K. Laha

Self-healing Effect of Boron Nitride Precipitation on Creep Cavitation in Austenitic Stainless Steel:

In this study, the precipitation of boron nitride (BN) on creep cavity surfaces during creep and its beneficial effect on the creep rupture properties are reported for a type 304 austenitic stainless steel. In the conventional 304 stainless steel, the trace of soluble S segregates strongly on the creep cavity surfaces and promotes its growth during creep. In the modified 304 stainless steel added with B, Ce, and Ti, the trace of soluble S is removed effectively by the formation of Ce and Ti sulfides, and the segregation of S on the creep cavity surface is replaced by the precipitation of the BN compound. As BN is very stable at high temperatures, its precipitation on the cavity surface is expected to suppress the creep cavity growth rate. It is suggested that the precipitation of BN on the creep cavity surface provides austenitic stainless steel with the function of self-healing for creep cavitation with an associated increase in the creep rupture strength and ductility.

Some chemical and microstructural factors influencing creep cavitation resistance of austenitic stainless steels

Creep cavitation in materials is greatly influenced by trace elements. To enhance creep cavitation resistance, the chemical composition of 304, 321, 347 austenitic stainless steels was modified with the addition of minute amounts of boron and cerium. The addition of boron and cerium to type 304 stainless steel led to an increase in its creep rupture life with an associated decrease in creep rupture ductility. The addition of boron and cerium to the titanium-containing 321 steel and niobium-containing 347 steel was found to increase their creep rupture life and ductility. Creep cavitation was highly suppressed in the 347 and 321 steels with the addition of boron and cerium. The chemistry of the grain boundary and creep cavity surface was analyzed by Auger electron spectroscopy. Extensive sulphur segregation was observed on the grain boundary and cavity surface of the steels without boron and cerium addition and even in the 304 steel containing boron and cerium. In the boron- and cerium-containing 347 and 321 steels, respectively, segregation of elemental boron and the BN compound on the cavity surface were observed. These segregations reduced cavity growth rate substantially in these steels and BN segregation was found to be more effective in reducing cavity growth rate than boron segregation. Cerium acts as a getter for soluble sulphur in the steels by precipitation of ceriumoxysulfide (Ce2O2S) to facilitate the segregation of boron on the cavity surface.

Reduced Activation Ferritic Martensitic Steel and Fabrication Technologies for the Indian Test Blanket Module in ITER

Abstract India is one of the countries associated with the development and testing of test blanket modules (TBMs) in ITER. Accordingly, India has taken up development of 9Cr-W-Ta reduced activation ferritic martensitic (RAFM) steel, which is the structural material chosen for TBMs, together with the associated manufacturing technologies required for TBM fabrication. With the objective of developing an India-specific RAFM steel, four heats of RAFM steel with tungsten and tantalum contents varying in the ranges 1 to 2 wt% and 0.06 to 0.014 wt%, respectively, were melted. The steel was melted through vacuum induction melting and vacuum arc refining routes with strict control over the amounts of elements that induce radioactivity (Mo, Nb, B, Cu, Ni, Al, Co, and Ti) and the elements that promote embrittlement (S, P, As, Sb, Sn, Zr, and O). Extensive characterization of the microstructure and mechanical properties of the steel was carried out. The ductile-to-brittle transition temperature of the steel increased slightly with increasing tungsten and tantalum content. The tensile strength of the steel was found not to change significantly with increasing tungsten content; however, it decreased marginally with increasing tantalum content, with a consequent increase in ductility. The creep rupture strength of the steel at 823 K was found to increase significantly with increasing tungsten content, whereas it decreased with increasing tantalum content. The low-cycle fatigue life of the steel at 823 K was found to increase with increasing tungsten and tantalum content; however, extensive cyclic softening was exhibited when the tungsten content was >1.4 wt%. RAFM steel containing 1.4 wt% tungsten and 0.06 wt% tantalum was found to have a better combination of strength and toughness and is specified as Indian RAFM (INRAFM) steel. The joining technologies adopted for the fabrication of a TBM are hot isostatic pressing to produce the first wall, followed by gas tungsten arc (GTA), electron beam (EB), laser, and laser hybrid welding for joining the rest of the TBM. Welding techniques for joining RAFM steel have been developed and characterized. The properties of the GTA welds met the full specifications of the requirement and were comparable to the properties of the base metal. This consumable has also been used to carry out hybrid laser welding successfully. A procedure for using EB welding to join plates of thicknesses up to 12 mm has been developed. Impact tests conducted on EB welds showed that the toughness of the weld metal in the as-welded condition is comparable to that of the base metal. A box structure that simulates one of the components of a TBM has been fabricated using EB welding to demonstrate the applicability of the process to component fabrication. Laser welding of 6-mm-thick plates of RAFM steel has also been carried out successfully, and the properties of the weld joints have been found to be satisfactory. This paper discusses the development of INRAFM steel and its properties and the current status of the fabrication technologies being developed for fabrication of the Indian TBM to be tested in ITER.

