PSI - Issue 43

Zuzana Molčanová et al. / Procedia Structural Integrity 43 (2023) 89– 94 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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Fig. 3 shows the XRD patterns obtained from different parts of the Alloy 625/P355NH bimetal: the blue curve on the top corresponds to the Alloy 625, taken approximately 2 mm away from the center line of the joint. The P355NH base metal was measured 2  m from the joint, but in the opposite direction (black curve at the bottom), while the center of the joint itself (represented by the red curve) is in between. Our analysis proves that the Alloy 625 consists of sole fcc-Ni solid solution while the P355NH consists of bcc-Fe (ferrite) and low amount (below 4 vol. %) of orthorhombic Fe 3 C (cementite - not visible in the pattern) phases. The material at the center of the joint is a mixture of the two phases, without the presence of any new phases created by the explosion. 3.3. Residual stresses in the material The amount of elastic strains in the as-prepared (explosively welded) material and in the materials after the heat treatment was calculated by applying the XRD 2 theory from the corresponding XRD patterns. In our analysis, we chose {222} Debye-Scherrer rings of the fcc Alloy 625 solid solution and {220} of the bcc-ferrite. Based on the calculated strains and by application of elastic module E and Poisson ratio  of the corresponding material, we were able to calculate the materials` residual stresses. Fig. 4 shows the normal components of the stress tensor σ 11 and σ 22 as a function of their position in the sample (yellow dashed line). The comparison of the figures shows that the largest residual stresses are in the as-prepared material. At its interface the clad fcc (Alloy 625) material is under tension by more than >700 MPa, while the ferrite bcc base material approaches tensile stresses of 500 MPa. It can be said that the bimetal after explosion welding is in a very high elastic stress state. Therefore, subsequent heating treatment is inevitable. The applied OC1 and OC2 significantly reduced the proportion of stresses, as shown in Figure 4. The OC1 reduces the stress tensor normal components σ 11 and σ 22 under 300 MPa while the OC2 , equalizes and homogenizes them even more with magnitudes below 200 MPa.

Fig. 4. a. Metallographic cross-section of the as-prepared Alloy 625/P355NH bimetal with residual stresses measured in its interface. b. residual stresses after the OC1 heat treatment. c. residual stresses after the OC2 heat treatment. 4. Conclusion In this paper we studied microstructure, phase composition and residual stresses of the bimetallic system nickel chromium-molybdenum alloy (Alloy 625) (clad) explosively welded to the ferritic pressure vessel P355NH ferritic steel (base). The material has potential application in the geothermal industry for transport of hot and highly corrosive media and as a material of heat exchangers, condensers, etc. • The microstructure of the Alloy 625 near the joint exhibits elongated grains with orientation copying the wavy structure of the joint. The joint exhibits equiaxial grains as a result of recrystallization with small cracks and cavities as result of imperfect welding and/or extensive residual stresses.