions in and around the sintering contacts can be influenced by strengthening, and on the other hand the core areas are unaffected. The properties of PM materials can be changed with a minimum use of alloying elements, because they are located at positions where their effect is highest. For the realisation of these requirements a simple option is the coating of a metal powder with another element and an adequate sintering process. EXPERIMENTAL Raw materials Iron powder, ASC 100.29, water-atomised, screened 180-63 µm Cr-Mo steel powder, Astaloy CrM, Fe-3 wt% Cr-0.5 wt% Mo, pre-alloyed, wateratomised, screened 180-63 µm These powders were coated with two different amounts of copper (3 or 8 wt%). Therefore the powders were put into an aqueous electrolyte, which contains coppersulfate and sulphuric acid, both with a concentration of 10 [g/l]. After stirring several minutes the basic materials were partially dissolved and copper was deposited by a cementation process on the particle surfaces (Fig.1.). The copper amount was controlled by the coppersulfate content in the electrolyte. In the case of 3% Cu the layer is very thin and not clearly visible (Fig.1.a), but for both concentrations the particle surface is completely covered with copper, which let the powders appear copper-coloured. The cementation process of the CuFe system was studied by atomic absorption spectroscopy (AAS) and described in literature [6]. From this data it is known that after 3 minutes the Cu2+-concentration in the electrolyte is about 20 ppm. Also the coated iron powder was analysed: the copper amount in the iron Susanne Strobl, Christian Gierl, Thomas Konegger, Technische Universität Wien, Vienna, Austria Miriam Kupková, Margita Kabátová, Institute of Materials Research of SAS, Košice, Slovak Republic
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was measured by chemical and gravimetric methods. The deviation from the desired content was ± 0.5%. The powders, which were modified in this way, were blended with 0.6% lubricant and partially with 0.5wt% graphite. After cold compaction with 600 MPa the bending, respectively tensile bars were sintered in pusher furnaces under the following conditions: carbon-free samples: 1120°C, 60 min, H2 / CrM additionally with ferroaluminiumgetter carbon-containing samples: 1120°C, 60 min, N2, dewaxing (600°C, 30 min, N2)
(a) ASC-3Cu
(b) ASC-8Cu
Fig.1. Coated iron powder ASC with 3 or 8 wt% copper, LOM. RESULTS AND DISCUSSION In Table 1 the basic properties of the studied materials are presented. ASC (with carbon and carbon-free) with low Cu-contents shows higher densities and with an increased grade of copper a significant swelling took place, while density decreased. This typical behaviour for PM-iron is called “copper swelling” and is suppressed when carbon is added [1, 2, 3]. In the case of CrM shrinkage can be always observed. Tab.1. Properties of the studied materials. Material
Sintered Dimensional TRS Tensile density change [MPa] strength [g/cm³] [lin.%] [MPa] ASC 1 7.13 -0.13 200 ASC-0.5C 1 7.10 -0.03 330 ASC-3Cu 7.13 +0.17 524 232 ASC-8Cu 6.91 +1.35 641 322 ASC-3Cu-0.5C 7.07 +0.14 938 455 ASC-8Cu-0.5C 7.00 +0.61 982 557 CrM-3Cu 6.97 -0.21 924 465 CrM-8Cu 7.03 -0.31 1030 588 CrM-3Cu-0.5C 6.96 -0.46 1777 965 CrM-8Cu-0.5C 7.09 -0.90 865 480 1 from Literature [7] sintering conditions: ASC: 1120°C, 30 min, 75H2/25N2 ASC + 0.5C: 1120°C, 30 min, 90N2/10H2 + 0.6%CO2
Apparent hardness HV10 50 113 83 132 161 184 133 191 361 385
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In general a strengthening of pure iron with copper or Cr and Mo could be observed. This is a good deal more pronounced by the addition of carbon. So the values for bending, tensile strength and apparent hardness increase by alloying the iron-base (Tab.1.). In the case of CrM-8Cu-0.5C the decrease of strength properties can be explained by the microstructure and the fracture surface. The changes in the mechanical properties are in close relation to the microstructures. The carbon-free specimens have a ferritic structure, where the Cu-rich areas can be seen clearly in Fig.2. – darker and more attacked by the etching reagent nital. They are strengthened by copper precipitation and by the formation of solid solution with iron [4].
(a) ASC-3Cu
(b) ASC-8Cu
(c) CrM-3Cu
(d) CrM-8Cu
Fig.2.a-d. Microstructures of sintered ASC and CrM with 3 or 8 wt% Cu (nital), LOM. In the microstructure of both investigated basic systems, which were alloyed with more Cu, free copper occurs because at the sintering temperature of 1120°C more than 8wt% can be dissolved in iron [5], but during cooling the solubility decreases: copper is precipitated along the grain boundaries - in the outer regions of the Cu-enriched areas. More free copper was observed in the Fe-Cr-Mo material (compare Fig.2.b and d). Cr and Mo are stabilising the α-iron, which is the room temperature modification, and it seems as if they increasingly displace Cu out of the Fe lattice. When carbon is added the microstructures of the steels are changed substantially (Fig.3.): ASC with Cu and 0.5C consists of ferrite and perlite, whereas in the case of higher copper content it is very fine structured (Fig.3.a, b). Again, the Cu-rich areas are darker and
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the influence on hardness, transverse rupture and tensile strength is obvious (Tab.1.). Finally in Fig.3.c, d the structures of CrM are shown: the microstructure becomes martensitic with the higher copper amount and bainitic-martensitic for CrM-3Cu-0.5C, where the mechanical properties achieve maximum values (Tab.1.). It is not easy to distinguish between lower bainite and martensite, but the differences in microhardness, apparent hardness, bending, tensile strength and fracture surfaces indicate this. Also here the Cu-microgradient was located in the darker areas and at such an increased Cu content free copper is visible (compare Fig.3.b and d).
