No abstract

To obtain the superior-high-temperature creep strength, a transformation of a fine-grained structure to large grains due to abnormal grain growth or recrystallization is an important process in oxide-dispersion-strengthened (ODS) alloys. The processing of steel is enabled with powder metallurgy, which utilizes powders consisting of a Fe-Al metal matrix with a large O content, prepared with mechanical alloying, and their hot consolidation due to rolling. The thermomechanical characteristics of new ODS alloys with a Fe-Al matrix are investigated in terms of the changes in the grain-size distribution. The recrystallization and grain growth were quantified after heating up to 1200°C, which is the typical consolidation temperature for standard nanostructured ferritic steels. The results show that new ODS alloys are significantly affected by the thermomechanical treatment leading to microstructural changes. Recrystallization is mostly affected by decreasing the deformation and increasing the holding time, which leads to a growth of the grain size. Keywords: grain growth, ODS alloys, steel, Fe-Al, Al2O3Da bi pridobili najvi{jo temperaturo mo~i lezenja, je transformacija drobnozrnate strukture v ve~ja zrna ali celo v abnormalno velika oziroma rekristalizacija pomemben proces pri zlitinah, oja~anih z oksidno disperzijo (angl. ODS). Obdelava jekla je omogo~ena z metalurgijo v prahu, ki pomaga prahom, ki vsebujejo metalno matrico Fe-Al z veliko vsebnostjo kisika, pripravljeno z mehanskim legiranjem in njihovo vro~o konsolidacijo pri valjanju. Termomehanske karakteristike novih ODS-zlitin s Fe-Al matrico so bile preiskovane, ker je pri{lo do sprememb v velikosti porazdelitve zrn. Rekristalizacija in rast zrn sta bili ovrednoteni po segretju do 1200°C, ki je tipi~na temperatura konsolidacije za nanostrukturirana feritna jekla. Rezultati ka`ejo, da na ODS-zlitine zelo vpliva termomehanska obdelava, ki pripelje do sprememb v mikrostrukturi. Na rekristalizacijo najve~krat vpliva deformacija in pove~anje~asa zadr`anosti, ki povzro~i rast velikost zrn.

The toughness of high-strength steels is improved when their martensitic matrix is combined with the presence of retained austenite. The stability of retained austenite is crucial, both upon cooling to room temperature and during subsequent cold deformation. One of the advanced heat treatment methods, which produces larger amounts of retained austenite in martensitic matrix thanks to an appropriate combination of parameters, is the Q&P process (Quenching and Partitioning). It produces microstructures consisting of martensite and retained austenite which exhibit strengths above 2000 MPa and elongation levels of more than 10%. The steel which was designed for this experiment was alloyed with manganese, silicon, molybdenum and chromium and had a low Ms temperature. This experimental heat treatment led to a martensitic microstructure with retained austenite, with ultimate strengths in the range of 2200-2400 MPa and A 5mm elongations of 7-10%. The largest fraction of retained austenite, according to X-ray diffraction analysis, was 10%. The behaviour of retained austenite under mechanical load was studied by means of compression tests to various strain levels. The resulting microstructures were observed using optical and scanning electron microscopes..

<p class="AMSmaintext"><span lang="EN-GB">Various ways are sought today to increase mechanical properties of steels while maintaining their good strength and ductility. Besides effective alloying strategies, one method involves preserving a certain amount of retained austenite in a martensitic matrix. The steel which was chosen as an experimental material for this investigation contained 2.5% manganese, 2.09% silicon and 1.34% chromium, with additions of nickel and molybdenum. An actual closed-die forged part was made of this steel. This forged part was fitted with thermocouples attached to its surface and placed in its interior and then treated using the Q&amp;P process. Q&amp;P process is characterized by rapid cooling from a soaking temperature to a quenching temperature, which is between the Ms and the Mf, and subsequent reheating to and holding at a partitioning temperature where retained austenite becomes stable. The quenchant was hot water. Cooling took place in a furnace. Heat treatment profiles were constructed from the thermocouple data and the process was then replicated in a thermomechanical simulator. The specimens obtained in this manner were examined using metallographic techniques. The effects of cooling rate on mechanical properties and the amount of retained austenite were assessed. The resultant ultimate strength was around 2100 MPa. Elongation and the amount of retained austenite were 15% and 17%, respectively. Microstructures and mechanical properties of the specimens were then compared to the real-world forged part in order to establish whether physical simulation could be employed for laboratory-based optimization of heat treatment of forgings.</span></p>

<p>The requirements placed on closed-die-forged parts of advanced steels have been increasing recently. Such forgings demand an innovative approach to both design and heat treatment. It is important to obtain high strength and sufficient ductility in closed-die forgings. High strength, mostly associated with martensitic microstructure, is often to the detriment of ductility. Ductility can be improved by incorporating a certain volume fraction of retained austenite in the resulting microstructure. Among heat treatment processes capable of producing martensite and retained austenite, there is the Q&amp;P process (Quenching and Partitioning). This process is characterized by rapid cooling from the soaking temperature to the quenching temperature, which is between Ms and Mf, and subsequent reheating and holding at the partitioning temperature. Thus, strength levels of more than 2000 MPa combined with more than 10% elongation can be obtained. This experimental programme involved steels with 2.5% manganese. Forgings of these steels were heat treated using an innovative process in order to obtain an ultimate strength of more than 2000 MPa combined with sufficient elongation. Thanks to a higher manganese level, the Mf was depressed as low as 78°C, and therefore quenching was carried out not only in air but also in boiling water. Holding at the partitioning temperature of 180°C, when carbon migrates from super-saturated martensite to retained austenite, took place in a furnace. The effects of heat treatment parameters on the resulting mechanical properties and microstructure evolution in various locations of the forging were studied.</p>