Influence of microstructure on SSC and SOHIC in base materials and welded joints of high strength sour service steels

The main objective of the project is to assess the effect of relevant metallurgical parameters on the resistance to Sulphide Stress Cracking (SSC) and Stress-Oriented Hydrogen Induced cracking (SOHIC) of welded joints in spiral pipes, linepipes and pressure vessels, throughout the validation of laboratory testing methodologies Because many different rolling parameter combinations were used while rolling the X60 trial coils, a toughness investigation was made about the effects of the rolling parameters on the toughness properties. Trial coils of both Salzgitter and Rautaruukki were used in the investigation. In principle, two different rolling practices were used. In one main group the reheating temperature, roughing finishing temperature, strip rolling temperature and coiling temperature and coiling temperature were lower than in the other main group. As results of two different practices grain size level was smaller in the "low temperature-case". So that the main conclusion of the investigation was that DWTT and Charpy V toughness were much better using the lower temperature practices. As a contrary to the good toughness results, the low rolling temperature possibly is one reason for the HIC cracks found close to the surface of the base material and especially close to the spiral weld of the pipes rolled using the lower temperature practice. The cracks were very rare in the pipes rolled and coiled at higher temperature generally in the rare case HIC cracks (of the base material) are always close to the centreline. With reference to the results of full ring tests and four point bend tests (at 72% and 95% AYS) using samples out of base material and of girth wells no cracks were found in any full ring tests. SOHIC behaviour of the pipes was evaluated by pulsating tests because in the gas lines pulsating gas pressure also has an effect on the stress/strain situation in large-diameter pipes and can end in pipe collapse. The results demonstrate reducing the number of cycles to breaking by higher times for loading and relieve, independent of strain level. On the other hand the time under load (to breaking) increases by higher times for loading and relieve. Concerning the grade X80 steel, after mechanical and microstructural characterization, on account of the indifferent HIC results further investigations with this material were aborted after consultation with the other project partners. Considering the H 2 S corrosion behaviour of the Q&T steels selected by CSM no cracks were observed after HIC tests on parent materials and on GMAW and SMAW girth welds. The SSC tests with constant load results (according NACE TM 01-77 method A) are good on parent material and on GMAW welded joints (until 90% AYS), for both steels. SMAW welded joints didn't fail until 70% AYS in steel with lower content of chromium, nickel and molybdenum, and until 80% AYS in the other steel. SSC tests according to NACE TM0177 Method B didn't fail until 100% of parent material A.Y.S on both parent materials and welds. SCC results by pulsating tests demonstrate that both parent materials didn't fail until 100% AYS, in all loading time conditions. SMAW welded joints of steel with lower content of chromium, nickel and molybdenum didn't fail until 80% AYS with all loading time conditions (until 100% AYS in the other steel).