PSI - Issue 36

Valentin Ilyushenko et al. / Procedia Structural Integrity 36 (2022) 100–105 V. Ilyushenko, T. Maydanchuk, E. Lukianchenko, S. Kozulin, S. Marynenko / Structural Integrity Procedia 00 (2021) 000 – 000 5

104

Table 2. Mechanical properties of the investigated copper Temperature,  С Ultimate tensile strength, MPa

Elongation, %

20

172. 6…174 .4 105.6…117. 0

46. 3…53 .0 40. 8…41 .7 10. 6…11 .4

350 750

4. 8…5 .6

As it is visible from the Table 2, the strength characteristics of the metal under study at 20 °C, as shown in the work of Smiryagin et al. (1974) are almost identical to those of cast copper. However, at temperatures above 350 °C , the investigated copper is embrittled. Analyzing the possible reasons for the formation of cracks in the surface layer of copper chambers, it was suggested that low-melting impurities (both controlled (Bi, Pb, Sb, S) and uncontrolled (Se, Te)) have a negative effect on the strength properties of copper at elevated temperatures. The presence of up to 0.0033% selenium in the metal, the solubility of which, as shown in the work of Massalsky (1986-1987), in copper is negligible (~ 0.001% at 500 ° C), can lead to the formation of a brittle Cu-Cu 2 Se eutectic, which worsens plasticity of copper and its weldability, as shown in the works of Ilyushenko and Lukyanchenko (2013), and Anoshin and Ilyushenko (2018). It should be noted that the presence of 0.04% P and 0.044% Fe in copper (see Table 1) reduces its thermal conductivity, as shown in the work of Smiryagin et al. (1974). Since the end of the chamber is located inside the furnace, where the temperature reaches 1600...1800 °C, a decrease in thermal conductivity leads to overheating of the chamber surface, which accelerates its destruction. Since during operation the nozzle of the chamber is in contact with air and a mixture of oxygen with natural gas is supplied through it, grain boundaries and already formed cracks can also be enriched with oxygen and hydrogen. The presence of microcracks, an aggressive environment, together with the thermal-deformation conditions of the copper chambers leads to intergranular corrosion and destruction of the end part of the chamber, up to the appearance of leaks in the water-cooling channels. As a result of the studies carried out, in our opinion, the mechanism of crack formation in the end part of the chambers is as follows: selenium, which is a surface- active element with a low melting point (217 °C) and a small distribution coefficient during copper crystallization, is concentrated along the grain boundaries, which leads to the effect of adsorptive reduction of plasticity and strength. The presence of tensile stresses associated with the thermal deformation conditions of the operation of the copper chambers of the ASF, as well as the coarse-crystalline structure of cast copper, leads to the formation of microcracks, which further propagate through the thickness of the metal. From many years of practice, it is well known that copper grade ASTM C12900 has satisfactory weldability, and these parts are quite repairable. As a result of the experiments, the technology of restoring worn-out copper parts by helium-arc welding was selected. Using the developed technology and special metal-cored wire, which ensured the production of directional metal with high thermal conductivity, the tightness and geometric dimensions of the ASF copper chambers were restored (Fig. 5), as shown by Ilyushenko et al. (2019).

Fig. 5. Exterior view of the recovered copper chamber.