Deformation behavior of two continuously cooled vanadium microalloyed steels at liquid nitrogen temperature

The aim of this work was to establish deformation behaviour of two vanadium microalloyed medium carbon steels with different contents of carbon and titanium by tensile testing at 77 K. Samples were reheated at 1250°C/30 min and continuously cooled at still air. Beside acicular ferrite as dominant morphology in both microstructures, the steel with lower content of carbon and negligible amount of titanium contains considerable fraction of grain boundary ferrite and pearlite. It was found that Ti-free steel exhibits higher strain hardening rate and significantly lower elongation at 77 K than the fully acicular ferrite steel. The difference in tensile behavior at 77 K of the two steels has been associated with the influence of the pearlite, together with higher dislocation density of acicular ferrite. [Projekat Ministarstva nauke Republike Srbije, br. OI174004

Structure and fracture of medium carbon microalloyed steel

У раду су испитиванана два средњеугљенична челика за ковање, микролегирана ванадијумом, који се међусобно разликују по садржају титана и угљеника. V– микролегирани челик садржи 0,26%C, означен je као „V19“, и Ti–V– микролегирани челик микролегиран је титаном и садржи 0,31%C, означен као „TiV22“. Циљ рада био је да се испитају трансформационе карактеристике средњеугљеничних микролегираних челика при континуираном хлађењу на ваздуху, директно са температуре аустенитизације, да се одреде услови постизања структуре ацикуларног ферита и да се одреде критични параметри лома цепањем на температури течног азота за ову структуру. Челици су испоручени као топловаљане шипке. Анализом микроструктура које су постигнуте при континуираном хлађењу на мирном ваздуху, директно са различитих температура аустенитизације, одређен је термички режим припреме узорака за испитивање. Припремљени узорци челика са структуром ацикуларног ферита испитивани су на температури течног азота једноосним затезањем и савијањем у четири тачке. За испитивање савијањем у четири тачке коришћене су епрувете са зарезом по Грифитсу и Овену. Температура течног азота одабрана је ради испитивања у условима потпуно кртог лома и одређивања критичног напона лома. Одређивање критичног напона лома обухвата фрактографију и моделовање савијања у четири тачке методом коначних елемената. Резултати једноосног затезног испитивања на температури течног азота послужили су као улазни подаци за моделовање. Моделовањем је израчуната расподела напона и деформација у узорку, у тренутку лома, при савијању у четири тачке. У опсегу температура аустенитизације између 850°C и 1300°C, за припрему узорака за испиттивање изабрана је температура од 1250°C. Након хлађења на мирном ваздуху са 1250°C, структура челика легираног ванадијумом и са нижим садржајем угљеника (челик V19) састоји се од ферита, перлита и ацикуларног ферита, док се структура челика легираног титаном и ванадијумом, са вишим садржајем угљеника (челик TiV22), претежно састоји од ацикуларног ферита...Two medium carbon vanadium microalloyed foring steels with different content of carbon and titanium were investigated. Broad goals were to examine transformational behavior of the steels on continuous cooling at still air directly from austenitisation temperatures, to determine optimal regime to produce predominantly acicular ferrite structure and to examine it both by means of mechanical behavior and fracture properties at liquid nitrogen temperature. Main goal was to establish micromehanism of cleavage fracture of the steels with predominantly acicular ferrite structure, and to measure critical parameters of cleavage fracture – critical fracture stress and effective fracture surface energy. The influence of vanadium and nitrogen on microstructure and mechanical properties of medium-carbon steels has been studied by means of metallography and mechanical testing. Vanadium addition to the low nitrogen steel suppresses the formation of ferrite-pearlite following the low reheating temperatures and microstructure consists of bainitic sheaves. For high reheating temperatures, dominantly acicular ferrite structure in both the low nitrogen and the high nitrogen vanadium steels is obtained. The results suggest that vanadium in solid solution promotes the formation of bainite, whereas the effect of nitrogen is related to the precipitation of VN particles in austenite with high potency for intragranular nucleation of acicular ferrite and to the precipitation of V(C,N) particles in ferrite with high potency for precipitation strengthening. Addition of both vanadium and nitrogen considerably increases the strength level. The aim of this work was to establish deformation behaviour of two vanadium microalloyed medium carbon steels with different contents of carbon and titanium by tensile testing at 77 K. Samples were reheated at 1250°C/30 min and continuously cooled at still air. Beside acicular ferrite as dominant morphology in both microstructures, the steel with lower content of carbon and negligible amount of titanium contains considerable fraction of grain boundary ferrite and pearlite..

