Investigation of deformation and heat treatment effects on properties of PM 92·5W–5Ni–2·5Fe heavy alloys B. Katavic ´ and Z. Odanovic ´ microconstituent’s behaviour during deformation and heat treatment was investigated as well. This paper presents the effects of vacuum heat treat- ment under different cooling conditions on mechan- ical and structural properties of forged heavy alloys, EXPERIMENTAL such as 92·5W–5Ni–2·5Fe and 92·5W–5Ni–2·5Fe Material microalloyed with Co. The tungsten composition The commercially produced W–Ni–Fe heavy alloys and in the c phase has proved to be higher and more those microalloyed with Co (W–Ni–Fe +Co) were exam- homogenous in the rapidly cooled alloys than in the ined. The as received W–Ni–Fe alloy was sintered and slowly cooled ones. The effects of chemical compos- annealed (NE), and samples were prepared in the form of ition inhomogeneity on mechanical and structural round rods, 36 mm in diameter. The W–Ni–Fe +Co alloys properties of alloys were also analysed and discussed. were received in sintered (S), and sintered then annealed The results of tensile and toughness testing have (SA) condition. These samples were prepared in the form shown an increase in ductility and toughness, while of round rods, 20 mm in diameter. the strength of heat treated alloys decreased in com- The chemical composition was determined by an atomic parison with the strength of forged alloys. The absorption spectrophotometer (AAS). Carbon, sulphur and fracture analysis has shown that in the sintered and oxygen content was determined by a Lecco apparatus. The rotary forged alloys, intergranular fracture of the chemical compositions of the alloys are presented in Table 1. tungsten phase and transgranular fracture of the c phase occurred, respectively. The fracture of these Rotary forging and heat treatment phases after heat treatment was characterised by The S and SA rods were rotary forged with compression transgranular morphology. PM/1070 strains of 20%, and the NE rods were rotary forged with compression strains of 30%. These rods were preheated up The authors (odanovic@ptt.yu) are at the Military to 673–723 K in a heat resistant air furnace. The rotary Institute of Technology, Vinogradski venac 18/2, forging deformation step was 0·0021–0·0033 m s-1 . The 11000 Belgrade, Serbia & Montenegro. Manuscript hammer stroke speed was 2 strokes/s and the forging force received 27 May 2003; accepted 27 November 2003. was 1177·2 kN. The forged rods were heat treated in a heat resistant Keywords: Heavy metal, Powder metallurgy, Rotary furnace with molybdenum heaters, under a vacuum of forging, Heat treatment, Mechanical and structural 2–3 Pa. The furnace temperatures were automatically properties, Fracture analysis controlled in a range of ±5 K. The samples were annealed under the following conditions: heating at 1423 K for 3·6 ks © 2004 Institute of Materials, Minerals and Mining. Published followed by furnace cooling; heating at 1423 K for 3·6 ks by Maney on behalf of the Institute. followed by oil cooling and heating at 1473 K for 7·2 ks fol- lowed by furnace cooling. The heating rate was 0·17 K s-1 and the furnace cooling rate was 0·25 K s-1 . The oil temp- erature cooling was set at 298 K. INTRODUCTION TESTING The heat treatment of heavy alloys in vacuum or inert Mechanical properties gases has the following advantages: removal of dissolved The tensile tests were performed on the round specimens hydrogen from the matrix phase,1–6 increase of homogeneity with a gage length of 30 mm, according to the DIN 50125 in the alloy chemical composition7–9 and change of the c standard. The tests were carried out on a 200 kN tensile phase structure.10,11 The purpose of this treatment is to machine, with a tension rate of 1·2 ×10-5 ms-1 . The improve mechanical properties. The heat treatment usually impact energy was determined by the 348 nm Charpy includes rapid cooling in water, oil or argon, thus preventing pendulum, equipped with an oscilloscope. The dimensions impurity segregation on the W/W, W/c and c/c interfaces, of the unnotched specimens were 10 ×10 ×55 mm. The as well as causing a decrease or a complete removal of Vickers hardness, HV30 was measured on the rod cross- composition inhomogenity and the prevention of inter- sections. metallic compound precipitation on the interface and grain boundaries.4,7–10 Low cooling rates, as in furnace cooled alloys with a high content of hydrogen, are applied.2–4,8 Microstructure The objective of this work was to examine the effects of The microstructure of the heavy alloy specimens was vacuum heat treatment on the dependence of chemical analysed by optical microscope at the magnifications composition inhomogenity and mechanical and structural of 100 and 500 times. The samples were prepared by properties of the forged rods of 92·5W–5Ni–2·5Fe and mechanical grinding and polishing. The structure was revealed by etching in a solution consisting of 5 cm3 of 92·5W–5Ni–2·5Fe microalloyed with Co heavy alloy. The Powder Metallurgy 2004 Vol. 47 No. 2 191 DOI 10.1179/003258904225015509