Ti-modified Austenitic Stainless Steel Research Articles

Influence of Large Strain Hot Deformation on Microstructural Evolution in Alloy D9

In this work, a Ti-modified austenitic stainless steel used in the core of fast reactors is subjected to large strain deformation. The steel is deformed to 200% effective strain through high-temperature torsional loading in temperature domain 1173–1473 K. Three strain rates, namely 0.01 s−1, 0.1 s−1 and 1 s−1, are used for this deformation, so that the combined effect of deformation temperature and strain rate on material response can be studied. The torsional loading conditions are correlated with microstructural mechanisms of work hardening, dynamic recrystallisation (DRX) and post-DRX grain growth, each of which are found to bear a different relation with processing parameters. The microstructural response to this thermo-mechanical treatment is contrasted with the response to a heat treatment that involves equivalent exposure to high temperature. It is found that the steel is amenable to large strain hot deformation, and a variety of microstructures can be generated through control of the hot deformation parameters.

Role of Twinning on Dynamic Recrystallization and Microstructure During Moderate to High Strain Rate Hot Deformation of a Ti-Modified Austenitic Stainless Steel

This article discusses the role of twinning on dynamic recrystallization (DRX) and microstructural evolution during moderate to high strain rate (0.1 to 100 s−1) hot deformation (1173 to 1373 K (900 to 1100 °C) range) in a Ti-modified austenitic stainless steel (alloy D9). The extent of DRX increased with increasing strain rate and temperature in the range of hot working parameters employed in the present study. The acceleration of DRX with strain rate is attributed to increased rate of dislocation accumulation during high strain rate deformation as well as adiabatic temperature rise. The DRX grains were found to be twinned and a linear relationship was observed between the area fraction of DRX grains and the fraction of Σ3 boundaries. Analysis of misorientations revealed that the majority of these Σ3 boundaries are newly formed coherent twin boundaries during DRX. Interaction of pre-existing Σ3 boundaries that may regenerate new Σ3 boundaries did not seem to occur frequently during DRX. The majority of the twin boundaries are found within the DRX grains, signifying that these annealing twins are mainly formed by “growth accidents” during the expansion of the DRX grains. It is suggested that annealing twins play an important role during nucleation and subsequent expansion of the DRX process in alloy D9.

Origin and Role of Σ3 Boundaries during Thermo-Mechanical Processing of a Ti-Modified Austenitic Stainless Steel

The origin and role of S3 boundaries during dynamic recrystallization (DRX) and grain boundary engineering (GBE) of a Ti-modified austenitic stainless steel (alloy D9) is studied. Hot deformation tests were carried out on solution-annealed (SA) specimens to study the DRX behavior whereas a series of cold deformation and annealing were performed on SA specimens to realize GBE microstructure. A linear relationship between the area fraction of DRX and the number fraction of Σ3 boundaries was observed during hot deformation. This high fraction of Σ3 boundaries could account for the formation of coherent annealing twins by “growth accidents” during DRX. For certain combinations of cold deformation and annealing, a significant increase in S3 boundaries was observed. In contrast to hot deformation, majority of these new S3 boundaries during cold deformation and annealing were formed by geometrical interactions between the pre-existing Σ3 boundaries. The role of the S3 boundaries during DRX and on tailoring microstructure through grain boundary engineering approach is discussed.

Grain boundary engineering in alloy D9 through thermo-mechanical processing: influence of process variables and aspects of micro-mechanisms

By employing low-strain one-step thermo-mechanical processing (OTMP) and iterative thermo-mechanical processing (ITMP) we developed grain boundary engineered microstructure in a Ti-modified austenitic stainless steel (alloy D9). In OTMP, small amount of strain (0, 5, 10 and 15%) was imparted on solution annealed sample and subsequently annealed at various temperatures (1173–1273 K) for different time periods (0.5, 1 and 2 h). A pre-strain of 10–15% followed by annealing at 1273 K for 0.5–2 h has been found to be the suitable OTMP to increase the fraction of Σ3 n boundaries significantly. ITMP employing 10% thickness reduction followed by annealing at 1273 K for 0.5 h revealed fluctuations in the evolution of Σ3s. ITMP employing 2.5% thickness reduction per iteration, on the other hand, resulted in continuous increase in Σ3 boundary fraction and a moderate increase in Σ9 and Σ27 boundaries. The role of Σ3 boundaries on the mechanical properties of GBE processed specimen was studied by correlating the hardness with grain size evaluated with and without considering twin boundaries. The paper also discusses the micro-mechanisms involved during low-strain GBE processes in austenitic stainless steel.

