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Some aspects of deformation, precipitation, and recrystallization behavior in medium carbon V-microalloyed and low carbon Nb/Ti-microalloyed steels are presented in the paper. Changes in microstructure are explained together with methods of quantification. The temperature of No-recrystallization (Tnr) is defined as a milestone to show the onset of retardation of recrystallization while the apparent activation energy for hot working shows the extent of this retardation. In the case of high cooling rates, this method is not sufficiently sensitive and Trl (recrystallization limit temperature) and Trs (recrystallization stop temperature) must be evaluated from softening data. Paper presented the possibility to estimate Tnr temperature on six stands finishing train at Hot Strip Mill in HBIS Iron and Steel Serbia, Smederevo as well as the activation energy for static recrystallization, QSRX, derived from Tnr temperatures.

The aim of this work was to evaluate the influence of welding speed on microstructure, mechanical properties, as well as Intergranular corrosion (IGC) susceptibility of EN AW-5083 alloy welded using FSW. FSW was performed using constant rotation speed of 750 rev/min, while welding speed was varied from 73 mm/min, 150 mm/min to 190 mm/min. Results show that increase of welding speed lead to decrease of the size of the nugget. This behavior is attributed to decrease of heat input during welding. Accordingly, lower grain size provides increase of the strength. All tested welds are resistant to IGC, evaluating by mass loss according to NAMLT test, and resistivity slightly decrease with increasing welding speed.

Isothermal transformation characteristics of a medium carbon Ti-V microalloyed steel were investigated using light microscopy, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), and by uniaxial compressive testing. Samples austenitized on 1100 degrees C were isothermally treated in the range from 350 to 600 degrees C and subsequently water quenched. The final microstructure of the samples held at 350 degrees C consisted of bainitic sheaves and had compressive yield strength, approximately from 1000 MPa, which is attributed to high dislocation density of low bainite. At 400 and 450 degrees C, acicular ferrite became prevalent in the microstructure. It was also formed by a displacive mechanism, but the dislocation density was lower, leading to a decrease of compressive yield strength to approximately 700 MPa. The microstructure after the heat treatment at 500 degrees C consisted of coarse non-polygonal ferrite grains separated by pearlite colonies, principally dislocation free grains, so that the compressive YS reached a minimum value of about 700 MPa. The microstructure of the samples heat-treated at 550 and 600 degrees C consisted of pearlite and both grain boundary and intragranular ferrite, alongside with some martensite. After 600 s, austenite became stable and transformed to martensite after water quenching. Therefore, the presence of martensite increased the compressive YS to approx. 800 MPa.

Copper-based alloys and composites, owing to their convenient properties, are being considered essential materials in various industries. Since copper possesses an ability to develop high corrosion resistance, putting it in the domain of a desirable material in the manufacturing of valves, pipes, and also systems that carry industrial gases and aqueous fluids. Its usage is also invaluable for cables and electrical wires. This review paper describes diversity in copper alloy processing techniques (powder and ingot metallurgy) which are alongside the phase transformation kinetics interpreted and explained in detail. Furthermore, the focus is put on the copper alloys, as well as the kinetics present in these systems, with the application being highlighted. Correlation between physical properties and phase transformation kinetics in copper alloys is made. It is shown that if certain alloying elements are to be added, different properties could be improved. The effect of phase precipitation on phase transformation kinetics of copper alloys is shown by studying the Cu–15Ni–8Sn alloy.

The present study estimated the cracking phenomenon in as-plasma-sprayed hydroxylapatite coatings (HACs) after they were being subjected to the severe cutting conditions in the direction perpendicular to the coating/substrate interface. In order to evaluate the effects of substrate preheating on the occurrence of micro-cracks, the HACs were deposited at different initial substrate temperatures (TS = 20, 100 and 200°C). The changes in phase composition and HA splat morphology with TS were observed and were correlated with the cracking occurrence. The results showed that severe cutting conditions introduced a localized cracking in the regions of HACs dominantly attributed to the brittle hydroxyl-deficient amorphous calcium phosphate (ACP) phase. This effect was particularly observable in the HACs deposited without preheating of substrate. On the other hand, the preheating of substrate reduced the presence of micro-cracks and caused insignificant changes in the average local phase composition. In HACs deposited with preheating of substrate, the HA splats (of which HACs are composed) were thinner and recrystallized HA regions seemed smaller in size and more evenly distributed. These results implied potentially important roles of the HA splat formation mechanism on the distribution of ACP and recrystallized HA regions in the as-plasma-sprayed HACs and the cracking resistance of HACs. [Project of the Serbian Ministry of Education, Science and Technological Development, Grant no. TR 34022]

