Ti-modified Steel Research Articles

Implications of microstructure, Taylor factor distribution and texture on tensile properties in a Ti-added Fe-Mn-Al-Si-C steel

Structure-property relationship in a Ti-added medium Mn multicomponent steel has been studied and compared with a Ti-free variant of same composition. Both the steels with initial ferrite-austenite microstructure were subjected to cold rolling, followed by annealing treatment at 1073K for 3 and 50 min. The Ti-modified steel shows good strength-ductility combination after 50 min of annealing. In contrast, a reverse trend is observed in the Ti-free variant where strength-ductility relationship ameliorates after 3 min annealing. The remarkable strength in the Ti-free variant originates from the higher volume fraction of Mn5(Al,Si)C precipitates having larger lattice misfit, higher fraction of deformed grains and finer ferrite grain size. The presence of Ti as a microalloying element in the Ti-modified steel instigates the formation of Ti-rich intermetallic carbide precipitates which refine the ferrite grain size during extended annealing. However, the evolution of such precipitates and refinement of ferrite grains do not significantly improve its tensile strength owing to the coarser size and lower lattice misfit of the precipitates, lesser fraction of deformed grains and predominant presence of cube fibers. The Taylor factor distribution within the range of (~2–5) is also studied to understand its implications on the texture-property relationship of the developed steels. It is observed that the γ-fiber fraction of ferrite phase increases due to higher number of grains within the Taylor factor range of (~3–4) and this, in turn, enhances the elongation in both the Ti-free (3 min annealed) and Ti-added (50 min annealed) steels.

Development of Stretch Flangeable Grade Steels by Inclusion Engineering Approach

The demand for good stretch flangeability in high-strength automotive steels has necessitated due attention on steel cleanliness and processes. The presence of elongated MnS stringers and centerline segregation of nonmetallic inclusions adversely affects the transverse mechanical property. In the present investigation, stretch flangeability of industrially produced steel, with and without Ca treatment, has been compared with Ti-added steel, produced at 40 kg scale without Ca treatment. The study brings out the influence of centerline segregation and modification of nonmetallic inclusions on HER performance. The microstructural analysis of industrial steel revealed that the method of hole formation (punching/drilling) determines crack initiation mechanism. The inclusions, along with the second phase present in the centerline, drive the crack propagation. An addition of Ti in liquid steel promotes formation of Al-Ti-O-based oxide, prior to MnS precipitation, during solidification and acts as nucleation site for MnS inclusion. This obviates the elongated stringer-type MnS inclusions. The centerline segregation-free Ti-modified steel, prepared at the laboratory scale, showed consistent HER property compared with industrial steel. It is also concluded that calcium treatment can be circumvented by modifying the sulfide stringer with the addition of Ti in the steel for further improvement in HER property.

Effect of titanium additions upon microstructure and properties of precipitation-strengthened Fe-Ni-Al-Cr ferritic alloys

Various amounts of Ti (0, 2, 4 and 6 wt%) is added to a ferritic alloy with a nominal composition of Fe–10Cr–10Ni–6.5Al–3.4Mo–0.25Zr–0.005B (wt%) (FBB8). The microstructure and composition of the matrix and precipitate phases are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For the Ti-modified steels, the bcc ferritic matrix is strengthened by submicron L21-Ni2TiAl-type precipitates which contain (i) Fe-inclusions with the precipitates' overall diameters ranging from 100 to 500 nm for both FBB8-4 wt%Ti and FBB8-6 wt%Ti, or (ii) B2-NiAl sub-precipitates with an average diameter of 50–100 nm for FBB8-2 wt%Ti. By contrast, the Ti-free FBB8 alloy contains B2-NiAl precipitates with Fe-inclusions. The four FBB8-Ti alloys were subjected to creep experiments at 700 °C in the stress range of 60–300 MPa. Threshold stresses for all studied compositions were observed, ranging from 69 to 179 MPa, with the most creep-resistant alloy being FBB8-2Ti with L21/B2 precipitates. Based on these mechanical results and detailed electron microscopy observations, the creep mechanism is rationalized to be general dislocation climb with repulsive elastic interaction between coherent precipitates and the matrix dislocations.

Effect of yttrium addition on the microstructure and mechanical properties of ODS RAF steels

An oxide dispersion strengthened (ODS) alloy with nominal composition Fe–14Cr–2W–0.3Ti–0.24Y (wt.%) was produced by mechanical alloying using elemental powders, and subsequent hot isostatic pressing. The microstructure of the material and characteristics of the oxide particle dispersion were investigated by electron microscopy. The effect of heat treatments on the microhardness and tensile properties at room temperature was also studied. The results show that a fine dispersion of Y–O-rich nanoparticles is achieved, together with larger (Cr, Ti)-rich precipitates. The absence of Ti is apparent in the majority of these nanoparticles, in contrast with reported results for ODS Ti-modified steels processed with Y2O3 addition.

