Quantitative microstructural analysis of pressure vessel steel for nuclear applications

The reactor pressure vessel (RPV) is a key component within a nuclear reactor, as it contains fission products and acts as a primary pressure boundary. The hostile conditions these components are subjected to means that the initial mechanical properties and microstructure at the start of life need to be well understood to enable design and safety allowances for their evolution in service to be made. RPVs have historically been manufactured from large thick section forgings, produced from ASME SA508 type steels and fabricated through submerged arc welding. Where the welded joints are paramount to maintaining the integrity and safe operation of the nuclear plant. However, through the collaboration of the Generation IV initiative [1] new materials and joining processes are being researched and trialled to prolong the reactor life.

The effect of forging prior to heat treatment is a key feature to understand and it is important to determine what happens to the microstructure following the forming process prior to heat treatment. Through the use of a trial forging it was possible to establish that the effect of forging was not uniform across the whole component, with areas of high and low stress and strain identified. However, the effect of location within the forging was found to have less of an effect on the final properties achieved, compared to the effect of heat treatment.

Heat treatment is carried out following the forging process to achieve the mechanical properties required for the application of reactor pressure vessels. However, to determine what happens to both the mechanical properties and microstructure and to investigate the relationship between the two, sections of the trial forging were subjected to different cooling rates and tempering times. It was found that air cooling leads to the formation of ferrite throughout the microstructure and this has a detrimental effect on the mechanical properties achieved. However, water quenching the component does not give the material time for ferrite grains to form and instead results in a lath structure of bainite. Water quenching combined with a tempering heat treatment that results in a Larson Miller Parameter of 19 was found to give preferred properties that are substantially higher than that of the other thermal treatment conditions examined.

Narrow gap tungsten inert gas (NG-TIG) welding has also been investigated as a replacement for submerged arc welding of reactor pressure vessels. It was found that the NG-TIG weld provided a more refined microstructure in the heat affected zone and fusion regions of the samples compared to the heterogeneous microstructure achieved with submerged arc welding. This also contributed to the substantially improved mechanical properties, including increased ultimate tensile strength and hardness in each region of the weld microstructure.

This research has established an understanding of an SA508 Grade 3 forging and the effect heat treatment and cooling rates have on the microstructure and mechanical properties that can be achieved with this grade of material. A new welding technique for the fabrication of reactor pressure vessels has also been determined to be an improved process, in which the microstructural refinement and enhanced properties achieved will potentially help to prolong the life of the RPV in the next generation of nuclear reactors.