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Research Papers: Materials and Fabrication

The Consequences of Macroscopic Segregation on the Transformation Behavior of a Pressure-Vessel Steel

[+] Author and Article Information
E. J. Pickering

Department of Materials Science and Metallurgy,
University of Cambridge,
Pembroke Street,
Cambridge CB2 3QZ, UK
e-mail: ejp57@cam.ac.uk

H. K. D. H. Bhadeshia

Department of Materials Science and Metallurgy,
University of Cambridge,
Pembroke Street,
Cambridge CB2 3QZ, UK
e-mail: hkdb@cam.ac.uk

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received July 26, 2013; final manuscript received January 6, 2014; published online February 27, 2014. Assoc. Editor: Osamu Watanabe.

J. Pressure Vessel Technol 136(3), 031403 (Feb 27, 2014) (7 pages) Paper No: PVT-13-1126; doi: 10.1115/1.4026448 History: Received July 26, 2013; Revised January 06, 2014

It is important that the material used to produce high-integrity pressure vessels has homogeneous properties which are reproducible and within specification. Most heavy pressure vessels comprise large forgings derived from ingots, and are consequently affected by the chemical segregation that occurs during ingot casting. Of particular concern are the compositional variations that arise from macrosegregation, such as the channels of enriched material commonly referred to as A-segregates. By causing corresponding variations in microstructure, the segregation may be detrimental to mechanical properties. It also cannot be removed by any practically feasible heat treatments because of the large scale on which it forms. Here we describe an investigation on the consequences of macrosegregation on the development of microstructure in a pressure-vessel steel, SA508 Grade 3. It is demonstrated that the kinetics of transformation are sensitive to the segregation, resulting in a dramatic spatial variations in microstructure. It is likely therefore that some of the scatter in mechanical properties as observed for such pressure vessels can be attributed to macroscopic casting-induced chemical segregation.

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Figures

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Fig. 1

Prominent macrosegregation mechanisms in steel ingot casting: (a) interdendritic fluid flow caused by thermosolutal convection (+ symbols denote positive segregation, i.e., regions enriched in solute, whilst the—symbols denote depleted regions), and (b) equiaxed grain sedimentation in the melt

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Fig. 2

(a) Different types of macrosegregation typically found in large ingots. Positive segregation is denoted by + symbols representing regions enriched in solute, and negative by—for solute-depleted regions. Similar figures can be found in Refs. [1,3] and [4]. (b) Schematic of material removed (shaded) from ingot to produce a typical large shell forging.

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Fig. 3

Section of SA508 Grade 3 forging received, roughly prepared, and macroetched. Dark regions are areas of positive segregation. Any horizontal striations are likely caused by the grinding procedure, not macrosegregation.

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Fig. 4

Optical microscopy. (a) Sample with A-segregation, showing region of allotriomorphic ferrite between two enriched areas. (b) Shows the transition from enriched material, ferrite, and widmanstätten ferrite. (c) Region of material without evidence of a-segregation showing band of ferrite within upper-bainite bulk. (d), (e), and (f) show the widmanstätten ferrite, allotriomorphic ferrite, and positively segregated microstructures, respectively.

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Fig. 5

Scanning electron microscopy: (a) secondary-electron image of enriched region and (b) corresponding back-scattered electron image. (c) and (d) are secondary-electron images of the allotriomorphic ferrite and widmanstätten ferrite microstructures, respectively. Blocky areas of retained austenite are evident in (c), and a similar formation is labeled in (b).

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Fig. 6

Transmission electron microscopy: (a) widmanstätten ferrite plates at low magnification, showing carbide-free laths separated by pearlitic regions. (b) Lower bainite colonies with coarse carbides. (c) Thin bainitic laths and extensive carbide precipitation within a-segregated material. (d) Twinned martensite within a martensite–austenite island.

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Fig. 7

Qualitative EPMA mapping showing optical micrograph of region of interest (with A-segregate, allotriomorphic ferrite, and widmanstätten ferrite moving bottom left to top right) alongside maps for Si, Mn, and Mo

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Fig. 8

Plot of Mn concentration in the solid (interfacial value) against fraction solid for the steel examined in the study, under non-equilibrium (Scheil [30]) solidification conditions. The dashed line gives the bulk alloy composition. Partition coefficients and liquidus slopes used for calculations were taken from the relevant phase diagrams. Note that the true conditions for solidification of low-alloy steels are likely to be between the ideal equilibrium and non-equilibrium cases.

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