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Tensile properties and fracture mechanism of a polycrystalline IN-100 superalloy have been investigated in the range from room temperature to 900 degrees C. Optical microscopy (OM) and transmission electron microscopy (TEM) applying replica technique were used for microstructural investigation, whereas scanning electron microscopy (SEM) was utilized for fracture study. High temperature tensile tests were carried out in vacuumed chamber. Results show that strength increases up to 700 degrees C, and then sharply decreases with further increase in temperature. Elongation increases very slowly (6-7.5%) till 500 degrees C, then decreases to 4.5% at 900 degrees C. Change in elongation may be ascribed to a change of fracture mechanism. Appearance of a great number of microvoids prevails up to 500 degrees C resulting in a slow increase of elongation, whereas above this temperature elongation decrease is correlated with intergranular crystallographic fracture and fracture of carbides.

Cleavage fracture of the medium carbon V-microalloyed steel with structure consisting of acicular ferrite, pearlite and grain boundary ferrite has been investigated by means of four-point bending of the notched samples at -196 degrees C. It was found that cleavage fracture initiation has not been related to the coarse second phase particles cracking. Calculated values of the effective surface energy of 49 J/m(2) and critical cleavage fracture stress normalized by yield stress of 1.84 are in agreement with the results for the steels with ferrite-pearlite and bainite structures.

The aim of this work was to establish the deformation behavior 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 for 30 min and continuously cooled in still air. Beside acicular ferrite as the 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 a 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., Čelici mikrolegirani vanadijumom i titanom razvijani su sa ciljem da zamene kaljene i otpuštene srednjeugljenične čelike. Iako ove čelike karakteriše dobra kombinacija čvrstoće i žilavosti, s druge strane zahtevaju naknadnu termičku obradu, koja značajno povećava troškove proizvodnje. Tehnologija dobijanja kontinuirano hlađenih čelika treba da obezbedi mikrostrukturu zahtevanih osobina posle hlađenja na vazduhu. Iz tog razloga je ovim čelicima promenjen sastav, a dobre osobine se obezbeđuju prisustvom acikularnog ferita. Acikularni ferit karakteriše nukleacija unutar prethodnog austenitnog zrna i velika razlika u kristalografskoj orijentaciji pojedinih zrna, odnosno feritnih pločica. Ova isprepletana struktura obezbeđuje povećan otpor kretanju prskotine i žilavost koja je na nivou žilavosti kod tradicionalnih kaljenih i otpuštanih čelika. U radu su ispitivana dva kontinuirano hlađena mikrolegirana čelika sa dodatkom vanadijuma. Struktura čelika bez titana i sa manjim sadržajem ugljenika ('A19') sastoji se od kontinuirane mreže proeutektoidnog ferita uz koji se izdvojio perlit, za kojim sledi acikularni ferit izdvojen unutar nekadašnjeg austenitnog zrna. Struktura čelika sa titanom i većim sadržajem ugljenika ('B22') sastoji se gotovo u potpunosti od acikularnog ferita. Deformaciono ponašanje na 77 K ispitivano je jednoosnim zatezanjem. Ukupno izduženje uzoraka čelika B22 znatno je veće nego kod čelika A19 (10 u odnosu na 5%). Čelik B22 takođe poseduje veću granicu tečenja i zateznu čvrstoću, što je razumljivo s obzirom na veći sadržaj ugljenika. Značajna razlika u duktilnosti na temperaturi tečnog azota u vezi je sa prisustvom perlita kod čelika A19, u odnosu na strukturu acikularnog ferita kod čelika B22. Brzina deformacionog ojačavanja se povećava sa povećanjem udela perlita u strukturi, čime deformacija postaje otežana, dok istovremeno konkurentni proces nastajanja mikroprskotina na krtim cementitnim lamelama ili česticama sekundarnih faza, brzo dovodi do krtog l

The aim of this work was to determine TTT diagram of medium carbon V-N micro-alloyed steel with emphasis on the development of intragranular ferrite morphologies. The isothermal treatment was carried out at 350, 400, 450, 500, 550 and 600°C. These treatments were interrupted at different times in order to analyze the evolution of the microstructure. Metallographic evaluation was done using optical and scanning electron microscopy (SEM). The results show that at high temperatures (≥ 500°C) polygonal intragranulary nucleated ferrite idiomorphs, combined with grain boundary ferrite and pearlite were produced and followed by an incomplete transformation phenomenon. At intermediate temperatures (450, 500°C) an interloced acicular ferrite (AF) microstructure is produced, and at low temperatures (400, 350°C) the sheave of parallel acicular ferrite plates, similar to bainitic sheaves but intragranularly nucleated were observed. In addition to sheaf type acicular ferrite, the grain boundary nucleated bainitic sheaves are observed.

The effect of austenitizing temperature and Cr, Mo and Mn addition on microstructure and mechanical properties of V microalloyed medium carbon steel has been studied by means of metallography and mechanical testing. The addition of Cr, Mn and Mo leads to a decrease in yield strength (YS) by approximate 100 MPa in comparison to the base steel. It is assumed that Mn and Mo increase hardenability by promoting the formation of bainitic sheaves (BS), i.e. by suppressing the formation of ferrite-pearlite and acicular ferrite (FP-AF). Cr at the level used in this work is not that effective. Presence and packet size of bainitic sheaves decrease the Charpy V-notch impact energy at 20 degrees C (CVN20) in comparison to ferrite-pearlite and acicular ferrite microstructures.

