S.L. Mannan

Solidification cracking in austenitic stainless steel welds

Solidification cracking is a significant problem during the welding of austenitic stainless steels, particularly in fully austenitic and stabilized compositions. Hot cracking in stainless steel welds is caused by low-melting eutectics containing impurities such as S, P and alloy elements such as Ti, Nb. The WRC-92 diagram can be used as a general guide to maintain a desirable solidification mode during welding. Nitrogen has complex effects on weld-metal microstructure and cracking. In stabilized stainless steels, Ti and Nb react with S, N and C to form low-melting eutectics. Nitrogen picked up during welding significantly enhances cracking, which is reduced by minimizing the ratio of Ti or Nb to that of C and N present. The metallurgical propensity to solidification cracking is determined by elemental segregation, which manifests itself as a brittleness temperature range or BTR, that can be determined using the varestraint test. Total crack length (TCL), used extensively in hot cracking assessment, exhibits greater variability due to extraneous factors as compared to BTR. In austenitic stainless steels, segregation plays an overwhelming role in determining cracking susceptibility.

Creep behaviour of post-weld heat-treated 2.25Cr-1Mo ferritic steel base, weld metal and weldments

Abstract The evaluation of the creep rupture behaviour of 2.25Cr-1Mo ferritic steel at 773 and 823 K over a stress range of 130–300 MPa has been carried out. The material conditions examined include base material, weld metal and weldments (comprising base, weld and heat-affected zones). Specimens for creep testing were taken from single-V-weld pads, fabricated by manual metal arc welding using basic coated 2.25Cr-1Mo electrodes, and were given a post-weld heat treatment (973 K for 1 h). Microstructure and hardness in the as-welded, post-weld heat-treated and creep-tested conditions were evaluated. The heat-affected zone consisted of coarse-grain bainite, fine-grain bainite and intercritical structure regions. Generally, the weld metal exhibited significantly higher creep rupture strength than the base material, while the composites showed inferior creep strength. Ductility was found to decrease with increasing rupture time for all the material conditions. The rupture elongation exhibited by the base material was significantly higher than that shown by weld metal and composite specimens. Composite specimens crept at a faster rate while weld metal deformed at a slower rate than base metal. The applied stress (σ) dependence of the secondary creep rate (es was found to follow a power-law relationship of the form e = Aσ n . At all test conditions, transgranular fracture was observed. Failure in the composite specimens occurred in the intercritical structure of the heat-affected zone. Interrupted creep tests at 823 K on composite specimens have revealed progressive localization of creep deformation in the intercritical structure prior to fracture. An attempt has been made to establish equations that can predict the weld joint creep properties on the basis of the base and weld metal properties.

Ball indentation studies on the effect of aging on mechanical behavior of alloy 625

Abstract The effect of aging on mechanical behavior of Alloy 625 was investigated using the non-destructive Stress–Strain Microprobe (SSM) system. SSM is based on an automated ball indentation (ABI) technique, and involves multiple indentations by a small spherical indenter at a single penetration location under strain-controlled conditions. The technique permits evaluation of mechanical properties such as yield strength, ultimate tensile strength, strength coefficient and strain hardening exponent. Alloy 625 was aged at six different temperatures in the range of 873–1173 K for 500 h each. ABI tests were carried out at room temperature and at 473 K. The variation of yield and ultimate tensile strengths with aging temperature exhibited a peak in strength following aging at 973 K. The peak stress was 1.5 times the strength of the unaged material, and the strength after aging at 1173 K was nearly equal to that of the unaged material. The peak in strength is attributed to the precipitation of the γ” phase. The fall in strength due to aging above 973 K is attributed to the precipitation, growth and dissolution of δ -phase precipitates. These studies demonstrate that ABI can be used as a non-destructive technique to determine changes in mechanical properties of nickel base alloy components due to aging.

Development of fuels and structural materials for fast breeder reactors

Fast breeder reactors (FBRs) are destined to play a crucial role in the Indian nuclear power programme in the foreseeable future. FBR technology involves a multi-disciplinary approach to solve the various challenges in the areas of fuel and materials development. Fuels for FBRs have significantly higher concentration of fissile material than in thermal reactors, with a matching increase in burn-up. The design of the fuel is an important aspect which has to be optimised for efficient, economic and safe production of power. FBR components operate under hostile and demanding environment of high neutron flux, liquid sodium coolant and elevated temperatures. Resistance to void swelling, irradiation creep, and irradiation embrittlement are therefore major considerations in the choice of materials for the core components. Structural and steam generator materials should have good resistance to creep, low cycle fatigue, creep-fatigue interaction and sodium corrosion.The development of carbide fuel and structural materials for the Fast Breeder Test Reactor at Kalpakkam was a great technological challenge. At the Indira Gandhi Centre for Atomic Research (IGCAR), advanced research facilities have been established, and extensive studies have been carried out in the areas of fuel and materials development. This has laid the foundation for the design and development of a 500 MWe Prototype Fast Breeder Reactor. Highlights of some of these studies are discussed in this paper in the context of our mission to develop and deploy FBR technology for the energy security of India in the 21st century.

