Abstract
Six kinds of Cu-Ni alloys with different sulfur contents were investigated at room temperature for tensile properties. The effects of tensile rate and sulfur content on the yield strength, tensile strength, elongation, and reduction of area of the alloys were studied. The changes of metallographic structure with sulfur content were studied by scanning electron microscope (SEM), energy disperse spectroscopy (EDS) and metallographic structure analysis. The distribution and deformation of sulfur precipitate and the influence law for the plasticity of Cu-Ni alloy were analyzed. The origin and occurrence of alloy fracture process and the influence mechanism of sulfur content on plasticity were analyzed, providing a theoretical basis for subsequent cold deformation process.
Science Press
In iron-based and nickel-based alloys, sulfur, as a harmful element, must be strictly controlle
However, there are few relevant literatures about the influence of sulfur on copper alloy and Cu-Ni alloy. In this research, the effect of sulfur on the tensile properties of C71500 Cu-Ni alloy was studied. The composition of sulfide and its failure mechanism to properties were investigated. According to different standards, the Cu-Ni alloys with different sulfur contents were developed. Through the tensile test, the influence of sulfur on the structure, properties and processing technique of Cu-Ni alloy was analyzed, providing a theoretical basis for the reasonable control of sulfur content in the Cu-Ni alloy, which is conducive to the optimization of subsequent cold deformation process for Cu-Ni alloy.
According to GB/T 8890-2015 “Seamless copper alloy tube for condenser and heat-exchanger”, the maximum sulfur content of C71500 Cu-Ni alloy should be less than 0.01wt%. According to ASTM-B111/B111M-18a “Standard Specifica-tion for Copper and Copper-Alloy Seamless Condenser Tubes and Ferrule Stock”, if the product is for the subsequent welding applications, the maximum sulfur content is 0.02wt%. According to EN 12451-2012 “Copper and Cop-per Alloys-Seamless, Round Tubes for Heat Exchangers”, the maximum sulfur content of C71500 Cu-Ni alloy is 0.05wt%, and if the product is for subsequent welding applications, the maximum sulfur content is 0.02wt%. There-fore, six kinds of C71500 Cu-Ni alloys with different sulfur contents were prepared. The sulfur content was controlled at <0.0003wt%, 0.0063wt%, 0.0179wt%, 0.0383wt%, 0.0648wt%, and 0.0784wt%. To eliminate the influence of carbon and oxygen, the contents of carbon and oxygen were controlled below 0.001wt%.
Tensile test specimens with diameter of 5 mm cut along the longitudinal direction of forged plates were used, according to ASTM B11 for specific dimensions. The test temperature was 23±5 ℃, and the constant true strain rates were 0.0001, 0.001, 0.01 and 0.1
The original structure of the Cu-Ni alloy is a single phase equiaxed crystal structure, as shown in
Fig.1 SEM images of Cu-Ni alloys with different sulfur contents at forged state: (a) 0wt%, (b) 0.0063wt%, (c) 0.0179wt%, (d) 0.0383wt%, (e) 0.0648wt%, and (f) 0.0784wt%
Fig.2 shows various mechanical properties of C71500 alloys with different sulfur contents at different true strain rates. It can be seen from Fig.2a that the yield strength of the alloy is relatively low when the sulfur content is less than 0.0003wt%, which is 225.03~250.16 MPa. As the sulfur content increases, the yield strength of the alloy increases rapidly. When the sulfur content increases to 0.0063wt%, the yield strength of the alloy increases rapidly to 288.26~297.14 MPa. At low strain rates (0.0001 and 0.001
It can be seen from Fig.2b that the tensile strength of Cu-Ni alloy with sulfur content <0.0003wt% is very low (365.04~385.75 MPa). The tensile strength of the alloy increases significantly after adding sulfur, indicating that sulfur element is conducive to improving the alloy strength. When the sulfur content increases to 0.0063wt%, the tensile strength of the alloy increases rapidly to 392.06~407.14 MPa. However, when the sulfur content is 0.01wt%~0.04wt%, there is a cold strength groove in the alloy, and the alloy strength rapidly reduces to 385.53~399.39 MPa. When the sulfur content continues to increase, the alloy strength slowly reduces, and the tensile strength of the alloy increases with the increase of strain rate. The increment of tensile strength for alloy at different strain rates is 0~20 MPa.
The elongation of sulfur-containing and sulfur-free Cu-Ni alloys is very different. As shown in Fig.2c, the elongation of the alloy decreases sharply after the addition of sulfur element. With the increase of sulfur content, the elongation of the alloy shows an overall downward trend. However, under the condition of high strain rate of 0.1
It can be seen from
Fig.3 SEM images of fracture morphology of alloys with different sulfur contents at deformation rate of 0.0001
It can be seen from
Fig.4 Metallographic morphologies of longitudinal section of fracture for alloys with different sulfur contents at deformation rate of 0.0001
From the sulfide morphology at the bottom of fracture dimple, it can be seen that there are a large number of sulfide inclusions in the fracture dimple, which are broken in the deformation process, indicating that the inclusions show no plastic deformation at room temperature. The effect of sulfide on the metal matrix and the fragmentation state of sulfide after deformation can be more macroscopically observed through the longitudinal section morphology of specimen, as shown in Fig.5. It can be found that the composition of sulfide inclusion is Mn, Fe, Ni, Cu, and S. By quantitative analysis, the atomic percentage of S is equal to the total atomic percentage of Mn, Fe, and Ni. Therefore, the compound can be named as (Mn, Fe, Ni)S.
1) With the increase of deformation rate, the tensile strength and yield strength of the C71500 Cu-Ni alloy increase, while the plasticity decreases.
2) The addition of sulfur improves the alloy strength, but the plastic failure of the alloy is evident. The reason for the decrease of plasticity is that the sulfur-containing compounds (Mn, Fe, Ni)S show no plasticity at room temperature and form micro pores after crushing, thus leading to stress concentration and uneven distribution of stress.
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