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

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H. Pitcher / Hydrogen corrosion of pressure-vessel steels

of an ammon i a synthesis facility was dest royed after 10 months of service as a result of the burst ing tube plate chamber . The pressure vessel was 17 200 mm in length, 1200 mm in inside diameter in the area of the tube plate chamber , and 175 mm in chamber wall thickness. The parts that had sustained damage were made from the quenched and tempered 24 CrMo 10 steel that was resistant to hydrogen at tack at elevated temperatures and pressures; pr ior to put t ing the 15 Mo 3 cool ing tubes in place by welding, the 24 CrMo 10 steel under- went double- layer weld cladding wi th a filler metal compat ible wi th the tube steel. The cracks or iginated at the highly stressed junc t ion between the tube plates and the chamber shell, i.e. they started from incipient cracks which al ready existed pr ior to the start-up of the cooler dur ing bui ld-up welding and were not detected by non-dest ruct ive test ing dur ing its manufacture. Internal stresses in addi t ion to al terna- ting loads as a result of changes in service pressure caused plastic flow to occur in areas of already existing incipient cracks. Exist ing in the synthet ic gas made by the Habe r -Bo s ch process at a part ial pressure of roughly two- thi rd of the service pressure which in the present case was up to 300 bar, molecular hydrogen dissociated at the active surfaces and after absorpt ion strain-in- duced HSCC occurred.

suscept ible to attack. In connec t ion wi th safety mea- sures in nuclear power plants laboratory tests on welded samples have conf i rmed that cracking may occur in high- temperature deionized water, i.e. in the absence of promotor s [14,15]. It is in par t icular the hardened coarse-grained area of the heat -af fected zone in its as-welded condi t ion that shows susceptibility. Cracking can be avoided if the welds are stress relieved. In case of st rain- induced HSCC also the oxygen-content of the high- temperature water is a decisive parame t er [16]. Af t er one year of service, a wide crack 530 mm in length was detected in the heat -af fected zone of a weld in a power plant boi ler drum. The drum was made from the heat-resistant 13 MnN iMo 54 structural steel; its wall thickness was 28 ram, the drum was stress rel ieved after fabricat ion. In accordance wi th rules and regu- lations, the drum contained condi t ioned feed water at 257 ° C and 44 bar. Metal lographic and f ractographic tests revealed that the crack or iginated in the heat -affected zone of a repaired ci rcumferent ial weld (fig. 14). Inadver tent ly, this area was not stress relieved, thus leaving a maxi- mum hardness of 400 HV 10 in the heat -affected zone. Unde r cor rosion condi t ions, the incipient crack grew unt i l approximately 3 mm in residual wall thickness were left. At the uncleaned fracture surface, up to the zone near the residual fracture, there are clearly def ined layers of magnet i te with the typical pat tern of oc- tahedrai shape. The fracture st ructure of the cleaned surfaces presents the pat tern of a low-duct i l i ty quasi- cleavage fracture with secondary cracks which differs great ly f rom the duct i le pat tern of the final dimpled rupture zone. The damaged par t was proper ly repai red and subsequent ly stress relieved. A simi lar probl em occurred in a boi ler vessel, of which the metal lographic pictures are shown in fig. 15. The damaged vessel had 1400 mm in external diameter , 14 mm in wall thickness, and 4500 mm in cyl indrical length. It was made f rom a Ni -V- a l l oyed f ine-grained

8. HSCC in hot water

In steel vessels containing high temperature water, e.g. feedwater vessels in nuclear and convent ional power plants, magnet i te is formed according to the overal l react ion

3Fe + 4H20 ~ Fe304 + 8H.

Hydrogen or iginat ing in this process can cause HSCC provided that the steel concerned presents a structure

(c) magnetite octahedras at the crack

Fig. 14. HSCC in boiler drum; (a) macrograph of the surface, (b) transverse rmcrosection, surface.