(a) ASC-3Cu-0.5C
(b) ASC -8Cu-0.5C
(c) CrM-3Cu-0.5C
(d) CrM-8Cu-0.5C (picral)
Fig.3.a-d: Microstructures of sintered ASC and CrM with 3 or 8 wt% Cu +0.5C (Nital), LOM. In general the morphology and distribution of the pores is influenced by copper and carbon: at higher Cu grades and 0.5 %C the pores are more rounded and isolated (Fig.4a, b).
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(a) ASC-3Cu
(b) ASC -8Cu-0.5C
Fig.4.a-b: Pores of sintered ASC-3Cu and ASC-8Cu-0.5C (polished), LOM. The fracture behaviour is demonstrated in Fig.5. and 6. The fracture of carbon-free ASC-Cu samples is a typical ductile one and the sintering contacts are well defined (Fig.5.a, b). This corresponds with the microstructural analyses and mechanical properties. Generally the same can be said about CrM-3Cu, but the dimples are finer than with ASC and the material is well sintered as well (Fig.5c). The appearance of the CrM-8Cu fracture surface changes, although the microstructure is also ferritic. It is a mixed fracture – ductile with a considerable amount of cleavage (Fig.5d).
(a) ASC-3Cu
(b) detail of (a)
(c) CrM-3Cu
(d) CrM-8Cu
Fig.5.a – d: Fracture surfaces of carbon-free ASC-3Cu and CrM with 3 or 8 wt% Cu, SEM.
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Again, when carbon is added to ASC-Cu- and CrM-Cu-systems significant changes in the mircostructures take place as well as the mechanical properties, and consequently the fracture behaviour are strongly influenced. The fracture of ASC-3Cu-0.5C is a mixed one: ductile with dimples and some cleavage facets (Fig.6.a). If the copper content is increased cleavage becomes predominant and also some quasi-cleavage can be observed (Fig.6.b). The failure of CrM-3Cu-0.5C can be described as ductile with very fine dimples, some quasi-cleavage, but no cleavage facets (Fig.6.c). Finally, the fracture surface of CrM with 8Cu and carbon (Fig.6.d) shows a mixed fracture: small amounts of ductile dimples, cleavage, quasi-cleavage and a considerable amount of intergranular fracture.
(a) ASC-3Cu-0.5C
(b) ASC-8Cu-0.5C
(c) CrM-3Cu-0.5C
(d) CrM-8Cu-0.5C
Fig.6.a – d: Fracture surfaces of sintered ASC and CrM with 3 or 8 wt% Cu and 0.5C, SEM. CONCLUSIONS Iron-based powders were coated successfully with 3 respectively 8 wt% copper and after sintering at 1120°C a microgradient structure was achieved. There is a close relation between microstructure, mechanical properties and fracture behaviour. Copper as well as carbon have a strengthening effect on the analysed systems, but the influence of carbon is more pronounced. The microstructures of ASC-based materials change from ferritic to perlitic-ferritic by adding carbon and the CrM-based systems show ferrite, and with 0.5C bainitemartensite and martensite. With 8 wt% Cu free copper can be observed. With an increase of Cu and carbon the pores are more rounded and isolated.
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The highest strength is found for the bainitic-martensitic Cr-Mo-steel with 3% Cu and 0.5% C. The failure of the carbon-free materials is predominately ductile. With carbon the character of the fracture surfaces becomes more brittle: mixed fracture with cleavage facets, quasi-cleavage and ductile parts can be observed.
Acknowledgements This work was supported by Austria-Slovak Project “Sintered iron-based alloys with microgradient structure” SK-AT-01306 / 08-2006 and by the Slovak Grant Agency for Science (VEGA grant 2/6208/26. REFERENCES [1] German, RM., D’Angelo, KA: International Metals Reviews, vol. 29, 1984, no. 4, p. 249 [2] Dautzenberg, N., Dorweiler, HJ.: Powder Metallurgy International, vol. 17, 1985, no. 6, p. 279 [3] James, BA.: Powder Metallurgy, vol. 28, 1985, no. 3, p. 121 [4] Houdremont, E.: Handbuch der Sonderstahlkunde. Springer Verlag, 1956 [5] Massalski, TB.: Binary Alloy Phase Diagrams. Metal Park OH : ASM International, 1990 [6] Strobl, S.: Dissertation. Wien, 1996 [7] Höganäs Iron and Steel Powders for Sintered Components. Höganäs AB, Sweden, 2002