A Study on Microstructural Evolution and Dynamic Recrystallization During Isothermal Deformation of a Ti-Modified Austenitic Stainless Steel

Dynamic recrystallization (DRX) behavior in hot deformed (by uniaxial compression in a thermomechanical simulator in the temperatures range 1173 K to 1373 K [900 °C to 1100 °C]) Ti-modified austenitic stainless steel was studied using electron back scatter diffraction. Grain orientation spread with a “cut off” of 1 deg was a suitable criterion to partition dynamically recrystallized grains from the deformed matrix. The extent of DRX increased with strain and temperature, and a completely DRX microstructure with a fine grain size ~4 μm (considering twins as grain boundaries) was obtained in the sample deformed to a strain of 0.8 at 1373 K (1100 °C). The nucleation of new DRX grains occurred by the bulging of the parent grain boundary. The DRX grains were twinned, and a linear relationship was observed between the area fraction of DRX grains and the number fraction of Σ3 boundaries. The deviation from the ideal misorientation of Σ3 boundaries decreased with an increase in the fraction of Σ3 boundaries (as well as the area fraction of DRX) signifying that most Σ3 boundaries are newly nucleated during DRX. The generation of these Σ3 boundaries could account for the formation of annealing twins during DRX. The role of Σ3 twin boundaries on DRX is discussed.

One-step and iterative thermo-mechanical treatments to enhance Σ3 n boundaries in a Ti-modified austenitic stainless steel

The article discusses the results of a study on low-strain thermo-mechanical (one-step and iterative) processing to enhance Σ3 n boundaries in a Ti-modified austenitic stainless steel (alloy D9). Solution annealed (SA) specimens were subjected to 10% thickness reduction by rolling followed by annealing at 1173, 1223, and 1273 K for 0.5, 1, and 2 h. Anomalous grain growth with moderate increase in Σ3 n boundaries was observed after annealing at 1,173 K for 0.5 to 2 h. Prolific multiple twinning with minimum deviation of Σ3 and Σ9 boundaries from ideal orientation was achieved after annealing at 1,273 K for 0.5 to 2 h. A significant disruption in random boundary connectivity was obtained in these conditions due to the presence of large number of Σ3-Σ3-Σ9/Σ3-Σ9-Σ27 triple junctions. Iterative processing (up to 4 cycles) employing 10% thickness reduction followed by annealing at 1,273 K for 0.5 h revealed fluctuations in the evolution of Σ3 boundaries. The Σ3 fraction increased after 2nd and 4th iteration and there is a drop after 3rd iteration. This was attributed to the increased driving force for grain boundary migration due to dislocation pile-up at twin boundaries during earlier iterations. A two step iterative processing comprising of 10% deformation followed by annealing at 1,273 K for 0.5 h is the recommended thermo-mechanical processing to achieve enhanced fraction of Σ3 n boundaries (~73%) in alloy D9.

In situ studies of evolution of microstructure with temperature in heavily deformed Ti-modified austenitic stainless steel by X-ray diffraction technique

The mechanism of the evolution of the deformed microstructure at the earliest stage of annealing where the existence of the lowest length scale substructure paves the way to the formation of the so-called subgrains, has been studied for the first time by X-ray diffraction technique. The study has been performed at high temperature on heavily deformed Ti-modified austenitic stainless steel. Significant changes were observed in the values of the domain size, both with time and temperature. Two different types of mechanism have been proposed to be involved during the microstructural evolution at the earliest stages of annealing. The nature of the growth of domains with time at different temperatures has been modelled using these mechanisms. High-resolution transmission electron microscopy has been used to view the microstructure of the deformed and annealed sample and the results have been corroborated successfully with those found from the X-ray diffraction techniques.

Dynamic Recrystallization in a Ti Modified Austenitic Stainless Steel During High Strain Rate Deformation

The article discusses the dynamic recrystallization (DRX) behavior in a 15Cr-15Ni-2.2Mo-Ti modified austenitic stainless steel (commonly known as alloy D9) during high strain rate deformation. Hot compression tests were conducted in a Gleeble thermomechanical simulator in the temperatures range 1173 K–1373 K with a strain rate 10 and 100 s−1 to different strains. Hot deformed microstructures were characterized using Electron Back Scatter Diffraction (EBSD) technique. Recrystallized grains were partitioned from the deformed grains employing grain orientation spread approach. Extent of dynamic recrystallization (DRX) was found to be minimal at 1173 K. DRX was predominantly found to happen at and above 1273 K which increases with increase in strain. The fraction of Σ3 and Σ9 boundaries was found to increase with %DRX. The origin and the possible role of these boundaries on DRX in alloy D9 are also explained.

