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Research Papers: Materials and Fabrication

Coalesced Martensite in Pressure Vessel Steels

[+] Author and Article Information
Hector Pous-Romero

Department of Materials Science & Metallurgy,
University of Cambridge,
Cambridgeshire CB2 3QZ, UK
e-mail: hp323@cam.ac.uk

Harry Bhadeshia

Department of Materials Science & Metallurgy,
University of Cambridge,
Cambridgeshire 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 24, 2013; final manuscript received December 4, 2013; published online February 27, 2014. Assoc. Editor: Marina Ruggles-Wrenn.

J. Pressure Vessel Technol 136(3), 031402 (Feb 27, 2014) (6 pages) Paper No: PVT-13-1121; doi: 10.1115/1.4026192 History: Received July 24, 2013; Revised December 04, 2013

An alloy commonly used for large pressure vessels, known as SA508 Grade 3, has a microstructure after heat treatment consisting of a mixture of tempered bainite and martensite at fast cooled regions near surfaces subject to water quenching. These two phases are conventionally recognized to consist of fine platelets, each of which is approximately 0.2 μm in thickness, enhancing strength and leading to good toughness properties. We have discovered in our experimental work that there are circumstances where the adjacent platelets of a similar orientation can coalesce as the austenite transforms, to produce much coarser structures which are believed to be detrimental to toughness. An examination of published micrographs reveals that such coalesced regions existed but were not noticed in previous studies. The mechanism of coalescence is described and methods to ameliorate the coarsening are discussed.

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Figures

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Fig. 1

SEM of the SA508 Grade 3 studied in this work showing clear bimodal size distribution of martensitic plates. Sample austenitized at 1200 °C for 48 h and water quenched. The arrows indicate regions where structures have coalesced.

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Fig. 2

Austenite grain size revealed by thermal etching. (a) Austenitized at 860 °C for 1 h. (b) Austenitized at 1150 °C for 10 min. (c) Austenitized at 1200 °C for 48 h. Arrows point at thermal grooves defining austenite grain boundaries.

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Fig. 3

Dilatometric curves after austenitization at 1150 °C for 10 min, showing the transformation behaviour during continuous cooling at 4, 7, 10, and 30 °C s−1. Hardness Vickers (Hv) values are also shown.

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Fig. 4

SEM micrographs of SA508 Grade 3 steel austenitized at 1150 °C for 10 min showing coalesced martensite. (a) Cooled at 10 °C s−1, (b) cooled at 30 °C s−1, (c) high magnification image of (b) showing serrated edges.

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Fig. 5

Calculated driving force versus transformation temperature for SA508 Grade 3 steel calculated using MUCG83

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Fig. 6

Size differences in coalesced martensite plates for different austenitic grain sizes. (a) Austenitized at 860 °C for 1 h. (b) Austenitized at 1200 °C for 48 h.

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Fig. 7

Unnoticed case of coalesced martensite in the HAZ of a SA508 Grade 3 steel, resulting in an unexplained very low value of impact energy [22]

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Fig. 8

SEM micrographs of SA508 Grade 3 steel austenitized at 1150 °C for 10 min showing coalesced martensite after tempering

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