T.K. Sinha

Delayed hydride cracking in Zr–2.5Nb pressure tube material☆

Abstract Delayed hydride cracking (DHC) is one of the localized forms of hydride embrittlement caused by hydrogen migration up the tensile stress gradient. In this work, DHC velocity was measured along the axial direction of the double melted, cold worked and stress-relieved Zirconium–2.5Niobium pressure tube material in the temperature range of 162–283 °C. The DHC crack growth was monitored using the direct current potential drop (DCPD) technique. The calibration curves between the normalized DCPD output and the normalized crack length at different test temperatures were also used to determine the DHC velocity. A simple model capable of explaining the observed features of DHC is proposed. The model explains the basis for the occurrence of incubation period associated with DHC crack initiation. Activation energy associated with the DHC in this alloy was found to be 56 kJ/mole.

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.

Effect of dynamic strain ageing on the tensile properties of a modified 9Cr–1Mo steel

Tensile testing of a modified 9Cr–1Mo steel in two microstructural conditions (710T– normalized at 1100°C, tempered at 710°C, and 550T – normalized at 1100°C tempered at 550°C) in the temperature range 25–450°C, under strain rates of 2.3×10-5–2.3×10-3s-1, exhibited serrated flow curves, with serrations appearing almost at the onset of deformation and disappearing before ultimate strengths were attained. The serrated flow curves (characteristics of dynamic strain ageing) were accompanied by increased ultimate strengths, loss of ductility and negative strain-rate sensitivity, relative to the ambient temperature properties. However, the increase in ultimate strength and the reduction in ductility were much larger for 710T specimens, as compared to 550T ones. In the dynamic strain ageing regime, the work-hardenability of 710T specimens increased rapidly while that of 550T specimens remained practically unaffected. Based on the microstructural consideration and the observed activation energy of 45 kJ mol-1, it is proposed that serrations are initiated by a nitrogen atmosphere formation on the waiting dislocations by a pipe diffusion mechanism, and they disappear by diffusion to the precipitate sinks during deformation. Because the fine alloy carbide precipitates in 550T specimens are more effective sinks than those of 710T ones, they can cause much faster depletion of the atmosphere, resulting in a much smaller effect of dynamic strain ageing on the tensile properties of 550T specimens.

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.

Hydride blister formation in Zr-2.5wt%Nb pressure tube alloy

Abstract Hydride blisters were grown over a period of 5–91 days under controlled thermal boundary condition using Zr–2.5wt%Nb pressure tube sections. Rectangular plate type specimens were hydrided to hydrogen concentration in the range of 20–250 ppm by weight and homogenized at 400 °C. These specimens were held in a specially fabricated jig capable of producing the required thermal gradients. The bulk specimen and the cold spot temperatures were maintained in the range of 270–400 °C and 40–100 °C respectively. Depending on the thermal gradients employed, two types of blister morphology were identified. The type I blister was single, round and located at the cold spot region whereas the type II blister consisted of several small blisters along a ring around the cold spot. Microstructural examination of the blister cross-section revealed three regions; a single-phase region consisting of hydrides, a region consisting of matrix containing both radial and circumferential hydrides, and another region consisting of matrix and circumferential hydrides. An attempt was made to rationalize the observed radial–circumferential hydride platelet orientation. Hydride blister growth rates were found to vary strongly with hydrogen concentration and bulk specimen temperature. The observed time for blister growth was found to be in agreement with the Sawatzkys model [31] .

Serrated flow in a modified 9Cr-1Mo steel

During high temperature tensile testing of a modified 9Cr-1Mo steel in tempered martensitic condition, serrated plastic flow, implying the occurrence of dynamic strain aging, was observed. The serrated stress-strain curves were unique in the sense that the serrations appeared almost at the beginning of deformation and disappeared before the ultimate strengths were attained. Moreover, the strains to the disappearance of serrations varied systematically with the test temperature and strain-rate. A brief literature survey revealed that in studies on Nickel alloys and ferritic-martensitic steels, the disappearance of serrated flow was found to be a thermally activated process and that the phenomenon manifested in the higher temperature regime by either a progressively larger strains to the onset of serrations or a progressively smaller strain to the disappearance of serrations of the flow curve. However, it appears that the type of serrated flow curves observed in this study, has not yet been reported in any other ferritic-martensitic steels. It was thus felt necessary to carry out a detailed investigation of dynamic strain aging in this steel. In this paper, the possible mechanism controlling the appearance of serrations and their disappearance of the stress-strain curves are discussed.