Void swelling in ion irradiated (15Ni–14Cr), Ti-modified stainless steel: A study using positron annihilation and step height measurements

The void swelling behaviour of (15Ni–14Cr) Ti-modified steels simulated by heavy ion irradiation, has been investigated using step height measurements and positron annihilation spectroscopy. 18% cold-worked samples were pre-implanted with helium at 170 keV and 275 keV energies so as to create a uniform helium concentration of 100 appm around the 588 nm depth region. This was followed by a 2.5 MeV Ni ion irradiation to create a peak damage of ~ 84 dpa (displacement per atom) at various irradiation temperatures between 700 and 970 K. The gross swelling which has occurred throughout the ion range is determined by step height measurements. Defect-sensitive positron annihilation lineshape S-parameter measurements show clear changes consequently to void formation as a function of sample depth. From the variation of the average S-parameter as a function of irradiation temperature, the peak swelling temperature has been deduced. The changes observed in step height measurements are compared with positron annihilation measurements and the results are discussed.

Austenite Grain Stability of Titanium-Modified Carburizing Steel

It is important to suppress grain coarsening during the carburizing treatment of automobile components such as gears and shafts so that high strength is maintained and heat treatment distortion is minimized. Two manufacturing methods are often used for the cost reduction. One is the increasing carburizing temperature. The other is the adoption of cold forging instead of hot forging. These methods are likely to result in grain coarsening in the case of conventional steels. It is well known that the key factors to control the grain growth are the initial austenite grain size and the volume and mean size of precipitates. In this study, the grain growth property of Ti-modified steel was investigated to confirm the influence of precipitates, and compared with those of Nb-modified and conventional steels. The influence of forging temperature on the grain growth property in a Ti-modified steel was also investigated.

Strong interface adhesion in Fe/TiC

As an aid to understanding the superior toughness of Ti-modified steels provided by fine Ti(C, N) particles, first-principles full-potential linearized augmented plane wave (FLAPW) density functional calculations were performed on the Fe matrix/TiC particle interface. It was found that at equilibrium a strong covalent bonding between Fe–C is formed at the interface, and the magnetic moment of the interface Fe (1.98 μ B ) is reduced from that of the tetragonally strained structure (2.51 μB). We then calculated with a rigid separation model the separation energy curve and the force separation law for the Fe–C debonding process at the interface, which predicts 2.45 J m−2 for the work of separation and 30.66 [GPa] for the force maximum. We also found that the strong Fe–C bond provides an interfacial fracture strength equal to that of the pure bcc Fe matrix. A clear picture is given for the microscopic origin of this strong metal/ceramic adhesion based on density of states (DOS) considerations. For a more realistic understanding of the Fe–C bonding, structural optimization calculations were performed at each separation distance. The effect of relaxation was found to be larger at short separation distances than in the large separation region, which leads to a crossover behavior in the separation energy curve from the elastically deformed to the clearly separated regime at a critical distance (∼1.75 Å), and to a discontinuity in the force separation law. Despite this large relaxation effect, the work of separation, 2.52 J m−2, is not changed much from that of rigid separation.

Effect of vanadium and titanium on mechanical properties of chromium-tungsten steels

Increasing the vanadium content from 0.1 to 0.50% in a 2.25Cr-2W-0.1C steel (all concentrations are in wt%) increased the yield stress 20% and resulted in a higher ductile-brittle transition temperature (DBTT). When vanadium was increased to 0.5%, a further slight increase in strength occurred with a large increase in DBTT. Thus, optimum strength and impact toughness is achieved at an intermediate vanadium concentration. With the addition of 0.02% Ti to 2.25Cr-0.25V-0.1C, 2.25Cr-2W-0.1C, and 2.25Cr-2W-0.25V-0.1C steels, yield stress decreased 10 to 30%. An increase in impact toughness accompanied the strength loss. The toughness may have been affected by a decrease in the prior austenite grain size. There was little difference in the DBTT of the Ti-modified steels tempered at 700 or 750°C. The use of a Ti-modified steel tempered at 700°C might offset the strength advantage of steels without titanium, which have to be tempered at the higher temperature.

Microstructural evolution of austenitic stainless steels irradiated in spectrally tailored experiment in ORR at 400°C

Abstract Several different heats of austenitic stainless steel, including Japanese-PCA(JPCA), were irradiated in the spectrally tailored ORR experiment at 400°C to 7.4 dpa. The levels of helium generated were 155 appm for JPCA (16Ni, 30 wppm B) and 102 appm for standard type 316 steel (13Ni). The mean He: dpa ratio throughout the irradiation falls between 15 and 20 appm He/dpa, which is close to the He/dpa values expected for fusion. Swelling was measured by transmission electron microscopy and by precision immersion densitometry. All the CW alloys showed swelling that was at or below the detection limit of the densitometer (0.1%). No measurable swelling was detected in the SA JPCA alloy, while the highest value of 0.8% was observed in the SA high-purity alloy. One Ti-modified steel with low C also showed a relatively high swelling value of 0.5%, while standard type 316 steel showed only 0.15% swelling. TEM observation gave consistent but slightly larger values of swelling.

