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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Keywords: bimetal; Alloy 625; P355NH; explosion welding; stress analysis; XRD 2

1. Introduction Geothermal energy refers to the heat stored in the accessible part of the Earth`s crust. It includes all the energy stored in the Earth which can be extracted and used as a renewable “green“ energy source Barbier et al. (2002). In a number of areas where geothermal energy is used, highly corrosion-resistant materials are required in order to preserve the typical life span of geothermal power plants over 20 years. As the industry progresses towards deeper wells and more corrosive conditions, the standard product must be replaced by more advanced materials. Carbon steel has commonly been used as a corrosion-resistant material in geothermal applications because of its low cost and availability. But in lower-pH geothermal fluids, alternate materials or coatings are required to prevent material degradation and failures. During the 1980s and early 1990s, various studies evaluated the causes of material failures in geothermal environments to improve geothermal power production. Nowadays, commercially available carbon steel, low-alloy or chromium-molybdenum (Cr-Mo) steels, martensitic and ferritic stainless steels, high-nickel (Ni) alloys, and titanium (Ti) are used for geothermal powerplant components working at high temperatures (>260°C) Kaya et al. (2005). Another type of materials is the layer bimetallic system. These materials consist of a clad layer made of noble material capable of withstanding a highly corrosive environment and a base material made of less noble material providing the system with the necessary strength, and also having other technologically advantageous properties, e.g. good weldability. Such layered materials can be prepared by explosion welding Findik (2011). For geothermal applications, we examined the bimetallic system consisting of the nickel-chromium-molybdenum alloy (Alloy 625) (clad) welded on the ferritic pressure vessel steel P355NH (base material). Since this material was prepared by explosion welding, it is assumed that a considerable amount of explosive energy is stored in the material in the form of elastic strains. The stress level of the material should be therefore known, and in case its intensity is high and/or inappropriately distributed, the material should be subjected to a suitable treatment in order to relieve it, preventing a potentially dangerous influence on strength, dimensional stability and reduction of fatigue lifespan. This paper presents the microstructure, phase composition, and distribution of residual stresses of one particular bimetallic system which is the nickel-chromium-molybdenum alloy (Alloy 625) cladded on the ferritic pressure vessel steel P355NH base material. 2. Experimental procedure 2.1. Material A sheet of pressure vessel steel P355NH of the following chemical composition (wt. %): Fe:97.75, C:0.18, Mn:1.19, Si:0.35, Ni:0.22, Cu:0.2, Al:0.041, Cr:0.02, Nb:0.02, P:0.015, Mo:0.004, V:0.003, Ti:0.003, S:0.002, B:0.0002 was clad (explosively welded) by nickel-chromium-molybdenum alloy (Alloy 625) Ni:61.19, Cr:21.5, Mo:8.7, Fe:4.6, Nb:3.32, Si:0.19, Ti:0.18, Al:0.16, Mn:0.05, Co:0.02, C:0.018, Ta:0.01, Cu:0.018, P:0.005 and S:<0.002. The thickness of the ferritic base material was 10 mm and the clad 3 mm. Dimensions of the welded sheets were 800 x 520 mm. Both the materials were explosively welded by the Zakład Technologii Wyso koenergetycznych, EXPLOMET Gałka, Szulc sp. jawn a company, Opole, Poland under mild conditions. The material after the explosion welding is named in the text the as-prepared material. The following heat treatment has been consecutively applied on the as-prepared material: • heat treatment OC1 : the material was put into furnace preheated to 300  C and heated to 610  C with the heating rate 120  C/hour. There it was annealed for 90 min and cooled down with the furnace to room temperature at the rate approximately 120  C/hour. • heat treatment OC2 : the material was put into furnace preheated to 300  C and heated to 910  C with the rate 150  C/hour where it was annealed for 30 min. and cooled down in calm air to room temperature.