J.K. Chakravartty

Effect of serrated flow on deformation behaviour of AISI 403 stainless steel

Abstract AISI 403, a martensitic stainless steel in the quenched and tempered condition has been subjected to uni-axial tension test at a range of strain rates (2×10−5–10−2 s−1) and a range of temperatures (25–500°C) to investigate the effect of temperature and strain rate on its mechanical properties. Serrated flow behaviour, predominately of type A and B, has been observed in the temperature range of 250–375°C and at strain rates lower than 10−3 s−1, which suggests the presence of dynamic strain aging (DSA) in this parametric domain. In this domain, while an increase in ultimate tensile strength and work hardening have been observed, the yield strength has been found to be nearly independent of temperature. A loss in ductility, due to the presence of DSA has also been seen in this material. The solute responsible for serrated flow has been identified to be chromium, which is the major alloying addition in this material. McCormick’s method has been employed to determine the activation energy from the knowledge of critical strain for the appearance of serrations as a function of strain rate and temperature. The magnitude of activation energy is approximately 50% of that for bulk diffusion of chromium in body centered cubic iron suggesting that a mechanism other than volume diffusion is involved.

A study of internal hydrogen embrittlement of steels

Abstract A novel procedure for hydrogen charging and studying the Internal Hydrogen Embrittlement (IHE) of steels is described here. A cylindrical notched tensile sample with an extended end is employed for hydrogen charging. The extended portion of the sample forms the cathode in an alkaline bath and a constant uni-axial tensile load is applied during hydrogen charging. The stress gradient set up by the notch, which is not in contact with the electrolyte, enhances the hydrogen concentration at various trapping sites of the matrix beyond the solubility limit. Subsequent to charging, the specimen is kept under the same load as that during charging, for another 24 h to stabilize the population of hydrogen within the specimen matrix. At the end of this stage, the specimen is tensile tested to failure at room temperature. Two different steels namely maraging and mild steels have been chosen to study the effect of hydrogen ingress on mechanical properties. While an increase in tangent modulus (linear portion of the stress–strain diagram), yield strength, work hardening rate and ultimate tensile stress (UTS) has been observed on hydrogenation, a decrease in total elongation has been noticed for both the steels studied. Fractographic investigation has revealed that the fracture mode is predominantly ductile dimple (failure by micro-void coalescence) in both the materials and that the hydrogen reduces the size of the dimples. The observations of this investigation are significant in two respects: firstly, it demonstrates the efficacy of a hydrogen charging method for steels which can introduce hydrogen to a level much higher than its solubility limit and secondly, it reports for the first time enhancement of modulus and work hardening by hydrogen charging. These observations have been rationalized on the basis of current understanding on the effect of hydrogen on plastic properties and hypothesis of the models of IHE. It is suggested that the trapping of hydrogen by dislocations and other structural features of the matrix and the mutual interactions of their strain fields can account for the observed effects on yield strength, tangent modulus, work hardening rate, UTS and ductility.

Characterization of hot deformation behaviour of Zr---2.5Nb---0.5Cu using processing maps

The characteristics of hot deformation of beta-quenched Zr-2.5Nb-0.5Cu in the temperature range 650-1050 degrees C and in the strain rate range 0.001-100 s(-1) have been studied using hot compression testing. For this study, the approach of processing maps has been adopted and their interpretation done using the Dynamic Materials Model. The efficiency of power dissipation given by [2m/(m + 1)], where m is strain rate sensitivity, is plotted as a function of temperature and strain rate to obtain a processing map. The processing map for Zr-2.5Nb-0.5Cu within (alpha + beta) phase field showed a domain of dynamic recrystallization, occurring by shearing of alpha-platelets followed by spheroidization, with a peak efficiency of 48% at 750 degrees C and 0.001 s(-1). The stress-strain curves in this domain had features of continuous flow softening and all these are similar to that in Zr-2.5Nb alloy. In the beta-phase field, a second domain with a peak efficiency of 47% occurred at 1050 degrees C and 0.001 s(-1) and this domain is correlated with the superplasticity of beta-phase. The beta-deformation characteristics of this alloy are similar to that observed in pure beta-zirconium with large grain size. Analysis of flow instabilities using a continuum criterion revealed that the Zr-2.5Nb-0.5Cu exhibits flow localization at temperatures higher than 800 degrees C and strain rates higher than about 30 s(-1) and that the addition of copper to Zr-2.5Nb reduces its susceptibility to flow instability, particularly in the (alpha + beta) phase field.

Hot-working characteristics of Zircaloy-2 in the temperature range of 650–950°C

Abstract The hot-working characteristics of Zircaloy-2 have been studied in the temperature range of 650 to 950°C and in the strain-rate range of 10−3 to 102 s−1 using power dissipation maps which describe the variation of the efficiency of power dissipation, η = 2m /(m + 1) where m is the strain-rate sensitivity of flow stress. The individual domains exhibited by the map have been interpreted and validated by detailed metallographic investigations. Dynamic recrystallization occurs in the temperature range of 730 to 830°C and in the strain-rate range of 10−2 to 2 s−1. The peak efficiency occurs at 800°C and 0.1 s−1 which may be considered as the optimum hot-working parameters in the α-phase field of Zircaloy-2. Superplastic behaviour, characterized by a high efficiency of power dissipation is observed at temperatures greater than 860°C and at strain rates lower than 10−2 s−1. When deformed at 650°C and 10−3 s−1, the primary restoration mechanism is dynamic recovery, while at rates higher than 2s−1, the material exhibits microstructural instabilities in the form of localized shear bands.

