Elastic-plastic behavior of high strength steel welded joint under static and dynamic loading

Abstract

Welding is one of the most common process in industrial or production engineering practice for components joining. Its main advantages are high speed in manufacturing combined with low costs and, usually, a high degree of flexibility, integrity and reliability. Nevertheless, welding is a highly complex metallurgical process and, therefore, weldments are susceptible to cracking, flaws and residual stresses which may lead to structural failure and life time reduction. Welded structures have wide range of applications in many industries. Steel grade DIN 15NiMoCrB4-5, which is a low-carbon microalloyed steel alloyed with Nickel and Molybdenum, which is used in line pipes for transporting oil and natural gas has been investigated. Its yield stress is 668 MPa and the UTS are close to 820 MPa. Catastrophic structure failure made of this steel grade due to fatigue is reported to cause sudden damage and loss to human-lives, as well as property. Hence, from end application point of view, knowledge on fracture toughness is of immense importance. Several tests were conducted on the welded joints. Fracture mechanics toughness testes using three point bending specimens were done. Obtained results of the applied force and CMOD were processed in order to determine the value of KIC, CTOD or J-integral. The pre-crack was created using fatigue and the crack propagation was measured using the unloading compliance method. Calculation of critical values ��� are compared with the values obtained by using FEM. The FEM model was defined on the basis of experimental condition in program Ansys. The goal of this research was to determine effect of welded joint geometry (different zone sizes) and crack location on fatigue life. First welded joints were made using the MIG procedure and investigated in terms of mechanical and fatigue properties. The focus of this study was on experimental and numerical analysis, with special focus on the heat-affected zone.

Zavarivanje je jedan od najčešćih procesa spajanja komponenata u industrijskoj ili proizvodnoj praksi. Njegove glavne prednosti su brzina izrade kombinovana sa niskom cenom i obično sa visokim stepenom fleksibilnosti, integriteta i pouzdanosti. Bez odzira na to, zavarivanje je izuzetno složen metalurški postupak i prema tome, zavareni spojevi su osetljivi na stvaranje prslina, grešaka i zaostale napone, što može dovesti do otkaza konstrukcije i skraćenja veka. Zavarene konstrukcije imaju široku primenu u mnogim granama industrije. U ovom radu je ispitan čelik 15NiMoCrB4-5 čvrstoće popuštanja 668 MPa i zatezne čvrstoće blizu 820 MPa. Ovaj materijal je niskougljenični mikrolegirani čelik legiran niklom i molibdenom, koji se koristi za izradu cevovoda za transport nafte i prirodnog gasa. U takvoj primeni, katastrofalni lom izazvan zamorom uzrokuje iznenadna oštećenja imovine i pogibiju ljudi. Stoga je, sa stanovišta krajnje primene, poznavanje žilavosti loma od ogromne važnosti. Urađena su eksperimentalna ispitivanja na zavarenim spojevima. Od ispitivanja mehanike loma urađeno je ispitivanje žilavosti loma na epruvetama za savijanje u tri tačke. Obrađeni su rezultati izmerene primenjene sile i CMOD da bi se utvrdila vrednost KIC, CTOD ili J-integrala. Stvaranje zamorne prsline i propagacija prsline su merene metodom popuštanja. Izračunate kritične vrednosti, JIC, su upoređene sa vrednostima dobijenim pomoću MKE. Model MKE je definisan na bazi eksperimentalnih uslova u programu Ansys. Cilj ovog istraživanja je bio da se utvrdi uticaj geometrije zavarenih spojeva (različite veličine zona) i položaja prsline na zamorni vek. Najpre su zavareni spojevi napravljeni MIG postupkom i ispitani su u pogledu mehaničkih osobina i osobina zamora. Fokus izučavanja je bio na eksperimentalnoj i numeričkoj analizi, sa posebnim osvrtom na zonu pod uticajem toplote.