M. Nandagopal

Effect of nitrogen content on dynamic strain ageing behaviour of type 316LN austenitic stainless steel during tensile deformation

Abstract The effect of nitrogen content on the dynamic strain ageing (DSA) behaviour of type 316LN austenitic stainless steel has been studied. The nitrogen content was varied from 0·07 to 0·22 wt-%. The tensile tests were carried out over a temperature range of 300–1123 K and at three strain rates in the range 3×10−3–3×10−5 s−1. Serration was observed in the load elongation curves in the intermediate test temperature range and has been considered due to DSA phenomenon. The critical strain to onset of serrated flow increased with increase in nitrogen content and strain rate. The temperature for onset of DSA and the temperature of disappearance of DSA were found to increase with the increase in nitrogen content. The variations in tensile strength and work hardening rate of the steel with temperature exhibit peak values in the intermediate temperature range and have been attributed due to DSA phenomenon. The activation energy for DSA, estimated based on the temperature and strain rate dependences of the strain to onset of serrated flow, was found to increase from 111 to 218 kJ mol−1 with the increase in nitrogen content from 0·07 to 0·22 wt-% and the increase has been attributed to the possible enhanced interaction of the DSA causing interstitial nitrogen with substitutional chromium.

Evaluation of variation of tensile strength across 316LN stainless steel weld joint using automated ball indentation technique

Tensile strength variation across 316LN stainless steel fusion welded joint comprising of base metal, deposited weld metal and heat affected zone (HAZ) has been evaluated by Automated Ball Indentation (ABI) technique. Automated Ball Indentation tests were conducted on the various zones of the steel weld joint at 300, 523 and 923 K. The flow curves obtained from ABI results were consistent with corresponding conventional uniaxial tensile test results. The HAZ exhibited higher tensile strength than the other regions of the steel weld joint at all investigated temperatures. The ratio of ultimate tensile strength to yield stress (YS), which represents the work hardening behaviour, increased with an increase in temperature for the base metal and HAZ; whereas it remained nearly the same for the weld metal.

Influence of temperature on tensile flow and workhardening behaviour of modified 9Cr–1Mo steel

Abstract Tensile true stress (σ)–true plastic strain (ϵ) and workhardening behaviour of modified 9Cr–1Mo steel have been analysed using flow constitutive relationships proposed by Hollomon, Ludwik, Ludwigson, Swift and Voce for wide range of temperatures (300–1023 K) at a strain rate of 3×10−4 s−1. Relationship proposed by Voce has been found to describe stress–strain data adequately at all temperatures. Variation of instantaneous workhardening rate (θ) with stress (σ) displayed the two stage workhardening behaviour. It was characterised by a rapid decrease in the transient stage followed by gradual decrease in the stage III of workhardening. Three distinct regimes have been exhibited in the variation of workhardening parameters of constitutive relationships and workhardening rate with temperature. The decrease in flow stress and workhardening parameters with increasing temperature indicated the dominance of dynamic recovery at high temperatures. At intermediate temperature (523–723 K) plateau has been observed and considered to be one of the manifestation of dynamic strain aging (DSA). Tensile properties of the steel have been predicted well from the parameters of voce constitutive relationship.

Effect of notch root radius on tensile behaviour of 316L(N) stainless steel

Abstract Type 316L(N) stainless steel (SS) is used as the major structural material for high temperature components of sodium cooled fast reactors. The influence of notch root radius on the tensile behaviour of 316L(N) SS under multi-axial stress state was investigated. Double U-notches with five different kinds of notch geometry were incorporated symmetrically into the tensile testing specimens by changing the notch root radius while keeping the gross diameter, net diameter and notch depth as the same for all the notches. The notch root radius was varied as 0·25, 0·5, 1·25, 2·5 and 5 mm. Tensile tests were carried out on the notched specimens at room temperature (298 K) and at 923 K at a constant strain rate of 3×10−3 s−1. The tensile strength and yield strength of notched specimen of 316L(N) SS increased with decrease in notch radius at both the temperatures and the notch severity was less pronounced at high temperature. The fractured notch surface was analysed using scanning electron microscope and unfractured notch was sliced along the axis and observed under optical microscope. Finite element analysis was performed on the models of notched specimens with various notch root radii. These results showed that Von Mises equivalent stress which was derived from triaxial stresses decreased with decrease in notch radius. The shift of location of peak values of maximum principal stress and hydrostatic stress towards the axis of the specimen, leading to formation of cracks, occurred at a lower nominal stress when the notch radius was increased.

