Procena integriteta i veka zavarenih spojeva mikrolegiranih čelika povišene čvrstoće pri dejstvu statičkog i dinamičkog opterećenja

Abstract

Integritet zavarenih spojeva predstavlja veoma značajan faktor pri eksploataciji zavarenih konstrukcija, usled njihove heterogenosti i sklonosti ka pojavi i rastu prslina, kao posledica samog postupka zavarivanja. Stoga se može reći da zavareni spoj predstavlja najkritičniji deo ovakve konstrukcije, i u slučaju posuda pod pritiskom je neophodno posebno obratiti pažnju na njihov kvalitet. U praksi, u većini slučajeva do otkaza zavarenih (kao i mnogih drugih) konstrukcija dolazi usled zamora, koji je još uvek u određenoj meri neistražena tema u oblasti integriteta konstrukcija. U ovom konkretnom slučaju je analiziran mikrolegirani niskougljenični sitrnozrni normalizovani čelik povišene čvrstoće za posude pod pritiskom, P460NL1. Iako zamor u principu ne predstavlja značajn problem kod posuda pod pritiskom, predmetni čelik se koristi u proizvodnji cisterni za transport amonijaka, što znači da je tokom eksploatacije izložen promenljivom (dinamičkom) opterećenju. Stoga je odlučeno da se ispita otpornost zavarenih spojeva od ovog materijala na nastanak i rast zamorne prsline, kako eksperimentalno, tako i numerički, primenom proširene metode konačnih elemenata. U okviru ovog istraživanja je urađen veliki broj eksperimenata, koji su obuhvatili ispitivanje zatezanjem, savijanjem u tri tačke, merenje temperature tokom zavarivanja, tvrdoće i žilavosti, kao i metalografska i fraktograska ispitivanja, kako bi se detaljno odredile mehaničke osobine materijala i zavarenog spoja, kao i njihove mikrostrukture i njihov uticaj na dobijene rezultate. Pri ovim ispitivanjima su korišćene epruvete isečene iz ploče dimenzija 500x500x14 mm, pri čemu su za odgovarajuća ispitivanja na ovim epruvetama napravljeni zarezi, u zoni uticaja toplote, koja je predstavljala oblast zavarenog spoja na kojoj je bio glavni fokus ovog istraživanja. Ispitivanja su krenula od pretpostavke da su razlike u temperaturi tokom zavarivanja na suprotnim krajevima ploče dovele do različite geometrije samog zavarenog spoja, kao i da se brzina rasta zamorne prsline menjala u zavisnosti od toga kroz koje oblasti je prslina prolazila tokom svog rasta. U tu svrhu su epruvete za ispitivanje žilavosti i zamora (savijanjem u tri tačke) podeljene u četiri grupe, u zavisnosti od kraja ploče iz kojeg su uzete, odnosno od položaja zareza u zoni uticaja toplote. Pored eksperimentalne analize, u ovoj disertaciji je prikazana i numerička simulacija ponašanja epruveta opterećenih savijanjem u tri tačke, uzimajući u obzir različito ponašanje zasebnih oblasti zavarenog spoja (osnovni metal, metal šava i ZUT). Numerički proračun je urađen primenom proširene metode konačnih elemenata (PMKE), koja je upravo i razvijena devedesetih godina prošlog veka sa ciljem da omogući simulaciju rasta zamorne prsline. Simulacije su bile zasnovane na eksperimentalno određenim vrednostima koeficijenata Paris-ove jednačine, C i m za svaku zonu kroz koju je prslina prošla pri svakom ispitivanju. Dobijeni rezultti su poređeni međusobno u odnosu na razlike među samim epruvetama, u pogledu položaja zareza i debljine ZUT.

Structural integrity of welded joints represents a significant factor in welded joint exploitation, due to their heterogeneity and tendency towards crack initiation and propagation, resulting from the welding procedure itself. Hence, it can be said that the welded joint represents the most critical part of such structures, and in the case of pressure vessels, it is necessary to ensure high welded joint quality. In practice, failures of welded (and other) structures are more often than not caused by fatigue, which even today still represents a relatively unexplored topic within the field of structural integrity. In this case, the analysis of a micro-alloyed low-carbon fine-grained normalised high strength pressure vessel steel, P460NL1 is presented. Even though fatigue is generally not a considerable problem for pressure vessels, the material investigated in this research is also used in manufacturing of ammonia transportation tanks, which implies it is subjected to variable (dynamic) load during exploitation. Hence, it was decided to assess the resistance of welded joints made of this material to fatigue crack initiation and propagation, both experimentally and numerically, using the extended finite element method. The researach performed within the scope of this dissertation involved a large number of experiments, including tensile and three-point bending tests, measuring of temperature during welding, hardness and toughness, as well as metalography and fractography tests, in order to determine the mechanical properties of the materials and the welded joints in detail, along with their microstructures and their influence on the obtained test results. Specimens cut out of a welded plate with dimensions of 500x500x14 mm were used for the experiments, whereas certain tests required the making of notches in the specimens, inside the heat affected zone, and this welded joint region was the focus of the research. The experimental tests were based on the assumption that temperature differences that were measured on the opposite ends of the plate during the welding process resulted in different welded joint geometry, along with the assumption that fatigue crack growth rate changes depending on the regions through which the crack passed during its propagation. For this purpose, specimens used in toughness and fatigue (three-point bending) tests were divided into four groups, depending on the part of the plate from which they were taken, as well as the notch position in the heat affected zone. In addition to the experimental analysis, this dissertation also shows the numerical simulation of the behaviour of specimens subjected to three-point bending, taking into account different properties of individual welded joint regions (parent material, weld metal and the HAZ). Numerical calculations were performed using the extended finite element method (XFEM), which was developed in the early nineties for the exact purpose of enabling the numerical simulation of fatigue crack propagation. Simulations were based on the experimentally determined values of Paris law coefficients, C i m, for every region through which the crack propagated during each test. Obtained results were compared to each other, in terms of differences in the specimens themselves, i.e. the notch position and HAZ thickness. Obtained results have indicated good compliance with the experimental ones, which verified the application of extended finite element method in this case.