During the fabrication processes of boiler components, heating, forming and welding processes are commonly applied to the materials used. The effect of heating and strain induced by the these fabrication processes are very important in maintaining the integrity of creep strength enhanced steels, because it is well known that the mechanical properties of normalized and tempered high strength steels are relatively sensitive to thermal and work history. In particular, while Gr.91 steel has been used extensively for boiler and pressure components over the last two decade, the heat treatment and forming procedures for this steel are not fully established based on fundamental transformation and strain behavior. This paper deals with the phase transformation behavior of Gr.91 during tempering at around the Ac1 temperature, and the hardness/creep properties of Gr.91 steel heated at temperatures from just below Ac1 to above Ac3 combined with strain.

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