PSI - Issue 48

Gašper Fašun et al. / Procedia Structural Integrity 48 (2023) 19–26 Fašun et al/ Structural Integrity Procedia 00 (2023) 000–000

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Fig. 1. Schematic representation of the barrel. Internal pressure distribution and a cross section of the barrel are shown with indications of all test samples used and their localizations and orientations. 2. Material properties A 35NiCrMoV12-5 steel was analysed (designated by the manufacturer SIJ Metal Ravne as KATO1), whose chemical composition can be found in the Table 1. KATO1 is a high-strength steel specifically designed for use in structural elements that are exposed to significant loads during their service life. This steel is produced through the electro-slag remelting (ESR) process, which ensures a high level of purity.

Table 1. Chemical composition of KATO1 steel in weight % [9]. C Si Mn Cr Ni Mo V Others 0.39 0.29 0.25 1 3.25 0.6 0.15 Max. Cu=0.15

Two different heat treatments were applied. The temperatures Ac3 and Ac1 were 740°C and 670°C, respectively. Both resulting materials (A and B) underwent a four-step heat treatment process, starting with normalization above the Ac3 temperature. Material B was normalized at a temperature 10°C higher than material A. Next, both materials were quenched at the same temperatures, followed by two tempering steps at temperatures below Ac1. Material B was tempered at a temperature 100°C higher than material A in the first step, while no temperature difference existed in the second step. The different temperatures during the heat treatment process resulted in different microstructures, leading to variations in the mechanical and fracture-mechanical properties of the materials The cut-out location of the specimens from the barrel for the metallographic analysis is marked with the number 4 on the cross-section in Fig. 1. Fig. 2 shows the microstructure of each material (A and B). The improved microstructure is tempered martensite, which is uniform throughout the entire cross-section. The grain size was estimated to be similar for both materials, with a grade of 6, equivalent to a size of d = 45-70 µm. The size of non metallic inclusions was estimated based on the standardized K method according to the comparative tables of DIN 50602 [8]. Material A had twice the number of non-metallic inclusions compared to material B, with inclusion sizes