Advanced Materials Research Vol. 794

Abstract: This paper summarizes the results of the studies on fracture mechanics characterisation of SS 316L(N) and its welds. The results presented include the fracture toughness and FCG properties of the base and weld materials at different temperatures. Influence of nitrogen content on the base material properties is discussed. Further, the effects of long-term ageing at different temperatures on the fracture and FCG behaviour of the welds are presented and discussed. The weld metal has been subjected to extended thermal ageing, and a detailed study has been undertaken to characterize the (i) FCG properties and (ii) quasistatic J-R curves for the indigenously developed SS 316(N) weld material at both ambient and service temperatures. The ageing conditions covered include the advanced ageing according to the RCC-MR design code, i.e, > 4000 h at 923 K and the low temperature ageing, i.e., 643-823 K the operating range for the SS 316L(N) components in PFBR. The results are discussed in detail in the light of microstructural changes taking place in the weld metal and their influence on the operating micromechanisms.

Abstract: The deformation and fracture behaviour of AISI 403, a tempered martensitic stainless steel for end fitting application of Pressurised heavy water reactor is being reported. The deformation behaviour studies entailed characterisation of tensile behaviour in the temperature range 77 - 873 K for the as recieved and the fine grained Nb modified variant of AISI 403. the study of elevated tensile behaviour in the two steels has been undertaken with the purpose of characterising the strain rate - temperature domain of the occurrence of dynamic strain aging (DSA) phenomenon. In both steels, while the temperature range for the manifestations of characteristic anomalies in the tensile curve due to DSA was observed within 523 - 673K, the strain domain for the fine grained Nb modfied variety was significantly higher as comapred with the as recievied variety. The low temperature tensile tests for the as recieved AISI 403 revealed the presence of Pseudo=alloy softening in the temperature range 273 - 193 K. The effect of high DBTT of the AISI 403 steel was shown by the fracture toughness tests in the J-integral format at room temperature that displayed significant scatter in smaples with high in-plane and out of plane constraint. Smaples with lower constraint showing stable crack growth were further tested at high temperature to obtain the temperature dependence of initiation fracture toughness and propagation touhgness. Within the DSA tempertaures a sharp decline in the fracture properties were observed. A mechanistic interpretation for the manifestations of the various observed phenomena is presented.

Abstract: The influence of magnetic interactions on high temperature thermodynamic stability of Fe-Cr binary system has been analysed in the light of accurate isothermal calorimetry measurements (400-1473 K) on Fe-20wt.%Cr alloy. The onset of two successive principal transformations namely, (i) α(Fe-rich bcc)+α(Cr rich bcc)α(HT bcc) at 702±10 K; and (ii) α_ferroα_para at 925 ±10 K, with their associated enthalpy effects (Δ^oH_mag = 2 kJ mol ^-1; C_p^mag = 20 J mol^-1 K^-1) have been clearly delineated by the measured enthalpy variation with temperature. A precise quantification of magnetic contribution to high temperature thermodynamic stability has been attempted using physically based modelling approach.

Abstract: Synchrotron X-ray microtomography(SR-μCT) scans have been carried out on sample coupons extracted from the fracture specimens of a 10.86% Cr heat resistant steel exposed to crep deformation at 873K over stresses of 120, 150, and 180 MPa. The 3D cavitation characteristics in terms of void volume fraction, numbwer density and size distribution as a function of the applied stress has been determined by quantitative analysis of the reconstructed tomograohy slice datasets. The relationship between heterogenous spatial distribution of creep voids and variation in rupture life has been exploited in terms of microstructural sites during the onset of creep embrittlement.

Abstract: The paper aims at demonstration of reduction in residual stress, distortion and sensitisation in austenitic stainless steel pipe welds. The welding processes considered are Gas Tungsten Arc Welding (GTAW) and Shielded Metal Arc welding (SMAW) along with Hot wire GTAW with narrow gap welding technique. It was shown that residual stress is significantly reduced in case of hot wire GTAW. The reduction in residual stress is due to the low heat input and high deposition rate. Lower heat input leads to higher cooling rate giving more margins on sensitization in heat affected zone (HAZ). This has been confirmed by measuring temperature and cooling rate during welding in HAZ of the weld joints. Susceptibility to sensitization of the welds has been compared by carrying out ASTM E262 Practice A and E along with Electo-Potentiokinetic Reactivation (EPR) method. The results of the tests show that values are within acceptable limit for both the cases. However, hot wire GTAW has marginally better sensitization resistance. Fatigue crack growth rate and fracture resistance behavior of the above mentioned weld joints have been compared by carrying out tests on the specimens (Compact Tension) and full scale pipe welds with crack. Fatigue crack growth rate and fracture toughness of the weld joints (hot wire GATW) is superior to conventional GTAW and SMAW. The paper presents the details of the above mentioned studies.

