Issue 35

W. Ozgowicz et alii, Frattura ed Integrità Strutturale, 35 (2016) 434-440; DOI: 10.3221/IGF-ESIS.35.49 435 In this paper, the impact of deformation temperature on the mechanical properties and structure of low-alloy copper containing nickel, silicon and chromium of the CuNi2Si type [5] was investigated. In particular, its temperature range of reduced plasticity as well as the impact of deformation temperature and condition of the structure of the alloy on the mechanism of its cracking was determined. M ATERIALS AND M ETHODS he investigations were carried out on copper alloy CuNi2Si with grade designation mark CNCS (Hovadur). The alloy was delivered as a rod of 30mm in diameter and 200mm in length. The ø30 rod was forged into specimens of 14mm in diameter at 850°C with air cooling. After forging, the tested alloy showed tensile strength R m =450MPa, yield point R p0,2 =365MPa, elongation A=25%, reduction of area Z=48% and hardness HV10=110. The chemical composition of the tested alloy is presented in Tab. 1. Alloy designation Concentration of elements, % wt CuNi2Si (CNCS) acc. EN-Norm Ni Si Cr others Cu acc. to check analysis 1.99 0.59 0.43 0.3 remainder acc. to PN- EN 12163 1.6 - 2.5 0.4 – 0.8 0.40 0.3 remainder Table 1 : Chemical composition of CuNi2Si alloy. After forging, the tested alloy was supersaturated in water from 850°C, 940°C and 1000°C. The supersaturation temperature was determined according to the analysis of binary systems of phase equilibrium of copper with nickel, silicon and chromium [6,7]. Based on the results of initial metallographic observations and hardness measurements, the optimum supersaturation temperature of 940°C was selected. The average grain size in α solution after supersaturation from that temperature was approx. 30µm. For static hot tensile tests the specimens were supersaturated from 940°C. The operations of heating and soaking for supersaturation were carried out in Thermolyne chamber furnace of 4.4 kW equipped with a controller to ensure temperature measurements with accuracy of ± 1°. The total heating and soaking time in supersaturation process was 60 min. After supersaturation, the mechanical properties of the CNCS alloy were as follows: R m =271MPa, A=66%, Z=83% and HV10=55. In order to accomplish the purpose of this paper, the investigations of chemical composition by spectrographic method, testing of mechanical properties in the range of temperatures between 20°C and 800°C, metallographic investigations with light microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM), and hardness measurements were performed. Mechanical properties of the CuNi2Si alloy were tested on a universal testing machine INSTRON 4505 using specimens with a threaded grip of 6mm in gauge diameter and 26mm in length. The hot tensile tests were carried out in the temperature range between 20°C and 800°C at a tensile rate (v r ) of 2 mm/min, corresponding to the strain rate of   = 1.2 ·10 3 s -1 , in the protective atmosphere of argon. The operations of heating and soaking of specimens to be stretched were performed in a chamber furnace with zone-controlled temperature measured with a Pt-PtRh thermocouple. The microprocessor furnace control system ensured temperature measurement accuracy of ± 2°C. Hardness was measured by Vickers method with a hardness testing machine HASUSER by applying the load of 10N to specimens in an as-forged condition and after supersaturation. Micro-hardness was measured on microsections of the CNCS alloy after tension at 200°C, 400°C, 550°C and 800°C using a micro-hardness testing machine PMT3 by applying the load of 50G for 15s. Metallographic investigations of the CuNi2Si alloy were carried out on longitudinal microsections after hot forging, supersaturation and hot tensile tests in the temperature range between 20°C and 800°C. In order to display their structure, the microsections were etched in a reagent containing 5g of ferric chloride (FeCl 3 ), 10cm³ of hydrochloric acid (HCl) and 90cm³ of ethyl alcohol (C 2 H 5 OH). The metallographic observations were performed using a light microscope Leica T