Formability of a high-strength Al Mg6.8 type alloy sheet

Room temperature formability testing was performed on an AlMg6.8 type alloy sheet with a fully recrystallized structure (average grain diameter similar to 18 mu m) and after partial annealing with a retained deformed structure. The yield strengths attained after full recrystallization and after partial annealing, were 175 and 283 MPa respectively. Such an increase in strength is followed by formability degradation, maximized around the plain strain state to either 42%, as obtained using the limiting dome height test (LDH), or 35% after using forming limit curves (FLC). A comparison with known high-strength formable alloys has shown that the tested alloy in the recrystallized condition has a better stretch formability (at the same or even higher yield stress level), while in the unrecrystallized-partially annealed condition it has a lower formability, limiting its application to moderate forming requirements for very high-strength parts

Stress state effect on dynamic strain aging and surface markings during stretching of AlMg7 alloy sheet

Stress state effect on dynamic strain aging (DSA) and the related surface appearance was analyzed in uniaxial and biaxial room temperature stretching of AlMg7 alloy sheet. In uniaxial tension the serrated yielding was found pertinent at the entire strain range for the tested material. Stretcher markings, usually noted as A ("flamboyant") type, formed during yield point elongation, while type B ("parallel bands") appeared at higher strains, due to the Portevin Le Chatelier effect. Stretching the tested sheet material over an hemispherical punch, and simulating a different degree of biaxiality, the dynamic strain aging (DSA) appeared to be suppressed by increasing the biaxiality, and accordingly the surface morphology was also changed in a complex manner

Deformation behavior of the AlMg4.5Cu0.5 type alloy sheet

Deformation behavior of the AlMg4.5Cu0.5 type alloy sheets was examined after cold rolling and annealing in the range of temperature from 220°C to 520°C, water quenching and natural aging. The low temperature annealed samples, characterized by fine grained recrystallized structure and high content of Cu-Mg precipitates, have shown a superior hardening ability in uniaxial tension and formability in biaxial stretching. The natural aging didn't bring any important changes of the properties in respect to the as-quenched condition of the tested material

Effect of annealing temperature on the formability of Al-Mg4.5-Cu0.5 alloy sheets

Formability of Al-Mg4.5-Cu0.5 alloy sheets was tested after annealing at different temperatures (220-520 degrees C) and water quenching. The formability (normalized limiting dome heights, LDH/R) showed only a small variation for all the samples with recrystallized structure, i.e. the LDH/R-e(2) curves were within similar to 13% in the range of plane strain deformation. The samples annealed at 280 degrees C experienced a formability level at the upper limit of the band containing all the LDHIR values. The formability rating seems to be the result of the complex interaction of the strain hardening ability and strain rate sensitivity (SRS) due to different structural features developed during different annealing conditions

Experimental and numerical research on the failure of railway vehicles coupling links

Two modes of failure are analysed for coupling links, the mechanical connections that transmit traction forces between vehicles along the length of a train. Coupling links are designed to withstand the dynamic loads in normal operation but may break if applied loads exceed safe operating limits. Fractography of two failed links revealed that one had failed through brittle fracture, and one through ductile fracture. Metallography indicated that one sample was hardened and tempered with typical tempered martensite microstructure and the other was normalized with a ferrite-pearlite microstructure. A numerical model of a coupling link is developed and validated against experimental data. Results indicate that the failed links experienced loads above the permitted limit

Principal component analysis of morphological descriptors for monitoring surface defects induced by thermal shock

Pattern recognition techniques are applied to various morphological descriptors to monitor the formation and propagation of surface defects of materials subjected to thermal shock. A low-cement high-alumina castable was synthesized, cured, sintered, and exposed to thermal stability testing using the water quench test. After a certain number of thermal shock cycles, photographs of the samples' surfaces were taken and subjected to image analysis. The influence of the sintering temperature on the morphology of the detected defects was studied using principal component analysis (PCA) as a pattern recognition technique that is the most informative for extracting possible differences. The morphological descriptors of the defects correspond to the previous results regarding the influence of sintering temperature on the structure of a castable during thermal shocks