In order to accurately and reliably evaluate the cavitation erosion resistance of materials using a cavitating jet generator, the effects of the hydrodynamic parameters and the nozzle geometry on the erosion process were investigated. Since the behavi our of a high speed submerged cavitating jet is also depending on the working conditions; their influence is also discussed based on the evaluation of cavitation erosion process . The erosion rate was used as an indic a tor for cavitating jet behaviour . Speci mens of commercial - purity copper were subjected to high speed submerged cavitating jets under different initial conditions, for certain time periods. The force generated by jet cavitation is employed to initiate the erosion in surface. The tested specimens were investigated with a digital optical microscope and a p rofilometer . It was found that erosion becomes more pronounced with decreas ing cavitation number s , as well as with increas ing exit jet velocit ies . The n ozzle configuration and hydrodynamic param eters have strong influence s on the erosion rate , eroded area and depth of erosion . A comparison between the obtained results explains some of the mechanisms involved in cavitation and erosion p rocesses and their relation to the tested parameters . Mathemat ical expressions which combine these parameters with the erosion rate are obtained . These parameters are very important in order to control the cavitation as a phenomenon and also to control the performance of the cavita ting jet generator .

When the testing of large quantities of bimetallic joints of aluminium (Al) and copper (Cu) is considered, consumption of Cu, as a raw material, is of significant importance. For tensile testing of Al/Cu joints, it is recommended to produce joints by using a longer initial length of Cu bars, whereas for the examination of micro-structure or other properties of these joints, Cu bars of shorter lengths can be used. In order to introduce the lower consumption of Cu for the testing of larger quantities of Al/Cu joints, it is necessary to determine whether the usage of different initial lengths of Cu bars produces the same micro-structural effects in joints obtained by using the same processing parameters or not. In this study, micro-structures and micro-hardness profiles of Al/Cu bimetallic joints produced by using continuous drive friction welding and noticeably different initial length of Cu bars will be analyzed. Also, mentioned properties of these joints in the open-die forged condition will be examined.

The aim of the present study was to investigate the influence of substrate preheating on the structural properties of hydroxyapatite coatings (HACs) deposited by using the high power (52 kW) laminar plasma jet. The deposition experiments were performed within the 20-200 degrees C temperature range at different stand-off distances. The structural properties in the thickness direction and at the surface of the HACs were investigated by using the micro-Raman spectroscopy, scanning electron microscopy coupled with energy dispersive spectrometry, and X-ray powder diffractometry. The deposition without the preheating of the substrate produced HACs with crystallinity gradient in the thickness direction. At the stand-off distance of 80 mm, the preheating of the substrate at 200 degrees C practically eliminated the crystallinity gradient. At distances from the coating/substrate interface shorter than similar to 100 mu m, the increase of crystallinity with the preheating of the substrate was dominantly attributed to the recrystallization of hydroxyl-rich ACP into HA. At longer distances and higher initial substrate temperatures ( GT 100 degrees C), the crystallinity changes were negligible, whereas the recrystallization of hydroxyl-deficient ACP into oxyapatite (OA) was also possible. The X-ray diffractometry indicated the deposition conditions under which a minimum residual stress was achieved. The results of the present study strongly suggested the relation between the ACP - GT HA remystallization process and the bonding strength of the HACs. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

