A toughness study of the weld heat-affected zone of a 9Cr–1Mo steel

Abstract

The toughness of the weld heat-affected zone microstructures of a 9Cr–1Mo steel has been studied with simulated samples. From the Charpy impact test results, the highest toughness in terms of the highest upper-shelf energy and the lowest ductile-to-brittle transition temperature have been observed in the intercritical region. The lowest toughness was observed in the coarse prior-austenitic grained martensite region adjacent to the fusion line. The microstructural effect on the toughness has also been discussed.

Introduction

9Cr–1Mo steel in the normalised and tempered condition is used for steam generator structural material in conventional and nuclear power plants. High-temperature creep strength, coupled with adequate corrosion resistance and long-term microstructural stability, has made this material suitable for application in steam generators [1], [2], [3], [4]. In the normalised and tempered condition, the steel possesses finely dispersed carbides in a tempered martensitic matrix [4], [5]. However, the heat-affected zone (HAZ) produced by welding undergoes microstructural variations from the fusion line to the base metal (BM) and gives rise to concern specially from a fracture point of view. In this region, the study of fracture toughness becomes difficult due to the practical problems in fabricating samples of proper sizes from the narrow microstructural bands existing in the HAZ. However, studies of other mechanical properties have been conducted using simulation techniques [6], [7], but for the 9Cr–1Mo steel, information pertaining to the fracture toughness of different HAZ microstructures is relatively scarce. In this work, microstructural effects on the fracture toughness have been studied using simulated HAZ specimens.

HAZ of 9Cr–1Mo steel is composed of a coarse prior-austenitic grained martensitic (CPAGM) structure adjacent to the fusion line followed by fine prior-austenitic grained martensite (FPAGM) and intercritical region (ICR) merging with the tempered BM [8], [9], [10]. These microstructures have been reproduced by a simulation technique starting with the normalised and tempered BM. For the simulation of different microstructures, steel blanks were isothermally heat treated at different temperatures, followed by rapid cooling. Charpy impact tests have been carried out with the samples simulating different microstructural regions present in the HAZ. In the present paper, the microstructural effects on the fracture properties of the HAZ of 9Cr–1Mo steel have been discussed.