The impact of the welded joints made of X8CrNiTi18–10 stainless steel on the reliability estimation of pipes

The chrome-nickel stainless steels of austenitic class applied in chemistry and energy industry are often used in the production of exceptional structures employed in an environment aggressive and dangerous for human life. Therefore, it is particularly significant for durability and reliability requirements. The article explores cracks that appeared in a tube made of X8CrNiTi18–10 austenitic steel. The examined pipe has worked for 90 000 hours under high temperature and pressure in an aggressive media. To establish reasons for developed cracks, chemical composition, strength, hardness testing and metallographic structures have been examined. The performed investigations will prevent from potential emergencies and help with a more accurate assessment of the pipes made of particularly this class of steel in order to ensure operational reliability and durability in the future. Article in Lithuanian. Nerūdijančiojo plieno X8CrNiTi18–10 suvirintųjų jungčių įtakos vamzdžių eksploataciniam patikimumui tyrimas Santrauka. Chemijos ir energetikos pramonėje iš austenitinės klasės chromnikelinių nerūdijančių plienų dažnai gaminamos svarbios konstrukcijos, naudojamos agresyviose ir pavojingose žmogaus gyvybei aplinkose, todėl joms keliami ypač aukšti ilgaamžiškumo ir patikimumo reikalavimai. Pateikiamas iš chromnikelinio austenitinės klasės X8CrNiTi18-10 plieno vamzdyje atsiradusių įtrūkių tyrimas. Šis vamzdis 90 000 valandų buvo naudotas agresyvioje terpėje, veikiant aukštai temperatūrai ir slėgiui. Įtrūkių atsiradimo priežastims nustatyti buvo atlikti cheminės sudėties, stiprumo, kietumo bandymai ir metalografiniai struktūrų tyrimai. Šie bandymai ir tyrimai padės užkirsti kelią galimoms avarijoms, tiksliau prognozuoti ir įvertinti vamzdžių, pagamintų iš šios klasės plieno, eksploatacinį patikimumą ir ilgaamžiškumą ateityje. Reikšminiai žodžiai: X8CrNiTi18–10, įtrūkiai, austenitinis plienas

The structure and properties of 5 % Cr-0.5 % Mo steel welded joints after natural ageing and post-weld heat treatment

The article deals with the ageing and heat treatment influence on operational reliability of 5 % Cr-0.5 % Mo steel welded joints and pipeline elements. Separate components of the pipelines in the refineries are manufactured from chromemolybdenum 5 % Cr-0.5 % Mo alloy steel. Reduction of internal stresses in chrome-molybdenum steel welded joints can be provided only by thermal treatment. Other methods of stress relieving are not acceptable for these steels. Presented materials analyses the impact of heat treatment on the microstructure and operational reliability of this steel. The objects of research are heat-treated welded joints of piping elements operated at high temperature for an extensive period of time, where degradation of mechanical properties has been observed. When value of the heat treatment temperature or time are exceeded, the structure degradation process is taking place, carbides coagulate within the boundaries of ferrite grains, they form coarse carbide colonies combined into long chains. Mechanical properties – tensile strength and impact strength decrease. Detailed analysis of these objects and the interpretation of the received results allow to select the most appropriate heat treatment parameters for the investigated steel structures

The structure and mechanical properties of the high chromium and nickel content cast alloy after long duration work in high temperature

In the petrochemical industries cast steel tube coil sheets with high chromium and nickel content are used. The operation range of these alloys are 533 to 1150°C. A paramount for these alloys are: - corrosion and scaling re-sistance at high temperatures; - dimensional stability, i.e. resistance to warping, cracking and thermal fatigue; - resistance to plastic flow, i.e. creep strength. Tube coil sheet of furnace resulting in aggregate failures have been con-sidered. Life span of the studied specimens obtained from tube coil sheet was 3 years. After a few years of exploita-tion several hollow cracks on the heater tube-sheet were identified. Pre-oxidation of the tubes surface are possible and it has not only favourable effects: during cooling cracks may form in the oxide layer. The aim of the per-formed research was to characterize the microstructure and mechanical properties of the investigated alloy after ageing at the temperature of 900°C. The high temperature determine elements diffusion in the structure, emergence of chromium and niobium carbides and other precipitates and cause the changes in the mechanical properties