V. Ganesan

Ball Indentation Studies on the Effect of Nitrogen on the Tensile Properties of 316LN SS

Abstract Type 316L(N) stainless steel (SS) containing 0.02–0.03 wt% carbon and 0.06–0.08 wt% nitrogen is used as the major structural material for the components of fast reactors. Research is underway to improve the high-temperature mechanical properties of 316LN SS by increasing the nitrogen content in the steel above the level of 0.08 wt%. In this investigation, ball indentation (BI) technique was used to evaluate the effect of nitrogen content on the tensile properties of 316LN SS. BI tests were conducted on four different heats of 316LN SS containing 0.07, 0.11, 0.14 and 0.22 wt% nitrogen in the temperature range 300–923 K. The tensile properties such as yield strength and ultimate tensile strength increased with increase in nitrogen content at all the investigated temperatures. These results were consistent with the corresponding uniaxial tensile test results. These studies showed that BI technique can be used to optimize the chemical composition during alloy development by evaluating tensile properties with minimum volume of material.

Creep Deformation and Rupture Behavior of Single- and Dual-Pass 316LN Stainless-Steel- Activated TIG Weld Joints

Abstract Creep deformation and rupture behavior of single-pass and dual-pass 316LN stainless steel (SS) weld joints fabricated by an autogenous activated tungsten inert gas welding process have been assessed by performing metallography, hardness, and conventional and impression creep tests. The fusion zone of the single-pass joint consisted of columnar zones adjacent to base metals with a central equiaxed zone, which have been modified extensively by the thermal cycle of the second pass in the dual-pass joint. The equiaxed zone in the single-pass joint, as well as in the second pass of the dual-pass joint, displayed the lowest hardness in the joints. In the dual-pass joint, the equiaxed zone of the first pass had hardness comparable to the columnar zone. The hardness variations in the joints influenced the creep deformation. The equiaxed and columnar zone in the first pass of the dual-pass joint was more creep resistant than that of the second pass. Both joints possessed lower creep rupture life than the base metal. However, the creep rupture life of the dual-pass joint was about twofolds more than that of the single-pass joint. Creep failure in the single-pass joint occurred in the central equiaxed fusion zone, whereas creep cavitation that originated in the second pass was blocked at the weld pass interface. The additional interface and strength variation between two passes in the dual-pass joint provides more restraint to creep deformation and crack propagation in the fusion zone, resulting in an increase in the creep rupture life of the dual-pass joint over the single-pass joint. Furthermore, the differences in content, morphology, and distribution of delta ferrite in the fusion zone of the joints favors more creep cavitation resistance in the dual-pass joint over the single-pass joint with the enhancement of creep rupture life.

High temperature tensile properties of 316LN stainless steel investigated using automated ball indentation technique

Abstract Type 316LN stainless steel (SS) is the principal structural material for the components of sodium cooled fast reactors operating under elevated temperature conditions. In order to assess the degradation in strength of service exposed components using a small specimen testing technique such as automated ball indentation (ABI), it is necessary to carry out prior detailed ABI studies on the virgin material. In this investigation, the tensile behaviour of as-received 316LN SS were investigated at several temperatures in the range 298–973 K using ABI technique. The load-depth of indentation data measured from ABI tests was analyzed using semi-empirical relationships to obtain the tensile properties. The yield stress and the flow curves were determined by correlating ABI results with corresponding uniaxial tensile test results. Trend curve for tensile strength with temperature, as estimated from ABI tests, exhibited a plateau region in the temperature around 823 K, similar to uniaxial tensile tests. The variations of strength coefficient, strain hardening exponent, yield ratio, hardness and uniform ductility with temperature were evaluated from ABI tests. The ABI technique was found to estimate the influence of temperature on tensile properties sensitively.

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.

Creep properties and design curves for nitrogen enhanced grade of 316LN stainless steel

Abstract 316L(N) stainless steel (SS) containing 0·02–0·03 wt-% carbon and 0·06–0·08 wt-% nitrogen is the principal material for the high temperature structural components of the prototype fast breeder reactor in India. In order to increase the economic competitiveness of sodium cooled fast reactors (SFRs), there is a strong desire to increase the design life from the current level of 40 years to at least 60 years for the future reactors. As a part of the efforts to develop materials with superior mechanical properties suitable for longer design life, the influence of nitrogen at concentrations higher than 0·07 wt-%, on the high temperature mechanical properties of type 316L(N) SS is being studied. Four heats of 316L(N) SS, containing 0·07, 0·11, 0·14 and 0·22 wt-% nitrogen have been evaluated extensively in terms of their tensile, creep, low cycle fatigue and creep fatigue interaction properties. Based on these studies, the nitrogen content has been optimised at 0·14 wt-%. This nitrogen enhanced grade of steel (NE316LN SS) was found to have significantly better tensile, creep, low cycle fatigue and creep-fatigue properties as compared to the PFBR grade of 316L(N) SS. This paper presents the influence of nitrogen on the creep deformation, damage and fracture behaviour of NE316LN SS. Design of high temperature SFR components is made on the basis of RCC-MR design code. The creep properties of NE316LN SS have been analysed in terms of the procedures for generation of the design code. Time–dependent design curves have been generated.