Age hardening of cold-worked Zr-2.5wt% Nb pressure tube alloy

Abstract Age-hardening of a cold-worked Zr-2.5wt% Nb pressure tube alloy at ageing temperature, 300–500°C was studied with crystallographic texture as a variable. Results showed that hardness, yield and ultimate tensile strengths increased with increasing ageing temperature up to about 400°C, while ductility remained practically unaffected by ageing. Further, the observed increase in both hardness and strength, was more pronounced in the direction parallel to the tranverse direction of tube than in the direction parallel to the longitudinal direction. X-ray diffraction indicated the formation of small volume fraction of ω-phase at all ageing temperatures. Transmission electron microscopic examination revealed partial decomposition of β-Zr phase and formation of fine spheroidal precipitate at an ageing temperature of 400°C, while ageing at 500°C, caused formation of cell structure and coarsening of the precipitate. Based on these observations, it was concluded that age-hardening of cold-worked 2r-2.5wt% Nb pressure tube alloy is a competition between precipitation-hardening and softening by recovery and the phenomenon is strongly influenced by texture.

The morphology and ageing behaviour of δ-ferrite in a modified 9Cr-1Mo steel

Dual phase (martensite + δ-ferrite) microstructures were developed in a modified 9Cr-1Mo steel, by austenitising at 1523–1623 K, followed by water-quenching. These duplex structures were thermally aged at 973 K for ageing periods varying from 30 min to 21 h. Morphological aspects of δ-ferrite phase and its response to age-hardening were studied by optical, scanning electron and transmission electron microscopy, X-ray diffraction, electron probe microanalysis and microhardness testing. It was observed that austenitizing at 1523 K produced fine, acicular δ-ferrite while the δ-ferrite formed by austenitising at higher temperatures (1573–1623 K) were massive, irregular-shaped and banded. Moreover the presence of 8-ferrite caused an abnormally strong (110) reflection, observed in X-ray diffraction patterns of martensite plus δ-ferrite structures. This behaviour is thought to be due to development of (110) texture in δ-ferrite phase. Thermal ageing at 973 K caused age-hardening of δ-ferrite with a peak hardness attained after 3.6 ks of ageing. Electron microscopic results suggest that the observed hardening was caused by the formation of Fe2Mo Laves phase.

Low temperature deformation behaviour of ASTM A-203D nuclear structural steel

The steel A-203D in martensitic, tempered martensitic and ferritic-pearlitic microstructural conditions, was deformed in tension at temperatures between 77 to 300 K and at a strain rate of 6.67×10−5 sec−1. The thermal component of the flow stress and the activation parameters were measured as a function of temperature. It was observed that the microstructure did not affect either the thermal part of the flow stress (σ*) or the activation parameters (V* and ΔH) and that its effect was felt only on the athermal component (σμ) of the flow stress. Further the relations of the activation parameters with stress, strain and temperature were found to be consistent with Dorn-Rajnak theory of Peierls mechanism of plastic deformation. In addition measurements of slow-bend and impact transition were also carried out for ferritic-pearlitic structure. Based on these observations, it is shown that the impact transition temperature of this structure can be emperically correlated with the thermal activation parameters.

Effect of prior cold rolling and test temperature on stress-strain rate behaviour of a Zr-2.5Nb alloy

Zr-2.5 wt% Nb alloy sheet, obtained by unfolding and straightening a pressure tube, was further cold rolled upto 39% reduction in thickness to investigate the effect of cold working on the stress (σ)-strain rate (έ) behaviour over a strain rate range of ∼2 × 10−5 to 5 × 10−3 s−1 and a temperature range of 625 to 700 °C. Irrespective of the amount of rolling, the log σ vs log έ plots exhibit superplastic behaviour with strain rate sensitivity index, m, as high as 0.8, which decreases to 0.2 at higher strain rates. On the other hand, the activation energy for deformation, Q, increases from 171.1 kJ/mol for superplastic deformation to 249 kJ/mol in Region III. The tendency for improved superplasticity (m) is seen upon cold working by 22% or more at the test temperatures 675 and 700 °C.