PSI - Issue 68

Jan Kec et al. / Procedia Structural Integrity 68 (2025) 1091–1097 Jan Kec / Structural Integrity Procedia 00 (2025) 000–000

1093

3

L-T CT

longitudinal orientation of specimens

compact tension

2. Experiment 2.1. Material and methodology

A segment of longitudinally submerged arc welded pipe (LSAW) with an outer diameter (OD) of 1422 mm and a wall thickness (WT) of 21.5 mm was used to produce the standardized specimens. The pipe material was X70ME designation according to ISO 3183. It is a modern X70ME steel with low carbon content, high metallurgical purity and microalloyed with vanadium, niobium and titanium. Chemical analysis was carried out on optical emission spectrometer SPECTROMAXx, see Table 1. The microstructure consists of polygonal ferrite, acicular ferrite and bainite. The microstructure of LSAW was formed by grain boundary ferrite, acicular ferrite and bainite. A more detailed investigation of the microstructure is beyond the scope of this paper.

Table 1. Chemical composition of investigated X70ME pipeline steel

Element content [wt. %]

C

Si

Mn 1,59 max. 1,70

V

Nb

Ti

P

S

Measured

0,07 max. 0,12

0,20 max. 0,45

0,003 max. 0,110

0,040 max. 0,080

0,023 max. 0,070

0,017 max. 0,025

0,004 max. 0,015

Required chemical composition of X70ME steel according to ISO 3183

Pre-charging of the specimens was carried out in high-pressure vessels Parr Instrument, capable of withstanding hydrogen at pressures up to 137 bar. The vessels are type 4555 with an internal volume of 18,75 litres. The vessels are fitted with a special split flange with Teflon o-ring, a set of nozzles, manometer, safety membrane, pressure gauge and PT100 sensor. The pressurization of the hydrogen was performed slowly to avoid excessive heating of the internal volume of hydrogen in the vessel. The pressurization was recorded continuously with a sampling rate of 5 Hz. Upon reaching a pressure of 100 bar, the pressure and temperature recording is performed at intervals, with a 5 second measurement every 5 hours at a recording frequency of 5 Hz. The pre-charging was carried out at room temperature, 100 bar pressure and for 3000 and 6000 hours. The room temperature TT was performed on an Instron 5500R test machine in accordance with ISO 6892-1. The constant strain rate method A1 was used with strain rates of 0,015 mm/mm/min up to a total strain of 0,8% followed by a strain rate of 0,402 mm/mm/min. An Instron 2620-602 extensometer was used to measure the strain. Cylindrical specimens were prepared for TT with a gauge diameter and length of 8 mm and 40 mm, respectively. Two specimens were taken from the pipe in circumferential (T-L) and longitudinal (L-T) orientation. The TT specimens were pre-loaded to 67% of the SMYS. The pre-loaded tensile stress corresponding to an operating pressure of 100 bar. The hoop stress σ θ = 331 MPa was calculated using the Barlow formula and corresponds to an operating pressure (p) of 100 bar, OD and WT, see the equation (1). The pre-loading force corresponding to the tensile stress was calculated according to equation (2). The pre-preloading jig is a relatively simple perforated cylinder that allows access of the hydrogen/methane blend to the elastically pre-loaded tensile specimen, see Fig. 1. (a). ! = 2 . . = 1 2 0 . . 2 1 1 4 , 2 5 2 ≅ 331 (1) = → = ! . = 331. . 4 64 = 16 638 ≅ 16,6 (2) The CVN impact test at 0 °C was carried out in accordance with ISO 148-1 on a Charpy pendulum impact striker PSWO 30 with a capacity of 300 J and a radius of 2 mm. Standardized specimens 55 mm long and of square cross-