R. Kishore

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.

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.

Effect of carbon on the ductilisation of electron-beam welds in molybdenum

Abstract The ductility of unwelded and electron-beam welded molybdenum sheet has been evaluated by means of notch-bend testing at temperatures ranging from − 190 to 260°C. The effect of carbon additions on the ductile-brittle transition temperature and fracture behaviour of molybdenum has also been determined. The results indicate that carbon is very effective in lowering the ductile-brittle transition temperature of welded and unwelded specimens. It has been demonstrated that a balance exists between the ductilising effect of carbon and the embrittling effect of oxygen segregating to grain boundaries. For most effective ductilisation, an optimum amount of carbon is required to offset the embrittling effect of oxygen. It has been shown that the brittle fracture mode changes from intergranular to transgranular cleavage when optimum ductilisation is achieved. The decrease in the transition temperature has been explained in terms of the enhancement of the grain boundary crack nucleation stress with additions of carbon.

Strengthening of a 12CrMoV turbine blade steel by retempering

Commercially available martensitic 12CrMo, 12CrMoV, and HCrMoVNb steels in the tempered condi-tion are often used to fabricate turbine blades. Tempering, done at the processing stage itself, is usually carried out at a temperature approximately 100 to 150 ° above the service temperature. At the service temperature, the steel undèrgoes a second tempering. The effect of this second tempering at a lower tem-perature on the strength of 12CrMoV steel is delineated and discussed in this paper.