Micromechanical modeling of brittle transcrystalline fracture in medium-carbon microalloyed steels

[G. Jovanović]

111 str.

Tehničke nauke - Inženjerstvo materijala / Technical sciences - Material Science and Engineering

U radu su ispitivana dva komercijala srednjeugljenična čelika za kovanje mikrolegirana titanom i vanadijumom – TiV-čelik (0,099%V, 0,002%Ti i 0,256%С) i V-čelik (0,123%V,0,011%Ti i 0,309%С). Cilj rada je da modelovanjem dodatno produbi razumevanje i detaljnije opiše mikromehanizam krtog loma ovih srednjeugljeničnih mikrolegiranih čelika sa dominantnom strukturom acikularnog ferita (AF). Mikrostrukture oba čelika su dobijene termičkom obradom na 1250 °C u trajanju od 30 minuta u atmosferi argona, i kontinuiranim hlađenjem na mirnom vazduhu i ispitivane su primenom svetlosnog mikroskopa i skenirajućeg elektronskog mikroskopa. Mehanička ispitivanja savijanjem u četiri tačke i jednoosnim zatezanjem vršena su na temperaturi tečnog azota. Modelovanje je rađeno metodom konačnih elemenata softverom ABAQUS, a kvantitativna analiza mikrostrukture i merenje debljine cementita rađeno je softverom MIPAR. Izmereni udeo cementitnih pločica između iglica AF za V čelik iznosi 3,78% (maksimalne debljine 0,948 μm), dok za TiV-čelik 18,30% (maksimalne debljine 1,109 μm). Prema modelovanim raspodelama napona debljina izmerenih cementitnih pločica u AF nijedan od identifikovanih uključaka nisu utvrđeni kao izvor inicijacije loma. Prosečna vrednost efektivne površinske energije loma V-čelika iznosi 58,5 Jm-2 dok za TiV-čelik iznosi 55,6 Jm-2. Mikromehanizam loma u oba ispitivana čelika može se svesti na dva koraka, u skladu sa Miler-Smitovim mehanizmom: I- Usled velike deformacije pri vrhu zareza dolazi do inicijacije loma stvaranjem inicijalnih mikroprslina lomom cementitnih lamela unutar perlita. II- Nastala mikroprslina lako se rasprostire kroz susedne perlitne kolonije i feritna zrna slične kristalografske orijentacije formirajući početnu mikroprsline kritične veličine.

Two commercial medium carbon forging steels microalloyed with titanium and vanadium – Ti-V- steel (0.099%V, 0.002%Ti and 0.256%S) and V-steel (0.123%V, 0.011%Ti and 0.309%S) were examined. The aim of the work is to deepen understanding and describe micromechanism of brittle fracture in the two medium carbon steels with a dominant structure of acicular ferrite (AF). The microstructure of both steels was obtained by thermal treatment at 1250 °C for 30 minutes in an argon atmosphere, continuous cooling in still air and were examined using light microscopy and scanning electron microscopy. Four-point bending and uniaxial tension tests were performed at liquid nitrogen temperature. Modeling was performed with finite element method using ABAQUS software, and quantitative analysis of microstructure and cementite thickness was done using MIPAR image analysis software. The measured volume fraction of cement plates in AF for V-steel is 3.78% (maximum thickness 0.948 μm), while for TiV-steel 18.30% (maximum thickness 1.109 μm). According to the modeled stress distributions, the thickness of the measured cementite plates in AF, as well any of the identified inclusions, there were not detected to be the source of fracture initiation. The average value of the effective fracture surface energy of V-steel is 58.5 Jm-2, while for TiV-steel it is 55.6 Jm-2. The micromechanism of fracture in both tested steels can be reduced to two steps, according to Miller-Smith's mechanism: I- Due to the large plastic deformation in front of the notch, fracture initiation occurs by creating microcracks by cracking the cementite lamellae inside pearlite. II- The resulting microcrack propagate easily through adjacent pearlite colonies and ferrite grains of similar crystallographic orientation, forming the initial facet of critical size.

Metoda konačnih elemenata, mikromehaničko modelovanje, srednjeugljenični mikrolegirani čelici, čelici za kovanje, acikularni ferit, lom mehanizmom cepanja, krti transkristalni lom, mikromehanizam loma, kritični napon loma

Finite element analysis, micromechanical modeling, Medium carbon microalloyed steels, forging steels, acicular ferrite, cleavage fracture, brittle transcrystalline fracture, fracture micromechanism, critical fracture stress

669.141.3.046.51:539.422.22(043.3)

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