Abstract: Stainless steels of different types and grades are being developed world over to meet ever increasing demand for enhanced materials performance. Advanced stainless steel products have applications in a variety of industries including nuclear, defence, space, chemical, oil and gas, medical and appliance. It is understood that the properties of the alloys strongly depend not only on the chemical composition but also on their microstructure, which in turn depends on parameters in the manufacturing process. Therefore, the challenge to the manufacturing industry is not only selection of optimum composition but also relevant manufacturing processes to meet the requirements. Designing a manufacturing process and optimum process schedules using experimental trials is both time consuming and expensive. Modeling and simulation play an important role to reduce these times effectively. This paper presents important points to be considered to produce clean steels and highlights the applicability of modeling techniques that can be effectively applied in a manufacturing industry. Some of the case studies that are included in the paper are Computational Fluid Dynamics model to understand gas atomisation Finite element modeling of compound tube extrusion In conclusion, the power of modeling and simulation to understand manufacturing processes is highlighted.

Abstract: Boron content and its distribution play a significant role in modifying the metallurgical and mechanical properties of many steels and alloy at lower level of concentration. Precipitation of boron at the grain boundaries, have shown to improve the creep strength in titanium stabilized steel, high temperature ductility in low carbon corrosion resistant steel and the hardenability in low carbon steel in general. Titanium-stabilized steel (DIN 1.4970), was developed as a possible material for fast breeder sodium-cooled nuclear reactor core components for its superior creep strength, high micro-structural stability and elevated void swelling resistance. It is well known that, helium produced during neutron irradiation through the ¹⁰B(n,α)Li⁷ reaction, affects the mechanical properties and the amount of void swelling in nuclear reactor materials. Two nos. of Ti-stabilized steel samples with 40ppm boron and 2ppm boron (DIN 1.4970 & DIN1.4970LB steel) were analyzed for boron re-distribution behavior during different thermo-mechanical treatment using a technique known as Neutron Induced Alpha Autoradiography (NIAA). This technique is a well known technique, and widely used for revealing the spatial distribution of boron in the materials with a resolution approaching to ppm level. This technique has also been used to detect the influence of copper addition on boron distribution pattern in steel specimen. Mapping of boron autoradiography of Low carbon steels containing 20ppm of boron with and without copper was able to demonstrate this behavior. Boron track mapping of Low carbon steel without copper, in solution annealing treatment, show the uniform distribution of boron throughout the matrix, whereas when the similar steel with 1.48% copper was mapped, it shows the precipitation of boron at the grain boundaries.

