Nanostructured bainite-austenite steels are applied in the armours construction due to their excellent combination of strength and ductility which enables to lower the armour weight and to improve the protection efficiency. Mechanical properties of the bainite-austenite steels can be controlled in the wide range by chemical composition and heat treatment. In the paper the results of investigation comprising measuring of quasi -static mechanical properties, dynamic yield stress and firing tests of bainite-austenite steel NANOS-BA R are presented. Reported results show that the investigated bainite-austenite steel can be used for constructing add-on armour and that the armour fulfils requirements of protection level 2 of STANAG 4569. Obtained reduction in weight of the tested NANOS-BA R plates in comparison with the present solutions is about 30%. Keywords: nanostructured steel, bainite-austenite steel, armours, mechanical properties Nanostrukturalne stale bainityczno-austenityczne stosowane do konstrukcji osłon balistycznych ze względu na znakomitą kombinację wytrzymałości i ciągliwości umożliwiają obniżenie masy własnej osłon i podwyższenie ich skuteczności ochronnej. Właściwości mechaniczne stali bainityczno-austenitycznych mogą być kontrolowane w szerokim zakresie poprzez modyfikację składu chemicznego i parametrów obróbki cieplnej. W artykule przedstawiono wyniki badań właściwości mechanicznych wyznaczanych w testach statycznych i dynamicznych oraz wyniki prób przestrzeleniowych. Przedstawione wyniki badań wskazują, że stale bainityczno-austenityczne (NANOS-BA R ) mogą zostać wykorzystane do konstrukcji opancerzenia o masie własnej mniejszej o 30% w stosunku do rozwiązań stosowanych obecnie dla wymaganego 2 poziomu ochrony według STANAG 4569.
Intense research on the metallurgy of iron alloys have recently resulted in the development of technologies enabling the making of high-carbon nanobainitic steels. Because of their chemical composition, the above-named steels belong to hard-to-weld materials. To identify the possibility of welding such steels using arc-based methods and welding in the solid state it was necessary to make test joints and perform related metallographic tests. The test results revealed that it is possible to obtain both fusion and pressure welded joints if the process was performed in the softened state, i.e. before the final heat treatment. Afterwards, joints should be subjected to a heat treatment ensuring the obtainment of a previously assumed microstructure and required hardness distribution in the weld and HAZ.
The age hardening behaviour of MS350 maraging steel both in undeformed specimens and in specimens deformed immediately after ageing was studied. Experiments of short-time (15, 30 and 60 s) ageing at temperature up to 600• C were performed. It was found that after ageing at 480-500• C for 15-60 s without deformation the hardness reached the 450-525 HV. The time of ageing at 600• C to reach hardness 600 HV was 15 seconds. No precipitation in specimen aged at 500• C for 60 s was found in TEM and HRTEM investigations. Deformation immediately after ageing intensifies the process of hardening caused by temporary increase of temperature and increase of dislocation density. The hardness of specimens aged for 30 s and then deformed at 480-550• C was similar to the hardness at peak aged conditions (480 • C/4h) and reached 600-630 HV. Material produced in industrial trials of cold forward flow forming and following laboratory ageing was analyzed. Increase of hardness from 370 HV to 590-630 HV for specimens taken from thin walled flow formed tube as a result of ageing at 460-490• C for 30 minutes occurred.Keywords: maraging steel, short-time ageing, flow forming, precipitation kinetics, HRTEM Przedstawiono wyniki badań procesu starzenia stali maraging w gatunku MS350 dla próbek nieodkształconych i odkształ-conych bezpośrednio po starzeniu. Przeprowadzono eksperymenty starzenia krótkotrwałego (15, 30 i 60 s) w temperaturze do 600• C. Stwierdzono, że po starzeniu w zakresie temperatury 480-500• C w czasie 15-60 s bez odkształcenia, twardość osiągnęła 450-525 HV. Czas starzenia w temperaturze 600• C do osiągnięcia twardości 600 HV wynosił 15 s. Nie stwierdzono wydzieleń w próbkach starzonych w temperaturze 500• C przez 60 s w badaniach z zastosowaniem wysokorozdzielczego transmisyjnego mikroskopu elektronowego. Odkształcenie bezpośrednio po starzeniu intensyfikuje proces umacniania wydzieleniowego w skutek chwilowego wzrostu temperatury oraz wzrostu gęstości dyslokacji. Twardość próbek starzonych przez 30 s i odkształ-canych w zakresie temperatury 480-500• C była zbliżona do twardości uzyskiwanej dla warunków starzenia w celu uzyskania twardości maksymalnej (480• C/4godz.) i wynosiła 600-630 HV. Przeprowadzono badania materiału wytwarzanego w próbach przemysłowych zgniatania obrotowego i starzonego w warunkach laboratoryjnych. Dla próbek pobranych z rury cienkościennej uzyskano wzrost twardości z 370 HV do 590-630 HV w wyniku starzenia w zakresie temperatury 460-490• C przez 30 minut.
The paper presents results of examinations of properties and microstructure of maraging steel grade MS350 (18Ni350) produced by a novel heat treatment method called "short-time ageing". It has been found that maraging steel after application of the short--time ageing achieves unique properties, in particular good combination of strength and impact toughness. After short-time ageing for time of heating up to 600 seconds at temperature of 550°C hardness in the range from 48 to 56 HRC, tensile strength ranging from 2000 to 2250 MPa, yield strength from 1930 to 2170 MPa and total elongation in the range 7-8% as well as notch impact toughness of 20 J/cm 2 at temperature minus 40°C were obtained. Results of microstructure examination in transmission electron microscope (TEM) with application of high resolution technique (HRTEM) have shown presence of Ni 3 Mo nano-precipitates of orthorhombic crystallographic structure. Precipitates were characterized by rod-like shape and were homogenously distributed in martensitic matrix of steel with high density of dislocations. The average size of cross-section of precipitates was ca. 4 nm while length reached several dozen of nm.
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