PSI - Issue 43

Marie Ohanková et al. / Procedia Structural Integrity 43 (2023) 300–305 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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Finally, fracture surfaces of selected tensile, impact and creep specimens were fractographic analyzed using light and electron microscopy, respectively. ( )( ) ( )( ) ( ) ( )( ) As Sn Sb CEF P As Si P Sb Sn K Mn As X P Sb Sn Si P Sn Sb Mn J Si P Sn J Mn 8.16 3.57 2.43 10 10 5 4 100 10 5 4 10000 * 10000 + + = +  + + + = +  = + + +  = + + +  = + + (1)

3. Results and disscussion 3.1. Embrittlement factors

Table 2 shows the susceptibility of the studied material to embrittlement, where factors J, J* and X, K represent the susceptibility of base and weld metals to thermal ageing embrittlement, respectively. Thereto, CEF represents the susceptibility to creep embrittlement during long-term exposure. As you can see, the weld metal of exposed segment is the most susceptible to ageing embrittlement, but not to creep embrittlement in a comparison to both base metals. In contrary, a non-exposed sample shows the best results in all determined factors. In work of Detemple et al. (2011) is recommended to have J factors of low-alloyed materials below 100 and X-factor below 15 that is actually fulfilled only by the non-exposed sample.

Table 2. A comparison of embrittlement factors of studied material.

Material

J-factor

J*-factor

X-factor

K-factor

CEF

BM1 WM BM2

173 239 151 102

210 337 186 144

17.5 19.6 16.4 14.9

1.3 2.6 1.2 1.1

0.123 0.100 0.115 0.071

NE

3.2. Mechanical testing Results of mechanical testing are graphically presented in Fig. 1.

A comparison of tensile tests and Charpy impact tests of discussed materials, see Fig.1a,b, with the Czech material standard of 15 128 steel showed that non-exposed sample (NE) meets the requirements for yield strength, tensile strength, ductility and notch toughness in the full range, although for strength characteristics (both limits) at room temperature only with a small margin. In contrast, the mechanical properties of exposed sample, such as tensile strength and yield strength, are at the lower limit or just below the tolerance interval. The impact work value at temperature of 20 °C is very low for both base materials and weld metal (less than half of the required minimum value). The transition temperature evaluation for the exposed sample corresponds on average to abo ut +60 ° C, which indicates a really significant embrittlement, while the transition temperature of the non-exposed sample is around 0 ° C. All three parts of exposed sample (BM1, WM, BM2) are brittle at room temperature, see Fig. 1b, but above temperature of +130 ° C they are fully tough. And although they do not achieve such high notched toughness (at + 100 °C around 150 J cm -2 ) compared to the non-exposed sample (at + 100 °C around 300 Jcm -2 ), at the operating temperature, a sufficient value of notched toughness (above 100 Jcm -2 ) safely meeting the standard can be expected. The creep behavior of discussed materials is also slightly different at high stress levels compared to the Czech material standard for 15 128 steel, as can be seen in Fig. 1c. This graph illustrates the stress master curve for 15 128 steel, published also by F ürbacher (2006-), depending on Larson -Miller parameter with a constant specified as 16.9