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Influence of weld thermal cycle and post weld heat treatment on the microstructure of MarBN steel

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journal contribution
posted on 20.06.2019 by J. Guo, X. Xu, Mark Jepson, Rachel Thomson
Martensitic steels strengthened by Boron and Nitrogen additions (MarBN) were developed for high temperature/ high stress service in power plant for periods of many years and are being considered as a promising candidate for the replacement of the more conventional Grade 91/92 steels. In the present study, extensive microstructural observation of physically simulated Heat Affected Zone (HAZ) MarBN material has been carried out after dilatometry simulations to link the variation in microstructure with weld thermal cycles. The microstructure in the MarBN HAZ has been observed to vary from a refined equiaxed morphology to a duplex microstructure consisting of refined grains distributed on the pre-existing Prior Austenite Grain Boundaries (PAGBs) as the peak temperature of the weld thermal cycle decreases. The temperature range corresponding to the formation of the duplex grain structure coincides with the temperature regime for the dissolution of the pre-existing M23C6 carbides. An even distribution of the M23C6 carbides within the martensitic substructure was also observed after Post Weld Heat Treatment (PWHT), which is beneficial for the creep performance of the weld HAZ. The MX precipitates are more resistant to thermal exposure and are not completely dissolved until the peak temperature reaches 1573 K (1300 °C). The Nb-rich MX precipitates are the predominant type observed both after weld simulations and PWHT. The hardness between the materials experienced with the thermal cycles with different peak temperature does not significantly vary after PWHT conducted in an appropriate condition, which is likely to mitigate an unfavoured stress condition in the localised area within the HAZ.

Funding

We would like to acknowledge the support of the Engineering and Physical Research Council (EPSRC) for their support for the project through: Flexible and Efficient Power Plant: Flex-E-Plant (Grant numbe EP/K021095/1) and EPSRC and Innovate UK through: IMPULSE - Advanced Industrial Manufacture of Next-Generation MARBN Steel for Cleaner Fossil Plant (Grant number: EP/N509942/1).

History

School

  • Aeronautical, Automotive, Chemical and Materials Engineering

Department

  • Materials

Published in

International Journal of Pressure Vessels and Piping

Volume

174

Pages

13-24

Citation

GUO, J. ... et al., 2019. Influence of weld thermal cycle and post weld heat treatment on the microstructure of MarBN steel. International Journal of Pressure Vessels and Piping, 174, pp. 13-24.

Publisher

© the Authors. Published by Elsevier BV

Version

VoR (Version of Record)

Publisher statement

This work is made available according to the conditions of the Creative Commons Attribution 4.0 International (CC BY 4.0) licence. Full details of this licence are available at: http://creativecommons.org/licenses/ by/4.0/

Acceptance date

23/05/2019

Publication date

2019-05-28

Copyright date

2019

Notes

This is an Open Access Article. It is published by Elsevier under the Creative Commons Attribution 4.0 Unported Licence (CC BY). Full details of this licence are available at: http://creativecommons.org/licenses/by/4.0/

ISSN

0308-0161

Language

en

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