A thermo-viscoplastic constitutive model to predict elevated-temperature flow behaviour in a titanium-modified austenitic stainless steel

The experimental stress–strain data from isothermal hot compression tests over a wide range of temperatures (1073–1473K), strains (0.1–0.5) and strain rates (0.001–1s−1) were employed to formulate a suitable constitutive model to predict the elevated-temperature deformation behaviour in a Ti-modified austenitic stainless steel (alloy D9). It was observed that the Johnson–Cook (JC) model in its original form is inadequate to provide good description of flow behaviour of alloy D9 in the above hot working domain. This has been attributed to the inadequacy of the JC model to incorporate the coupled effects of strain and temperature and of strain rate and temperature. A modified constitutive model based on the Zerilli–Armstrong model has been proposed for considering the effects of thermal softening, strain rate hardening and isotropic hardening as well as the coupled effects of temperature and strain and of strain rate and temperature on flow stress. The proposed modified constitutive model could predict the elevated-temperature flow behaviour of alloy D9 over the specified hot working domain of alloy D9 with good correlation and generalization.

Studies on twinning and grain boundary character distribution during anomalous grain growth in a Ti-modified austenitic stainless steel

Solution-annealed alloy D9 (a 15Cr–15Ni–2.2Mo–Ti-modified austenitic stainless steel) samples were subjected to annealing in the temperature range 1173–1273 K with and without 5% prior deformation. Anomalous grain growth (AGG) was observed in these processing conditions. Electron back scattered diffraction (EBSD) analysis revealed that the proportion of Σ3 boundaries increased during AGG. This was attributed to the formation of new annealing twins during grain growth. Fraction of Σ27 boundaries remained constant in all processing conditions, whereas the fraction of Σ9 boundaries increased marginally at higher annealing temperatures. This indicates that only a few interactions between Σ3 boundaries happened during grain growth. The AGG in alloy D9 during annealing is attributed to inhomogeneous distribution of local strain in material coupled with intrinsic misorientation dependant boundary mobility. The inhomogeneous grain size distribution in the solution-annealed specimen would have provided the ‘size-advantage’ so that bigger grains could grow abnormally during annealing.

Study of Hot Cracking Behaviour of 14Cr-15Ni-2.5Mo Ti-Modified Fully Austenitic Stainless Steels using Varestraint and Hot Ductility Tests

Studying the hot cracking behaviour of 14Cr-15Ni-2.5Mo Ti-modified austenitic stainless steel is an important weld-ability consideration because this material solidifies in the prior austenitic mode without any residual delta ferrite and also contains titanium. Hot cracking in this material is attributed to the formation of low melting phases in the solidifying weld metal and in the heat-affected zone that lead to cracking due to shrinkage stresses and restraint imposed on the weld joint. Different heats of this fully austenitic alloy, containing 0.022–0.025% P, 0.75–1.00% Si and 0.21–0.42% Ti, as well as 316L stainless steel (SS) were investigated. Both Varestraint and hot ductility tests were used to evaluate the hot cracking susceptibility of these alloys. Longitudinal Varestraint tests were carried out at four strain levels of 0.5, 1.0, 2.0 and 4.0%, and the brittleness temperature range (BTR) was evaluated during testing. The Varestraint test results indicated that this material has a very high hot cracking susceptibility during autogenous welding, as the total crack length and maximum crack length in both the weld and heat-affected zone as also the BTR values were very high compared to that of SS 316L. Hot ductility tests were also conducted on using a Gleeble thermo-mechanical simulator to determine the nil ductility temperature (NDT). The NDT of this material is lower than SS316L, with the material containing higher titanium and silicon. This paper discusses the results of the hot cracking behaviour of this fully austenitic SS material using the longitudinal Varestraint and hot ductility tests.

