PSI - Issue 72

Nedeljko Vukojević et al. / Procedia Structural Integrity 72 (2025) 157 –163

158

Japan, with initial studies conducted by Japanese scientists, Matsubara (1994), Iseki and Kumon (1994), and this research has continued to the present day, Tsiza (2012). Earlier research and practical experiences in application, Prime (1999), have shown that the commonly most used material for making dish ends is hot-rolled carbon steel plates in the range of 200 to 800 N/mm² tensile strength. Production experiences related to the manufacture of dish ends from the same material show that the experience and training of workers are significant for achieving the most favorable production parameters for dish ends, Shakir et al. (2016), Vukojević et al. (2007). In addition to the known inf luential parameters for manufacturing dish ends through the incremental sheet forming process, the behavior of the welded joints, which represent a heterogeneous part of the dish end, remains unclear. This area is a site of local stiffness changes, hardness variations, and local weakening due to the presence of increased residual stresses. All these influences are different and may be critical acceptance criteria from a safety perspective, Singh et al (2023), Vukojević et al. (2024) .

Nomenclature D a

outer diameter of vessel thickness of vessel thickness of samples

[mm] [mm] [mm] [MPa] [MPa]

s, t,

R m R eH

tensile strength yield strength contraction impact energy

A elongation

[%] [%]

Δ

KV HV  res K IIc

[J]

hardness

[HV] [MPa] [mm] [bar] [MPa·m

residual stress fracture toughness specimen thickness internal pressure safety factor according

1/2 ]

B p S 

[-] [-]

weld joint coefficient (0,85  1,0)

2. Experimental research Research is focused on steel S235, which is one of the most commonly used for making pressure vessels, and on steel P460, which is increasingly replacing S235 in use. For experimental samples, circular steel sheets of appropriate diameters, material types and plate thicknesses were chosen, designed to provide comprehensive combinations of treated parameters, Ištvanić (2022). Since the materials have good weldability, welding technology was made acc. EN ISO 4063-121(2010) and EN14171 (2002). All joints were made in an "X" (double V) shape, with preheating at 100°C and an inter-pass temperature of 200°C. Testing of the samples was performed according to standard procedures. The thinning of the plates, particularly noticeable in thicker plates was observed. This phenomenon causes an increase in thickness toward the outer edges of the dish end due to material extrusion during the formation of the toroidal part. Incremental deformation of the roundel was performed on a hydraulic press with interchangeable tooling and pressure force, Fig.1. The press type used is a P2MF 200x4 - Sertom, Milan. For the experimental purposes, four dimensionally different rundels were created: Ø1500x6 mm; Ø1500x14 mm; Ø3000x6 mm; Ø3000x14 mm. The materials used for the tests were P460 NL1 NT and S235 JR+N. Samples were produced in two series:  Series I: Samples from the starting plate,  Series II: Samples from the plate after plastic processing.