Abstract: Austenitic stainless steels are the major material of construction for the fast breeder reactors in view of their adequate high temperature mechanical properties, compatibility with liquid sodium coolant, good weldability, availability of design data and above all the fairly vast and satisfactory experience in the use of these steels for high temperature service. All the Nuclear Steam Supply System (NSSS) components of FBR are thin walled structure and require manufacture to very close tolerances under nuclear clean conditions. As a result of high temperature operation and thin wall construction, the acceptance criteria are stringent as compared to ASME Section III. The material of construction is Austenitic stainless steel 316 LN and 304 LN with controlled Chemistry and calls for additional tests and requirements as compared to ASTM standards. Prototype Fast Breeder Reactor (PFBR) is sodium cooled, pool type, 500 MWe reactor which is at advanced stage of construction at Kalpakkam, Tamilnadu, India. In PFBR, the normal heat transport is mainly through two secondary loops and in their absence; the decay heat removal is through four passive and independent safety grade decay heat removal loops (SGDHR). The secondary sodium circuit and the SGHDR circuit consist of sodium tanks for various applications such as storage, transfer, pressure mitigation and to take care of volumetric expansion. The sodium tanks are thin walled cylindrical vertical vessels with predominantly torispherical dished heads at the top and bottom. These tanks are provided with pull-out nozzles which were successfully made by cold forming. Surface thermocouples and heaters, wire type leak detectors are provided on these tanks. These tanks are insulated with bonded mineral wool and with aluminum cladding. All the butt welds in pressure parts were subjected to 100% Radiographic examination. These tanks were subjected to hydrotest, pneumatic test and helium leak test under vacuum. The principal material of construction being stainless steel for the sodium tanks shall be handled with care following best engineering practices coupled with stringent QA requirements to avoid stress corrosion cracking in the highly brackish environment. Intergranular stress corrosion cracking and hot cracking are additional factors to be addressed for the welding of stainless steel components. Pickling and passivation, Testing with chemistry controlled demineralised water are salient steps in manufacturing. Corrosion protection and preservation during fabrication, erection and post erection is a mandatory stipulation in the QA programme. Enhanced reliability of welded components can be achieved mainly through quality control and quality assurance procedures in addition to design and metallurgy. The diverse and redundant inspections in terms of both operator and technique are required for components where zero failure is desired & claimed. This paper highlights the step by step quality management methodologies adopted during the manufacturing of high temperature thin walled austenitic stainless steel sodium tanks of PFBR.

Abstract: The austenitic stainless steels are used in nuclear spent fuel reprocessing and waste management plants and the process fluid is nitric acid at temperature up to boiling point. However incorporation of oxidizing ions e.g. fission products as well as corrosion products, in nitric acid stream make the environment highly corrosive to stainless steels. Present work aims to investigate role of process parameters and material parameters (composition and microstructure) on corrosion behaviour of stainless steels. The process parameters studied are temperature, acid concentration and oxidizing ions. It has been shown that the potential attained on stainless steel is a function of acid concentration and temperature and is further strongly affected by addition of oxidizing ions. This developed potential determines the corrosion behaviour of stainless steel. Increasing the temperature and concentration of nitric acid and concentration of oxidizing species increased the developed potential. Potentials were applied to types 304 L (nitric acid grade - NAG), 304 L (commercial purity) and 310 L stainless steels in boiling 6 M nitric acid for a period of 48 h. The corrosion rates measured in such experiments were plotted as a function of applied potential. The form of corrosion was established by microstructural examination. A clear demarcation was observed between uniform corrosion and intergranular corrosion at a potential of 960-980 mV_SCE. Above this potential range corrosion rate increased exponentially and the form of corrosion is shown to be intergranular corrosion. Below this potential range, uniform and low rate of corrosion occurred. The influence of microstructure (step, dual and ditch) of type 304 L was also studied and is described in this paper.

Abstract: Austenitic stainless steels have received much attention in recent years due to their excellent combination of corrosion, mechanical and wear properties. They are finding wide applications in chemical, power, oil, refinery, biomedical, marine sectors and other industries where both good mechanical properties and excellent corrosion and wear resistances are demanded. In the spent nuclear fuel reprocessing plants and waste storage and processing plants involving nitric acid as the main process medium, type 304L stainless steels (SS) are employed as work horse materials for manufacturing more than 90% of the plant components. Though these alloys form a protective Cr₂O₃ passive film over the surface in nitric acid under plant operating conditions, they undergo various types of corrosion failures in service. Welding and other metallurgical parameters including alloying elements, cold working, heat treatment etc. degrade the performance of the alloy in service. For qualifying the alloy for plant applications, ASTM A262 practice A and C are currently employed, however, long term performance under simulated plant operating conditions is necessary to understand the failure modes and life prediction of components. Today, nitrogen represents an economically, environmentally, attractive and versatile alloying element to steels and stainless steels. The beneficial effect of nitrogen alloying in stainless steels are manifolds, including solid solution strengthening, precipitation effects, phase control and corrosion and wear resistances. Recent years have seen a rapid development of these alloys with improved properties owing to advances in alloy processing technologies. The objective of the lecture is to bring out the various corrosion issues in reprocessing plants, short term laboratory versus long term field corrosion data, modeling for life prediction, effect of redox ions, nitrogen alloying, welding and corrosion damage, etc. and highlight the remedial actions to overcome the shortcomings due to corrosion issues.