Stress-strain relations of HFIR-irradiated austenitic stainless steels

Tensile specimens of austenitic stainless steels were irradiated up to 50 dpa at temperature of 573, 673 and 773 K and HFIR. True stress-strain relations derived from the load-displacement curves were analyzed in terms of induced effective strain and strain hardening behavior. The strain hardening exponent of the material was only slightly affected by irradiation at 573 and 673 K. Irradiation at 773 K changes work hardening characteristics. This might have resulted from fine MC precipitation and/or helium produced during irradiation. Fracture strain decreases with dose, and the dose dependence was pronounced at 773 K. Heat-to-heat variation was not large between two Ti-modified steels.

Leak-before-break: an integrated approach for high energy piping: Chexal, V.K., Norris, D.M. and Server, W.L.Int. J. Pressure Vessels Piping 1988, 34, (1–5), 237–254

Several different heats of austenitic stainless steel, including Japanese-PCA(JPCA), were irradiated in the spectrally tailored ORR experiment at 400°C to 7.4 dpa. The levels of helium generated were 155 appm for JPCA (16Ni, 30 wppm B) and 102 appm for standard type 316 steel (13Ni). The mean He: dpa ratio throughout the irradiation falls between 15 and 20 appm He/dpa, which is close to the He/dpa values expected for fusion. Swelling was measured by transmission electron microscopy and by precision immersion densitometry. All the CW alloys showed swelling that was at or below the detection limit of the densitometer (0.1%). No measurable swelling was detected in the SA JPCA alloy, while the highest value of 0.8% was observed in the SA high-purity alloy. One Ti-modified steel with low C also showed a relatively high swelling value of 0.5%, while standard type 316 steel showed only 0.15% swelling. TEM observation gave consistent but slightly larger values of swelling.

Effects of titanium and zirconium on void swelling in electron-irradiated austenitic stainless steel

Solution-annealed Type 316 stainless steels modified with 0.3 wt% titanium or zirconium were electron-irradiated up to a dose of 50 dpa at temperatures of 723 to 823 K in a high voltage electron microscope. The swelling produced by the irradiation at 823 K to the maximum dose in the Ti-modified steel containing 0.02 wt% carbon is 0.5%. which is remarkably low compared with 14.4% for the standard 316 steel. The swelling is further decreased by increasing the carbon content from 0.02 to 0.07 wt% and no voids are observed in the steel with 0.07 wt% carbon at 823 K. In the Zr-modified steel with 0.02 wt% carbon, no swelling occurs at temperatures of 773 to 823 K. An increase in carbon content to 0.05 wt% induces a swelling of 0.8% in the Zr-modified steel at 823 K to a dose of 50 dpa. Increasing the titanium and zirconium by 0.1 wt% in the steel with 0.01 wt% carbon also suppresses the swelling at 823 K.

Modeling microstructural evolution in fusion reactor environments

Interactions between dislocation, cavity and precipitate evolution in austentic stainless steels irradiated in fission reactors are analyzed. Cavity evolution is modeled using the critical bubble concept: helium bubbles grow stably with the incremental addition of helium up to a critical size, thereafter they grow rapidly as voids. Dislocations and precipitates are preferred sites for bubbles and influence the critical size, hence, incubation period. The models predict broad trends in the swelling data base, as well as many detailed observations. The calibrated models predict a nonmonotic swelling response to increases in helium, with a peak at intermediate values; this suggests that fusion environments will generally have lower incubation times and higher swelling than found for fission reactor irradiations. The mechanisms controlling swelling in Ti-modified steels are also examined; partitioning of helium to a fine distribution of stable MC phases is found to be the key factor in retarding swelling.

Effect of pre-implanted helium on cavity formation in modified 316 stainless steels during electron irradiation

Cavity formation and dislocation structural evolution during electron irradiation were investigated in modified 316 stainless steels implanted by helium. In standard 316 steel, helium implantation caused to increase in cavity number density and reduced the size at whole temperature range of 623 to 873 K. However, implanted helium in Si and Ti-modified steels enhanced strongly the cavity nucleation at lower temperature (<770 K), and suppressed cavity formation at higher temperature (>820 K). Moreover, initial dislocation structure was also affected by the helium. Especially black dotts were observed with high density and not changed with further irradiation eventhough at higher temperature. From these results it is suggested that helium could affect on the both of dislocation loop and cavity formation, and the suppression effect of helium on cavity formation would originate from the loops of high number density, in which they would act as a neutral sink for point defects.