The aim of this work was to investigate the combined effect of small difference in pin geometry, together with rotation and welding speed on the weldability, mechanical and structural properties of FSW 2024-T351 Al plates. The only difference in tool pin design was the shape of thread: regular and rounded. Specimens were welded using rotation rate of 750 rev/min and welding speeds of 73 and 93 mm/min. Specimens were defect free, with good or acceptable weld surface, in all four cases. Modification in pin design showed strong influence on macro structure and hardness distribution. Weak places are identified as low hardness zone, close to the nugget zone and are in good agreement with fracture location in tensile testing. Weld efficiency, as a measure of weld quality, is better in case of 310 tool, while UTS values can differ up to 13% for the equal welding parameters. Therefore, it can be assumed that small modification in tool design, particularly in pin geometry, can have great influence on weld formation and mechanical properties. U radu je ispitivano kakav zajednički, kombinovani uticaj brzina rotacije alata, brzina zavarivanja i mala promena u geometriji trna alata imaju na zavarljivost, mehaničke i strukturne karakteristike sučeonog spoja Al legure 2024-T351 zavarene primenom FSW (zavarivanje trenjem alatom) postupka zavarivanja. Zavarivane su ploče dimenzija 260 mm×65 mm×6 mm. Ploče su pre zavarivanja mašinski obrađene i kruto stegnute za potpornu ploču. Zavarivanje je izvedeno u pravcu valjanja ploča, na alatnoj glodalici prilagođenoj postupku FSW. Dužina svakog zavarenog spoja iznosila je oko 210 mm. Korišćena su dva alata sa veoma slično profilisanim trnovima. Razlika u dizajnu trna alata je u obliku zavojnice, normalna (klasična) kod alata oznake 310 odnosno zaobljena za alat 310-O. Zavarivanje je izvedeno pri brzini rotacije alata od 750 o/min i brzinama zavarivanja 73 i 93 mm/min. Ukupno su zavarene četiri kombinacije spojeva. U svim slučajevima dobijen je zavareni spoj bez prisustva grešaka, sa glatkom odnosno delimično hrapavom površinom zavara. Prisutne su tri različite oblasti u spoju - grumen, zona termomehaničkog uticaja i zona uticaja toplote. Promene u dizajnu trna alata imaju znatnog uticaja na makrostrukturu spoja i raspodelu tvrdoće kroz spoj. Mapiranjem spoja, na osnovu raspodele tvrdoće, određene su oblasti sa najnižim vrednostima tvrdoće a nalaze se u neposrednoj blizini grumena, tačnije na liniji dodira grumena i zone termomehaničkog uticaja. Položaj oblasti najmanje tvrdoće je u saglasnosti sa mestom preloma zateznih epruveta. Efikasnost spoja, kao ocena kvaliteta spoja, je veća kod spojeva zavarenih alatom 310. U zavisnosti od alata, pri istim parametrima zavarivanja dobijene su različite vrednosti zateznih čvrstoća a razlika iznosi do 13%. Pokazano je da mala modifikacija u dizajnu trna alata ima velikog uticaja na formiranje spoja i njegove mehaničke karakteristike.

A significant increase in the research activity dedicated to high manganese TWIP steels has occurred during the past few years, motivated by the breakthrough combination of strength and ductility possessed by these alloys. Here a short review of the relations between microstructure and mechanical properties is presented. This summarized knowledge explains why TWIP steel metallurgy is currently a topic of great practical interest of especially automotive industry. Dvojnikovanjem Izazvana Plastičnost (TWinning Induced Plasticity - TWIP) u čelicima je fenomen koji je omogućio razvoj zasebne grupe čelika za koju se pretpostavlja da će imati masovnu upotrebu, naročito u autoindustriji. Mikrostruktura ovih čelika na sobnoj temperaturi se sastoji od austenita koji je stabilisan značajnim dodatkom mangana. U toku deformacije dolazi do dvojnikovanja koje značajno povećava otpor deformaciji, uz zadržavanje odlične plastičnosti, tako da su čvrstoće od 1300-2000 MPa praćene izduženjem od minimalno 45%. Sa vremena istraživanja su više usmerena na ispunjenje tehnoloških zahteva, kao što su zavarljivost, koroziona postojanost, mašinska obradljivost i sl.

In this work, the structural and mechanical properties of friction stir welded dissimilar butt joints of AA2024-T351 to AA5083-H112 aluminum alloys were evaluated. Plates of 6 mm thick were friction stir welded with optimized tool. In the first case, plate of AA2024 was positioned on the advancing and plate of AA5083 was on the retreating side of the weld. In second case, position of the plate was opposite. Specimens were welded using rotation rate of 750 rpm and welding speed of 73 and 93 mm/min. Specimens were defect free, with good or acceptable weld surface. The effects of rotation and lateral speed of the welding tool were related to structural features and mechanical properties. The structural appearances of concentric rings in the nugget zone were controlled by tool speed and plate position. Structural analysis of the joint cross-section, revealed the distinct lamellar bands. The joints consistently failed on the AA5083 aluminum alloy plate. Failures were observed through the heat-affected zone at distance of approximately 21 mm from weld center. U radu su predstavljeni rezultati strukturnih i mehaničkih ispitivanja sučeonih spojeva raznorodnih aluminijumskih legura AlCu4,5Mg (AA2024) i AlMg4,5Mn (AA5083) zavarenih postupkom FSW. Primenom optimizovang alata za zavarivanje, spojene su ploče debljine 6 mm. Međusobni položaj ploča u spoju je variran tako da je u prvom slučaju ploča od AA2024 pozicionirana na strani napredovanja a ploča od AA5083 na povratnoj strani spoja. U drugom slučaju položaj ploča je bio obrnut. Korišćeni su sledeći parametri zavarivanja: brzina rotacije alata nije menjana i iznosila je 750 obr/min a brzina zavarivanja iznosila je 73 i 93 mm/min. Dobijeni su spojevi bez prisustva grešaka i sa glatkom odnosno prihvatljivo ravnom površinom spoja. Analiziran je uticaj parametra zavarivanja na dobijenu strukturu i mehanička svojstva spoja. Oblik grumena i u njemu prisutnih koncentričnih krugova, zavisi od brzine zavarivanja i položaja ploča u spoju. Do loma uvek dolazi na strani spoja gde je pozicionirana ploča od legure AA5083. Karakteristično je da se lom uvek dešava na rastojanju od oko 21 mm od centra spoja.