Constitutive equations to predict high temperature flow stress in a Ti-modified austenitic stainless steel

The experimental stress–strain data from isothermal hot compression tests, in a wide range of temperatures (1123–1523 K) and strain rates (10 −3–10 2 s −1), were employed to develop constitutive equations in a Ti-modified austenitic stainless steel. The effects of temperature and strain rate on deformation behaviors were represented by Zener-Holloman parameter in an exponent type equation. The influence of strain was incorporated in the constitutive analysis by considering the effect of strain on material constants. The constitutive equation (considering the compensation of strain) could precisely predict the flow stress only at 0.1 and 1 s −1 strain rates. A modified constitutive equation (incorporating both the strain and strain rate compensation), on the other hand, could predict the flow stress throughout the entire temperatures and strain rates range except at 1123 K in 10 and 100 s −1. The breakdown of the constitutive equation at these processing conditions is possibly due to adiabatic temperature rise during high strain rate deformation.

Assessment of microstructure stability of cold worked Ti-modified austenitic stainless steel during aging using ultrasonic velocity measurements and correlation with mechanical properties

As ultrasonic velocity is sensitive to the changes in texture, it is a more reliable technique than mechanical property measurements for assessment of microstructural stability (recrystallization behaviour) of cold worked alloy where recrystallization is coupled with precipitation. Hence, ultrasonic velocity measurements have been employed for studying the influence of Ti/C ratio on the microstructural stability of cold worked Ti-modified austenitic stainless steel during isochronal aging. In this alloy precipitation of TiC is known to retard recovery and recrystallization. The variation in ultrasonic velocity with aging temperature exhibited a three stage behaviour at all three frequencies employed (2, 10 and 20 MHz) and correlated well with the microstructural changes. Based on the microstructural investigations, the three stages have been identified to be recovery, progress of recrystallization and completion of recrystallization. There was one to one correspondence between the variation in the hardness, strength values and the variation in the ultrasonic velocity values as a function of aging temperature in assessing the microstructural changes, except when the interaction between the TiC precipitation and recrystallization is stronger.

Characterization of Microstructural Changes During Annealing of Cold Worked Austenitic Stainless Steel Using Ultrasonic Velocity Measurements and Correlation with Mechanical Properties

A technique that is sensitive to change in texture is more reliable than mechanical property measurements for the study of recrystallization behavior in alloys where recrystallization is coupled with precipitation. Hence, ultrasonic velocity measurements have been employed to characterize microstructural changes during annealing of cold worked Ti-modified austenitic stainless steel where precipitation of TiC is known to retard recrystallization. Mechanical property measurements, such as strength and hardness, could not distinguish the recovery and recrystallization regimes, and it was difficult to determine the exact time or temperature for onset of recrystallization. The variation of velocity with annealing time or annealing temperature exhibited a three-stage behavior. The velocity exhibited a slight increase in the recovery region followed by a sharp and continuous decrease in the recrystallization region and reached saturation on completion of recrystallization. Based on the microstructural investigations, the three stages were identified to be recovery, progress of recrystallization, and completion of recrystallization. Velocity measurements could sense the onset, progress, and completion of recrystallization more accurately compared with that of hardness and strength measurements.

Substructural recovery in a cold worked Ti-modified austenitic stainless steel during high temperature low cycle fatigue

Total axial strain controlled fatigue tests have been conducted in air at 823 and 923 K to ascertain the influence of cold work on LCF behaviour of a 15Cr–15Ni, Ti modified austenitic stainless steel, designated as Alloy D9. The LCF behaviour of the 20% cold worked alloy was compared with that of solution annealed alloy. A symmetrical triangular waveform at a constant strain rate of 3×10 −3 s −1 was employed for all the tests performed over the strain amplitude in the range of ±0.25 to ±1.00%. The cyclic stress response varied as a complex function of microstructure, temperature and strain amplitude. The cyclic stress response of solution-annealed alloy was generally characterized by initial hardening to the maximum stress followed by a regime of nearly stable peak stresses. The degree of initial hardening was higher at 823 K due to dynamic strain ageing effects. The cold worked alloy displayed a gradual softening prior to the attainment of saturation stress response stage. The plastic strain fatigue resistance of the cold worked alloy was inferior compared to that in solution-annealed condition at both 823 and 923 K. At low strain amplitudes, the cold worked alloy exhibited better total strain fatigue resistance at 923 K. The observed variations in fatigue life and cyclic stress response behaviour have been explained on the basis of crack initiation, development of substructure, precipitation behaviour and evolving changes in plastic strain during cycling.

