PSI - Issue 13

Evy De Bruycker et al. / Procedia Structural Integrity 13 (2018) 226–231 Evy De Bruycker/ Structural Integrity Procedia 00 (2018) 000 – 000

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(Husemann et al., 1999). The main objective during development of these steels was to create tube steels which could be fabricated without Post Weld Heat Treatment (PWHT). They also showed improved creep strength at higher temperatures, compared to the original 10CrMo9-10 (T22). On the basis of wide-ranging analyses and tests, carried out by various organisations (Vaillant et al. 2005; Bendick et al. 2007) the T24 material has been qualified for Code application. As an outcome of this qualification action, the T24 material and all corresponding procedures and standards have been specified in detail by EN and VdTÜV. In 2010, however, extensive T24 tube weld cracking during the commissioning phase of several newly built boilers was observed. As the dominant root cause, Hydrogen Induced - Stress Corrosion Cracking (HI-SCC) was reported and the evaporator was identified as the most critical area (Ludenbach 2012). HI-SCC is a form of environmental hydrogen embrittlement which results from hydrogen being absorbed by solid metals. The three requirements for hydrogen-induced stress corrosion cracking to occur are: 1. Susceptibility of the material on account of chemical composition, microstructure or surface finish (internal oxide layer) 2. Presence of high tensile stresses, e.g. from strong external load and/or residual stresses (welding) 3. Presence of an aggressive chemical medium (H, possibly in conjunction with a promoter e.g. H 2 S, HF, …) Since hydrogen seemed to play an important role in the T24 failures, an investigation into the interaction of the T24 material with hydrogen under different conditions was launched, in order to compare its hydrogen embrittlement susceptibility with that of the T12 steel commonly used for older boiler evaporators.

Nomenclature BM

Base Material

D b

Apparent diffusion coefficient

HAZ Heat Affected Zone HH High Hardness L Thickness of the material PWHT Post Weld Heat Treatment t b Breakthrough time TDS

Thermal Desorption Spectrometry

2. Experimental set-up 2.1. Material Two materials were compared: the classical T12 ferritic evaporator steel and the advanced T24 ferritic/bainitic evaporator steel. The compositions of the two materials used in this investigation are shown in Table 1. Table 1. Chemical composition in wt% of the investigated material (ICP-LECO for T24, test certificate for T12). Material C Mn Si S P Cr Ni Mo Cu V Nb Al Ti T12 0.120 0.45 0.240 0.004 0.007 0.83 0.07 0.50 0.10 - - 0.011 - T24 0.092 0.56 0.24 0006 0.010 0.58 0.06 0.97 0.05 0.21 0.021 0.024 0.090

Both materials were tested in different conditions:  As delivered base material:

T12 BM T24 BM  Worst case simulation of weld heat affected zone with high hardness: T12 HAZ HH (30' at 1050°C, water quenching, 10' at 600°C) T24 HAZ HH (20' at 1200°C, water quenching)  Simulation of a normal weld heat affected zone: T12 HAZ (30' at 1050°C, water quenching, 20' at 650°C)