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. Increasing nitrogen at the same vanadium level promotes the acicular ferrite formation. 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, while CVN20 impact energy increases on changing the microstructure from bainitic ferrite to the fine ferrite-pearlite and acicular ferrite.

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. Increasing nitrogen at the same vanadium level promotes the acicular ferrite formation. 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. 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, while CVN(20) impact energy increases on changing the microstructure from bainitic ferrite to the fine ferrite-pearlite and acicular ferrite.

Two microalloyed steels (Nb/Ti low carbon and V medium carbon) were tested by means of continuous cooling multipass tests on high temperature torsion tester, with aim to investigate the recrystallization kinetics. Multistage continuous cooling torsion tests (modified Boratto test for determination of Tnr temperature), have been used to evaluate the interpass recrystallization data. The determination of activation energy for static recrystallization, Q(SRX),. was limited to temperatures above Tnr temperatures, i.e. temperature range in which microalloying elements are in solid solution. Tnr temperatures were, in all cases, related to equal amount of recrystallized fraction, enabling plotting lnt vs. l/T dependence. Activation energies determined for both steels have close values and show good agreement with previously published data.

Physical and metallurgical basis of historical steel development and some modern solutions for specific applications, together with predictions and projections of future development are reviewed in this paper. Undoubtedly, in future, steel will remain absolutely most used construction material due to the best relationship properties/price. Dominant place is reserved for steels for welded constructions, and their further development will be provided by application of concept of Total Thermomechanical Control Processing (TTCMP). Simultaneously, predictions consider intensive Development in the field of welding procedures, filler materials, modeling of TMCP and welding, with aim to provide fabrication of welded constructions with improved safety., U radu su izložene fiziko-metalurške osnove istorijskog razvoja čeličnih materijala, primeri savremenih rešenja čelika za specifične namene i pretpostavke daljeg razvoja. Nedvosmisleno, u budućnosti, čelični materijali će ostati suvereno najrasprostranjeniji konstrukcioni materijal, zbog najboljeg odnosa kvalitet/cena. Dominantno mesto će zauzimati čelici predviđeni za izradu zavarenih konstrukcija, a njihov razvoj će biti omogućen konceptom totalne termomehaničke prerade. Uporedo, predviđa se intenzivan razvoj na polju tehnologija zavarivanja, dodatnih materijala, matematičkog modeliranja termomehaničke prerade i zavarivanja, sa ciljem da se obezbedi izrada zavarenih konstrukcija povišene sigurnosti.

Two-stage linear In[sinh(alpha sigma)] vs. 1/T relations indicative of double Q(HW) behavior, are obtained from anisothermal multipass flow curves generated by testing Nb/Ti microalloyed steel in torsion tin the range 1250-800 degrees C), except for the case of the high cooling rate-short interpass time tests, which gave rise to single-stage plots, indicative of single QHW behavior. Above the T-nr the QV (apparent activation energy for hot working corresponding to the upper temperature range is little affected by test variables (interpass time and cooling rate) and type of deformation (isothermal continuous and anisothermal multipass deformation give virtually equivalent Q(H)(W)(U), values above the T-nr). However, below the T-nr the O-HW(L) corresponding to the lower temperature range, becomes sensitive to test variables, a nd may show considerable deviation from O-HW(L) Obtained in continuous tests. Therefore, the temperature dependence of the flow stress, below the T-nr relevant to a multipass hot working operation, can be well described only by the Q(HW)(L) Obtained from the multipass flow curves, because it is interpass time dependent. The interpass time dependence of both the Q(HW)(L) and the T-nr,, can be divided into three regions. Within the short interpass time region (1.8 to 10 sec), the two parameters show opposite trends, while beyond 10 sec they correlate well. Within the 1.8 to 10 sec region the T-nr,, is assumed to be controlled by Nb in solid solution, while the O-HW(L) is controlled by both solute and precipitation effect. Beyond 10sec precipitation is the only controlling mechanism. In addition to the T-nr derived from Mean Flow Stress (MFS) and in [sinh(alpha sigma)] vs. 1/T plots, the recrystallization limit, T-rl which correlates well with both T-nr,,'s, and the recrystallization stop, T-rs temperatures are derived from % Fractional Softening (FS) vs. 1/T plots.

The effect of test variables on the apparent activation energy for hot working of a medium carbon V-microalloyed steel, Q(HW), and the critical recrystallization temperature, T-nr, have been studied by means of isothermal continuous and anisothermal multipass torsion test. The Q(HW) is found to be temperature dependent. Above the T-nr, the Q(HW)(U) is close to that of austenite self diffusion, Q(SD), irrespective of the type of test or test variables. Below the T-nr, the Q(HW)(L) is not only considerably higher, but that based on multipass flow curve exibits a strong interpass time dependence. With increasing interpass time from 1.8 to 10 s the Q(HW)(L) and the T-nr increase and decrease respectively. The former is assumed to be controlled by solute drag, while the latter both by solute drag and precipitation pinning. Beyond 10 s, the two parameters exhibit the same trend, and the precipitation pinning is assumed to be the only recrystallization inhibiting mechanism.