Ultrasonic velocity measurements for characterizing the annealing behaviour of cold worked austenitic stainless steel

Precise measurements of shear wave velocities have been made in 20% cold worked and annealed samples of alloy D9 (Ti-modified austenitic stainless steel) to characterize the microstructural changes during annealing. The variation in wave velocity with annealing time exhibited a three stage behaviour. In the first stage, a slight increase in the velocity during recovery regime has been observed. This is followed by a significant increase in the velocity in the recrystallization regime (second stage) and reaching saturation on completion of recrystallization (third stage). Microstructural observations using optical metallography confirmed these three stages. The maximum variation in velocity is observed only when the polarization or the propagation direction of the shear wave is alingned with the cold working direction. Variation in shear wave velocity during annealing process, in general, is just opposite to that of the variation in longitudinal wave velocity. A number of velocity measurements were made by changing the propagation and polarization directions of the shear waves with respect to the cold working direction. Based on these measurements, a suitable velocity ratio parameter is suggested for determining the degree of recrystallization during annealing of cold worked alloy D9.

The kinetics of formation and growth of TiC precipitates in Ti-modified stainless steel studied by positron annihilation spectroscopy

The formation and growth of TiC precipitates in Ti-modified austenitic stainless steel (D-9 alloy) is monitored by positron lifetime spectroscopy. From isochronal annealing studies various recovery stages are identified. TiC precipitates are found to be more stable in 20% cold worked alloy than in a 17.5% cold worked sample. From the isothermal annealing studies, it is found that TiC precipitation is controlled by dislocations. The limited temperature dependence of dislocation controlled TiC precipitation is governed by an apparent activation energy of 1.6 eV. In 20% cold worked alloy, TiC precipitates are found to be stable against growth even after 1000 h of annealing at 923 K. For higher annealing temperatures, TiC precipitate coarsening occurs due to recrystallisation.

Tensile flow and work-hardening behavior of a Ti-modified austenitic stainless steel

The flow-stress data of a 15Cr-15Ni-2.2Mo-Ti modified austenitic stainless steel in the temperature range 300 to 1023 K was analyzed in terms of Ludwigson and Voce equations. The parameters of these equations were critically examined with respect to the effect of Ti/C ratio and test temperature. It was found that the Ludwigson equation described the flow behavior adequately up to the test temperature of 923 K, whereas the Voce equation could be employed in the full temperature range. The peaks/plateaus observed in the variation of these parameters as a function of temperature in the intermediate temperature range have been identified as one of the manifestations of dynamic strain aging (DSA). Also, the variation of these parameters with temperature clearly could bring out the different domains of DSA observed in this alloy. The work-hardening analysis of the flow-stress data revealed that in the DSA regime, the onset of stage III hardening is athermal.

In-situ observation of an austenitic stainless steel weld joint during helium irradiation

Microstructural evolution during helium-ion irradiation at 773 K in a weld metal sample (containing 10% δ-ferrite) of Ti-modified austenitic stainless steel was observed in-situ using a transmission electron microscope. Very fine helium bubbles formed at high number density in both δ-ferrite and austenite by a dose of 3 × 10 19 ions m −2. However, the different microstructural evolution developed in the two phases with increasing dose. Fine bubbles in δ-ferrite rapidly grew with increasing dose and coalescenced at doses beyond 9 × 10 19 ions m −2. Tiny bubbles in austenite remained very stable during irradiation to a dose of 2 × 10 20 ions m −2. The number density of bubbles was about one order of magnitude larger in austenite than that in δ-ferrite, and increased with increasing dose. Swelling became much larger in δ-ferrite than in austenite, as a result. This is the inverse phenomenon to the conventional result that swelling is lower in ferrite than in austenite. Sigma phase formed by radiation-enhancement at grain boundaries between δ-ferrite and austenite and at the interfaces within δ-ferrite at a dose 9 × 10 19 ions m −2 and grew with increasing dose. The chemical composition of σ-phase formed during irradiation showed Cr and Mo enrichment, and Fe and Ni depletion compared with σ-phase formed thermally.

The influence of temperature on swelling behavior in electron irradiated austenitic stainless steels

A study of the influence of temperature on swelling behaviour in electron irradiated austenitic stainless steel has been performed with a high voltage electron microscope (HVEM) in the temperature range 400 to 720°C. The specimen materials were solution annealed (SA) 316 stainless steel (SS), cold-worked (CW) 316 SS and Ti-modified austenitic stainless steel (Ti-mod, SS). The electron energy in HVEM was 1 MeV. The results of mean void density, mean void diameter, void swelling rate and incubation dose vs dose and temperature are presented. The irradiation temperature influenced the microstructure, overall sink strength and relative strength of neutral sinks to that of biased sinks in the specimens and so made the void nucleation, void growth and swelling behaviour different in 316 SA, 316 CW and Ti-mod, SS.