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

J Guo,X Xu,Mark A.E Jepson,R.C Thomson

doi:10.1016/j.ijpvp.2019.05.010

J Guo, X Xu + Show 2 more

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https://doi.org/10.1016/j.ijpvp.2019.05.010

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Abstract

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.

Highlights

MarBN steel, based around the general composition of 9Cr-3W-3CoVNbBN, is a recently developed material and is a promising candidate for the replacement of the more conventional 9–12% Cr steels for the applications of hot section components including tubes, pipes and headers within thermal powerplant
When Tp reached an intermediate temperature of ∼1473 K (∼1200 °C), a more homogeneous microstructure consisting of equiaxed austenite grains was present in the simulated heat affected zone (HAZ) microstructure, Fig. 2 (e)
The microstructure in the simulated weld specimens can be linked with the microstructure in the HAZ of real welds

Summary

Introduction

MarBN steel, based around the general composition of 9Cr-3W-3CoVNbBN, is a recently developed material and is a promising candidate for the replacement of the more conventional 9–12% Cr steels for the applications of hot section components including tubes, pipes and headers within thermal powerplant. Compared to the more conventional materials, including the Grade and Grade steels, MarBN steel has demonstrated a superior creep strength and improved oxidation resistance [1,2,3]. This is achieved by an increased content of solid solution elements in combination with a balanced content of minor elements to provide an additional precipitation strengthening effect [4]. Within other 9–12%Cr steels, creep failure in these welds typically occurs in the region close to the boundary between the heat affected zone (HAZ) and the parent metal [8,9], which is termed as ‘Type IV’ failure [10]

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