Influence of carbon and nitrogen on the creep properties of type 316 stainless steel at 873 K

Abstract This paper is concerned with the creep properties of a nitrogen-alloyed type 316L (316LN) stainless steel (SS) at 873 K in the stress range 215–335 MPa. A comparison has been made with the creep properties of type 316 SS having a similar grain size and chemical composition with respect to the major alloying elements. Type 316LN SS showed a higher rupture life, a lower steady state creep rate and a higher rupture ductility at all stress levels. The improvement in creep properties has been attributed to the combined influence of precipitation strengthening by fine intragranular and intergranular carbides and strengthening arising from the nitrogen in solid solution. Correlations between metallographic observations and the creep properties as well as the role of interstitial elements in these properties are discussed.

Low cycle fatigue and creep-fatigue interaction behavior of 316L(N) stainless steel and life prediction by artificial neural network approach

Low cycle fatigue (LCF) behavior of solutionized 316L(N) stainless steel (SS) has been studied at various temperatures, strain amplitudes, strain rates, hold times and in 20% prior cold worked condition. The alloy in general showed a reduction in fatigue life with, increase in temperature, increase in strain amplitude, decrease in strain rate, an increase in duration of hold time in tension and with prior cold work. The LCF and creep-fatigue interaction (CFI) behavior of the alloy was explained on the basis of several operative mechanisms such as dynamic strain ageing, creep, oxidation and substructural recovery. The capability of artificial neural network (ANN) approach to life prediction under LCF and CFI conditions has been assessed by using the data generated in the present investigation. It is demonstrated that the prediction is within a factor of 2.

Processing maps for hot working of commercial grade wrought stainless steel type AISI 304

The hot-working behaviour of commercial grade wrought stainless steel type AISI 304 is characterized using processing maps developed on the basis of the dynamic material model and hot compression data in the temperature range 600–1250°C and strain rate range 0.001–100 s–. The material exhibits a dynamic recrystallization (DRX) domain in the temperature range 1000–1200°C at a strain rate of 0.01 s−1. Optimum hot workability occurs at 1100°C and 0.01 s−1, which corresponds to a peak efficiency of 32% in the DRX domain. Finer grain sizes are obtained at 1000°C and all strain rates. In comparison with low interstitial stainless steel type AISI 304 L, the DRX occurs at lower strain rate and temperature. This result is attributed to the strong primary effect of interstitial carbon enhancing the rate of DRX nucleation. Flow instabilities occur in the entire region above the DRX domain. Flow localization occurs in the regions of instability at temperatures lower than 1000°C and ferrite formation is responsible for the instability at higher temperatures.

A toughness study of the weld heat-affected zone of a 9Cr-1Mo steel

Abstract The toughness of the weld heat-affected zone microstructures of a 9Cr–1Mo steel has been studied with simulated samples. From the Charpy impact test results, the highest toughness in terms of the highest upper-shelf energy and the lowest ductile-to-brittle transition temperature have been observed in the intercritical region. The lowest toughness was observed in the coarse prior-austenitic grained martensite region adjacent to the fusion line. The microstructural effect on the toughness has also been discussed.

Validation of processing maps for 304L stainless steel using hot forging, rolling and extrusion

The development of a microstructure in 304L stainless steel during industrial hot-forming operations, including press forging (mean strain rate of 0.15 s(-1)), rolling/extrusion (2-5 s(-1)), and hammer forging (100 s(-1)) at different temperatures in the range 600-1200 degrees C, was studied with a view to validating the predictions of the processing map. The results have shown that excellent correlation exists between the regimes exhibited by the map and the product microstructures. 304L stainless steel exhibits instability bands when hammer forged at temperatures below 1100 degrees C, rolled/extruded below 1000 degrees C, or press forged below 800 degrees C. All of these conditions must be avoided in mechanical processing of the material. On the other hand, ideally, the material may be rolled, extruded, or press forged at 1200 degrees C to obtain a defect-free microstructure.

Influence of prior thermal ageing on tensile deformation and fracture behaviour of forged thick section 9Cr-1Mo ferritic steel

Abstract Tensile tests were performed on specimens in quenched and tempered (Q+T) and thermally aged (TA) conditions over a wide temperature range (300–873 K) to assess the influence of prior thermal ageing on tensile deformation and fracture behaviour of forged thick section 9Cr–1Mo ferritic steel. Prior thermal ageing at 793 and 873 K for durations ranging from 10 to 5000 h did not cause a significant change in room temperature tensile properties. However, a marginal decrease in yield strength and reduction in area were observed for specimens aged for longer durations at 793 and 873 K. Prior thermal ageing at 793 K for 5000 h and at 873 K for 1000 and 5000 h produced significant reduction in strength values at intermediate temperatures (523–723 K) compared to that observed at high temperatures. At intermediate temperatures, the alloy in all heat treatment conditions exhibited serrated flow, a manifestation of dynamic strain ageing (DSA). The significant loss of strength in thermally aged conditions at intermediate temperatures has been attributed to reduced propensity to DSA. The elongation to fracture values at temperatures in the range 300–873 K were affected little by prior thermal ageing, whereas the reduction in area exhibited a decrease in the value with increasing thermal ageing. The fracture mode remained transgranular ductile at all test conditions investigated in the present study. However, specimens aged for longer durations exhibited chisel fracture at room and intermediate temperatures due to split in the martensite lath boundaries. The influence of thermal ageing on room temperature tensile properties of the forging remained similar to that reported for thin section 9Cr–1Mo steel.