Influence of heat input and post-weld heat treatment on boiler steel P91 (9Cr–1Mo–V–Nb) weld joints Part 1 – Microstructure

Abstract

Steel P91 is known for its excellent high temperature properties. The achievement of optimum weld metal properties for steel P91 within the course of its extensive applications in power plants has however often caused concern. In the present work, three thick pipes of P91 steel were welded using three different levels of heat inputs within the range of 1·15–3·5 kJ mm⁻¹. A circumferentially multipass butt welded P91 steel pipe, typically used for high temperature applications in power plants was selected for this investigation. The achievements of optimum weld metal properties, which are closely linked to microstructure, are known to cause concern in such weldments. Two types of heat treatments were employed, subcritical post-weld heat treatment and normalising/tempering treatment. The microstructure was evaluated by optical, scanning electron microscope, X-ray diffraction and magnetic permeability. The results have shown a great influence of heat input and heat treatment on the microstructure. Martensite and ferrite were the main structures obtained. Bainite and δ-ferrite have been observed in the weld metal, heat affected zone and weld metal for all heat input/treatment conditions. The volume fraction of bainite and δ-ferrite increased with increase in heat input till a critical value slightly lower than 1·15 kJ mm⁻¹; then decreased with the increase in heat input. The normalising/tempering treatment resulted in a decrease in the volume fraction of δ-ferrite and bainite compared to the subcritical post-weld treatment which is conventionally used. This explains the enhancement in the toughness and creep properties of the steel presented in the second paper.

Keywords:

• Microstructures
• δ-ferrite
• Modified 9Cr–1Mo steel
• Welding
• Heat treatments
• Heat input

The authors wish to deeply thank the Project Services Company (PSC) for offering the materials and facilities available at their plant, without which this work would have not been possible.