Recrystalization Behavior of Coldworked 14Cr-15Ni-2Mo Austenitic Stainless Steel Under Tensile Deformation

Abstract Recrystallization behavior of 20% coldworked alloy D9 (14Cr-15Ni-2Mo austenitic stainless steel) during high temperature tensile deformation has been assessed. The alloy was produced by vacuum induction melting followed either by vacuum arc refining (VAR) or electroslag refining (ESR). Tensile studies were carried out at various temperatures between ambient and 1073 K at an interval of 50 K and at strain rate of 1.2 × 10−3 s−1. The peak in the stress-strain curve was observed for the tensile tests conducted at 973–1073 K. This is attributed to dynamic recrystallization during tensile deformation. The values of the apparent activation energy were estimated as 384 kJ/mol for ESR grade alloy D9 and 372 kJ/mol for VAR grade alloy D9 by hyperbolic-sine Arrhenius equation, which agree well with that required for dynamic recrystallization in stainless steels. TEM studies confirmed extensive recovery and dynamic recrystalization in both the alloys.

Effect of Prior Cold Work on Tensile Flow and Work Hardening Behavior of a Titanium Bearing Modified Austenitic Stainless Steel

The tensile flow and work hardening behavior of 14Cr-15Ni-2.2Mo-Ti austenitic stainless steel at 298 K and 973 K with different prior cold worked levels ranging from 16% to 24% were analyzed using the Ludwigson, Swift and Voce constitutive equations. Influence of prior cold work on various Ludwigson parameters is discussed. While the Swift equation is used to predict the prior cold work, the yield strength is estimated using the Voce equation. The work hardening analysis using plot showed stage II hardening in the case of the material without coldwork. A two stage hardening behavior consisting of rapidly decreasing transient stage followed by gradually decreasing stage III hardening is observed at all cold work levels.

Influence of Nitrogen on the Notch Sensitivity of 316LN Stainless Steel During Tensile Deformation

316L(N) stainless steel (SS) containing 0.07 mass% nitrogen is used as a major structural material for sodium cooled fast reactor components that are designed to operate at elevated temperatures with a design life of 40 years. With a view to increase the component design life to 60 years, studies are being carried out to develop a high nitrogen grade of 316L(N) SS designated as 316LN SS with superior high temperature tensile, creep and low cycle fatigue properties. Extensive studies on the mechanical properties, corrosion resistance and weldability are underway on 316LN SS containing nitrogen in the range of 0.07 to 0.22 mass%. In this investigation, influence of nitrogen on the notch sensitivity of 316LN SS during tensile deformation has been studied. The notched specimens contained a 60ı V-notch with a root radius of 0.1 mm and an effective gauge diameter of 2.8 mm. Tensile tests were conducted at room temperature and at 823 K on four different heats containing 0.07, 0.11, 0.14 and 0.22 mass% nitrogen using both smooth and notched specimens, at a strain rate of 3 10 3 s . Finite element analysis was performed to evaluate the values of triaxial stresses and strain developed in the notch region. The yield strength and ultimate tensile strength of 316LN SS were found to increase with the inclusion of notch for all the heats and at both the test temperatures. The ductility of notched specimens was found to be lower than that of the smooth specimens in all the cases. It was observed that the amount of nitrogen content had a significant effect on the tensile strength and ductility of 316LN SS.

Effect of nitrogen on tensile flow behaviour of type 316 LN austenitic stainless steel

Abstract The influence of nitrogen content on the tensile flow behaviour of type 316 LN austenitic stainless steel has been studied. Nitrogen content in the steel has been varied in the range 0·07 to 0·22 wt-%. Tensile tests were carried out over the temperature range of 300–1123 K at a nominal strain rate of 3×10−3 s−1. The tensile flow behaviour of the steels has been analysed based on the constitutive equation proposed by Voce. The Voce’s parameters of initial stress (σi) and saturation stress (σs) were found to increase linearly with increase in nitrogen content at all the test temperatures. Tensile properties of the steels were predicted from Voce constitutive equation parameters.

Influence of Steel Melting Processes on Tensile Properties of 14Cr-15Ni-Ti Stainless Steel

Abstract A titanium-modified 14Cr-15Ni-2Mo austenitic stainless steel, known as alloy D9, has been chosen as the material for the fuel cladding and hexagonal wrapper of the Prototype Fast Breeder Reactor presently under construction at Kalpakkam. The alloy is generally produced by double vacuum melting process consisting of Vacuum Induction Melting (VIM) followed by Vacuum Arc Remelting (VAR). An alternate route consisting of vacuum induction melting followed by electro slag refining (ESR) has been employed to produce the alloy with lower inclusion content Tensile studies were carried out at various temperatures between ambient and 1023 K at an interval of 50 K and strain rate of 1.2 × 10–3 s–1 on the steels in 20% coldworked condition. Tensile properties of both VAR grade and ESR grade material were found to be similar. The influence of the secondary processing routes on the mechanical properties of alloy D9 is studied.