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.

Small Specimen Test Techniques for Estimating the Tensile Property Degradation of Mod 9Cr-1Mo Steel on Thermal Aging

The degradation in mechanical properties of modified (mod) 9Cr-1Mo steel on thermal aging at 923 K has been studied using the small specimen test techniques—shear-punch and ball-indentation tests. Small volumes of material required for these test techniques make them unique tools for the assessment of service related degradation and failure analysis of structural components. A variety of heat treated microstructural conditions of mod 9Cr-1Mo steel is generated on which both conventional tensile test and shear-punch/ball-indentation tests are carried out. The relationship between conventional tensile properties and small specimen test results is established from these standardization experiments. Small specimens of Mod 9Cr-1Mo steel in the thermally aged conditions are tested using both shear punch and ball-indentation techniques and the tensile property changes are determined. Both these tests identically reflect and quantify the changes in the tensile properties with aging time which correlates well with the microstructural changes observed using optical and electron microscopic studies.

Type IV cracking behaviour of modified 9Cr-1Mo steel weld joints

Creep deformation and rupture behaviour of modified 9Cr–1Mo steel weld joints fabricated by single-pass activated TIG (A-TIG) and shielded metal arc welding (SMAW) processes have been investigated at 923 K over a stress range of 50–110 MPa after post-weld heat treatment. The formation of coarse M23C6 precipitates and recovery of martensite lath structure into subgrain in the ICHAZ of the weld joints resulted in hardness trough. The weld joints exhibited significantly lower creep rupture lives than the base metal at lower applied stresses. Creep rupture location of the weld joints was found to occur in the ICHAZ. The reduced ductile mode of failure was observed in the low stress level in comparison with high stress level. An extensive localised creep deformation, coarsening of M23C6 precipitates in the ICHAZ with creep exposure led to the premature Type IV failure of the joints. The coarsening of M23C6 precipitates was extensive in the mid-section of the ICHAZ than the sub-surface of the joints, and was more predominant in the SMAW joint. While A-TIG weld joint exhibited reduced creep cavitation and coarsening of M23C6 precipitates due to lower deformation constraints by adjacent regions in the ICHAZ. Hence, A-TIG weld joint exhibited higher creep rupture life than the SMAW joint.

Mechanistic approach for prediction of creep deformation, damage and rupture life of different Cr–Mo ferritic steels

Abstract A mechanistic approach based on finite element analysis of continuum damage as proposed by Kachanov has been used to assess and compare creep deformation, damage and rupture behaviour of 2·25Cr–1Mo, 9Cr–1Mo and modified 9Cr–1Mo ferritic steels. Creep tests were carried out on the steels at 873 K over a stress range of 90–230 MPa. Modified 9Cr–1Mo steel was found to have highest creep deformation and rupture strength whereas 2·25Cr–1Mo steel showed the lowest among the three ferritic steels. Creep damage in the steels has been manifested as the microstructural degradation. 2·25Cr–1Mo steel was more prone to creep damage than 9Cr–steels. Finite element estimation of creep deformation and rupture lives were found to be in good agreement with the experimental results.

Assessment of fatigue response of thermally aged reduced activation ferritic-martensitic steel based on finite element analysis

Abstract In the present investigation, effect of thermal ageing on low cycle fatigue (LCF) behaviour of Reduced Activation Ferritic Martensitic steel has been assessed by finite element analysis. The steel was thermally aged at 873 K for 3000 hour. Low cycle fatigue tests were carried out on both the as-received and thermally aged material at strain rate of 3×10−3 s−1 at 823 K, over strain amplitudes in the range of ± 0.25 to ± 0.8%. Continuous cyclic softening till final failure, except for initial few cycles especially at relatively lower strain amplitudes, was observed in both the material conditions. Thermal ageing resulted in marginally higher cyclic stress response accompanied by lower fatigue life. The differences in fatigue responses have been attributed to the coarsening of precipitates on thermal ageing. Finite element analysis has been carried out considering combined isotropic and kinematic hardening as material model to estimate the effect of thermal ageing on the response of material under LCF loading. Thermal ageing was found to decrease both the isotropic and kinematic hardening with appreciable effect on isotropic hardening. The predicted cyclic stress response and hysteresis loops were found to be in good agreement with the experimental data. The LCF life of the steel has been estimated based on the hysteresis energy approach.

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.