Dynamic strain-ageing of A203D nuclear structural steel

Abstract The present investigation deals with the dynamic strain-ageing behaviour of a nuclear structural steel, designated ASTM A203 grade D, in tempered martensitic and ferritic-pearlitic microstructural conditions. The serrated stress-strain curves, characteristic of this phenomenon have been observed in the temperature range 100–200°C, with nominal strain rates varying from 1.33 × 10 −5 to 6.66 × 10 −4 /s . It has been noted that dynamic strain-ageing causes a sharp rise in ultimate strength and work-hardening rate, a marked decrease in ductility and a negative strain-rate sensitivity of the flow stress. In this temperature range, the yield stress also increases with increasing temperature but the rise in ultimate stress is much greater than the rise in yield stress. The temperature and strain-rate dependence of the onset of serrations yields an activation energy of 63 kj/mol (15 kcal/mol), which suggests that the process is controlled by interstitial diffusion, probably of nitrogen, in ferrite. It appears that microstructure does not have any strong influence on the changes in mechanical properties of this steel during dynamic strain-ageing.

Characterization of hot deformation behaviour of Zr–2.5Nb in β phase

Hot deformation characteristics of β Zr–2.5wt%Nb in the temperature range 1225–1425 K and in the constant true strain rate range of 0.002–10 s−1 were studied by uniaxial compression testing in vacuum. A processing map using the strain rate sensitivity parameter was developed for β Zr–2.5Nb on the basis of flow stress data as a function of temperature and strain rate for a given strain. A domain of high strain rate sensitivity of 0.5 at 1375 K and 0.002 s−1, was obtained and the grain size in this domain was large and equiaxed, suggesting the occurrence of large grain superplasticity. Thermal activation analysis was employed to determine the experimental activation volume and enthalpy, from which it was suggested that the rate controlling mechanism involved the movement of dislocation jogs. A comparison of Zr–2.5Nb with Zr in β phase suggested that Nb shifts the domain peak to higher temperatures.

Assessment of hydrogen embrittlement of Zircaloy-2 pressure tubes using unloading compliance and load normalization techniques for determining J–R curves

Abstract The fracture toughness of unirradiated cold pilgered pressure tubes (PT) has been studied as a function of temperature and hydrogen levels. Two methods of J – R curve evaluation, from small curved compact tension specimen, have been employed. In addition to the single sample unloading compliance method (298–473 K), a recently developed load normalization technique, using LMN function, has been used for temperature upto 548 K. The effect of circumferential hydrides on fracture toughness parameters has been studied. The results of both the techniques are compared to find out the suitability of load normalization method in such hydrogen embrittlement studies. The results show large deleterious effects of circumferential hydrides at lower temperatures and gradual restoration of toughness with increase in temperature. The study also shows that load normalization can be used to evaluate J – R curve in the cases where slow stable crack growth takes place. Hydrogen embrittlement can be evaluated using load normalization method for temperatures higher than 423 K.

Deformation in Zr–1Nb–1Sn–0.1Fe using stress relaxation technique

Abstract Deformation behavior of Zr–1Nb–1Sn–0.1Fe was studied using the stress relaxation technique. Stress relaxation experiments were carried out over a range of temperatures (296–765 K) and for strains up to 0.12. The stress–time data were analyzed to obtain the activation volume and enthalpy. It was found that in the strain rate range of 10 −4 –10 −6 s −1 and in the temperature range of 296–570 K, the activation volume and enthalpy do not vary with strain. From this and the magnitude of the activation volume and its variation with thermal stress, either the Peierls stress or the dislocation–interstitial interaction is the rate controlling short range barrier to dislocation motion. The time independent stress component obtained using decremental unloading technique, called here as the remnant stress, was observed to have a large temperature dependence. By using a relation in which the activation free energy is a function of thermal stress, it was found that, in general, the remnant stress cannot be used to represent the athermal stress.

The study of microstructural defects and mechanical properties in proton-irradiated Zr–1.0%Nb–1.0%Sn–0.1%Fe

Abstract We had earlier analysed point defects in irradiated Zr–1.0%Nb–1.0% Sn–0.1%Fe samples by positron annihilation lifetime spectroscopy (PALS). The studies revealed the generation of irradiation induced di- and tri-vacancy clusters. In the present work, we have carried out X-ray diffraction line profile analysis (XRDLPA) and studies of mechanical properties like ultimate tensile strength (UTS), yield strength (YS) and percentage elongation (%E) on the same irradiated samples at different doses. XRDLPA reveals that order of dislocation density remains almost unchanged up to the experimental limit of irradiation dose. Attempts have also been made to correlate the changes in mechanical properties with irradiation-induced defects. YS of the alloy increases with irradiation due to locking, or pinning of dislocations with vacancy clusters. However, there is an anomaly in YS at a dose of 5×10 20 protons/m 2 which has been explained in the light of interaction of vacancies with solute atoms. Percentage elongation on the other hand, shows a monotonic fall with increasing dose.

Microstructural and mechanical properties of service exposed Alloy 625 ammonia cracker tube removed after 100 000 h

Abstract The present study examined the evolution of microstructure and mechanical properties of Alloy 625 ammonia cracker tubes of heavy water production plant which had been exposed to temperatures around 600°C for 100 000 h. Detailed investigations revealed considerable modification of microstructure, significant increase of strength and loss of ductility and toughness during service and the extent of degradations were found to be sensitive to service temperature at different locations of the tube. While the increase in strength was due to precipitation of γ′′ and Ni2(Cr,Mo), the loss in ductility resulted from the presence of carbides and needle shaped δ particles at grain boundaries. When the exposed material was tested at elevated temperatures, the material softened with an attendant increase in ductility and toughness. A solution annealing treatment at 1160°C for 2 h was found to restore the microstructural as well as mechanical properties similar to that of virgin material.