Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

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H. Pireher / Hydrogen corrosion of pressure-vessel steels

At climatic temperatures and under mechanical load with no plastic deformation, such hydrogen absorption from aqueous solutions under service conditions as tend to endanger the pressure-vessel material will generally occur only in the presence of promotors. Hydrogen absorption from gases within the aforementioned range of temperatures presupposes a very clean and active steel surface which does not exist in technical construc- tional parts. Catalytic effects such as those caused by palladium, for instance, can mostly be excluded under technical service conditions. Clean and active surfaces are likely to be brought about by creeping under alternating mechanical loads or in notches. Under plastic deformation with a low strain rate, hydrogen is picked up from all media able to offer hydrogen. Here, no promotors are required. The method mostly used to determine the HSCC resistance of a steel is to expose stressed tensile speci- mens in a suitable electrolyte solution at ambient tem- perature, for instance in accordance with NACE stan- dard TM-01-77 [10]. Because of the relatively small amount of equipment required, HSCC testing of steels and welds is often done on bend test specimens as well. Fig. 7 shows test results obtained from weldable structural and tube steels. Plotted on the x axis is the yield strength of the materials and on the y axis the threshold stress for HSCC after 720 hours of testing [5]. The steels can be divided into two strength categories which present a different behavior with respect to the HSCC resistance. For steels with a yield strength of up to 650 N / mm 2, the critical stress increases as the yield strength rises. Under the stringent test conditions, threshold stresses from 40 to 90% Rel have been de- termined. For high-strength steels with a yield strength above 650 N / mm 2, an inverse trend was noted. Here, the threshold stress for HSCC decreases as the yield strength rises, i.e. the steels become more susceptible to HSCC. In connection with the HSCC susceptibility of welds fig. 8 provides test results obtained from three-point bend test specimens. The specimens were taken from the top layer of welded joints and from single-pass welds in different types of steel plate. The test results are plotted against the maximum hardness of the heat- affected zone. Differentiation is made between speci- mens that present cracking in the heat-affected zone during the 400 h test period, and those that passed the corrosion test with no HSCC or with cracking of the 6. Promotor-enhanced HSCC at ambient temperature

Fig. 6. Hydrogen-induced cracking (HIC); (a) Blisters, (b) Stepwise cracks [11].

Metallurgical measures to be taken to improve the resistance to HIC are aimed at lowering the contents of sulphur and oxygen in steel, and at controlling the inclusion shape. In this respect, paramount importance is attached to the ladle treatment using calcium, a metallurgical process developed in the late sixties [7,8]. Further steps relate to the avoidance of hard banded structures by way of reducing the carbon and manganese contents or by preventing excessive carbon segregation during the cooling from the final austenizing of the steel. With a view of furnishing evidence of an increased resistance of steel to HIC, exposure tests are generally carried out in an H2S-containing acid medium. Subse- quent to an exposure which mostly lasts for 96 h, the test specimens are thoroughly checked for blistering and internal cracking. The method most frequently used is described in the NACE standard TM-02-84 [9]. Apart from the absorption of hydrogen by the steel, hydrogen-induced stress corrosion cracking (HSCC) also presupposes the existence of tensile stresses of a critical magnitude which depends upon the amount of hydro- gen absorbed. Cracking is preferably oriented in a direc- tion perpendicular to the direction of principal stress. Stress-induced HSCC exists whenever the hazard is exclusively the result of exceeded threshold tensile stress, in contrast to strain-induced HSCC where plastic defor- mation within critical strain rates is required. 5. Hydrogen-inducedstress corrosion cracking