PSI - Issue 68

Birhan Sefer et al. / Procedia Structural Integrity 68 (2025) 1129–1139 Sefer et al. / Structural Integrity Procedia 00 (2025) 000–000

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pass through the exhaust. Thus, understanding of the influence of H 2 on the mechanical degradation of SiMo51 material is still relevant and knowledge and understanding are expected to be of significant value. The aim of this work was to study the influence of highly pressurized H 2 on the mechanical degradation at room and elevated temperatures of conventional metallic materials, commonly used in diesel internal combustion engine and exhaust components. In that regard, slow strain rate tests (SSRT) were conducted using hollow specimen method at 200 bar pressure in argon and H 2 environment. To understand the influence of temperature and to compare the behaviour of the studied material SSRT tests were carried out at room (RT) as well as high temperature (HT) relevant to the service conditions of the respective materials. The SSRT was combined with fractographic and hydrogen analysis with goal to reveal whether hydrogen/material interaction and hydrogen absorption occurred as well as to identify/verify whether hydrogen embrittlement was present. 2. Materials and Methods Three materials were studied, NCF3015 Fe-Ni-base alloy in form of bar in hot rolled condition, compact graphite iron (CGI), and SiMo51 ductile iron, both in cast condition. The NCF3015 bars where heat treated in vacuum furnace in two steps: 1) heated to 1050 °C and hold for 30 minutes and then followed by oil quenching; 2) reheating to 750 °C, hold at temperature for 4 hours, then air cool. Table 1 shows the nominal chemical composition in wt.% of the studied materials and Table 2 shows the typical values for yield strength (Rp0.2), tensile strength (Rm) and elongation of the materials at room temperature and high temperature.

Table 1. Nominal chemical composition in wt.% of the studied materials. Element (wt.%) C Si Mn Ti Cr

Cu

Ni

Al

Mo

Fe

SiMo51

3.0-3.5 3.6-3.9

4.2-4.8 1.7-2.4

<0.4 <0.5 <0.5

- -

<0.1 <0.1

<0.1

< 0.1

- -

0.8-0.12

Bal. Bal. Bal.

CGI

0.5-1.0

-

-

NFC3015

<0.08

<0.5

2.3-2.9

13.5-15.5

-

30.0-33.5

1.6-2.2

0.4-1.0

Table 2. Tensile properties of the materials at room and high temperature. Data provided by end users. Material Temperature (°C) Rp0.2 (MPa) Rm (MPa) Elongation (%) SiMo51 23 535 ± 4 647 ± 20 7±3 800 30 50 - CGI 23 329±4 445±6 2±0.3 300 264 358 2 NFC3015 21 655 1124 35 650 676 952 17

The microstructures of the materials are shown in Figure 1. The cast SiMo51 material consists of ferritic matrix with spheroidal graphite nodules. The cast ductile iron CGI has a similar microstructure consisting of graphite particles with different size and orientations embedded in ferritic matrix. Finally, the microstructure of the NCF3015 material was γ matrix with precipitation hardening phase γ' which consisted of Ni 3 (Al, Ti, Nb) precipitates providing the material high temperature strength and creep resistance as well as Nb- and/or Ti-rich carbides from M 23 C 6 type.