Medium carbon V-microalloyed steel continuously cooled from the austenitization temperature at still air, with predominantly acicular ferrite structure, has been investigated by means of four-point bending of notched Griffiths–Owens’s type specimens at liquid nitrogen temperature. Local fracture stress and plastic strain were determined by using finite element analysis and fracture surface examination using scanning electron microscope. It was revealed that cleavage fracture initiation, which takes place close to the notch root in the narrow zone of high plastic strains, is not related to any broken coarse second phase particles. It was assumed that microcracks nucleate by strain induced fracture of pearlite nodules. Two effective surface energy values of 24 and 42 J/m2 were estimated according to the Griffith’s equation, indicating the influence of crystallographic orientation between neighboring grains at the origin of fracture. Lower value was attributed to fracture of coarse ferrite–pearlite units with similar crystallographic orientation and higher value to fracture propagation through fine acicular ferrite matrix.

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.

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 aim of this paper is to investigate the possible application of the cavitation phenomenon as efficient method to modify the surface properties (e.g. the surface roughness) in the nano- and micro-levels. Aluminum alloy (AlSiMg) specimens were subjected to high speed submerged cavitating jets under various working conditions, for short time periods between 15 and 30 s. The force generated by the cavitating jet is employed to modify the surface roughness of the specimen. The target surface was analyzed with optical microscopy, white light interferometry, atomic force microscopy (AFM) and also with electrostatic force microscopy (EFM). The results show the possibility to use the cavitation bubbles as a nanofabrication method e.g. for shotless surface peening. With AFM, the deformation mechanism and the formation of planar or wavy slip were also investigated. EFM shows that the changes in the surface roughness also have a strong influence on the electrostatic field above a biased sample.

The aim of this work was to examine the dependence of the local surface structure (surface structural heterogeneity) on the local thickness (thickness uniformity) and structure properties along the thickness of hydroxyapatite coatings (HACs) deposited by using the high power (52 kW) laminar plasma jet. For the deposition process, a relatively small size of the feedstock hydroxyapatite powder (HAP) (33 mu m) was used. The HACs were deposited at different stand-off distances (SODs) (80, 100, and 150 mm) without preheating of substrates. The thickness measurements were performed by using the calibrated micrometer and light microscopy (LM). The surface micro-structure was analyzed by using the micro-Raman spectroscopy (MRS), whereas the micro-structure along the thickness of HACs was studied by using the MRS, X-ray powder diffractometry (XRPD), and LM (combined with chemical etching). The results showed a heterogeneous phase distribution on the surface of each HAC analyzed as well as the non-uniform thickness of HACs. Apparently, a correlation existed between the local surface structure and the local thickness of HACs. However, the MRS, XRPD, and LM analyses of micro-structure along the thickness of HACs indicated that the differences in the local surface structure could be related to the different recrystallization established during the plasma deposition process in the different parts along the thickness of each HAC analyzed. (C) 2016 Elsevier B.V. All rights reserved.

Experimental setup with a submerged cavitating jet has been used for the study of influences of material, exposure time and working fluid temperature on the erosion process. Each of the parameters has been varied separately, and the results of erosion are analyzed in detail. Additionally, comparison of experiments with nitrated and non-nitrated material has been made in order to study the enhancement (mostly reflected as the prolonged incubation time) of erosion resistance achieved by nitrating the specimen surface.

