Effect of copper on the cyclic crack resistance and heat resistance of graphitic steels
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EFFECT OF COPPER ON THE CYCLIC CRACK RESISTANCE AND HEAT RESISTANCE OF GRAPHITIC STEELS I. M. Andreiko, I. P. Volchok, O. P. Ostash, I. V. Akimov, and Yu. V. Holovatyuk
UDC 539.43
We study the effect of copper content (0.05 – 3.45 mass %) on the structure of the matrix and graphitic phase of graphitic steels (GS) annealed for globular perlite. It has been established that the effect of copper content on the physicomechanical characteristics of GS is ambiguous. An increase in the copper content up to 2.3 – 2.9% is accompanied by an increase in the characteristics of thermal conductivity, strength, cyclic crack resistance, and heat resistance. The thermal conductivity of GS grows with increase in the copper content (if it is more than 3%), but the mechanical characteristics fall. Thus, the heat resistance of GS is mainly determined by their crack resistance.
Some articles of metallurgical equipment, in particular, ingot moulds, rolls, etc., are subjected to the action of cyclically variable stresses in the course of operation, which is a result of their periodic heating and cooling. In the case of significant thermomechanical stresses, these articles can fail by means of crack initiation and propagation. One of the ways of enhancement of their durability is connected with the choice of the corresponding structural materials with the optimal combination of physicomechanical properties that determine their heat resistance. In the first place, we should mention here strength, which dictates the size of a construction, as well as crack resistance and thermal conductivity, which determine its durability under conditions of heat shocks. Ingot moulds and rolls are traditionally manufactured of cast irons, where the graphitic phase guarantees a high thermal conductivity; however, they have a low crack resistance [1]. Therefore, graphitic steels, which possess a higher crack resistance as compared with cast irons, can be promising for such articles [2]. In particular, as shown in [3], the heat resistance of rolls made of GS is almost twice as high as that for high-strength cast iron and hypereutectoid steel. Copper is an efficient alloying material, which increases the thermal conductivity of metals. Copper affects substantially the structure and mechanical properties of the metal base of cast irons and graphitic steels, in particular, their crack resistance. The objective of the present work is to find the optimal copper content in GS for the enhancement of their breaking strength under cyclic and thermocyclic loads. Material and Procedure of Investigations We studied GS having the following basic composition (in %): 1.30 – 1.33 C, 0.85 – 0.89 Si, 0.23 – 0.25 Mn, 0.39 – 0.44 Al, 0.024 – 0.031 P, and 0.022 – 0.027 S with different copper contents: 0.05, 0.81, 1.57, 2.27, 2.90, and 3.45% (Table 1). Copper accelerates the decomposition of cementite and favors graphitization [4]. Hence, we did not subject the GS under study to long-term graphitizing annealing [2], but only normalized them from 840°C and annealed for globular
