Issue 46

I. Čamagić et alii, Frattura ed Integrità Strutturale, 46 (2018) 371-382; DOI: 10.3221/IGF-ESIS.46.34 380 Impact of the testing temperature on the value of critical stress intensity factor, KIc, for notched specimens in HAZ from the side of the new and exploited PM is graphically illustrated in Fig. 10 (left), and the impact of the testing temperature on the value of the critical crack length, ac, also for the notched specimens in HAZ from the side of the new and exploited PM is graphically illustrated in Fig. 10, (right) [1]. 0 100 200 300 400 500 600 0 40 80 120 160 200 Heat affected zone - HAZ New HAZ Exploited HAZ Fracture toughness, K Ic , MPa m 1/2 Testing temperature, o C 0 100 200 300 400 500 600 0 10 20 30 40 50 60 Heat affected zone - HAZ New HAZ Exploited HAZ Critical crack length, a c , mm Testing temperature, o C Figure 10 : Change in value of K Ic depending on the testing temperature at HAZ (left) and change in value of a c (right). D ISCUSSION esting of the welded joint specimens by introducing the load transversely to welded joint provided necessary data regarding how selected welding technology and exploitation time impact the welded joint strength and welded joint components. The obtained results of testing the welded joint specimens by introducing load transversely to welded joint, tab. 5, indicate that all tested specimens have cracked in exploited PM. This information is of great importance because it indicates the weakening of PM which was in exploitation. The fracture of specimens in PM clearly indicates the character of the welded joint. This is “over-matching”, which means that the strength of the welded metal is higher than the strength of the parent metal, [1, 18]. Character of obtained tensile curves at room temperature corresponds to a ductile material with approximate share of homogeneous and non-homogeneous elongation at a ratio of 1/2:1/2. Here, homogeneous elongation is considered as elongation up to the maximum force, and non-homogenous elongation is considered as elongation from maximum force to fracture (unstable crack growth, i.e. necking). When testing the welded joint specimens at working temperature, there is a similar tendency of change in the properties of strength as with the testing at room temperature, but the difference occurs at the properties of strain (elongation). Namely, here we have the case where the ratio of homogeneous to non-homogeneous elongation is approximately 1/4:3/4, which is rather unfavourable from the aspect of exploitation properties. The reserve of homogenous plasticity of materials is considerably smaller, thus the hazard to PM of the consequences of potentially poor operation of the plant is real. By analysing the results obtained by tensile testing at room temperature of specimens taken from the sample of the new PM, given in tab. 6, it can be concluded that the testing results of the new PM are within the limits of values prescribed by the standard for that material, that is, values provided by a manufacturer in attest documentation. The obtained results of tensile testing of WM specimens given in tab. 7 confirm the properly selected welding technology, i.e. welding parameters. Yield stress and tensile strength satisfy values prescribed by the standard, whereas strain properties are much better than those given in the standard for this additional material, [19]. This phenomenon indicates a high-grade selected regime of thermal processing after welding. The behaviour of the HAZ in the loaded welded joint was conditioned by its small volume portion, as well as by the heterogeneity of the structure and different mechanical properties of certain HAZ areas. A well-made welded joint, designed according to the principle of higher WM strength, should break in PM, which is exactly what happened in presented tests, [1, 18]. Based on the obtained testing results of specimens taken from the new and exploited PM, WM and HAZ from the side of the new and exploited PM, it can be seen that with the increase of the testing temperature there is a decrease in the value of critical J Ic , integral, that is, fracture toughness, KIc. The value of critical crack length, ac, also decreases. T