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 °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/m2 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 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. [Projekat Ministartsva nauke Republike Srbije, br. OI174004]

The influence of R/v ratio on joint quality in 2024-T351 aluminum alloy was studied. Specimens were subjected to friction stir welding with the rotation rates of 750, 950 and 1180 r/min and welding speed between 73 and 190 mm/min, providing R/v ratio between 5.00 and 10.27. The welded joints were tested by means of both non-destructive (visual, penetrant and X-ray inspection) and destructive (metallographic, tension and hardness) testing. In all specimens typical zones are revealed, with corresponding differences in grain size. Tensile efficiency of the joints obtained is in the range of 52.2% to 82.3%. The results show that the best quality is obtained at R/v ratio of 8.06, 10.17 and 10.27. This behavior is attributed to the assumption that the material flows around the pin with an optimal speed, i.e. sufficient amount of material is available to fill the gap and prevent tunnel formation. R/v ratio also showed influence on hardness distribution, onion features and crack initiation/propagation zones.

The aim of this paper is to establish a possible application of the cavitation phenomenon as an efficient method to modify surface properties at the nano and micro levels. Commercial-purity copper was subjected to high submerged cavitating jets under different initial conditions, for time periods between 15 and 1,800 s. The force generated by jet cavitation is employed to modify the surface roughness in the order of nano and micro scales. The target surface was analyzed with optical as well as scanning probe electron microscopy. The results showed the possibility to use cavitation bubbles to establish a nanofabrication method for the surface preparation, shoot-less surface peening (nano/micro level). Also, the cavitation is assumed to be one of the miniaturized testing methods that have to be developed to reliably measure mechanical properties in small dimensions and to identify the behavior caused by the size dependence. With optical, SEM, and atomic force microscopy observation techniques in this study, the deformation mechanism and the formation of planar or wavy slip were also studied. The results indicate that even at short exposure times, observed roughness having a characteristic “serpentine” configuration can be related to the start of the plastic deformation of the specimen surface. Longer exposure times inevitably result in a greater number of jet–specimen interactions leading to specimen fracture.

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.

Amorphous polymer-derived Si - C - N ceramics can be doped with different elements ( Al , B etc.) through various pre-ceramic polymer routes. Thus, controlling of the high temperature oxidation resistance can be achieved on an atomic level. An important factor for silica layer growth is oxygen diffusion in protective thermally grown layers. In order to get insight of the oxygen diffusion mechanism, analysis should include both, bulk and short-circuit diffusion. XRD measurements of oxidized Si - C - N and SiC revealed the possibility that oxide layers were fully crystallized and are composed of nano-sized cristobalite-like grains. Secondary ion mass spectrometry depth profile analysis after 18 O 2-16 O 2 isotope exchange experiments on oxidized SiC indicated that short-circuit diffusion is probably grain boundary diffusion of molecular oxygen.

The relationship between microstructural parameters and cleavage fracture has been studied in (950–1300°C) air cooled vanadium microalloyed steels by means of impact testing, light microscopy and scanning electron microscopy. Large variations in impact energy are obtained as a function of test temperature and microstructure. The results show that accicular ferrite (AF) and a multiphase structure consisting of ferrite-perlite (FP) and 30–70% AF posses room temperature toughness superior to that of classical bainitic sheaves (BS) as revealed by impact energy level. However, AF is superior to FP-AF in term of energy transition temperature.

Two-stage linear ln[sinh(as)] vs 1/T plots, based on the stress data derived from either continuous or multipass flow curves generated by testing low carbon Nb/Ti microalloyed steel (Nb steel) and medium carbon V-microalloyed steel (V-steel) in torsion, indicative of double Q(HW) behavior, are revealed in all but the high cooling rate-short interpass time multipass tests, which gave rise to single-stage plots, indicative of single Q(HW) behavior. Above the Tor, the Q(HW)(U) is little affected by steel composition, type of test, or test variables. Below the Tnr, the Q(HW)(L) becomes sensitive to all variables, and the temperature dependence of the flow stress relevant to a multipass hot working operation, can be well described only by the Q(HW)(L) based on the multipass flow curves, because it is interpass time dependent. The interpass time dependence of both the Q(HW)(L) and the Tor can be divided into three regions. Within the short interpass time region (1.8 to 10 seconds), the two parameters show opposite trends, while beyond 16 seconds they correlate well. Within the 1.8 to 10 second region the Tnr is assumed to be controlled by solutes, while the Q(HW)(L) is controlled by both solute and precipitation effect. Beyond 10 seconds precipitation is the only controlling mechanism. Also, high Tnr does not necessarily mean a high Q(HW)(L) or vice-versa. It seems reasonable to assume that the Tnr temperature indicates the beginning of suppression of recrystallization, while Q(HW)(L) indicates the extent of suppression.