Modified 9Cr-1Mo steel finds increasing application in power plant construction because of its excellent high-temperature properties While it has been shown to be weldable and resistant to all types of cracking in the weld metal and heat-affected zone (HAZ), the achievement of optimum weld metal properties has often caused concern The design of appropriate welding consumables is important in this regard In the present work, plates of modified 9Cr-1Mo steel were welded with three different filler materials: standard 9Cr-1Mo steel, modified 9Cr-1Mo, and nickel-base alloy Inconel 182 Post-weld heat treatment (PWHT) was carried out at 730 and 760 °C for periods of 2 and 6 h The joints were characterized in detail by metallography Hardness, tensile properties, and Charpy toughness were evaluated Among the three filler materials used, although Inconel 182 resulted in high weld metal toughness, the strength properties were too low Between modified and standard 9Cr-1Mo, the former led to superior hardness and strength in all conditions However, with modified 9Cr-1Mo, fusion zone toughness was low and an acceptable value could be obtained only after PWHT for 6 h at 760 °C The relatively poor toughness was correlated to the occurrence of local regions of untransformed ferrite in the microstructure

Microstructure and Mechanical Properties of Weld Fusion Zones in Modified 9Cr-1Mo Steel

Three thick pipes of modified 9Cr–1Mo steel were welded using three different levels of heat inputs: low heat input 1·1 kJ mm−1, medium heat input 2·3 kJ mm−1 and high heat input 3·16 kJ mm−1. Two types of heat treatments were employed, namely subcritical post-weld heat treatment and normalising/tempering treatment. The influence of heat input and post-weld heat treatment on the microstructure of boiler steel P91weld joints has been investigated in the previous paper. In the present work hardness, tensile, impact toughness and stress rupture properties were evaluated. Our results show a great influence of heat input and heat treatment on mechanical properties. The best combination of properties was obtained in low to medium range of heat input, between 1 and 2·8 kJ mm−1 for both treatments and a 90% increase in time to rupture was observed for normalising/tempering treatment compared to subcritical post-weld treatment.

Influence of heat input and post-weld heat treatment on boiler steel P91 (9Cr–1Mo–V–Nb) weld joints Part 2 – Mechanical properties

This chapter provides a review on some important welding considerations that need to be considered to minimize chances for in-service failures of three of the most important steels (austenitic, duplex, and creep-resistant steels) used in manufacturing parts of power boilers. The service conditions for power boilers provides the three necessary conditions (susceptible microstructure, state of stress, and corroding environment) causing stress-corrosion cracking to end the life of parts. The review shows how failure is an end result of wrong material selection, or wrong welding conditions that do not count for possible metallurgical changes. In addition to proper material selection, the study highlights the significance of considering and understanding metallurgical related issues before adopting the welding route and the selection of the welding parameters.

H. Heuser

Papers

Welding of modified 9% Cr steel : T91/P91 with optimised filler metals and welding parameters